Eindhoven University of Technology

MASTER

Initiated chemical vapor deposition of organosiliconesfrom the growth mechanism to multilayer moisture diffusion barriers

Palmans, J.

Award date:2011

Link to publication

DisclaimerThis document contains a student thesis (bachelor's or master's), as authored by a student at Eindhoven University of Technology. Studenttheses are made available in the TU/e repository upon obtaining the required degree. The grade received is not published on the documentas presented in the repository. The required complexity or quality of research of student theses may vary by program, and the requiredminimum study period may vary in duration.

General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.

• Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain

Eindhoven University of Technology

Department of Applied Physics

Plasma & Materials Processing

Under supervision of:

G. Aresta M.Sc.

Dr. M. Creatore

Prof.dr.ir. M.C.M. van de Sanden

Initiated Chemical Vapor Deposition

of Organosilicones:

From the growth mechanism to multilayer

moisture diffusion barriers

J. Palmans

May 2011 PMP 11-03

i

ii

Abstract

The state-of-the-art in moisture permeation barriers for flexible devices, such as organic

photovoltaics and organic light emitting diodes, is an organic/inorganic multilayer

system. Despite the several results available in literature, the role of the organic interlayer

in affecting the global barrier system properties is not yet understood.

This thesis aims to gain insight into this role by selecting a model system composed of a

plasma-deposited SiO2-like barrier layer and, as organic interlayer, an organosilicone

synthesized by means of initiated chemical vapor deposition (i-CVD). Both plasma and i-

CVD layers make use of a single precursor (1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane,

V3D3) and the processes have been carried out in a novel vacuum chamber.

The implementation of in situ, real-time spectroscopic ellipsometry (SE) allows

monitoring the film growth of the V3D3 polymerization process. In particular, when

applied to the bulk polymer growth, a transition from a reaction kinetics-limited (with

activation energy of 65 ± 4 kJ/mol) to a mass transfer-limited regime has been found. The

deposition process is also found to be monomer adsorption-limited with an activation

energy of -39 ± 4 kJ/mol. Furthermore an increasing substrate temperature affects the

polymer structure by promoting the transition from an unstable, low cross-linked to a

stable, highly cross-linked layer, as pointed out by infrared spectroscopy. By making use

of adsorption isothermal studies of the monomer on the substrate, the i-CVD process

window has been detected, showing that an adsorbed monomer thickness ≤ 0.10-0.15 nm,

is required to achieve stable organosilicone films, exhibiting no thickness loss upon

evacuation.

When SE is applied to the initial i-CVD stages, isothermal adsorption/desorption studies

provide insight into the microstructure of the SiO2-like barrier layer underneath,

characterized by a residual porosity in the micro/meso transition region (pore radius ≤ 2

nm).

This result implicitly points out the role of the i-CVD organic interlayer in multilayer

barrier structures, i.e. the filling of the open micro/meso porosity of the inorganic barrier

layer, therefore, improving the intrinsic barrier quality of the SiO2-like film underneath.

This outcome nicely correlates with the superior water vapor barrier performances (a

barrier improvement factor of 2400 is reported with respect to the pristine polymer) of

multilayers based on the application of i-CVD organic interlayers with respect to fully-

PECVD developed multilayers.

iii

iv

Table of contents

1 Introduction ............................................................................................................... 1 1.1 Barrier technology ................................................................................................ 1

1.2 Goal and approach ................................................................................................ 4 1.3 Outline of the thesis .............................................................................................. 5

2 Chemical Vapor Deposition ..................................................................................... 7 2.1 Initiated Chemical Vapor Deposition ................................................................... 7

2.1.1 i-CVD vs. conventional polymerization processes ....................................... 7

2.1.2 Polymerization mechanism of i-CVD processes .......................................... 7 2.1.3 Key parameters of the i-CVD process ........................................................ 10

2.2 Plasma-Enhanced CVD of SiO2-like and SiOxCyHz films ................................. 17

2.2.1 Tuning the organic or inorganic character of RF PECVD grown layers .... 17

3 Experimental set-up and diagnostic tools ............................................................. 21 3.1 Experimental set-up ............................................................................................ 21 3.2 Diagnostic tools .................................................................................................. 25

3.2.1 Spectroscopic Ellipsometry ........................................................................ 25 3.2.2 Fourier Transform Infrared Spectroscopy .................................................. 28

3.2.3 Atomic Force Microscopy .......................................................................... 29 3.2.4 Rutherford Backscattering Spectrometry / Elastic Recoil Detection .......... 30 3.2.5 Water Vapor Transmission Rate measurement ........................................... 31

4 Results ...................................................................................................................... 35 4.1 Study of the i-CVD process ............................................................................... 35

4.1.1 (V3D3)- i-CVD polymer layers: a detailed IR analysis ............................... 35

4.1.2 Role of process parameters on p(V3D3) layers ........................................... 38

4.1.3 Adsorption/desorption study of V3D3 monomer ......................................... 41 4.2 Growth and process window definition of p(V3D3) layers ................................ 45

4.2.1 In situ spectroscopic ellipsometry studies of i-CVD polymer films –

Observation of film thickness reduction ..................................................... 45 4.2.2 Time evolution study of the thickness losses .............................................. 47

4.2.3 Effect of process parameters on the thickness losses .................................. 49 4.2.4 Process window definition .......................................................................... 53

4.3 Study of RF PECVD depositions from V3D3 ..................................................... 54

4.3.1 Deposition of SiO2-like and SiOxCyHz type layers from V3D3 monomer .. 55 4.3.2 Comparing i-CVD and PECVD organic layer properties ........................... 57

4.4 Multilayer diffusion barrier systems .................................................................. 58

5 Conclusions and recommendations ....................................................................... 65 5.1 Conclusions ........................................................................................................ 65 5.2 Recommendations .............................................................................................. 66

List of abbreviations and symbols ................................................................................. 67

Bibliography .................................................................................................................... 69 Acknowledgements ......................................................................................................... 75 Appendix A ...................................................................................................................... 77 Appendix B ...................................................................................................................... 79 Appendix C ...................................................................................................................... 81 Appendix D ...................................................................................................................... 83 Appendix E ...................................................................................................................... 85

v

1

1 Introduction

1.1 Barrier technology

Many industrial applications make use of polymers as they are versatile materials due to

their beneficial properties over other materials, such as metals and ceramics. Among

these advantages, a few are here reported: lightweight, flexibility, transparency, cost

reduction, and compatibility with roll-to-roll processing [1-4]

. This makes polymers useful

for a variety of applications, i.e. from automotive [1]

to packaging [5]

, from flexible solar

cells [6, 7]

to flexible electronics [1-4, 8, 9]

.

Although there is a wide range of applications for polymers, there are also some

drawbacks, namely its glass transition temperature Tg which limits the processing

temperature during the various production steps [4, 10, 11]

, and the high permeation rate to

gases and moisture which induces the device degradation and the shelf-life of the packed

food/drink. [2, 6, 8, 12]

.

The barrier requirements for industrial applications can vary considerably, as indicated in

Fig. 1-1. Since uncoated polymers can only provide transmission rates in the range of 1

g/m2day (water vapor transmission rate, WVTR) and 100 cm

3/m

2day·atm (oxygen

transmission rate, OTR), the research and development in the field of moisture and gas

barrier layer deposition and device encapsulation is very much active [13]

.

Fig. 1-1: Maximum allowed transmission rates (WVTR and OTR) for a variety of applications.

The state-of-the-art in encapsulation using single barrier layers is still limited (indicated by red

line).

Typically a thin (from a few tens up to few hundred nm) barrier layer consists of a

ceramic (SiOx, SiNx, AlOx) material deposited by means of various techniques (e.g.

Plasma enhanced-CVD, sputtering, evaporation, atomic layer deposition) [6, 8, 10, 12-16]

.

Although their bulk shows excellent water and oxygen permeation barrier properties (e.g.

~10-15

g/m2day for AlOx

[8]) the above-mentioned thin layers exhibit lower performances,

typically reaching a minimum in WVTR of 10-2

-10-3

g/m2day, depending on the testing

conditions (temperature and relative humidity) as well as layer properties (chemistry,

density, residual stress), therefore hampering the application of the single layer barrier

2

technology to high end devices (Fig. 1-1). As shown in Fig. 1-2, the thin layer barrier

performance strongly correlates with the density of defects present at the polymer surface

prior to deposition (pinholes, scratches, dust and anti-static and/or filler particles) or

developed during film deposition (dust and large oligomers generated in the plasma

phase) [7, 9, 17, 18]

. These defects (with a radius in the range of few tens of nm to μm)

represent discontinuity regions in the layer and preferred paths of permeation for water

and oxygen molecules with respect to the bulk permeation of the inorganic layer. The

defects may be conformally coated (encapsulated) by the barrier layer leading to orders

of magnitude improvement in terms of barrier performance up to a barrier layer thickness

defined as critical thickness, above which no further improvement is observed.

Fig. 1-2: Defect density and OTR as a function of the coating thickness of PECVD deposited SiO2

on 13 μm PET. The minimum OTR of 1.5 · 10-12

cc/m2-s-cm Hg corresponds to 0.1 cm

3/m

2day

[9].

In order to address the defect presence, two approaches can be generally followed. The

first consists in controlling the quality of the polymer surface in terms of roughness and

contamination, which in general requires the application of a so-defined smoothening

layer, e.g. the BarixTM

multilayer coating (Fig. 1-3), on the polymer surface and a clean

room working environment.

Fig. 1-3: SPM image of uncoated PET and after BarixTM

coating. Planarization of the surface

features is observed after the coating reducing the peaks from 150 Å to < 10 Å [1]

.

3

Table 1-1: Overview of multilayer barrier coatings and its barrier properties. Substrate/Multilayer

layer type

Deposition

method

Barrier

material

Organic

interlayer

WVTR

(g/m2day)

Reference

Polycarbonate/Ultrahigh

barrier hybrid* layer

PECVD SiOxNy SiOxCy 8.6(±0.3)·10-6

3

Polycarbonate/(SiOx/SiNx)n PECVD SiOx and

SiNx - 3.54 ·10

-5 20

Poly(ethylene

terephthalate)/(Polyacrylate/

Al2O3)

Evaporation Al2O3 Polyacrylate < 10-5

1

Poly(ethylene

terephthalate)/(SiOx/Hybrid)

Electron

beam

evaporation

SiOx Hybrid 0.0019 13

Poly(ethylene

terephthalate)/(AlOx/Hybrid

)

Magnetron

sputtering AlOx Hybrid 0.02 13

*Material consisting of a combination of (a minimum of) two materials having different nature, and

therefore it combines the properties of each material with the possibility of having an added value in terms

of optical, thermal and mechanical properties compared to the single materials.

The second approach is to engineer appropriate multilayer systems consisting of

inorganic barrier layers sandwiched between organic (often acrylate- or silicon- based)

layers [1-4, 8-10, 12, 15, 17, 19]

. Although this approach massively reduces the defect density and

increases the shelf-life of the encapsulated device, the optimization of a multilayer is

rather empirical as the mechanisms behind the improvement of the global barrier

performance are not yet unraveled. For example, it is often claimed that the organic

interlayer acts as a decoupling layer between the defects present in two inorganic barrier

layers, therefore, delaying the molecule permeation, as modeled in Fig. 1-4 [19, 20]

. This

may be a plausible explanation in the case a defect present in a layer does not develop

any further in the subsequent layer, and it is therefore dependent on the adopted

deposition methods for the inorganic and the organic layers. In case of a PECVD

approach for both layers, for example, this is very unlikely to occur, whereas if the

vacuum deposition is chosen (monomer condensation on the chilled polymer surface

followed by cross-linking via UV, plasma or electron beam), it can be argued that the

organic interlayer will either fill or conformally coat the source of defect (see Fig. 1-5) [8]

.

Nevertheless an improvement in barrier performance is observed also when a fully based

PECVD approach is adopted, as summarized in Table 1-1 [1, 3, 13, 21]

.

Fig. 1-4: Schematic illustration of the diffusion length l, through the polymer interlayer. The path

length is much longer than the polymer thickness t, which is also much smaller than the defect

spacing s [19]

.

4

Fig. 1-5: Smoothing effect of the BarixTM

multilayer coating as deposited on top of an OLED

device. Improvement is obtained by increasing the number of dyads [22]

.

1.2 Goal and approach

On the basis of the overview presented in 1.1, this master thesis work focuses the

attention on two different multilayer systems engineered by means of:

- plasma deposition of inorganic barriers and organic interlayers;

- plasma deposition of inorganic barriers and initiated chemical vapor deposition (i-

CVD) of organic interlayers,

where the i-CVD process is a dry, low temperature compatible, vacuum polymerization

process, which allows the full retention of the organic precursor chemistry.

In this project, the i-CVD process has been developed and studied in a novel vacuum

chamber in terms of bulk material properties as well as mechanisms of film growth

identified by means of in situ real-time surface diagnostics. Furthermore, the above-

mentioned multilayers have been compared in terms of water vapor diffusion barrier

performances and the role of the organic interlayer has been highlighted.

In particular, the following research questions were addressed:

Can the application of in situ spectroscopic ellipsometry and complementary

diagnostics allow the study of the i-CVD film growth mechanism?

How do the key process parameters affect the i-CVD film growth and can this

lead to the i-CVD process window definition?

Is the initial step in the i-CVD growth process, i.e. monomer adsorption, affected

by the substrate type and can it reveal the microstructure of these substrates?

How does the i-CVD growth process affect the multilayer moisture diffusion

barrier development and performance compared to a PECVD deposited

multilayer?

5

1.3 Outline of the thesis

6

7

2 Chemical Vapor Deposition

2.1 Initiated Chemical Vapor Deposition

2.1.1 i-CVD vs. conventional polymerization processes

The implementation of polymer substrates for (transparent) flexible devices is

accompanied by diverse thin film deposition technologies ranging from wet chemistry-

based processing to chemical vapor deposition (CVD), depending on the demand on the

thin film properties, up-scaling and deposition rate/cost issues. Limiting the present

discussion to the case of thin (from tenths of nm to m) organic film (e.g. polymer)

deposition, the above-mentioned techniques are considered to be complementary since

the wet chemistry approach allows a quantitative retention of the chemical structure of

the monomeric unit, while CVD-based techniques allow the tuning of the film chemistry

from full retention to a massive rearrangement of the chemical structure of the original

monomer unit with the additional advantage of being vacuum compatible. In particular,

initiated-CVD (i-CVD), a dry-chemistry, free-radical chain growth, polymerization

process, has been recently acknowledged [23, 24, 25]

as successful tool to deposit polymer

layers under vacuum conditions, in contrast to the conventional methods, and with full

retention of the monomer structure, exhibiting several advantages such as conformality

(of interest in several applications such as biopassive dielectric [23]

, antimicrobial [26]

and

low-k materials [27]

), solvent-less processing, and compatibility with thermally sensitive

substrates [23, 28, 29]

.

The application of the i-CVD process is especially interesting when compared to the

plasma-enhanced CVD (PECVD) polymerization processes which are also characterized

by growth steps involving radicals formed in the reactive plasma medium and which are

then transported towards the surface of the growing layer. The film chemistries (and the

layer properties in general) are virtually endlessly tunable since they depend on the

chemistry of the radicals (and therefore the dissociation paths in the plasma phase) and

the surface reaction probability (i.e. sticking and recombination).

In this master project work, both the i-CVD and plasma-based CVD processes will be

implemented, tested and investigated for the deposition of organosilicone-based polymer

layers in a novel vacuum deposition chamber.

2.1.2 Polymerization mechanism of i-CVD processes

The polymerization mechanism of the i-CVD process is basically similar to the

conventional polymerization methods, so consisting of three steps: initiation, propagation

and termination. The main difference is the vacuum compatibility of the process because

the two precursors, called monomer and initiator, are injected simultaneously as a gas

vapor into the deposition chamber. As indicated in the schematic of the reaction

mechanisms as proposed for the i-CVD process by Lau and Gleason [30, 31]

(Fig. 2-1),

primary radicals are generated by the decomposition of the labile bonds of the volatile

initiator molecules near the resistively heated grid. The monomer molecules on the other

hand remain unaffected as they are thermally stable at the applied grid temperatures (523-

773 K).

8

Fig. 2-1: The reaction mechanisms for the i-CVD polymerization process, starting from the gas

phase reactions, followed by the gas-to-surface processes and finally the surface reactions. The

polymerization process takes place during this last step [30, 31]

.

Both primary radicals and monomer units diffuse subsequently towards the substrate

surface due to a concentration gradient (for the initiator radicals) or a temperature

gradient (cooled substrate) (for the monomer units). The monomer adsorption onto the

cold substrate is dependent on the applied conditions (substrate temperature, pressure, gas

flow rates) as will be explained in more detail in section 2.1.3. The initiator radicals will

also adsorb onto the substrate surface where they can either diffuse on the surface, desorb

back into the vapor phase after some time, or they can react with a monomer unit or

polymeric chain. However, it has been demonstrated by [32] that the sticking probability

is related to the surface monomer concentration (which in the following section will be

related to the dimensionless parameter PM/Psat [30, 31]

) and it has been found to be similar

for various monomers. Thus, when reaching the surface, the primary radicals attack the

labile, vinyl groups of the monomer and subsequently initiate the polymerization process.

By the successive addition of monomer molecules the polymer chain can propagate. The

polymerization is assumed to propagate on the cooled substrate only, since it is

hypothesized that the high molecular weight and steric hindrance of the monomer can act

as a barrier (due to the low volatility) to the gas phase polymerization, implying that

radical generation is the only gas phase reaction. This hypothesis has been confirmed by

the studies of Chan and Gleason [28]

by combining i-CVD and Quartz Crystal

Microbalance (QCM) experiments (addressed in the section “Monomer adsorption”),

showing a linear relation for both deposition rate and molecular weight with the

equilibrium surface monomer concentration. Furthermore, a second requirement for gas-

phase propagation to occur would be the presence of bimolecular reactions at a vacuum

pressure of few tenths of a mbar. As the gas-phase concentrations are typically low (~10-5

9

M), the probability of having a third-body collision required for conservation of energy is

limited. The termination phase contains three possible reactions: coupling of polymer

chains, primary radical termination through the attack of a polymer radical and primary

radical recombination. The role of both the monomer adsorption and the substrate

temperature on the i-CVD process are highlighted in the next section as they are key

parameters for the i-CVD polymerization method.

In this work the deposition of organosilicone films is demonstrated by using a cyclic

siloxane monomer, 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3D3). To initiate the

polymerization reaction di-tert-butyl peroxide (d-TBPO) is used as initiator. An

illustration of the hypothesized polymerization process, considering only one vinyl group,

is shown in Fig. 2-2.

Fig. 2-2: Polymerization mechanism as proposed for the growth of p(V3D3) organosilicone films.

The final polymeric layer consists of siloxane rings, acting as cross-linking moieties for the

carbon backbone chains throughout the polymer matrix [23]

.

The polymer formation occurs through the successive addition of monomer units or

polymer chains across one of the three vinyl bonds present in the monomer. Because of

the trifunctionality of the V3D3 molecule, a densely cross-linked network is obtained

when the majority of the vinyl groups have reacted. For the chain termination the

recombination of growing polymer chains is less probable than the end-capping with

another peroxide radical due to the steric hindrance of the monomer [23]

. Therefore the

chain mobility of the polymer chains is further constrained if each monomer unit is

involved in multiple polymer chains. It will be shown in Chapter 4 that the ratio of long,

high molecular weight, cross-linked chains and short, low molecular weight chains can be

tuned by varying the key deposition parameters, especially the substrate temperature.

The polymerization of p(V3D3) by means of i-CVD can be demonstrated by comparing

the Fourier Transform Infrared (FT-IR) spectrum of the polymer with that of the liquid

monomer. Both monomer and polymer spectra were measured in this thesis’ work, and

are reported in Fig. 2-3. The assignment of the absorption bands for both monomer and

polymer is given in Appendix A, based on the assignments proposed in literature for

similar studies [23]

. By making the comparison between the spectra, information about the

polymerization process and the polymer chemical structure can be obtained. More

10

specifically, the two main characteristics of the i-CVD process can be demonstrated,

namely the cleavage of the labile vinyl bonds upon polymerization and the retention of

the monomer functionality ((SiO)3 siloxane ring and Si-CH3 groups) in the i-CVD

process. A detailed explanation of the absorption bands is provided in Chapter 4, in

which it is demonstrated which additional information can be obtained from the FT-IR

analysis.

4000 3600 3200 2000 1600 1200 800 400

CH2

{{

Si-CH3

Si-O-Si a

CHx

p(V3D

3)

Abs (

a. u.)

Wavenumber (cm-1)

V3D

3

CHx

Si-CH3

C=C

CH2

CH2

a

Fig. 2-3: FT-IR spectrum of the V3D3 monomer and the resulting i-CVD polymer. The presence of

the Si-O-Si absorption bands in both spectra indicates the retention of the monomer siloxane ring

structure and methyl groups bond to silicon. The cleavage of the vinyl bonds upon polymerization

is observed by the decrease of the vinyl related signals.

2.1.3 Key parameters of the i-CVD process

The principle of the i-CVD polymerization process is demonstrated in the previous

section. In order to understand how the polymerization process can be controlled, the key

parameters of the process are highlighted in this paragraph based on literature studies

performed so far. Basically there are three experimental parameters that play a key role:

the grid temperature, the surface monomer adsorption and related the substrate

temperature as it also affects the monomer adsorption.

Effect of the grid temperature on the polymerization process

In Fig. 2-4 a typical Arrhenius plot of the logarithm of the deposition rate as a function of

the reciprocal grid temperature is shown as reported in literature. A transition between

two regimes is observed and, in particular, in the work of O’Shaughnessy et al. [23]

this is

demonstrated for p(V3D3) layers, where a transition takes place around 673 K. This

temperature corresponds to a transition from a regime in which gas-phase reactions, i.e.

initiator radical formation in the proximity of the grid, are limiting the deposition process

(referred to as reaction-kinetics limited regime), to a regime in which mass-transport

becomes the limiting factor (mass-transport limited regime), i.e. the diffusion of both

monomer and initiator species towards the substrate. The apparent activation energy

11

corresponding to the reaction-kinetics limited regime, is determined by applying the

Arrhenius equation,

RTE

d

a

eAkR

~ (2.1)

where k represents the reaction rate, A is a pre-exponential factor, R is the gas constant

and T the temperature.

For the activation energy calculation in the reaction-kinetics limited regime one is

restricted to an apparent Ea, due to the inability of separating gas-phase concentrations

(affected by the filament temperature) of the species that can contribute to the deposition

rate, from the reactant concentration on the surface which would give rise to a kinetic rate

constant. Since these two competing processes are both contributing to the deposition

rate, the calculated activation energy cannot be entirely attributed to the kinetic rate

constant, implying the use of an apparent activation energy which considers the whole

process. A value of 28 kJ/mol is found compared to an activation energy of ~160 kJ/mol [33, 34]

for the initiator decomposition, suggesting that radical formation in the gas phase is

not likely the rate-limiting step in this polymerization process. Instead, the propagation

reaction is expected to dominate the polymerization rate. The discrepancy between the

apparent activation energy and the initiator dissociation energy was addressed by

Ozaydin-Ince and Gleason for the ethylene glycol diacrylate monomer [33]

by considering,

as a correction factor, the temperature profile from the grid to the substrate. Based on

simulations for the specific reactor length scales and design (flat reactor with flows

parallel to substrate) a logarithmic temperature profile was assumed away from the

filaments towards the cold substrate. By taking into account this temperature profile, the

logarithmic mean temperature was determined and used instead of the filament

temperature, in order to calculate the activation energy. A value of apparent activation

energy in the reaction-kinetics limited regime of 166 kJ/mol, (compared to a value of

93.2 kJ/mol obtained without considering the temperature profile correction) was found

in agreement with the initiator dissociation energy (~160 kJ/mol), indicating that in the

reaction-kinetics limited regime the generation of radicals is the rate limiting factor.

Fig. 2-4: Deposition rate for the p(V3D3) polymer growth as a function of the grid temperature as

reported by O’Shaughnessy et al. [23]

.

12

In order to preserve the monomer structure it has also been demonstrated that in the

mass-transfer limited regime, at too high filament temperatures (≥ 873 K) ring opening

can appear, meaning that the polymerization process propagates through linear Si-O

chains measured around 1080 cm-1

with FT-IR, instead of through the vinyl groups only.

Associated herewith is a decreasing intensity for the Si-O-Si ring absorption band around

1012 cm-1

as the probability of ring opening is higher than its retention. The operation at

lower grid temperature is therefore recommended if the monomer functionality is aimed

to be retained.

Effect of the surface monomer adsorption on the polymerization process

Another key parameter in the i-CVD polymerization method affecting the film growth is

the monomer adsorption, which can be represented by a dimensionless parameter PM/Psat

as illustrated by Lau and Gleason based on Quartz Crystal Microbalance (QCM)

experiments [30, 31]

with which the surface monomer concentration has been measured by

the change in molecular weight. The parameter PM/Psat is process controllable since it is

composed of the ratio between the monomer partial pressure PM ( pPtot

MM

, with ΦM

the monomer flow rate, Φtot the total flow rate and p the deposition pressure) and the

temperature dependent saturated vapor pressure Psat of the monomer. The saturated

monomer vapor pressure Psat can be calculated once the heat of vaporization ΔHvap is

determined based on the boiling point (T1 = 462.6 K at P1 = 1013.250 mbar) and the

vapor pressure at a given temperature (P2 = 0.571 mbar at T2 = 298 K) for the V3D3

monomer [35, 36]

, by the application of the Clausius-Clapeyron equation (Eq. 2.2).

122

1 11ln

TTR

H

P

P vap (2.2)

with R the gas constant. A heat of vaporization ΔHvap of 52 kJ/mol has been calculated.

By putting the values of ΔHvap, P2 and T2 in Eq. 2.2, the saturated vapor pressure can be

determined for every substrate temperature.

The relation between adsorbed monomer surface concentration and PM/Psat can be

generally described by numerous mathematical relations for adsorption isotherms, e.g.

the Brunauer-Emmett-Teller (BET) equation (Eq. 2.3) [37]

. This can be applied for the i-

CVD process to express the amount of surface monomer adsorption as a function of

PM/Psat.

sat

M

sat

M

sat

Mml

ad

PP

cP

P

PP

cV

V

111

(2.3)

where Vad is the total adsorbed volume, Vml is the monolayer adsorbed volume and c the

BET constant giving an indication of the magnitude of the adsorbent-adsorbate

interaction.

The adsorption/desorption experiments performed by using a QCM were based on the

monomer adsorption on a temperature-controlled gold-coated quartz sensor crystal. These

13

measurements revealed the normalized surface monomer concentration, determined from

the frequency change of the oscillating QCM, as a function of the QCM/substrate

temperature, monomer partial pressure or monomer vapor pressure [28, 30, 31]

. Since the

QCM and i-CVD experiments were performed under similar conditions showing similar

trends as a function of the QCM/substrate temperature, both deposition rate and

molecular weight have also been expressed as a (linear) function of the normalized

surface concentration, which is related to PM/Psat as well according to the BET equation.

The strong correlation between QCM and i-CVD experiments is also indicative of the

propagation process being most likely surface related.

Fig. 2-5: a) Deposition rate for the i-CVD of ethyl acrylate as a function of the monomer

adsorption given by PM/Psat, which has been varied by changing the substrate temperature [30]

.

The three curves correspond with various grid temperatures. b) Adsorption isotherm of butyl

acrylate measured by means of QCM and fitted to a BET equation [31]

.

An example of the dependence of the deposition rate on PM/Psat is shown in Fig. 2-5a for

the i-CVD of ethyl acrylate for various grid temperatures. In this small range of PM/Psat

the deposition rate shows a nonlinear, second order dependence on PM/Psat, explained by

the radical termination of the growing polymer chains, slowing down the polymerization

rate. However, the adsorbed volume/areal concentration can be related to PM/Psat as well

by performing the adsorption measurement on the QCM, for example for butyl acrylate

(Fig. 2-5b) as it shows a larger range of PM/Psat than for ethyl acrylate also containing the

nonlinear part for higher PM/Psat. A BET equation has been fitted to the obtained

adsorption isotherm.

The linear relation found for low PM/Psat can be explained by considering a limiting form

of Eq. 2.3 (Henry’s law limit) [38]

,

sat

Mml

PPad P

PcVVM

sat

M 0~ (2.4)

where [M] is the surface monomer concentration showing a linear proportionality with

the dimensionless parameter PM/Psat [30]

.

The applied BET equation is only valid for isotherms of type II (nonporous or macro-

porous solids), as is the case for the i-CVD experiments reported above. An overview of

(b) (a)

14

the six types of adsorption (physisorption) isotherms as classified by IUPAC, is presented

in Fig. 2-6 [39]

.

Fig. 2-6: Various types of adsorption isotherms [39]

.

The shape of the isotherms depends on the microstructure of the adsorbent and the

interaction with the adsorbate. By performing adsorption measurements the

microstructure of the considered solid can be determined from the isotherm shape.

Among the various isotherms three types: type I, II and IV, are of special interest in this

thesis. A linear relation for the amount adsorbed is found in many cases when

considering low surface coverage (PM/Psat), corresponding with Henry’s law limit as

mentioned before.

The type I isotherm, often referred to as Langmuir isotherm, has a steep initial increase

due to the filling of the micro-pores of the solid. When increasing the relative pressure

towards 1, the adsorption levels off at a constant value. This isotherm type is related to

micro-porous solids having relatively low external surfaces compared to the internal pore

surface.

An isotherm of type II also has the fast initial increase towards an inflection point (B),

indicative of monolayer adsorption. This is a typical isotherm related to nonporous or

macro-porous absorbents as it is not restricted to monolayer adsorption but can have an

unlimited multilayer adsorption for high relative pressure starting once the inflection

point is passed.

Related to the type II is the type IV isotherm, showing a similar initial adsorption part to

the type II isotherm, with the inflection point indicating the transition from monolayer to

multilayer adsorption. However, for high relative pressures there is a limited uptake and

hysteresis, due to differences in adsorption and desorption, associated with adsorption in

meso-pores instead of micro-pores for the type II isotherm.

15

Other adsorption isotherms of type III and V, containing a convex behavior indicative of

incomplete monolayer coverage due to the adsorbate-adsorbate interaction dominating

over the adsorbate-adsorbent interaction, are rare.

The type VI isotherms at last, showing a stepwise multilayer adsorption, are related to

uniform nonporous surfaces. The step-height represents the monolayer capacity for each

adsorbed layer and can remain constant for two or three adsorbed layers.

The presence of high pressure hysteresis in isotherms is determined by the pore structure

or network. In general four types of hysteresis are reported by IUPAC [39]

although the

understanding of the origin of these is still under debate. Besides this hysteresis at high

pressure, many systems contain low pressure hysteresis, related to the irreversible uptake

of molecules in pores having comparable width as the adsorbate molecule or with an

irreversible chemical interaction of the adsorbate with the adsorbent.

The importance of monomer adsorption in the i-CVD polymerization process will be

addressed in Chapter 4 as well as the choice of the substrate (Si or SiO2-like) underneath

as this can be relevant for multilayer diffusion barrier systems in which i-CVD and

PECVD are deposited alternately.

Since the data reported in literature requires the combination of QCM and i-CVD

experiments, as separate experiments do not provide sufficient information, in order to

link monomer adsorption to the dimensionless parameter PM/Psat and the deposition rate,

it is preferable to have a single diagnostic tool, in situ SE, that can monitor the i-CVD

process throughout all its stages: initial monomer adsorption, film growth and end of the

deposition process; This allows to gain insight into the relation between the monomer

adsorption, the layer growth and optical properties. The implementation of in situ SE to

study the i-CVD process will be addressed in Chapter 4. The real-time monitoring by

means of SE is advantageous over the combinatorial use of laser interferometry (real-time

monitoring of thickness) and ex situ SE [30, 33]

, as with in situ SE both optical properties

and thickness can be determined for every process step, providing better process control.

Effect of the substrate temperature on the polymerization process

Up to now the focus was directed to the monomer adsorption and deposition rate in

relation to PM/Psat. It was already shown that the monomer and total flow rate, and also

the deposition pressure can alter the monomer adsorption as they change the monomer

partial pressure. Instead, the substrate temperature can be varied as well in order to tune

the monomer vapor pressure and therefore the monomer surface concentration. At this

point the role of the substrate temperature is addressed briefly as a more detailed

discussion is provided in section 4.1.2. The two effects induced by the substrate

temperature which affect the polymerization process are the following:

1) With increasing temperature the adsorbed monomer surface concentration

decreases, when the monomer partial pressure is kept constant.

2) By increasing the temperature, the surface mobility of the adsorbed monomer

molecules is enhanced which (for the i-CVD process) will improve the

polymerization cross-linking.

16

The first effect of monomer adsorption is related to the PM/Psat parameter as already

demonstrated in literature. Since the monomer concentration at the surface will decrease

with the temperature, the initiation and propagation probability of the monomer,

respectively polymer chains, is reduced as the intermolecular distances are enlarged.

However, by increasing the temperature, thermal energy is transferred to the adsorbed

molecules enhancing the diffusion [40, 41]

if the thermal energy is sufficient to overcome

the existing diffusion barrier according to the Arrhenius equation,

Tk

E

B

diff

e

(2.5)

where Γ is the hopping rate, ν the vibrational frequency, Ediff the potential energy barrier

to diffusion, kB the Boltzmann constant and T the temperature. Due to the improved

surface mobility the cross-linking degree can be enhanced as well [42]

, since the

probability of two chain ends to meet, is increased [43]

. Because molecules always strive

to a state of minimum energy, on a plane substrate the adsorption also tends to develop

layer-by-layer instead of having local multilayer adsorption.

In order to check whether the monomer adsorption is a limiting factor in the i-CVD

process, the activation energy can be determined from a log-scale plot of the deposition

rate vs. the reciprocal temperature by applying the Arrhenius equation. This activation

energy has been shown to be negative for layers deposited by i-CVD, p.e. for p(V3D3)

(Fig. 2-7) [23]

. In general one would expect an increasing deposition rate for higher

temperatures, since more heat energy is added, facilitating the energy barrier, existing for

a reaction to occur, to be overcome. As a consequence of the inverse behavior, the

polymerization reaction can be considered as an adsorption-limited process.

Fig. 2-7: Relation between the deposition rate (log-scale) and the reciprocal substrate

temperature for p(V3D3) layers deposited by i-CVD. An Arrhenius fit is performed to determine

the activation energy of the process [23]

.

A detailed discussion of the polymerization process activation energy as function of the

substrate temperature is provided in Chapter 4 together with the comparison between the

results obtained in this thesis’ work and literature, for both i-CVD and plasma deposition

processes.

17

2.2 Plasma-Enhanced CVD of SiO2-like and SiOxCyHz films

The i-CVD process, as described in the previous paragraph, represents a special, dry-

chemistry, vacuum compatible technique, dedicated to the deposition of polymeric films

with the complete retention of the monomer chemistry (i.e the monomer functionality)

resulting in conformal film growth. A way of tailoring the monomer chemistry of the

deposited polymeric layer is represented by the plasma-enhanced CVD process

(PECVD). Indeed, in the case of the organosilicone-based layers, by controlling the key

plasma parameters such as, the input power, the gas composition, pressure and the

precursor (i.e. the monomer) dilution in the other reaction gases (for example, argon and

oxygen), the layer chemistry can be tuned in a wide range in order to obtain an organic

(i.e. SiOxCyHz) or an inorganic (i.e. SiO2-like) layer. The deposition of SiO2-like layers

by means of PECVD has been widely studied because of their specific layer properties

(optical transparency in the visible range, recyclability and microwave ability) which

made these layers a suitable candidate for the implementation in food and pharmaceutical

packaging, microelectronics as well as for anti-scratch and anticorrosion systems for

metals [5, 16, 25, 44]

. SiOxCyHz layers deposited by means of PECVD techniques find their

application as low-k materials, biomaterials and organic interlayers for moisture diffusion

barrier systems [45, 46]

.

In this paragraph a brief description on the tuning of the radio frequency (RF) PECVD

technique is presented, as this is a widely applied technique for the deposition of both

inorganic and organic films and which is also applied throughout this master thesis

project. The fact that only a single monomer (V3D3) is required for both plasma

processes, and its additional usage in the i-CVD process, makes it advantageous for the

combinatorial use of i-CVD and PECVD methods in order to deposit a multilayer

moisture diffusion barrier system. In the following section the tuning of the layer

properties between organic and inorganic is highlighted based on changes in the process

parameters. The general principles and fundamentals of a plasma and of the RF PECVD

technique, in comparison with other plasma deposition techniques, are reported in

Appendices B and C.

2.2.1 Tuning the organic or inorganic character of RF PECVD grown layers

The film growth during a plasma deposition process can be easily tuned from silicone

(SixOyCzHt) to SiO2-like by the deposition parameters [16, 25, 47-54]

. Since many

publications on hexamethyldisiloxane (HMDSO) are available, containing studies on the

monomer fragmentation, it can be considered as a good example to understand the

proceeding of a plasma deposition process. Since the film growth is determined to large

extent by the radical generation in the plasma phase, a schematic (Fig. 2-8) of the various

reacting species and reaction products is shown. The present molecular fragments can be

distinguished between the gas-phase and the surface reactions of an HMDSO plasma

diluted in oxygen [55-57]

.

18

Fig. 2-8: Schematic diagram of the reaction pathways in HMDSO plasma diluted in oxygen [56]

.

The generation of radicals due to partial fragmentation is caused by electron impact as the

plasma is sustained by applying a power. By choosing the power (density) the degree of

monomer fragmentation can be tuned as the electron energy and/or density is varied as

well as oxygen atom production. Various types of carbonated radicals can be produced

therefore in the gas-phase, i.e. CHx, SixOyCzHt, SiCxHy and SiOCxHy. The fragments can

subsequently reach the substrate surface and constitute the growing layer. An increasing

power can also give rise to additional fragmentation, resulting in especially lower mass

fragments, and so consequently leads to various species amongst others CH3, CH4 and

H2O. The role of the power is not only being an additional energy source for an enhanced

fragmentation, but it also improves the ion bombardment leading to etching of the

growing layer, resulting in the removal of mainly carbon. Since a layer can also contain

chain terminating silanol (Si-OH) groups, which introduce residual porosity into the

layer, an increase in the applied power can reduce the silanol content of the layer by

closing the build-in pores and therefore enhance the barrier performance as it will be

demonstrated in Chapter 4 that the incorporation of porosity affects the layer properties.

The layer density is also enhanced if the void fraction is reduced, generally achieved by

increasing the substrate temperature [58, 59]

. Since the fragmentation can vary significantly

dependent on the deposition conditions, the deposited layer can contain less or more

carbon and hydrogen, approaching a more inorganic or organic layer type. An illustration

19

(a) (b)

of the effect on the layer chemistry by varying the RF power is given in Fig. 2-9a for a

HMDSO/O2 gas mixture [57]

.

Fig. 2-9: a) Effect of the RF power on the layer chemistry for a constant O2/HMDSO ratio and

pressure. b) Role of the O2/HMDSO ratio on the layer chemistry keeping the power and pressure

constant [57]

.

Besides the power also the monomer dilution in oxygen affects the carbon contribution to

the growing layer. When the oxygen/HMDSO ratio is high (Fig. 2-9b) [57]

, the carbon

contribution can be reduced considerably as the oxidation efficiency (more SiO and CO2

production than Si and CH) and carbon etching are enhanced (middle and bottom part of

schematic in Fig. 2-8) [16, 25, 47]

. The layer’s character tends to be more SiO2-like [52]

,

while a low dilution results in organic type layers with a layer stoichiometry given by

SiOxCyHz. In order to obtain inorganic layers both high power and/or high monomer

dilution are desired, while the organic character is preserved for low dilution or even no

oxygen contribution, and a low power input.

Since specific monomers can possess certain functionalities (e.g. siloxane ring in V3D3),

these can only be preserved by carefully controlling the deposition conditions, although

in plasma processes the radical generation is based on fragmentation of the monomer

molecules in the gas-phase. The role of structural retention will be addressed in Chapter 4

when making the comparison between the i-CVD and PECVD polymerization processes.

For a polymeric layer the propagation consists of complex chemical reactions between

various, non-specific radical fragments on the surface. The repetition of monomer units

will therefore be less pronounced than in the i-CVD process which is based on the

propagation by the addition of unreacted monomer units adsorbed onto the surface. In

terms of cross-linking the plasma polymer generally possesses a lower degree although

this can be tuned by the substrate temperature [51, 60]

.

20

21

3 Experimental set-up and diagnostic tools

In this chapter the experimental set-up and the ex situ and in situ diagnostics are

described. The experimental set-up is designed to perform both i-CVD and PECVD

processes in a single deposition chamber. The film growth and optical constants are

monitored real-time by in situ spectroscopic ellipsometry (SE). Complementary to the in

situ diagnostic tools, the film properties are determined by ex situ SE. Information about

film chemistry and morphology are obtained by Fourier transform infrared transmission

spectroscopy (FT-IR) and atomic force microscopy (AFM) respectively. Rutherford

Backscattering Spectrometry (RBS) and Elastic Recoil Detection (ERD) yield

information about the chemical composition and density of the layer. Finally the barrier

properties, in terms of WVTR, of single and multilayer barriers are determined by means

of a Ca test.

3.1 Experimental set-up

The set-up is engineered and developed to combine both i-CVD and PECVD processes in

a single deposition chamber. A picture of the set-up in which two deposition processes

are performed, is shown in Fig. 3-1.

Fig. 3-1: Picture of the set-up in which both the i-CVD and PECVD processes are performed

combinatorially.

The deposition chamber has a cylindrical shape with a diameter of 40 cm and a height of

53 cm. In order to avoid monomer condensation and minimize monomer adsorption on

the chamber walls these are heated to a constant temperature of 80°C. The substrate

holder (15 cm x 15 cm), connected to a temperature control system based on a resistive

heating unit (Eurotherm) and three Peltier cooling elements, is earthed since it acts as

ground electrode during the PECVD process. The wire to substrate as well as the RF

electrode to substrate (ground electrode) distance can be varied by adjusting the table

height. The reactor is pumped by a rotary vane pump (Adixen) in combination with a

turbomolecular pump (Pfeiffer). The deposition pressure, controlled by opening/closing a

butterfly pressure control valve (VAT), is monitored through a combination of capacitive

22

(Pfeiffer), Pirani (Pfeiffer) and Penning (Pfeiffer) pressure gauges to cover the range

from base pressure (10-6

mbar) up to atmospheric pressure.

The monomer (1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane, V3D3, > 95% Gelest, Tb =

189°C) and the initiator (di-tert-butyl peroxide, d-TBPO, 98% Aldrich, Tb = 110°C),

contained in stainless steel bubblers (kept at a constant temperature of 100°C and 25°C,

respectively) are vaporized and their flow rates are metered through Vapor Source

Controllers (VSC 1150C, MKS Instruments). Monomer and initiator vapors are then

delivered to the deposition chamber through two lines connecting the vapor source

controllers to the vessel. To avoid vapor condensation, both monomer and initiator lines

are heated at a constant temperature of 120°C and 35°C, respectively.

The other reaction gasses Ar, O2 and He are also metered through mass flow controllers

(MFC 1179B, MKS Instruments).

Fig. 3-2: Gas/vapor injection system consisting of two separate rings. O2,,Ar or He are injected

through a closed ring, while V3D3 and d-TBPO are injected through two half rings. In both cases

the flows are directed along the diagonal of the substrate holder.

The monomer and initiator vapors and the other reaction gases are injected inside the

deposition chamber through a system (Fig. 3-2) consisting of two rings with four equally

sized holes each. The holes are pointing along the diagonal of the substrate holder, with

the upper ring positioned 5.5 cm above the substrate holder. Before entering the injection

ring, monomer and initiator vapors are pre-mixed in a buffer chamber, attached to the

ring inside the deposition chamber. Additional gasses during deposition are injected

through the second, full ring just below the other ring.

The key components in the deposition chamber are the grid heating system (i-CVD), and

the RF electrode with matching box (PECVD). In Fig. 3-3 schematic representations of

the set-up for both i-CVD and PECVD processes are shown.

The grid heating system consists of a single tungsten wire (Ø = 0.2 mm) mounted on an

insulated frame in order to make an array of 15 equally spaced (2 cm) lines. The wire

temperature is controlled by a custom built temperature control system described in

details in Appendices D (temperature control system) and E (grid temperature

calibration). The grid to substrate distance is fixed to 2 cm in this work.

23

Fig. 3-3: a) Schematic representation of the deposition chamber for the i-CVD process. b)

Deposition chamber during the PECVD process. Both schematics show the components:

temperature controlled substrate holder, gas injection systems, grid with heating filaments,

separate vacuum chamber, RF electrode with matching box, pumping lines and the in situ SE.

The grid system is attached to a magnetic arm and placed in a separated vacuum chamber

during the PECVD process in order to avoid deposition on the grid.

Next to the grid system for the i-CVD process, a radio-frequency (RF) power (13.56

MHz, RF VII, Inc) is applied to ignite the plasma during the PECVD process. The top

electrode is placed at a distance of 6.8 cm above the substrate. To maximize the forward

power and minimize the reflected power, a matching network is incorporated (Fig. 3-4).

This matches the impedances of the source and the discharge in order to deliver the

maximum power to the plasma.

Fig. 3-4: Schematic of the matching network incorporated in the set-up in order to match the

source and load impedance, typically 50 Ohms. This way the forward power is maximized and the

reflected power minimized during the PECVD process to achieve the maximum power delivered

to the plasma.

Due to the absence of a load lock in the set-up, an Ar plasma discharge (Ar flow rate = 30

sccm, P = 150 W, p = 0.3 mbar; t = 10 min) is run before every PECVD deposition, to

heat the reactor walls and remove adsorbed water. During the Ar plasma discharge a

(a) (b)

24

shutter is placed in front of the substrate to avoid any plasma-substrate interaction. The

shutter is also used to shield the substrate during the plasma ignition, before starting the

PECVD deposition process. It is only removed to start the deposition once the gas flow

rates, pressure and plasma conditions are stable.

For real-time film characterization by means of in situ spectroscopic ellipsometry, the

set-up is equipped with quartz windows transparent to the SE light beam. The angle of

incidence is ~71.5°, close to the Brewster angle of Si (~74°), SiO2-like and SiOxCyHz at

which the measurement sensitivity is highest [61, 62]

.

An overview of typically used deposition parameters for the PECVD process is given in

Table 3-1.

Table 3-1: Typical ranges of deposition parameters for organic and inorganic PECVD layers.

Layer

chemistry

V3D3

flow

rate

(sccm)

Ar flow

rate

(sccm)

O2 flow

rate

(sccm)

Pressure

(mbar)

Power

(W)

Tsub

(K)

Twall

(K)

PECVD SiOx-like 0.7 70 35 0.3 250 373 353

SiOxCyHz 0.7 100 - 0.45 50 373 353

For the study of the i-CVD process different series were deposited of which the

corresponding conditions are shown in Table 3-2. For every series one or two parameters

are varying, as will be highlighted when discussing the results.

Table 3-2: Deposition conditions for the p(V3D3) layers.

V3D3

flow

rate

(sccm)

d-TBPO

flow rate

(sccm)

Pressure

(mbar) PM/Psat

Tgrid

(K)

Tsub

(K)

Twall

(K)

Time

(min)

Time

evolution

study

10 2 0.7 0.374 673 313 353 5-40

Tsub series 10 2 0.7 0.374-0.039 673 313-353 353 20

25

3.2 Diagnostic tools

3.2.1 Spectroscopic Ellipsometry

Spectroscopic ellipsometry (SE) measures the change in light polarization upon reflection

on a sample surface (Fig. 3-5) [61, 62]

.

Fig. 3-5: Schematic representation of an incident light beam, consisting of both s- and p-

polarized light, which is reflected on the sample surface. The resulting light is elliptically

polarized due to changes in amplitude and phase of the s- and p-components [62]

.

The polarization state of the incident light beam (electromagnetic wave) can be separated

into an s- and p-component. The s-component corresponds to the oscillation direction

perpendicular to the plane of incidence and parallel to the sample surface. Oscillations

parallel to the plane of incidence are defined as the p component. The emitted linearly

polarized light changes upon reflection on the surface in elliptically polarized light

characterized by the s- and p- component. The change in polarization can be attributed to

the independent change in amplitude (intensity) and phase of both components. A

description of the polarized light after reflection is given by the fundamental ellipsometry

equation relating the Fresnel coefficients with two ellipsometric parameters, Δ and Ψ (Eq.

3.1).

s

pi

r

re )tan( (3.1)

The parameters rp and rs represent the amplitude of respectively the p- and s-component

of the polarized light after reflection. The ellipsometric parameter Δ corresponds to the

phase difference between the p- and s- polarizations while tan(Ψ) represents the ratio of

the amplitudes upon reflection.

The measured ellipsometric parameters, Δ and Ψ, determined from the reflected light, are

related to the complex, total refractive index ñ which is complex due to dielectric loss and

a non-zero direct current conductivity, and therefore can be defined as

inñ (3.2)

where n is the real part of the refractive index indicating the phase speed, and κ is the

imaginary part representing the light absorption in the material.

26

In case of transparent materials the Cauchy parameterization is well suited to describe the

wavelength dependent refractive index as in equation 3.3.

42

)(

CB

An (3.3)

A, B and C are related to the material optical properties and λ is the wavelength in μm (In

this thesis the wavelength is reported in nm as ). The Cauchy relation in Eq. 3.3 only

provides information about the refractive index n(λ). The extinction coefficient k(λ) is

described by the Urbach tail in Eq. 3.4.

1112400

)( e (3.4)

with α, β and γ representing the extinction amplitude, exponent factor and band energy

respectively. The band energy in this work is fixed at 400 nm.

In situ measurements are performed at an angle of incidence of ~71.5° and by covering

the visible and near infrared wavelength range by using a Xe arc light source (247-1034

nm, J.A. Woollam Co. M-2000U). For the ex situ ellipsometer, measurements are

performed in the visible and ultraviolet wavelength range (191-1034 nm, J.A. Woollam

Co. M-2000D), made possible by the combination of a QTH (Quartz Tungsten Halogen)

and D2 (Deuterium) light source. The data is acquired at single (70°) or multi-angle (65°-

75°, steps of 5°) option. For both ellipsometers the data acquisition and modeling are

performed with CompleteEase 4.27 analysis software and the refractive indices are

reported for a wavelength of 633 nm.

The optical model

The measured ellipsometric parameters, Δ and Ψ, do not yield direct information about

the film properties. Therefore an optical model, composed of a substrate and a Cauchy

layer (applicable for transparent materials) for the deposited material, is required to

describe the sample (Fig.3-6).

During this work single side polished crystalline silicon (Si) has been used as substrate

for both i-CVD and PECVD deposition processes. To accurately model the experimental

data the bare substrates are measured ex situ and in situ before the deposition. Both for

the in situ and ex situ acquired data, the optical model is comprised of a silicon substrate

layer, a SiO2 native oxide layer (thickness ~ 1.5-2 nm) and a Cauchy layer describing the

PECVD or i-CVD layer’s optical constants and thickness as described in Figure 3-6.

The fitting procedure of the above model consists of two steps. On first the substrate data

are modeled to define the thickness of the native SiO2 present on top of the silicon

substrate (in the case of the in situ data the silicon substrate temperature is also fitted).

Then the deposited layer’s (i-CVD or PECVD) optical constants and thickness are fitted

to the data, keeping fixed the previous fit results of the substrate. In the case of the

PECVD deposited SiO2 layer, the fit parameters are the layer thickness and the Cauchy

parameters A and B (C is included if it improves the fit while the extinction coefficient

κ(λ) = 0). For the i-CVD and PECVD organosilicone layers both refractive index (A,B

27

and C) and extinction coefficient (α and β) are fitted to the data together with the film

thickness.

Fig. 3-6: Model to describe the single layer deposition on a Si substrate.

Adsorption/desorption measurements

The monomer adsorption/desorption measurements on Si substrate are performed by

monitoring the changes in adsorbed thickness with in situ SE. The conditions of the

adsorption/desorption measurements are chosen similar to the deposition conditions of

the Tsub series (Table 3-2) except that the heating grid is turned off to avoid layer growth.

The adsorption/desorption measurements can be separated into two sets: one performed

under equilibrium and the other ones performed by mimicking the deposition conditions.

Since the measurements are starting from base pressure (~10-6

mbar) the adsorbed

thickness increases till the deposition pressure (0.7 mbar) is reached. In the case of the

equilibrium conditions pressure is increased stepwise (0.02 mbar per step) allowing the

pressure to stabilize and obtain the absorbed thickness for every step. For the mimicked

deposition conditions pressure is increased continuously up to the set point, similar to the

i-CVD deposition process. Both sets of adsorption/desorption measurements are

performed at a substrate temperature of 313 K. In the case of the mimicked conditions

higher substrate temperatures of 333 and 353 K are performed as well for comparison.

Since in situ SE is used to monitor the adsorption/desorption measurements, an optical

model is applied, similar to the one of the in situ monitoring of the i-CVD layer growth,

so comprised of: 1) silicon substrate; 2) SiO2 native oxide layer (~ 1.5-2 nm); 3) V3D3

monomer layer, modeled with the Cauchy dispersion being the thickness the only fitting

parameter whereas the monomer refractive index is fixed to its liquid value of 1.4215 (at

589 nm) as reported in literature [63]

.

For the adsorption/desorption measurements performed on dense and porous SiO2-like

layers, a similar procedure has been followed, according to the following steps:

1) the SiO2-like (dense/porous) layer has been deposited (according to the conditions

in Table 3-1);

2) after the deposition, the chamber has been evacuated to base pressure and the

temperature set to the adsorption/desorption measurements value (313 K; 333 K;

353 K);

3) then the adsorption/desorption measurements have been performed without

breaking the vacuum

Since the monomer (transparent in the investigated wavelength range) is supposed to

adsorb into the SiO2-like layer pores/defects, the same optical model (Cauchy), can be

applied. However, the SiO2-like layer thickness has been kept constant to the value found

28

after the deposition process, and only the A and B coefficient of the Cauchy layer are

used as fitting parameters.

3.2.2 Fourier Transform Infrared Spectroscopy

Fourier Transform Infrared (FT-IR) spectroscopy is a light absorbing technique revealing

information about the rotational-vibrational states of the molecules included in the

deposited layer. The atoms in the molecule are in periodic motion and vibrate at the so-

called vibration frequency. These vibration frequencies correspond with discrete energy

levels characteristic for specific vibrational bonds and hence are used to identify them.

An overview of common vibrational modes is shown in Fig. 3-7.

Fig. 3-7: Schematic overview of vibrational modes. Movements towards the front side out of the

plane are indicated with a + sign. Movements out-of-plane through the backside are marked with

a - sign.

To identify the different bonds a broadband infrared light source emits light towards the

sample at specific frequencies. The light is absorbed if its frequency matches the

vibration frequency of the chemical bond.

Fig. 3-8: Schematic diagram of a Michelson interferometer [62]

.

29

Initially an interferogram, from a Michelson interferometer (Fig. 3-8), is obtained due to

the interaction of the light with the sample. In the FT-IR set-up the sample is located in

front of the detector. The obtained interferogram contains the recorded light intensities as

a function of the optical path length. Subsequently this interferogram is Fourier

transformed into an intensity spectrum for a broad range of frequencies. To facilitate the

interpretation this transmission spectrum, intensity ratio between film and bare substrate,

can be transformed into an absorption spectrum via equation 3.5 based on Beer’s law,

lcI

IA )log(

0

(3.5)

where A represents the absorbance, I the transmitted intensity of the deposited layer, I0

the transmitted intensity of the bare Si substrate, ε is the molar absorptivity, c the

concentration of the absorbing species and l the path length of the light through the

sample [64]

.

The FT-IR measurements are performed with a Bruker Tensor 27 instrument, on both the

bare Si substrate, used as background, the deposited layer (i-CVD or PECVD) and also

the liquid monomer. Before a measurement is taken, the spectrometer is purged with

nitrogen for at least 15 minutes to minimize the signals due to water and carbon dioxide

absorption. The spectra are acquired with a resolution of 4 cm-1

for a number of scans

varying from 256 to 2000. The region of interest is located in the mid-infrared, 400-4000

cm-1

and is used for comparison of the spectra in terms of layer chemistry.

The spectra measured in this thesis, for both monomer and polymer, contain vital

information about the monomer/polymer chemistry, and therefore specific regions of the

spectra are fitted in order to obtain further information. The procedure of the fitting varies

between the monomer and polymer, as a Lorentzian function is applied for the liquid

monomer since collision broadening is dominating the vibrational modes, while a

Gaussian function is used for the polymer since Doppler broadening due to the thermal

movement of the molecules is dominant [65, 66]

. The fitting procedure is the following: on

first the position, width and area of the peaks are manually set until the cumulative fit

matches rather good with the absorption band. Since position and width are optimized,

the area of the peaks can be fitted as this parameter is the one used for determining the

layer properties such as the polymerization degree.

3.2.3 Atomic Force Microscopy

Atomic force microscopy (AFM) is used to characterize the sample surface and yield

information about the surface morphology by scanning the surface with a tip (Fig. 3-9).

Every tip is attached to a cantilever oscillating at its eigenfrequency. When approaching

the tip to the sample surface a topographic map is created by monitoring the changes in

vertical position while keeping the amplitude of oscillation constant. The amplitude of

oscillation will change upon atomic force interaction between tip and sample surface and

is monitored by using a laser and photodiode detector. From the obtained topographic

map, information such as the root mean square (RMS) surface roughness is obtained by a

roughness analysis.

For this study, measurements are performed with a Solver Pro Scanning Probe

Microscope (NT-MDT) in semi-contact mode using tips (NSG-10) with a radius of

30

curvature of 10 nm. Typically used scan areas are 2x2 and 10x10 μm2. The measurement

and analysis is performed with NOVA software (NT-MDT) in order to describe the

surface morphology of the sample.

Fig. 3-9: Schematic representation of the AFM containing the main components: Cantilever with

probe, laser and photodiode detector and the feedback electronics.

3.2.4 Rutherford Backscattering Spectrometry / Elastic Recoil Detection

Rutherford Backscattering Spectrometry (RBS) is an ion beam analysis technique

revealing information about the composition, density and concentration depth profile of

thin films [67]

. In RBS a beam of high energy (1-2 MeV) ions, in many cases He2+

, is

impinging on a sample (Fig. 3-10a). The ions are elastically backscattered by the heavier

nuclei in the sample and are detected in a well-defined angle. The detector, under a small

angle with respect to the incoming ion-beam, is counting the number of ions as a function

of their energy. The energy of the detected ions depends on both the depth and the mass

of the target atom. A heavier atom provides more energy for the backscattered ion due to

conservation of energy and momentum. The limitation in depth (1 μm) is related to the

energy loss due to the re-scatter off a second atom.

Fig. 3-10: a) RBS measurement set-up with a small detection angle compared to the incoming

ion-beam to detect the backscattered ions due to the atoms heavier than helium. b) ERD set-up to

detect the hydrogen atoms recoiled in a forward direction from the sample [67]

.

Complementary to the RBS analysis, Elastic Recoil Detection (ERD) is applied to

determine the hydrogen content of the sample (Fig. 3-10b). With RBS this is not possible

because the lighter H atoms are recoiled in a forward direction. The detector position

therefore is located under a wider angle, and instead of detecting the backscattered

(a) (b)

31

helium ions, the recoiled hydrogen atoms are detected. To avoid the detection of scattered

helium ions a stopper foil is placed in front of the detector allowing only the recoiled

hydrogen atoms to pass through. The depth range for ERD is limited to 0.3 μm.

3.2.5 Water Vapor Transmission Rate measurement

The Water Vapor Transmission Rate (WVTR), yielding information about the barrier

performance of a material, can be determined by means of the calcium test which, for this

thesis, is performed by Holst Centre [68]

. In general a calcium film of well-defined

thickness is deposited on top of a transparent substrate (e.g. glass) and subsequently

encapsulated with the barrier layer to be investigated. By using a back-light (with

constant illumination for the whole sample) the optical transmission of the sample is

detected by a CCD camera (Fig. 3-11). Initially the metallic Ca has a no or low

transmission which as a function of time becomes more transparent when oxidizing to

calcium monoxide due to reaction with water or oxygen. Because the light transmittance

of the sample is used as a measure of the amount of water permeating the barrier layer,

reaction 1 below can be considered the dominating one. The uptake of O2 in reaction 3 is

considered as negligible, because of the generally low oxygen transmission rate (OTR)

compared to its WVTR [69]

. Reaction 2 is also assumed not to contribute to water uptake

because this reaction does not take place initially, but requires the formation of CaO on

first, so the occurrence of reaction 1. By not considering reaction 2 the WVTR is slightly

underestimated. However, the detection limit of the Ca test is in the order of 10-6

g/m2day.

The possible reactions of the Ca taking place when moisture and oxygen permeate the

layers are:

22 HCaOOHCa (Reaction 1)

22 )(OHCaOHCaO (Reaction 2)

CaOOCa 221 (Reaction 3)

Due to the oxidation of the Ca, pinholes (visible as white spots) present in the barrier

layer can hamper the measurement of the intrinsic water vapor transmission rate which is

generally obtained from the Ca test. With intrinsic one means the transmission through

the material matrix, excluding diffusion through pinholes and defects. However, there can

still be a contribution of diffusion through smaller pinholes which did not yet lead to the

formation of visible white spots. When the amount of pinholes is too extensive, one is

restricted to the measurement of the extrinsic WVTR of the barrier material as the whole

measurement area is covered with pinholes and defects.

32

Fig. 3-11: Measurement set-up of the Ca test to determine the intrinsic barrier properties of the

deposited layer. A CCD camera detects the transmittance and provides the information to

analysis software which determines the WVTR and, the number and area of pinholes.

The calculation of the WVTR is based on the average transmitted light intensity as

recorded by the CCD camera for a specific measurement area. Every sample consists in

general of multiple Ca pads (9 or 4 dependent on the size of the sample), and each pad is

composed of 9 small squares which can be considered as one measurement area (In total

81 or 36 measurement areas are available). The grayscale is determined from black (non-

transmitting) and white (fully-transmitting) reference measurements taken for every

sample separately. The analysis software basically converts the transmittance of a

measurement area into a WVTR, and it also counts the number of pinholes and their area

for every measured area. As these pinholes are excluded from the area under

consideration, one obtains the intrinsic WVTR. The measurements are taken in intervals

of several days. An accurate WVTR can already be obtained for an oxidized layer of 1

nm.

In this thesis the approach is slightly different from the one mentioned above, in the sense

that a PEN-coated glass substrate is used on which the diffusion barrier system (either a

single layer or a multilayer stack) is deposited. Subsequently the Ca pattern (40 nm thick

in this thesis) and a transparent SiNx (intrinsic WVTR ~ 10-5

- 10-6

g/m2day)

encapsulation layer are deposited on top (Fig. 3-12). The encapsulation with SiNx allows

the WVTR measurement of the deposited barrier system since the moisture permeation

evolves from the PEN side as the glass is removed before the Ca test.

33

Fig. 3-12: Schematic representation of a sample with a SiO2-like layer to be investigated as a

barrier by means of the Ca test. The water is mainly penetrating from the bottom because of the

SiNx reference barrier layer. The pictures at the right show one calcium square of a deposited

sample consisting of nine pads. With time the Ca starts oxidizing which leads to the presence of

bright spots.

The condition at which the Ca test is performed is a controlled atmosphere (20°C), while

in between measurements the samples are stored in a climate chamber (20°C, 50%

relative humidity). The development of pinholes with time can be observed by the

increasing presence of bright spots in the example of a single SiO2-like barrier layer (Fig.

3-12).

PEN SiO2

SiNx

H2O

Ca

Time = 0 days Time = 3.9 days Time = 6.9 days

34

35

4 Results

4.1 Study of the i-CVD process

The i-CVD process and polymerization mechanism have been presented in Chapter 2

together with a literature review concerning the main process parameters affecting the

layer growth and properties. For the specific study of p(V3D3) layers different analysis

techniques are applied in this experimental work. The main diagnostic tool is Fourier-

Transform Infrared (FT-IR) spectroscopy, which allows investigating the layer chemistry

and the polymerization process. Complementary diagnostic tools, i.e. Spectroscopic

Ellipsometry (SE), Rutherford Backscattering Spectroscopy (RBS)/Elastic Recoil

Detection (ERD) and Atomic Force Microscopy (AFM), allow further characterization in

terms of thickness, optical properties, density, stoichiometry and surface roughness.

4.1.1 (V3D3)- i-CVD polymer layers: a detailed IR analysis

The development of the i-CVD polymerization process can be demonstrated by the

analysis of the IR absorption spectra of the p(V3D3). The FT-IR measurements provide

information on the chemical structure of the deposited layers and the polymerization

degree can be quantified.

The polymerization process develops through free radical (via thermal decomposition of

the initiator molecule) polymerization of the unsaturated bonds of the monomer, i.e. the

vinyl functionalities (see Fig. 2-2). Therefore a comparison between the polymer and

monomer (liquid phase) spectra is made (see Fig. 2-3) based on the assignment of the

observed IR absorption bands (See Table A-1 of Appendix A). The polymerization

develops through radical transfer on the vinyl groups, as it can be observed in Figs. 4-1a

and 4-1c by the quantitative decrease of the signals at 1408 cm-1

(CH2 asymmetric

deformation in the vinyl group), at 1597 cm-1

(C=C stretching), and in the CHx region

(CH2 asymmetric and symmetric stretching at 3057 cm-1

and 3018 cm-1

, CH asymmetric

and symmetric stretching at 2976 cm-1

and 2935 cm-1

) in the polymer spectrum with

respect to the liquid monomer. The absorption at 1408 cm-1

, still present in the polymer

spectrum, is also attributed to the deformation of the methylene group bonded to a silicon

atom [23, 29]

.

The other monomer functional groups, i.e. the methyl group (the asymmetric stretching

of the methyl group at 2962 cm-1

and the Si-CH3 symmetric bending signal at 1260 cm-1

)

and the cyclotrisiloxane ring (Si-O-Si asymmetric stretching at 995 cm-1

) are retained in

the polymer layer (Figs. 4-1b and 4-1c). However, a shift in peak position, associated

with the cyclotrisiloxane ring, from 1016 cm-1

in the monomer to 995 cm-1

in the

polymer, has been observed. The same peak position has been reported in literature for i-

CVD deposited p(V3D3), at 1012 cm-1

[23, 29]

, both in the monomer and the polymer

spectrum. A similar behavior has been shown for the polymerization of the homologous

of the V3D3, namely V4D4, which showed a shift of the cyclotretrasiloxane- related signal

from 1075 cm-1

(monomer) to 1065 cm-1

(polymer) [32]

. A possible reason for the shift of

the Si-O-Si peak to lower frequency can be the increased stress due to the ring tension

built in the cross-linked polymer bulk, compared to the liquid phase (i.e. liquid

monomer). The absorption band centered at 995 cm-1

can be deconvoluted into three

36

1600 1400 1200 1000

Wavenumber (cm-1)

CH2

Si-O-Si a CH

2

Si-CH3

s

CH2

a

Polymer

Ab

s (

a.

u.)

Monomer

C=C

3150 3100 3050 3000 2950 2900 2850 2800

Monomer

Ab

s (

a.u

.)

Wavenumber (cm-1)

s (=CH-)

a (-CH

3)

a (=CH-)

s (=CH

2)

s (-CH

3)

a (=CH

2)

Polymer

Experimental data

Cumulative fit

a (=CH

2)

s (=CH

2)

a (=CH-)

a (-CH

3)

s (=CH-)

a (-CH

2-)

s (-CH

3)

s (-CH

2-)

Experimental data

Cumulative fit

Abs (

a.u

.)

1300 1290 1280 1270 1260 1250 1240 1230

Wavenumber (cm-1)

(=CH)

s(Si-CH

3)

s(Si-CH

3)

(=CH)

Polymer

Monomer

(b) (a)

(c)

main peaks: one at 1002 cm-1

associated to the Si-O-Si asymmetric stretching of the

cyclotrisiloxane bonded to short, low molecular weight, polymeric chains (low tension

induced into the ring structure [70]

), one at 995 cm-1

associated to the same signal but of

the cyclotrisiloxane bonded to long, cross-linked polymeric chains (higher molecular

weight and higher tension induced in the ring structure [70]

) and a third peak assigned to

the wagging of the methylene bridge (Si-(CH2)x-Si [29]

).

Fig. 4-1: FT-IR spectra of polymer and monomer for the CHx region (a), Si-CH3 region (b)

including the deconvolution and the peak fitting. The vinyl bonds are indicated in blue, saturated

bonds are assigned in red. Monomer and polymer FT-IR absorption spectra in the range 1780-

925 cm-1

containing: Si-O-Si asymmetric stretching band (950-1150 cm-1

) and vinyl related

signals decreasing upon polymerization due to the cleavage of the vinyl bonds are shown in (c).

Finally, the possibility of ring opening can be excluded since this would imply a

broadening of the absorption band with a new contribution at 1080 cm-1

[23, 71]

, related to

the open, linear Si-O-Si chains, which is not present in the polymer spectrum.

Also in the polymer spectrum, new signals related to the chains generated through the

polymerization process, are present: the asymmetric and symmetric stretching of the

37

methylene groups at 2910 cm-1

and 2861 cm-1

, respectively, as well as the asymmetric

bending of the methylene groups centered at 1460 cm-1

.

Further insight into the polymerization process can be inferred by the deconvolution of

the CHx stretching and the Si-CH3 bending modes in the monomer and the polymer

spectra. The assignments based on the deconvolution results of the CHx region are

reported in Fig. 4-1a and in Appendix A for both monomer and polymer. After the

polymerization process has proceeded, the saturated groups, i.e. –CH3 and the methylenic

bridges: -CH2- become dominant. In the case of the Si-CH3 bending region, the

absorption band has also been deconvoluted into two contributions both for the monomer

and polymer (Fig. 4-1b): the main peak, used in the polymerization degree calculation,

associated with the symmetric bending of the Si-CH3 group (1261 cm-1

in the monomer;

1260 cm-1

in the polymer) with a shoulder at higher wavenumber (1273 cm-1

in the

monomer, 1271 cm-1

in the polymer) which can be attributed to the rocking of the =CH-

groups [72]

.To determine the degree of polymerization, the -CH2-asymmetric stretching at

3057 cm-1

, together with the Si-CH3 bending signal (as a reference) are used since this

last one is present in both polymer and monomer and remains unaffected by the

polymerization process, allowing the calculation despite the different absorption

intensity. Without the reference signal no quantitative comparison (monomer vs.

polymer) can be made. The polymerization degree therefore is defined as:

monomer

monomer

polymer

polymer

CHSiAreaCHspArea

CHSiArea

CHspArea

)()(

)(

)(

1100 (%)degreetionPolymeriza

3

2

2

3

2

2

(4.1)

Lorentzian and Gaussian functions have been used to fit the (liquid) monomer and the

polymer absorption bands, respectively. The use of the Lorentzian is justified as collision

broadening is dominating in the vibrational modes of the liquid. For the fitting of the

solid polymer spectra the Gaussian profile, due to Doppler broadening, is adopted as the

molecules undergo thermal motion leading to a Maxwellian velocity distribution and

therefore non-monochromatic absorption [65, 66]

.

From the results of the absorption band deconvolution, the degree of polymerization can

be determined according to Eq. 4.1, and this has lead to polymerization degrees of 81 (±

3) % and 85 (± 2) % for depositions carried out at 313 and 333 K respectively. For higher

temperatures the =CH2 signal was reduced further and within the noise level. A more

detailed analysis of the temperature effect on the layer properties will be addressed in

section 4.2.3.

38

1100 1050 1000 950

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

0.00035

exp data

(SiO)3 long chain

(SiO)3 short chain

Si(CH2)

xSi

fit

A

bs / thic

kness (

nm

-1)

Wavenumber (cm-1)

Fig. 4-3: Deconvolution of the SiOSi region: at ~1011 cm

-1 the Si-O-Si asymmetric stretching of

the cyclotrisiloxane bonded to short, low molecular weight, polymeric chains, at ~991 cm-1

the Si-

O-Si asymmetric stretching of the cyclotrisiloxane bonded to long, cross-linked polymeric chains,

at 970 cm-1

the wagging of the methylene bridge (Si-(CH2)x-Si).

The further analysis of the FT-IR spectra allows gathering information on the polymer

network structure as this can be inferred by the deconvolution of the Si-O-Si absorption

band indicated in Fig. 4-3 for an i-CVD layer deposited at 333 K. The presence of the Si-

O-Si (in the trisiloxane ring) absorption band has been reported to shift to about 1020 cm-

1 compared to linear siloxanes (ranging from 1130 to 1000 cm

-1), due to geometric effects

[70]. The peak has been deconvoluted into three absorption bands with increasing

frequencies: Si-(CH2)x-Si wagging of the vinyl groups; long, high molecular weight and

short, low molecular weight polymer chains [73]

. The role of the substrate temperature on

the deconvolution and as such on the polymer network will be illustrated in section 4-2.

4.1.2 Role of process parameters on p(V3D3) layers

Kinetic vs. mass transfer limited regime: the role of the grid temperature

The grid temperature is an important parameter when considering the growth rate of the i-

CVD process. This is due to the formation of radicals from the initiator molecules: by

increasing the grid temperature the efficiency of radical generation improves, leading to

higher growth rates following an Arrhenius relation, as already demonstrated by

O’Shaughnessy et al. [23]

. Both transition point and activation energy can be determined

from the data reported in Fig. 4-4 based on experiments performed in this work. The

substrate temperature has been set to 333 K, allowing a comparison of the outcome with

literature results.

39

0.0012 0.0014 0.0016 0.0018 0.0020 0.0022

0.02

0.05

0.14

0.37

1.00

2.72

7.39

Deposition rate

Linear fit

Rd (

nm

/min

)

1/Tgrid

(K-1)

Ea = 65 4 kJ/mol

833 714 625 556 500 455

Tgrid

(K)

Fig. 4-4: Log scale plot of the deposition rate (within an accuracy of 5% due to reproducibility)

as a function of the reciprocal of the grid temperature for the p(V3D3) layers deposited in this

thesis.

The transition point is found at a temperature of ± 623 K (Fig. 4-4), which is slightly

lower than the 673 K reported in literature for the same p(V3D3) layer. The difference is

likely related to the different reaction kinetics due to the deposition conditions (e.g.

difference in monomer flow rate can imply a different PM/Psat and therefore shift the

transition point [33]

) and reactor design (e.g. different flow profile, different temperature

gradient).

When considering the activation energy, determined from the linear fit of the reaction-

kinetics limited regime, it is larger than the literature value, namely 65 ± 4 kJ/mol

compared to 28 kJ/mol, but in both cases it is well below the activation energy of

decomposition of the d-TBPO (~160 kJ/mol), confirming that the generation of radicals

is not likely the limiting factor [33, 34]

. Furthermore, the temperature profile (between grid

and substrate) has been simulated (COMSOL Multiphysics software) and found to have a

polynomial shape. By applying the mean value theorem1 (with Tgrid and Tsub as the

boundaries) on the average temperature profile [74]

, the mean gas temperature has been

determined and the corrected apparent activation energy found to increase from 65 (± 4)

to 83 (± 6) kJ/mol. Since the value is still below the dissociation energy of the initiator,

the limitation in the polymerization process is not related to a limited amount of radical

generation, but to the kinetics of surface reactions (low reactivity, steric hindrance of the

monomer).

All the experiments carried out further and described in this work are performed at a grid

temperature of 673 K, implying that the deposition processes occur in the mass-transfer

limited regime.

1 Mean value theorem states: If a function f(x) is continuous on the closed interval [a, b] and differentiable

on the open interval (a, b), then there exists a point c in (a, b) such thatab

afbfcf

)()()(' .

40

Monomer adsorption and surface mobility: the role of the substrate temperature

In Chapter 2 the relation between monomer adsorption and PM/Psat has been illustrated as

well as the role of the substrate temperature, having a dual effect: affecting the monomer

adsorption (at constant PM) and enhancing the polymerization process. The effect of the

substrate temperature on the layer properties will be described in section 4.2, by applying

in situ spectroscopic ellipsometry during the layer growth.

The apparent activation energy (as explained in Chapter 2) is calculated from the log-

scale plot of the deposition rate versus the reciprocal substrate temperature (Fig. 4-5).

0.0028 0.0029 0.0030 0.0031 0.00321.00

2.72

7.39

Ea = -39 4 kJ/mol

Deposition Rate

Linear Fit

Rd (

nm

/min

)

1/Tsub

(K-1)

350 340 330 320 310

Tsub

(K)

Fig. 4-5: Log-scale plot of the deposition rate (standard deviation of 5%) versus the reciprocal

substrate temperature. The apparent activation energy of -39 ± 4 kJ/mol is calculated from the

slope of the Arrhenius equation used to fit the data.

By fitting the data to the Arrhenius equation [75]

, the apparent activation energy is

calculated from the slope of the fit and equals -39 ± 4 kJ/mol. The negative activation

energy is consistent with data reported in literature (Table 4-1), suggesting that the i-

CVD polymerization processes are adsorption-limited and diffusion dependent [29, 76]

.

Because of the low activation energy, this basically means that there is no energy barrier

to be overcome for the reaction to proceed. In particular, as the temperature increases, the

PM/Psat ratio decreases, therefore indicating a decrease in monomer surface concentration

as these are related.

Considering further the i-CVD processes given in Table 4-1 [23, 29]

, activation energies of

-23 and -41 kJ/mol for respectively p(V3D3) and copolymerized V3D3-HVDSO are

reported for adsorption-limited processes performed by the i-CVD polymerization

technique [23, 29]

. The difference between both polymer types can be related to the

propagation reaction which can proceed easier for the copolymer as the monomer units

have more vinyl groups (due to the HVDSO spacer molecules) in their neighborhood to

polymerize with. Since the steric hindrance is reduced relatively as the HVDSO

molecules can be directed in various directions, opposite to the V3D3, this results in a

higher deposition rate and therefore the lower activation energy. The p(V3D3) i-CVD-

polymer obtained in this thesis, having an activation energy of -39 kJ/mol, lies in the

41

same range (same order of magnitude) as the ones reported in literature for i-CVD,

although lying closer to the copolymer. The difference existing for the p(V3D3) i-CVD-

polymer with respect to literature can probably partly be ascribed to the different reactor

design as this has also been reported in literature to be a possible cause of mismatch in

film properties [78]

and also for the kinetic regime study this is mentioned as a possible

cause of mismatch. The fact that Ea is more negative indicates that the polymerization

process can proceed easier despite the adsorption-limiting kinetics.

Table 4-1: Overview of activation energies for i-CVD polymerization processes. Deposition

method i-CVD i-CVD i-CVD

Polymer layer p(V3D3)

(thesis)

p(V3D3) [23]

V3D3-

HVDSO [29]

Ea (kJ/mol) -39 -23 -41

4.1.3 Adsorption/desorption study of V3D3 monomer

From the previous study on the effect of the substrate temperature on the deposition rate,

the role of monomer adsorption has been briefly addressed. In this section we further

address the i-CVD monomer adsorption studies by means of in situ spectroscopic

ellipsometry.

Fig. 4-6: a) Adsorption/desorption isotherm on Si obtained by in situ SE under process conditions

and by considering equilibrium situations for every pressure step. b) Shape of a type II

adsorption isotherm typically for a nonporous or macro-porous solid in a range of PM/Psat from 0

to 1.

The first adsorption/desorption isotherm has been measured under equilibrium conditions

(Tsub = 313 K, monomer and initiator flow rate of 10 and 2 sccm, respectively), meaning

that for every pressure step the monomer adsorption is given sufficient time to reach the

equilibrium. The pressure is increased step-by-step starting from the base pressure (10-6

mbar) until the final deposition pressure of 0.7 mbar is reached. The

adsorption/desorption isotherm obtained under equilibrium conditions on c-Si substrates

(in the presence of a native SiO2 layer) is shown in Fig. 4-6a. From the adsorption curve

the inflection point, indicative of monolayer adsorption as included into the BET

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

Adsorption

Desorption

Ad

so

rbe

d t

hic

kne

ss (

nm

)

PM/P

sat

Monolayer

adsorption

(b) (a)

42

equation, is observed after a fast initial increase for low PM/Psat (< 0.1). Since the

adsorbed monomer thickness increases further, it implies that multilayer adsorption

develops. The shape of the adsorption curve points to a so-called type II isotherm (Fig. 4-

6b) (valid for adsorption on nonporous or macro-porous solids) [39]

and which can be

described by the BET equation.

Since for the i-CVD process the monomer adsorption is reported to be physisorbed onto

the substrate, the desorption is expected to be reversible. Instead, a hysteresis behavior

develops, which points out to chemisorption processes in the presence of the native SiO2

layer, and the (SiO)3 siloxane ring of the monomer.

Higher values of PM/Psat have not been investigated in order to avoid monomer

condensation to occur in the chamber. Concerning the BET equation as reported in

Chapter 2 (Eq. 2.2), the monomer adsorption can diverge for PM/Psat approaching unity.

Therefore, in order to determine the monolayer thickness, the BET equation used to fit

the adsorption data, is slightly adapted by replacing the adsorbed volumes by the

adsorbed layer thicknesses ( adadml tVtV and0 ) and by including an additional

correction factor k, in order to exclude infinite multilayer adsorption for PM/Psat = 1. The

monolayer thickness for the V3D3 monomer can now be determined by fitting the

adsorption data to the adapted BET equation as shown in Eq. 4.1,

sat

M

sat

M

sat

M

ad

PP

kcP

Pk

PP

kct

t

111

0

(4.1)

with c the BET constant indicative of the magnitude of the adsorbent-adsorbate

interaction. Since the range of PM/Psat (0-0.40) considered in these experiments is smaller

than 1, the correction factor k can be kept constant at 1. The fit of the adsorption data is

shown in Fig. 4-7a. The considered range is sufficient to apply the BET equation as the

inflection point is clearly present.

Fig. 4-7: a) BET fit to the adsorption isotherm obtained by means of in situ SE under deposition

conditions (substrate temperature of 313 K) revealing a monolayer thickness t0 = 0.497( ± 0.004)

nm. b) Molecular structure of the V3D3 in minimum energy configuration which is the most stable

state. The colors are assigned to the following: purple = Si, red = O, grey = C and white = H.

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

Experimental data

BET model

Ad

so

rbe

d t

hic

kne

ss (

nm

)

PM/P

sat(a) (b)

43

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Adsorption

Desorption

Adsorb

ed thic

kness (

nm

)

PM/P

sat

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Ads @ 313 K

Ads thickness @ 333 K

Ads thickness @ 353 K

Adsorb

ed thic

kness (

nm

)

PM/P

sat(a) (b)

The BET fit defines a monolayer thickness of 0.497 (± 0.004) nm at PM/Psat = 0.13, which

is close to the molecular radius of 0.404 nm of the V3D3, as estimated by using the

calculation method of the van der Waals Volume based on the sum of atomic and bond

contributions [79]

. Since this estimation does not take into account any geometry of the

molecule (it assumes a sphere, and therefore overestimates the dimension) and the fact

that the monolayer thickness seems to be comparable with the molecular radius (based on

the estimation of the van der Waals Volume), the preferred direction of adsorption for the

molecule is probably horizontal, with the siloxane ring parallel to the substrate [80-82]

.

In order to confirm this hypothesis of the molecular shape, the molecule has been drawn

(Fig. 4-7b) by using the ChemBioOffice 2010 (CambridgeSoft) software package. Since

the most stable state of a molecule is the one with a minimum energy, the siloxane ring

shows an almost planar structure with only a slight distortion of the Si-O-Si bonds.

The equilibrium conditions, considered in the previous experiment, are slightly different

from the real deposition conditions, and therefore additional adsorption/desorption

isotherms have been measured mimicking the deposition conditions (i.e. continuous

pressure increase) for several substrate temperatures (i.e. 313, 333, 353 K). At 313 K

(Fig. 4-8a), the adsorption isotherm is still of type II, but the amount of adsorption is

reduced since equilibrium is not reached at every pressure step. Also the hysteresis

behavior is reduced.

Fig. 4-8: a) Adsorption/desorption isotherms performed under mimicked deposition conditions at

a substrate temperature of 313 K. b) Adsorbed monomer thickness obtained for various substrate

temperatures under deposition conditions at 0.7 mbar.

Despite the lower adsorption (0.36 nm) measured at the final pressure of 0.7 mbar

compared to the equilibrium measurement (0.77 nm), the influence of the substrate

temperature is still visible since the adsorbed monomer thickness is found to decrease

with increasing temperature as the monomer adsorption decreases with a decrease in

PM/Psat (Fig. 4-8b).

So far the adsorption/desorption experiments have been performed on a bare, nonporous

Si substrate by considering the changes in adsorbed monomer thickness. Similar studies

on PECVD deposited SiO2-like layers (n = 1.45 and n = 1.41) have been carried out and

reported here with the purpose of highlighting how the monomer adsorption processes are

affected by the microstructure of the substrate. In this case, the porous SiO2-like layer

44

(low refractive index, n = 1.41) is accompanied by the presence of more residual porosity

and the development of Si-OH groups as highlighted by IR analysis (See Fig. 4.22a and c

of section 4.3.1 for examples). The resulting adsorption/desorption isotherms for both

dense and porous SiO2-like layers are reported (Fig. 4-9a and c) by considering the

change in refractive index which the SiO2-like layer undergoes upon

adsorption/desorption of the monomer unit into its porous structure. The increasing

refractive index trend is interpreted as the filling of the pores (n = 1) with the monomer (n

= 1.4215 at 589 nm).

Fig. 4-9: a) Adsorption/desorption measurements performed under mimicked deposition

conditions on a dense SiO2-like layer, at a substrate temperature of 313 K, b) Adsorption

isotherm of type I, c) Adsorption/desorption measurements performed under mimicked deposition

conditions on a porous SiO2-like layer, at a substrate temperature of 313 K, d) Adsorption

isotherm of type IV.

The experiments were all performed at a substrate temperature of 313 K. The dense SiO2-

like layer follows the type I isotherm (Fig. 4-9b) typical for micro-porous layers (pore

size < 2 nm) [39]

. The monomer adsorption at low PM/Psat shows a steep initial increase

indicating the fast filling of the micro-pores underneath with adsorbing monomer. After

this initial phase the monomer adsorption continues until it reaches a constant level of

adsorption, implying the completion of the micro-pores filling, but with a limited amount

of multilayer adsorption. In the case of the porous SiO2-like layer (Fig. 4-9c) it is

reasonable to think that a micro- to meso-porous transition occurs as the adsorption

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.401.412

1.413

1.414

1.415

1.416

1.417

1.418

Adsorption

Desorption

n (

63

3 n

m)

PM/P

sat(c) (d)

(b) (a) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

1.449

1.450

1.451

1.452

1.453

1.454

Adsorption

Desorption

n (

63

3 n

m)

PM/P

sat

45

isotherm is increasing instead of reaching a plateau and because of the presence of low

pressure hysteresis. Therefore the isotherm type is in between the type I and type IV (Fig.

4-9b and d). For clarification of the isotherm shape, the small PM/Psat range should be

extended. However, as the changes in refractive index are also small and close to the one

reported for the dense SiO2-like layer, it points to a micro-meso porous transition (pore

sizes ~2 nm).

The investigation of the initial steps in the i-CVD growth process by means of

ellipsometry allows drawing the following conclusions:

1. In situ adsorption/desorption studies of the deposition precursor allow to

characterize the microstructure of the layer/substrate underneath, therefore

providing complementary information (i.e. porosity characterization) to the

chemical and optical characterization carried out by (reflection) ellipsometry and

FT-IR spectroscopy;

2. The microstructure characterization by means of the above-mentioned studies

implicitly points out to the role of the i-CVD organic interlayer in multilayer

barrier structures, i.e. the filling of the open micro-meso porosity of the inorganic

barrier layer. This outcome will be addressed later when the evaluation of the

multilayer barrier performances will be discussed in section 4.4.

4.2 Growth and process window definition of p(V3D3) layers

As shown in 4.1.1, FT-IR spectroscopy provides access to information related to the

polymerization process in terms of layer chemistry, polymerization degree and cross-

linking, which are depending on the layer growth mode and the process conditions.

Besides the FT-IR, real-time monitoring of the film growth with SE provides insight into

the film growth, thickness development and optical properties. In addition to SE and FT-

IR, RBS/ERD and AFM measurements were performed for specific deposition conditions

in order to confirm the results obtained by applying SE and FT-IR. In the following

paragraphs the use of in situ SE is highlighted as a key and novel diagnostic tool in the

study of p(V3D3) layers.

4.2.1 In situ spectroscopic ellipsometry studies of i-CVD polymer films –

Observation of film thickness reduction

The application of in situ SE allows the monitoring of the i-CVD process from the initial

monomer adsorption, to the layer growth and finally to the end of the deposition process.

In the previous section, SE has been applied to investigate the initial adsorption process

of the V3D3 molecule on several substrates. In this section, SE will be applied to

investigate the poly-V3D3 bulk growth. In particular, an interesting observation will be

discussed: the film thickness reduction (Δt) occurring during the evacuation of the

deposition chamber after the i-CVD processing of V3D3-polymers under certain

deposition conditions. The condensation of the monomer on the growing polymer layer

can be excluded as the monomer partial pressure, 0.583 mbar, is kept below its vapor

pressure, 1.561 mbar (at 313 K). The absolute thickness reduction is defined as the

difference between the film thickness as result of the deposition process (as-deposited),

46

25 30 35 40 45 50

140

150

160

170

180

190

200

Thic

kness (

nm

)

Time (min)

End deposition

Chamber evacuation

0 10 20 30 40 500

50

100

150

200

Thic

kness (

nm

)

Time (min)

Setting P

Start deposition

End deposition Chamber evacuation

0 1 2 3 4 5 6 7 8 9 10 11 12 130.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Thic

kness (

nm

)

Time (min)

Set pressure

start deposition:

set grid temperature

10.5 11.0 11.5 12.0 12.5 13.00

2

4

6

8

10

12

Thic

kness (

nm

)

Time (min)

Start deposition

Set grid temperature

(b) (a)

(c) (d)

and the ex situ film thickness as measured with SE after chamber evacuation and

exposure of the film to the environment. An example of an in situ, real-time SE

monitoring is given in Fig. 4-10a.

Fig. 4-10: Example of growth profile of an i-CVD deposited layer as monitored in situ with SE

(a) showing the different phases of the growth: initial adsorption, film growth and the chamber

evacuation in which thickness losses can be observed under certain deposition conditions. The

initial adsorption is shown more closely in (b). The delay time existing between the setting of the

grid temperature and the start of the film growth is illustrated in (c). A close view of possible

thickness losses during chamber evacuation at the end of an i-CVD deposition is shown in (d).

First, the monomer adsorbs on the substrate when setting the working pressure (Fig. 4-

10b). The initial monomer adsorption takes place during this time interval and is

dependent on the deposition conditions, as studied in 4.1. The second region monitors the

polymerization process which develops as soon as the grid temperature is brought to the

set temperature, once the pressure is stable and the monomer surface concentration is

stabilized. The start of the film growth shows a short time delay since the grid requires

some time to reach the set temperature (Fig. 4-10c). During the growth process, the

pressure is kept constant resulting in a linear growth. The deposition is concluded by

closing the precursor flow meters and switching off the grid heating system. A third

region can then be observed in which the deposition chamber is gradually evacuated until

the base pressure of 10-6

mbar is reached. Once at base pressure, the system is

pressurized. Under certain deposition conditions, the thickness profile exhibits a

47

thickness loss upon chamber evacuation performed at the end of the deposition as shown

in Fig. 4-10d.

Observation of thickness losses

Previous studies reported in literature where p(V3D3) is investigated for its

biocompatibility and dielectric properties in neuroprosthetic devices [23, 29, 83, 84]

, have

shown that, under specific conditions, the film exhibits small visible islands of

micrometer-size disk-shaped voids. This is hypothetically attributed to the entrapment of

either short oligomers or single monomer units in the polymerized film, which are then

released upon chamber evacuation. The outdiffusion would then suggest an insufficient

cross-linking degree of the deposited polymer. However, neither thickness losses are

monitored, nor in situ, real-time studies are carried out to further investigate this

observation. Therefore, this thesis work further investigates the deposition of p(V3D3) by

monitoring the film growth with in situ SE in order to obtain direct information on the

growth process and optical properties of the film.

Differently from the work of O’Shaughnessy et al. (p(V3D3)), and Achyuta et al.

(polyV3D3-HVDSO), a SEM image (JEOL 7500FA, Fig. 4-11) of a deposited layer in our

setup characterized by a thickness reduction of 62 nm does not show the presence of any

micrometer-size disk-shaped voids [29, 83]

.

Fig. 4-11: SEM image (JEOL 7500FA, top view, amplification: 1200x) of a layer showing 62 nm

of thickness losses. Unlike the hypothesis of Achyuta et al., no visible disk-shaped voids are

observed [29]

.

4.2.2 Time evolution study of the thickness losses

The observation of the thickness losses (Δt) raises the question whether it is a bulk or a

surface related phenomenon. To investigate this phenomenon a study on the Δt evolution

as function of the deposition time is performed: this would highlight whether the Δt is

related to surface processes, i.e. the presence of an excessive adsorbed monomer on the

surface of the substrate or growing layer, independently of the deposition time/layer

thickness, or not.

10 μm

48

5 10 15 20 25 30 35 4010

20

30

40

50

60

70

80

90

t (n

m)

Deposition time (min)

5 10 15 20 25 30 35 4010

15

20

25

30

35

40

t%

Deposition time (min)(a) (b)

Fig. 4-12: a) Absolute thickness reduction increasing linearly with the deposition time. b)

Relative thickness reduction as a function of the deposition time (Reproducibility showed

accuracy within 3.3% and is therefore assumed as the maximum deviation).

In Fig. 4-12 the trend of the absolute and the relative thickness losses (is presented as a

function of the deposition time: the results suggest that the thickness losses are a bulk-

related phenomenon. Furthermore, when the relative thickness reduction is considered, a

plateau can be observed for sufficiently long deposition times: this suggested that, under

the hypothesis that the thickness loss is attributed to unreacted monomer, this latter

participates into polymerization processes also in the bulk of the growing layer. Within

20 minutes of deposition, equilibrium between reacted and unreacted monomer is reached

in the polymerization process.

5 10 15 20 25 30 35 402

4

6

8

10

12

Rd (

nm

/min

)

Deposition time (min)

Apparent Rd

Actual Rd

Fig. 4-13: Apparent and actual deposition rates calculated for the time/thickness evolution study

(Maximum deviation of 5% assumed based on reproducibility).

Alternatively to the thickness loss, the deposition rate Rd can be also studied (Fig. 4-13).

We refer to an apparent Rd when this is determined from the slope of the in situ SE-

derived thickness measurements, while the actual Rd is calculated from the thickness of

the layer measured ex situ. The apparent Rd is independent of the deposition time

49

Th

ickn

ess

because the measured thickness includes polymerized material, short oligomers and

unreacted monomer units.

A comparison between a continuous and a step-by-step deposition, reported in Figs. 4-14

and 4-15 supports the hypothesis on the propagation of the polymerization step in the

layer bulk.

Fig. 4-14: a) Film growth profile monitored by in situ SE for a continuous deposition of p.e. 20

minutes. b) Film growth profile monitored by in situ SE for a step by step deposition. Every step

consists of 5 minutes of deposition corresponding with a total deposition time of 20 minutes in

agreement with the continuous deposition.

The layers which have been obtained from alternating deposition and evacuation steps are

characterized by larger thickness losses. On the contrary, when the deposition is a

continuous process, the overall thickness loss is smaller because the polymerization

propagates into the bulk of the growing film.

0 2 4 6 8 10 12 14 16 18 20 220

10

20

30

40

50

60

70

Continuous deposition

5 min steps deposition

t(

nm

)

Deposition time (min)

Fig. 4-15: Comparison between the absolute thickness losses for a continuous and step-by-step

deposition with a similar total deposition time going from 5 to 20 minutes.

4.2.3 Effect of process parameters on the thickness losses

Besides the effect of the deposition time on the material reduction indicating the bulk

phenomenon, other deposition parameters are affecting the process under investigation.

The study of these parameters is based on in situ SE measurements and complementary

50

(a) (b) 0.0 0.1 0.2 0.3 0.4

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Tsub

= 313 K

Tsub

= 333 K

Adso

rbed thic

kness

(nm

)

PM/P

sat

Tsub

= 353 K

310 320 330 340 350 3600

5

10

15

20

25

30

35

40

t(

nm

)

Tsub

()

analysis (FT-IR spectroscopy, RBS/ERD), providing information about the process

window of the i-CVD process, using V3D3 and d-TBPO as monomer and initiator

respectively, and allowing to gain insight into the film growth mechanism.

Role of substrate temperature on p(V3D3) layers

An important deposition parameter affecting the polymerization process is the substrate

temperature, Tsub, as it has influence on both the monomer adsorption and the

polymerization process. To investigate the effect on the thickness reduction the substrate

temperature is varied from 313 up to 353 K. As demonstrated in Chapter 2 and in [30],

the substrate temperature affects PM/Psat, resulting in a PM/Psat range in this thesis

corresponding with 0.374-0.039, for the temperature range here investigated.

Fig. 4-16: a) Absolute thickness losses as a function of the substrate temperature showing a

decrease for increasing Tsub due to the enhanced polymerization process (Reproducibility showed

accuracy within 3.3%). b) Adsorbed thickness determined via adsorption isotherms for various

substrate temperatures by mimicking the deposition conditions.

The observed thickness losses (Fig. 4-16a) as function of the substrate temperature,

confirm the hypothesis that the thickness loss is correlated to the monomer adsorption

(Fig. 4-16b): as the temperature increases, the monomer adsorption decreases, as also

demonstrated in section 4.1.2.

51

310 320 330 340 350 3601.45

1.46

1.47

1.48

1.49

1.50

As-dep

Vacuum

nin

situ (

63

3 n

m)

Tsub

()

Fig. 4-17: Refractive index trend as a function of the substrate temperature. An increasing n is

observed for higher substrate temperatures indicating the densification of the polymer matrix in

correspondence with the reduced thickness losses for higher Tsub.

The data analysis reported till now concerns with the thickness loss. However, it is

necessary to understand how the thickness loss process affects the layer quality. More

information can be gained by studying its optical and chemical properties. The trend of

the refractive index, measured for both the as-deposited and in vacuum condition is

presented in Fig. 4-17 for depositions performed on Si substrates. The increase in

refractive index for both the as-deposited and vacuum conditions can indicate a layer

densification process with increasing substrate temperature. Similar results are obtained

for PECVD deposited layers, p.e. for low-k SiOCH composite layers [45, 77, 85]

. However,

RBS/ERD measurements show a constant layer density (varying between 1.06 (± 0.05)

and 1.15 (± 0.06) g/cm3) as differences are within the experimental error. Furthermore,

the composition is also constant, i.e. a ratio of C-to-Si equal to 3.2:1 is reported, close to

the stoichiometric ratio of 3:1 of the monomer unit. The slightly higher carbon content is

most likely due to the incorporation of initiator which causes the polymerization to be

initiated. Therefore, the change in refractive index can indicate a different polymer matrix

rearrangement upon a change in substrate temperature. As matter of fact, in 4.1.1 it has

already been shown that an increase in temperature is responsible for an increase in

polymerization degree.

52

1150 1100 1050 1000 950 9000.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

0.00035

0.00040

(SiO)3 (short polym chain)

Abs / thic

kness (

nm

-1)

Wavenumber (cm-1)

Tsub

= 313 K

Tsub

= 333 K

Tsub

= 353 K

Si(CH2)

x- Si

(SiO)3 (long polym chain)

Increasing Tsub

Fig. 4-18: FT-IR spectra of the substrate temperature series focused on the Si-O-Si region. A

transition from short to long chain polymers is observed for increasing substrate temperature.

Complementary to the in situ SE measurements, FT-IR spectra provide more insight into

the polymer structure. The region of interest is the one of the Si-O-Si asymmetric stretch

(900 cm-1

- 1150 cm-1

) (Fig. 4-18). The deconvolution of the Si-O-Si region into three

peaks is already demonstrated in 4.1.1 (Fig. 4-3). By performing such a deconvolution by

using Gaussian functions the ratio between the long and short chain contribution is

determined as a function of the substrate temperature (Fig. 4-19). In Table 4-1 the effect

of the various temperatures on the deconvolution results is also reported in terms of peak

width and area showing indeed a difference between 313 K, at which high thickness

losses appear, and the higher temperatures showing negligible thickness losses.

310 320 330 340 350 3600.00

0.10

0.20

0.30

0.40

0.50

0.60

A (

SiO

) 3 long c

hain

/ A

(S

iO) 3

short

chain

Tsub

()

Fig. 4-19: Ratio between the areas of the (SiO)3 long, high molecular weight cross-linked chain

and (SiO)3 short, low molecular weight chain.

As the substrate temperature increases, a transition in the polymer structure from short,

low molecular weight chains to long, high molecular weight chains occurs, up to a

substrate temperature of 333 K after which it stabilizes, in agreement with the trend in

53

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

i-CVD process

window

Ads thickness @ Tsub

= 313 K

Ads thickness @ Tsub

= 333 K

Ad

so

rbe

d m

on

om

er

thic

kn

ess (

nm

)

PM/P

sat

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400

10

20

30

40

Th

ickn

ess lo

sse

s

t (n

m)

PM/P

sat(a) (b)

refractive index (Fig. 4-17) and decrease in Δt (Fig. 4.16a). As the temperature increases,

the monomer surface concentration is reduced, furthermore its surface mobility is

enhanced, promoting the polymerization process and reducing the thickness losses. When

considering the RMS surface roughness of two layers (with similar thickness) deposited

at different substrate temperatures, corresponding to low and high Δt (e.g. 313 and 333

K) a decrease is observed from 0.94 (± 0.02) nm to 0.51 (± 0.02) nm, indicating a

morphology improvement as well.

Table 4-1: Fitting parameters of the Si-O-Si absorption band deconvolution. p(V3D3) 313 K p(V3D3) 333 K p(V3D3) 353 K

Si-(CH2)x-Si ω Position (cm-1

) 967 967 967

FWHM (cm-1

) 20 21 21

Area 2.6 (± 0.2)·10-3

3.7 (± 0.1)·10-3

3.9 (± 0.1)·10-3

(SiO)3 νa long

chain Position (cm

-1) 991 991 991

FWHM 17 23 23

Area 1.5 (± 0.1)·10-3

4.9 (± 0.1)·10-3

5.1 (± 0.1)·10-3

(SiO)3 νa short

chain Position (cm

-1) 1008 1011 1011

FWHM 45 38 39

Area 15.8 (± 0.1)·10-3

9.4 (± 0.1)·10-3

8.9 (± 0.1)·10-3

4.2.4 Process window definition

The previous studies carried out by means of SE allow defining the i-CVD process

window. The explored experimental conditions show that the layers exhibiting no or

negligible thickness loss (Fig. 4.20b) are characterized by an initial adsorbed monomer

thickness falling below the inflection point in the adsorption isotherm studies (Fig. 4-

20a).

Fig. 4-20: i-CVD process window identification based on the monomer adsorption (a) and the

thickness losses (b) as a function of PM/Psat.

Therefore, by tuning PM/Psat with the substrate temperature or any other process

parameter (i.e. precursor flow ratio, pressure), a well cross-linked polymer structure

exhibiting no thickness loss upon evacuation can be achieved by studying the adsorption

of the monomer unit on the substrate, which should be limited to an adsorbed monomer

54

0 2 10 120

50

200

Th

ickn

ess (

nm

)

Time (min)

Start deposition

Shutter off

5 10 20 25 300.0

0.5

1.0

1.5

2.0

Th

ickn

ess (

nm

)

Time (min)

Start deposition:

set grid

Set pressure:

pore filling

(b) (a)

thickness corresponding with the inflection point in the isotherm. So this means that the

adsorbed monomer thickness should be restricted to maximum 0.15 nm of adsorbed

monomer thickness, which is around one third of the monolayer thickness.

The real-time monitoring and analysis of the i-CVD process has allowed drawing the

following conclusions:

1. In situ SE provides insight into the different growth stages of the V3D3

polymerization process. This has revealed the presence of thickness losses upon

evacuation under certain deposition conditions, and additionally demonstrated

the thickness losses to be monomer adsorption related.

2. The PM/Psat parameter is identified as key parameter to promote a transition in

polymer structure from short, low molecular weight to long, cross-linked chains,

as highlighted by IR analysis of the Si-O-Si stretching mode absorption band.

3. The i-CVD process window for the V3D3 polymerization is defined from the

monomer adsorption study. By selecting the process parameters, PM/Psat and

therefore the monomer adsorption, can be tuned in a range below the inflection

point of the isotherm curve, i.e. approximately corresponding with 0.10-0.15 nm

of adsorbed monomer thickness, leading to stable V3D3 polymer layers.

4.3 Study of RF PECVD depositions from V3D3

The deposition of polymerized-V3D3 films has been discussed in the previous sections.

The same monomer can also be used for the deposition of organic (SiOxCyHz) and

inorganic (SiO2-like) layers in an RF PECVD process. The main difference in growth

process between the PECVD and i-CVD organic interlayer is the pore filling possible for

the i-CVD monomer during the initial monomer adsorption stage (Fig. 4-21b), whereas a

linear growth profile in the case of the PECVD process (Fig. 4-21a) is observed once the

shutter, shielding the substrate, is removed (when plasma conditions and pressure are

stable).

Fig. 4-21: a) Growth profile of the organic/inorganic PECVD layer as monitored with in situ SE.

The film growth is started by the removal of the shutter from the deposition chamber once the

plasma conditions and pressure are stable. b) Initial monomer adsorption able of filling the pores

of the barrier layer underneath preceding the actual i-CVD growth.

55

The film growth in the PECVD processes depends on reactions between the various

species, i.e. molecules, atoms, radicals, ions and electrons, striking the surface, lacking

the need for initial monomer adsorption as in the i-CVD process.

The deposition of a good quality SiO2-like layer will be of key importance as this layer,

in combination with the organic interlayer (either i-CVD or RF PECVD), will function as

a diffusion barrier for moisture and oxygen permeation. The film properties of both

organic and inorganic layers are investigated in the following paragraphs based on a

variation in the plasma key parameters, such as input power, pressure and precursor flow

rates, in order to obtain the most suitable conditions and layer properties with respect to

the application in moisture diffusion barrier systems.

4.3.1 Deposition of SiO2-like and SiOxCyHz type layers from V3D3 monomer

In order to deposit either organic or inorganic layers, it has been illustrated in Chapter 2

that the tuning of the layer chemistry and properties can be achieved by varying the key

parameters of the deposition process, i.e. the plasma power input, the oxygen-to-

monomer flow rate ratio and deposition pressure. For the deposition of dense SiO2-like

layers it was found in this work that the addition of Ar in the deposition process can

reduce the silanol (SiOH functionalities) contribution considerably (Fig. 4-22a and b),

most certainly related to the positive role of ion bombardment which allows film

densification. A decrease in SiOH concentration is generally accompanied by an increase

in film density, which leads to better barrier performances. Besides the addition of Ar, it

is generally reported, e.g. for HMDSO (briefly introduced in Chapter 2) that a high

oxygen-to-monomer flow rate ratio, high RF input power and low pressure are needed to

reduce the carbon incorporation and thus obtain dense, C-free coatings as the monomer

fragmentation is enhanced under these conditions [16, 47, 86]

. The addition of Ar allows a

more extensive ion bombardment (more ions available), whereas a higher power provides

more energy to the ions and thus accelerating them. By reducing the pressure, the mean

free path of the species is increased as well as the potential difference between plasma

and substrate, which also enhances the ion bombardment and consequently leads to

denser layers. By tuning the deposition parameters (deposition conditions in Table 3-1), a

dense (n = 1.45, t = 100 nm, low silanol content), carbon-free (or at least below the

detection limit of both FT-IR and RBS/ERD) inorganic SiO2-like layer (Fig. 4-22c) has

been obtained which is shown to be suitable for the barrier performance tests discussed

further on.

56

4000 3500 3000 2500 2000 1500 1000 500

0.0000

0.0004

0.0008

0.00120.0000

0.0004

0.0008

0.00120.0000

0.0004

0.0008

0.0012

V3D

3 = 0.7 sccm; O

2 = 35 sccm; Ar = 70 sccm

P = 250 W; p = 0.3 mbar

Wavenumber (cm-1)

V3D

3 = 0.7 sccm; O

2 = 35 sccm; Ar = 50 sccm

P = 200 W; p = 0.45 mbar

Abs / thic

kness (

nm

-1)

(a)

(b)

(c)

nin situ

= 1.42

nin situ

= 1.43

nin situ

= 1.45

SiOSi

SiOSi

Si-OH

Short and long Si-O-Si a

V3D

3 = 0.7 sccm; O

2 = 35 sccm

P = 200 W; p = 0.45 mbar

Si-OH

Fig. 4-22: FT-IR spectrum of SiO2-like layer deposited via PECVD in the absence of Ar (a), in

addition of Ar (b), and for increased power and reduced pressure (c).

In addition to these signals, a limited silanol (Si-OH) contribution is observed in the

dense film as presented by the absorption band around ~3663 cm-1

(H-bond free Si-OH) [47, 50, 52, 86]

and the absence of the related bending absorption band ~ 930 cm-1

.

Complementary RBS/ERD analysis has revealed a layer stoichiometry for O:Si of 2.1:1,

and a layer density of 2.02 (± 0.01) g/cm3, in close agreement with literature for dense

layers [5, 87]

.

The surface morphology of the best SiO2-like layer was investigated by AFM

measurements showing an RMS surface roughness of 0.34 (± 0.01) nm. The

role/development of the surface roughness concerning the implementation in a multilayer

diffusion barrier system will be discussed in section 4.4.

Since the SiO2-like layer will be implemented in the deposition of multilayer moisture

diffusion barrier systems, the WVTR has been measured (at Holst Centre Eindhoven) for

such a single, C-free SiO2-like layer, based on the intrinsic properties of the layer,

meaning that the permeation through defects and pinholes is not accounted for. This

evaluation of intrinsic barrier performances allows correlating the WVTR with the

microstructure of the inorganic layer, i.e. its density, Si-O-Si ring structure and presence

of micro/meso porosity. For a 100 nm SiO2-like layer, an intrinsic WVTR value of 2.8 (±

0.1)·10-4

g/m2day has been obtained which is a considerable improvement with respect to

the bare PEN (1.6 g/m2day). Since the intrinsic barrier properties cannot be compared

with literature as these are extrinsic values, the only reference is a 30 nm ETP-CVD

grown Al2O3 layer (currently performed within the PMP group at TU/e) showing an

57

intrinsic WVTR of 5.4·10-5

g/m2day, although the layer density is higher than the SiO2-

like layer (2.90 vs. 2.02 g/cm3).

4000 3500 3000 2500 2000 1500 1000 500

0.0000

0.0001

0.0002

Si-(CH3)

x

O-Si-O

CHx

Si-O-Si a

SiOxC

yH

z

V3D

3/O

2 = 0.7:0

Abs / thic

kness (

nm

-1)

Wavenumber (cm-1)

Si-H CH

2

Si-O-Si s

0.0000

0.0002

0.0004

0.0006

0.0008

H-Si-O

Si-(CH3)

x

SiOxC

yH

z

V3D

3/O

2 = 0.7:20

free Si-OH H-bonded Si-OH

Si-O-Si a

Fig 4-23: Effect of the oxygen dilution on the PECVD organic layer chemistry.

With a similar approach, a carbon-containing SiO2-like layer (SiCxHyOz) can be

deposited: in Fig. 4-23 the comparison is made between two organic type layers, one of

which is deposited with (20 sccm) and one without (0 sccm) oxygen flow rate. By the

addition of oxygen to the process, the carbon content, related to various Si-(CH3)x

vibrational modes, is reduced considerably. The CHx and Si-(CH3)x related absorption

bands appear at 3050-2800 cm-1

, 1462 cm-1

, 1410 cm-1

, 1275 cm-1

, 915-650 cm-1

, as

assigned in Fig. 4-23 [45, 48, 51, 88]

. Besides the carbon related absorption bands there is an

additional difference in the Si-O-Si absorption bands. For the high oxygen flow rate this

peak is rather sharp with a shoulder at higher wavenumber, and it is comparable to the

SiO2-like layer, whether in the absence of oxygen the absorption band is broader,

indicating a more heterogeneous chemical environment including next to Si-O-Si bonds

also Si-O-C bonds.

4.3.2 Comparing i-CVD and PECVD organic layer properties

The main difference in layer properties between the plasma and the i-CVD

polymerization process is related to the Si-O-Si absorption band as highlighted in Fig. 4-

24.

In the i-CVD process the siloxane ring is preserved as the polymerization proceeds

through the vinyl bonds, and therefore a narrow absorption band (1100-900 cm-1

) is

measured consisting of long and short chain contributions but for a preserved monomer

ring embedded into the polymer network (Fig. 4-24a). In addition a wagging signal is

observed at the low wavenumber side. Whereas, in the case of plasma deposition, the

siloxane ring is opened and the the Si-O-Si absorption band range (1200-900 cm-1

) is

extended (Fig. 4-24b). Although the network structure of a SiO2-like or SiOxCyHz layer

can vary considerably, the Si-O-Si stretching mode IR absorption band generally consists

of a main peak related to long Si-O-Si chains or small rings, and on the other hand

a)

58

1200 1150 1100 1050 1000 950

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

0.00035

Abs / thic

kness (

nm

-1)

Wavenumber (cm-1)

Exp. data

Open ring Si-O-Si long chains

Open ring Si-O-Si short chains

Si-O-C chains

Fit

1200 1150 1100 1050 1000 950

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

0.00035

Exp data

(SiO)3 long chain

(SiO)3 short chain

Si(CH2)

xSi

Fit

A

bs / thic

kness (

nm

-1)

Wavenumber (cm-1)

(a) (b)

possesses a lower intensity shoulder (at higher wavenumber), related to a disordered

network structure composed of short Si-O-Si chains or larger rings [16, 24, 44, 48-50]

. The

third peak is assigned to the presence of Si-O-C bonds typical for organic plasma

deposited layers. The ratio long-to-short chain is typically higher for dense layers

compared to porous layers as the cross-linking is improved reducing the void fraction.

Besides the main Si-O-Si absorption band the following absorption bands are observed as

well: Si-O-Si rocking (~450 cm-1

) and the Si-O-Si bending (~815 cm-1

). The presence of

these Si-O-Si related signals are indicating the formation of a silica-like network, mainly

composed of long Si-O-Si chains or small rings. Further proof of the ring opening can be

observed by the presence of H-terminated Si (~2156 cm-1

) as is the case for the non-

diluted SiOxCyHz spectrum in Fig. 4-23.

In terms of carbon incorporation, the p(V3D3) i-CVD-polymer shows a slightly higher

carbon content of 31 (± 5) % compared to 27 (± 5) % for the PECVD polymer. However,

the difference is within the error, so pointing to a difference mainly in the network

structure. If there is slightly lower carbon content in the case of the PECVD this is

probably due to the ion-bombardment and monomer fragmentation paths involved.

Fig. 4-24: a) Si-O-Si absorption band as measured for an i-CVD polymer. b) Si-O-Si absorption

band measured for a PECVD deposited polymer. A clear difference in absorbance and width of

the absorption band is observed.

In addition to the similar carbon content for both processes, the density obtained from

RBS/ERD data of the PECVD and i-CVD organic layers can also be considered as

similar, namely 1.06 ± 0.06 g/cm3 vs. 1.15 ± 0.06 g/cm

3. So far the properties are similar

except for the polymer chemistry. A final comparison that can be made is in terms of

RMS surface roughness for the PECVD and i-CVD layer, showing a surprisingly low

RMS value of 0.32 ± 0.03 nm for the PECVD layer, compared to a value of 0.51 ± 0.02

nm for the i-CVD- polymer. Based on the previous outcomes the implementation of both

layer types in a multilayer moisture diffusion barrier system is discussed in the following

section.

4.4 Multilayer diffusion barrier systems

Up to now single organic and inorganic layers have been discussed in terms of layer

properties, as they can be deposited by either i-CVD/PECVD (for organic films) or

59

PECVD (for inorganic films). A comparison can now be made between similar

multilayer stacks with either the i-CVD or PECVD organic layer.

Surface roughness study

The model systems, for which the surface roughness development and the smoothening

effect are studied, are the following (Fig. 4-25):

Fig. 4-25: Model systems studied for different layer order and various organic layer deposition

methods.

For the deposition of the i-CVD p(V3D3) layer the deposition temperature was chosen to

be 333 K as good layer properties in terms of negligible thickness losses, polymerization

degree, etc. were found.

In Fig. 4-26 an overview of the RMS surface roughness is illustrated according to the

model systems presented in Fig. 4-25, as these correspond with the various multilayer

barrier systems considered in this study. The single SiO2-like, SiOxCyHz and p(V3D3)

layers are used as references.

The RMS surface roughness of the SiO2-like layer further develops when deposited on

top of an organic layer (Model system b and Fig. 4-26: blue), independently of the

deposition technique of the organic layer.

Barrier layer: 100 nm SiO2 Organic interlayer: 200 nm SiOxCyHz layer by i-CVD or PECVD

Silicon substrate

Organic interlayer: 200 nm SiOxCyHz layer by i-CVD or PECVD Barrier layer: 100 nm SiO2

Silicon substrate

(a)

(b)

60

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

SiO 2

/ i-C

VD p

(V 3D 3

)

Silico

n re

f

SiO 2

/ PECVD S

iOCH

PECVD S

iOCH /

SiO 2

i-CVD p

(V 3D 3

) / S

iO 2

RM

S s

urf

ace r

oughness (

nm

)

i-CVD p

(V 3D 3

)

PECVD S

iOCH

SiO 2

Fig. 4-26: RMS surface roughness values for various multilayer barrier systems. A comparison is

made according to the two model systems of Fig. 4-25 (Blue: model b; Green: model a) and with

respect to the single layers (Red) and the silicon reference (Cyan).

When the organic layer is deposited on top of the SiO2-like layer (Model system a and

Fig. 4-26: green), a different behavior is observed. The roughness development of the

p(V3D3) i-CVD-layer is changing from 0.51 ± 0.02 nm (on bare Si) to 0.58 ± 0.02 nm (on

SiO2-like layer) whereas the SiOxCyHz PECVD-layer undergoes an increase from 0.32 ±

0.03 to 0.54 ± 0.01 nm. Based on these results, it can be suggested that the i-CVD layer

deposition shows more conformality in film growth and this is in agreement with the

results previously reported in 4.2, where the i-CVD layer is shown to be able to fill the

micro/meso pores present in the layer underneath and, therefore, promoting a

decelerating effect of the surface roughness development of the SiO2-like layer.

Barrier performance of multilayer systems

The barrier performance of several multilayer barrier systems has been checked by means

of the Ca-test. Therefore, the multilayer barrier system has been deposited on top of a

PEN coated glass substrate, and subsequently a Ca and SiNx (as barrier, WVTR = 1·10-6

g/m2day) layer have been deposited on top of the multilayer. Three different multilayer

barrier systems have been considered, as reported in Fig. 4-27. By performing the Ca-test

the intrinsic barrier properties can be determined, meaning that the permeation through

pinholes and defects can be separated from the permeation through the bulk of the layer.

However, when the presence of defects/pinholes is massive, the intrinsic barrier

properties can no longer be independently determined and only an overall permeation rate

can be provided. A qualitative evolution of the Ca oxidation (changing transmission)

with time, due to water permeation is shown in Fig. 4-28 in which both single (reference)

and multilayer barrier systems (Schematic overview in Fig. 4-27a and b), with either an i-

CVD or PECVD organic interlayer, are tested for the WVTR.

61

Fig. 4-27: a) Single barrier layer system used as reference, b) Multilayer barrier system which

can basically be considered as two sets: i-CVD/PECVD combined multilayer systems and fully

PECVD multilayer systems, c) Two dyad (organic-inorganic) multilayer barrier system.

The origin of the large amount of defects/pinholes into the barrier systems is investigated

further by taking a closer look with an optical microscope. It has been observed that

cracks only appear when depositing a SiO2-like layer on first on top of the PEN substrate

(Fig. 4-29a and b). The reason of the cracks is thought to be related to the build-in

(tensile) stress when growing the SiO2-like layer. When the organic layer is deposited on

top of the PEN substrate (according to Fig. 4-27c), prior to the SiO2-like layer, no cracks

have been observed (Fig. 4-29c), pointing out the fact that the organic interlayer

promotes a better adhesion of the inorganic barrier layer to the substrate.

Fig. 4-28: Images of CCD camera showing the time evolution of the Ca oxidation of the

processed single, reference layer and multilayer barrier systems for both i-CVD/PECVD

combined (a) and fully PECVD (b) deposited multilayer.

The large amount of defects/pinholes is probably related to the processing of the PEN

substrate. In addition, dust and filler particles can be incorporated as well into the PEN

substrate, giving rise to a higher density of defects/pinholes.

0 days 1.8 days 3.8 days

PEN / SiO2 / p(V3D3) / SiO2 / Ca / SiNx

PEN / SiO2 / Ca / SiNx

0 days 1.8 days 3.8 days

PEN / SiO2 / SiOxCyHz/ SiO2 / Ca / SiNx

PEN / SiO2 / Ca / SiNx (a) (b)

(a)

PEN

Barrier layer (100 nm)

(b)

PEN

Barrier layer (100 nm) Organic interlayer (200 nm)

Barrier layer (100 nm)

(c)

PEN

Barrier layer (100 nm) Organic interlayer (200 nm)

Barrier layer (100 nm) Organic interlayer (200 nm)

62

(a) (b) (c)

Fig. 4-29: a) Presence of cracks in a fully oxidized sample with a single SiO2-like layer, b) A

similar system with a single SiO2-like layer but still with some Ca (dark areas) left unoxidized,

also showing the presence of cracks, c) A multilayer barrier system consisting of a double i-CVD

/ PECVD (organic / inorganic) structure which does not reveal any cracks in the sample (Dark

areas represent unoxidized Ca).

In Fig. 4-30 a comparison of the WVTR values is made for specific single and multilayer

barrier systems as they are presented in Fig. 4-28 by the evolution of the Ca oxidation.

10-4

10-3

10-2

10-1

100

WV

TR

(g

/m2d

ay)

PEN /

SiO

2 / S

iOxC y

H z /

SiO

2

PEN /

SiO

2 / p

(V 3D 3

) / S

iO2

PEN /

SiO

2

PEN /

SiO

2

Unco

ated

PEN

i-CVD/PECVD

fully PECVD

Fig. 4-30: a) WVTR values for three types of barrier systems and compared for the i-

CVD/PECVD combined and fully PECVD technique. b) WVTR values (25°C, 98% R.H.)

measured for a PEN/SiO2 / p(HVDSO) / SiO2 multilayer barrier system [15]

.

A clear difference between the deposition techniques is observed as the Ca oxidation

proceeds faster (more bright spots) for the fully PECVD multilayer barrier system,

shortly after assembling (deposition of Ca and SiNx) the sample.

Starting from a single SiO2-like layer (deposited on PEN) as reference, the barrier

performance is improved by an order of magnitude when applying the i-CVD/PECVD

combined approach. The plasma deposited multilayer, on the other hand, does not

improve the WVTR compared to the single barrier layer. As addressed in 4.2, the growth

63

mechanism of the i-CVD organic layer includes the filling of the micro/meso porosity

present in the inorganic barrier layer; furthermore as shown earlier in 4.3, a decelerated

roughness development accompanies the deposition of the i-CVD layer on the SiO2-like

film. Therefore, it can be concluded that the better barrier performances in the presence

of the i-CVD layer are due to the improvement of the SiO2-like microstructure

underneath as well as to a more controlled surface morphology (i.e. limited surface

roughness evolution) for the deposition of the 2nd

SiO2-like layer. On the contrary, when

the multilayer is fully grown by PECVD, it can be expected that any micro/meso porosity

present in the 1st SiO2 layer will propagate further in the organic interlayer and in the 2

nd

SiO2 layer. Although the conformal growth of the i-CVD layer appears essential to

successfully deposit barrier systems, it should be noted that the i-CVD layer thickness

here under study is still rather limited (200 nm) and, therefore, the possibility for

conformal growth also on large area pinholes (in the range of hundreds of nm diameter)

should be checked.

Finally, when compared to recent literature results [15]

, the system here under

investigation shows the same improvement factor (10x) with respect to a single SiO2

layer, as in the case of a 400 nm thick p(HVDSO) sandwiched between two SiO2 layers.

However, the overall barrier improvement factor (i.e. with respect to the pristine

polymer) is in our case higher than 1000 (more specifically 2400), while in [15] it is

limited to a factor 10.

From the study of the role of the organic interlayer in a multilayer diffusion barrier

system the following conclusions can be drawn:

1. The implementation of the i-CVD technique gives rise to conformal layers when

deposited on top of an inorganic barrier layer, due to the micro/meso pore filling

ability of the i-CVD monomer. A decelerated roughness development is observed

for the organic i-CVD layer compared to its PECVD counterpart.

2. The conformality of the i-CVD layer, improving the microstructure of the barrier

layer underneath, correlates with the barrier performance being promoted by 3

orders of magnitude compared to the pristine polymer.

64

65

5 Conclusions and recommendations

5.1 Conclusions

The initiated-chemical vapor deposition technique has been setup and implemented in a

novel vacuum chamber compatible also with plasma- based deposition processes for the

engineering of multilayer organic/inorganic gas and moisture permeation barriers.

By means of an extensive diagnostic study of the i-CVD layers, the following

conclusions have been drawn:

The polymerization mechanism of the V3D3 molecule proceeds via cleavage of

the vinyl groups and retention of the siloxane ring, with a polymerization degree

of about 80%, as deduced by a detailed FTIR analysis;

The implementation of in situ (real-time) spectroscopic ellipsometry allows

following the different growth stages in the V3D3 polymerization process. In

particular, when applied to the polymer bulk growth, the determination of the

growth rate allows monitoring the transition from a kinetic-limited (with

activation energy of 65 ± 4 kJ/mol) to a mass transfer-limited regime.

Furthermore, the deposition process is found to be monomer adsorption- limited

with an activation energy of -39 ± 4 kJ/mol;

The PM/Psat parameter, which controls the monomer surface adsorption, has been

identified as key parameter to promote the transition of the polymer structure

from short, low molecular weight to long, cross-linked polymer chains, as

highlighted by the analysis of the Si-O-Si stretching mode IR absorption band;

When spectroscopic ellipsometry is applied to the monomer initial adsorption

steps, isothermal adsorption/desorption studies provide insight into the

microstructure of the layer/substrate underneath. In the specific case of

inorganic/organic multilayer systems, the SiO2-like layer is characterized by a

residual open porosity in the micro-meso transition region (pore radius ≤ 2 nm);

The isotherm monomer adsorption studies also allow defining the i-CVD process

window for the V3D3 polymerization. Stable V3D3 polymer layers exhibiting no

material loss upon evacuation can be engineered when the selected process

parameters allow the PM/Psat parameter and, therefore, the adsorbed monomer

thickness to be in a range below the inflection point of the isotherm curve, i.e.

approximately corresponding to 0.1 nm of adsorbed monomer thickness;

The microstructure characterization by means of the above-mentioned studies

implicitly points out to the role of the i-CVD organic interlayer in multilayer

barrier structures, i.e. the filling of the open micro/meso porosity of the inorganic

barrier layer. This outcome nicely correlates with the superior barrier

performances of multilayers based on the application of i-CVD organic interlayers

with respect to fully PECVD developed multilayers. This is mainly attributed to

the improvement of the SiO2-like layer microstructure underneath, upon filling of

the micro/meso porosity which promotes an overall increase of the barrier

66

properties. A final barrier improvement factor of more than 1000 is reported with

respect to the pristine polymer.

5.2 Recommendations

For a full understanding of the i-CVD process and the multilayer diffusion barrier

systems a few recommendations are given for further research as listed below.

The studies performed in this master thesis are generally considering Si as a

substrate. For implementation of the processes in flexible applications the

substrate should be replaced by for example poly(ethylene naphthalate) (PEN), in

order to perform similar studies on the bulk layer properties of the i-CVD and

PECVD deposited layers. Further morphology studies are required as well

concerning the surface roughness development on polymeric substrates as this is

important with respect to the implementation in multilayer moisture diffusion

barriers.

Since the focus of this thesis is on the i-CVD organic layer, the role of the

monomer structure on the i-CVD bulk polymer properties and the barrier

performance of the multilayer diffusion barrier system, can be understood by

applying a monomer with different functionalities (e.g. linear instead of siloxane

ring).

The i-CVD/PECVD combined multilayer barrier systems considered in this thesis

work make use of good quality i-CVD layers consisting of a long chain, cross-

linked polymer network. Instead the ratio long-to-short chains can be tuned and

therefore the effect of a short chain dominated polymer network on the barrier

performance can be studied for comparison as the layer structure and

conformality of the i-CVD layer could be affected resulting in changing barrier

performance as well.

The multilayer diffusion barrier systems require additional analysis, in situ SE

data modeling and XPS measurements, in order to study the possible interphase

development between the organic and inorganic layers as this can be expected due

to ion bombardment and etching during the plasma deposition processes. In

addition, the introduction of cracks when depositing the barrier layer at first on

top of the PEN substrate needs further understanding since only a hypothesis of

stress induced between the SiO2 and PEN is provided so far. Reducing the

contamination of the PEN substrate (delivered by Holst Centre) which is mainly

incorporated during the production and processing steps (defects, dust, filler

particles), is recommended in order to allow for intrinsic WVTR measurements.

Possible solutions are the use of a different substrate, either glass/Ca or

glass/PEN/planarization layer, on top of which the multilayer is deposited.

67

List of abbreviations and symbols

A Fitting parameter of refractive

index in Cauchy model / Pre-

exponential factor / Absorbance

AC Alternating Current

AFM Atomic Force Microscopy

Å Angstrom

B Fitting parameter of refractive

index in Cauchy model

BET Brunauer - Emmett - Teller

C Fitting parameter of refractive

index in Cauchy model

c BET constant / Concentration of

absorbing species

CVD Chemical Vapor Deposition

D Diffusivity

e Electron charge

ERD Elastic Recoil Detection

ETP Expanding Thermal Plasma

Ea Activation energy

Ediff Potential energy barrier for

diffusion

(F-)OLED (Flexible) organic light emitting

device

FT-IR Fourier Transform Infrared

Spectroscopy

FWHM Full Width Half Maximum

HMDSO Hexamethyldisiloxane

HVDSO Hexavinyldisiloxane

I Transmitted intensity of

deposited sample

IR Infrared

I0 Transmitted intensity of bare Si

i-CVD Initiated Chemical Vapor

Deposition

k Dielectric constant / reaction

rate

kJ Kilojoule

kB Boltzmann constant

K Kelvin

L Plasma dimension

l Path length of light through

sample

Me Methyl

MeV Megaelectronvolt

me Electron mass

n Real part of refractive index

ne Electron density

ni Ion density

ñ Complex refractive index

OTR Oxygen transmission rate

p Pressure

PECVD Plasma enhanced chemical

vapor deposition

PEN Poly(ethylene naphthalate)

PET Poly(ethylene terephthalate)

PPECVD Pulsed Plasma Enhanced

Chemical Vapor Deposition

PM Monomer partial pressure

Psat Saturated vapor pressure

QCM Quartz Crystal Microbalance

QMS Quadrupole Mass Spectrometry

R Ideal gas constant

RBS Rutherford Backscattering

Spectroscopy

RF Radio frequency

RMS Root mean square

Rd Deposition rate

rp Amplitude of p polarized light

after reflection

68

rs Amplitude of s polarized light

after reflection

SE Spectroscopic Ellipsometry

SEM Scanning Electron Microscopy

SPM Scanning Probe Microscope

T Temperature

d-TBPO Di-tert-butyl peroxide

Tg Glass transition temperature

Tgrid Grid temperature

Tsub Substrate temperature

Twall Chamber wall temperature

t Thickness

tad Adsorbed monomer thickness

t0 Monolayer thickness

VSC Vapor Source Controller

Vad Adsorbed volume

Vml Adsorbed monolayer volume

V3D3 1,3,5-trivinyl-1,3,5-

trimethylcyclotrisiloxane

WVTR Water vapor transmission rate

XPS X-ray Photoelectron

Spectroscopy

α Fitting parameter of extinction

coefficient in Cauchy model

β Fitting parameter of extinction

coefficient in Cauchy model

γ Band energy in Cauchy model

δ Bending mode

δa Asymmetric bending mode

δs Symmetric bending mode

ε Molar absorptivity

ε0 Vacuum permittivity

κ Imaginary part of refractive

index (Absorption coefficient)

λ Wavelength

λD Debye length

ν

Stretching mode / vibrational

frequency

νa Asymmetric stretching mode

νs Symmetric stretching mode

νen Electron-neutral collision

frequency

νpe Plasma frequency

ρ Rocking mode

ω Wagging mode

ωs Symmetric wagging mode

ωpe Angular plasma frequency

Γ Hopping rate

Δ Phase difference between p and

s polarized light

ΔHvap Heat of vaporization

Δt Thickness loss

ΦM Monomer flow rate

Φtot Total flow rate

Ψ Angle determined from

amplitude ratio between p and s

polarized light

[M] Monomer concentration

Ø Diameter

69

Bibliography

[1] P. E. Burrows, G. L. Graff, M. E. Gross, P. M. Martin, M. K. Shi, M. Hall, E.

Mast, C. Bonham, W. Bennett, M. B. Sullivan, Displays, 2001, 22, 65-69.

[2] M. Schaepkens, T. W. Kim, A. G. Erlat, M. Yan, K. W. Flanagan, C. M. Heller,

and P. A. McConnelee, Journal of Vacuum Science and Technology A, 2004,

22(4), 1716-1722.

[3] T. W. Kim, M. Yan, A. G. Erlat, P. A. McConnelee, M. Pellow, J. Deluca, T. P.

Feist, R. Duggal, M. Schaepkens, Journal of Vacuum Science and Technology A,

2005, 23(4), 971-977.

[4] T.-N. Chen, D.-S. Wuu, C.-C. Wu, R.-H. Horng, H.-F. Wei, L.-Y. Jiang, H.-U.

Lee, Y.-Y. Chang, Vacuum, 2010, 84, 1444–1447.

[5] A. Bieder, A. Gruniger, and R. von Rohr, Surface and Coatings Technology,

2005, 200, 928-931.

[6] J. Fahlteich, M. Fahland, W. Schönberger, N. Schiller, Thin Solid Films, 2009,

517, 3075–3080.

[7] G. Dennler, C. Lungenschmied, H. Neugebauer, N.S. Sariciftci, M. Latrèche, G.

Czeremuszkin, M.R. Wertheimer, Thin Solid Films, 2006, 511 – 512, 349 – 353.

[8] J. Affinito, D. Hilliard, 45th Annual Technical Conference Proceedings

of the Society of Vacuum Coaters, 2004, p. 563-593.

[9] J. S. Lewis and M. S. Weaver, IEEE Journal of Selected Topics in Quantum

Electronics, 2004, 10(1), 45-57.

[10] E. Langereis, M. Creatore, S. B. S. Heil, M. C. M. van de Sanden, and W. M. M.

Kessels, Applied Physics Letters, 2006, 89, 081915.

[11] M. Yan, T. W. Kim, A. G. Erlat, M. Pellow, D. F. Foust, J. Liu, M. Schaepkens,

C. M. Heller, P. A. McConnelee, T. P. Feist, and A. R. Duggal, Proceedings of

The IEEE, 2005, 93(8), 1468-1477.

[12] D.G. Howells, B.M. Henry, J. Madocks, H.E. Assender, Thin Solid Films, 2008,

516, 3081–3088.

[13] S. Logothetidis, A. Laskarakis, D. Georgiou, S. Amberg-Schwab, U. Weber, K.

Noller, M. Schmidt, E. Küçükpinar-Niarchos, and W. Lohwasser, The European

Physical Journal-Applied Physics, 2010, 51, 33203 (1-5).

[14] A. G. Erlat, R. J. Spontak, R. P. Clarke, T. C. Robinson, P. D. Haaland, Y. Tropsha, N. G.

Harvey, and E. A. Vogler, Journal of Physical Chemistry B, 1999, 103, 6047-6055.

[15] A. M. Coclite, G. Ozaydin-Ince, F. Palumbo, A. Milella, K. K. Gleason, Plasma

Processes and Polymers, 2010, 7, 561–570.

[16] A. M. Coclite, A. Milella, R. d'Agostino, F. Palumbo, Surface and Coatings

Technology, 2010, 204, 4012-4017.

[17] A. S. da Silva Sobrinho, G. Czeremuszkin, M. Latrèche, and M. R. Wertheimer,

Journal of Vacuum Science and Technology A, 2000, 18(1), 149-157.

[18] H. Chatham, Surface and Coatings Technology, 1996, 78, 1-9.

[19] G. L. Graff, R. E. Williford, and P. E. Burrows, Journal of Applied Physics, 2004,

96(4), 1840-1849.

[20] J. Lewis, Materialstoday, 2006, 9 (4), 38-45.

70

[21] T. N. Chen, D. S. Wuu, C. C. Wu, C. C. Chiang, Y. P. Chen, and R. H. Horng,

Journal of The Electrochemical Society, 2006, 153(10), F244-F248.

[22] Vitex Systems Inc. (2009), Available at

http://www.vitexsys.com/index.html (accessed 4th

March 2011).

[23] W. S. O’Shaughnessy, M. Gao, and K. K. Gleason, Langmuir, 2006, 22, 7021-

7026.

[24] A. K. H. Achyuta, A. J. White, H. G. Pryce Lewis, and S. K. Murthy,

Macromolecules 2009, 42, 1970-1978.

[25] Y. Mao and K. K. Gleason, Langmuir, 2004, 20, 2484-2488.

[26] T. P. Martin, K. L. Sedransk, K. Chan, S. H. Baxamusa, and K. K. Gleason,

Macromolecules, 2007, 40, 4586-4591.

[27] N. J. Trujillo, Q. Wu, and K. K. Gleason, Advanced Functional Materials, 2010,

20, 607–616.

[28] K. Chan and K. K. Gleason, Macromolecules, 2006, 39, 3890-3894.

[29] A. M. Coclite, G. Ozaydin-Ince, R. d’Agostino, and K. K. Gleason,

Macromolecules, 2009, 42, 8138-8145.

[30] K.K.S. Lau, and K.K. Gleason, Macromolecules, 2006, 39, 3688-3694.

[31] K.K.S. Lau, and K.K. Gleason, Macromolecules, 2006, 39, 3695-3703.

[32] S. H. Baxamusa and K. K. Gleason, Chemical Vapor Deposition, 2008, 14, 313-

318.

[33] G. Ozaydin-Ince and K.K. Gleason, Journal of Vacuum Science and Technology

A, 2009, 27, 1135-1143.

[34] P.J. Skrdla, International Journal of Chemical Kinetics, 2004, 36, 386-393.

[35] Material Safety Data Sheet, Gelest Inc., CAS NO. 3901-77-7.

[36] SciFinder, CAS NO. 3901-77-7.

[37] S. Brunauer, P. H. Emmett, E. Teller, Journal of the American Chemical Society,

1938, 60, 309-319.

[38] W. F. Beach, Macromolecules, 1978, 11, 72-76.

[39] K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J.

Rouquerol, and T. Siemieniewska, Pure and Applied Chemistry, 1985, 57, 603-

619.

[40] M. Song, D. J. Hourston, Y. Pang, Progress in Organic Coatings, 2000, 40, 167–

173.

[41] S. Pai and D. Doren, Surface Science, 1993, 291, 185-191.

[42] J. S. Young and C. N. Bowman, Macromolecules, 1999, 32, 6073-6081.

[43] T. R. E. Simpson, B. Parbhoob, J. L. Keddie, Polymer, 2003, 44, 4829–4838.

[44] A. Milella, M. Creatore, M. A. Blauw, M. C. M. van de Sanden, Plasma

Processes and Polymers, 2007, 4, 621–628.

[45] T.K.S. Wong, B. Liu, B. Narayanan, V. Ligatchev, R. Kumar, Thin Solid Films,

2004, 462–463, 156– 160.

[46] A. Francescangeli, F. Palumbo, R. d’Agostino, C. Defranoux, Plasma Processes

and Polymers, 2009, 6, 132-138.

71

[47] E. Vassallo, A. Cremona, L. Laguardia, E. Mesto, Surface and Coatings

Technology, 2006, 200, 3035-3040.

[48] D. D. Burkey, K. K. Gleason, Journal of The Electrochemical Society, 2004, 151,

F105-F112.

[49] D. Barreca, A. Gasparotto, C. Maccato, E. Tondello, G. Rossetto, Thin Solid

Films, 2008, 516, 7393-7399.

[50] C. Voulgaris, A. Panou, E. Amanatides, D. Mataras, Surface and Coatings

Technology, 2005, 200, 351-354.

[51] A. M. Coclite, A. Milella, F. Palumbo, F. Fracassi, R. d’Agostino, Plasma

Processes and Polymers, 2009, 6, 512–550.

[52] C. Vallée, A. Goullet, A. Granier, A. van der Lee, J. Durand, C. Marlière, Journal

of Non-Crystalline Solids, 2000, 272, 163-173.

[53] T. B. Casserly, K. K. Gleason, Plasma Processes and Polymers, 2005, 2, 679–

687.

[54] A. Francescangeli, F. Palumbo, R. d’Agostino, Plasma Processes and Polymers,

2008, 5, 708-717.

[55] M. R. Alexander, F. R. Jones, and R. D. Short, Journal of Physical Chemistry B,

1997, 101, 3614-3619.

[56] D. Magni, C. Deschenaux, C. Hollenstein, A. Creatore, and P. Fayet, Journal of

Physics D: Applied Physics, 2001, 34, 87–94.

[57] M. Creatore, F. Palumbo, and R. d’Agostino, Plasmas and Polymers, 2002, 7 (3),

291-309.

[58] M. Creatore, M. Kilic, K. O’Brien, R. Groenen, M.C.M. van de Sanden, Thin

Solid Films, 2003, 427, 137–141.

[59] M.F.A.M. van Hest, B. Mitu, D.C. Schram, M.C.M. van de Sanden, Thin Solid

Films, 2004, 449, 52–62.

[60] S. M. Gates, D. A. Neumayer, M. H. Sherwood, A. Grill, X. Wang, and M.

Sankarapandian, Journal of Applied Physics, 2007, 101, 094103.

[61] H. Fujiwara, Spectroscopic Ellipsometry: Principles and Applications, John

Wiley and Sons Ltd.: West Sussex 2007.

[62] P. H. W. Robeerst, System Integration Project II:

Diagnostics in plasma processing, 2006, Eindhoven University of Technology.

[63] H. Ahmed, Journal of Chromatography A, 2007, 1169, 179-192.

[64] H. Günzler, H-U. Gremlich, IR Spectroscopy: An Introduction, John Wiley and

Sons Ltd.: Weinheim 2002.

[65] W. F. Maddams, Applied Spectroscopy, 1980, 34 (3), 245-267.

[66] K. S. Seshadri and R. Norman Jones, Spectrochimica Acta, 1963, 19, 1013-1085.

[67] W. A. Bik, AccTec BV, http://www.acctec.nl/ (accessed 12th

December 2010).

[68] P. van de Weijer and T. van Mol, Characterisation of thin-film barrier layers for

protection of organic Light-Emitting Diodes, White paper Fast2Light, 2009.

[69] B. E. Deal and A. S. Grove, Journal of Applied Physics, 1965, 36, 3770–3776.

[70] A. L. Smith, Analysis of silicones, 4th

ed.; John Wiley and Sons Ltd.: New York

1974; Vol.41.

72

[71] B. Cruden, K. Chu, K. Gleason, and H. Sawin, Journal of The Electrochemical

Society, 1999, 146, 4590-4596.

[72] J.E. Drake and A. Vahradian and B. M. Glavincevs, Canadian Journal of

Chemistry, 1983, 61, 2712.

[73] B. Arkles, Silicone Compounds Register and Review, 1987, Petrarch Systems.

[74] H. Jeffreys, and B. S. Jeffreys, Methods of Mathematical Physics, 3rd

ed.;

Cambridge University Press, Cambridge, England, 1988.

[75] H. S. Fogler, Elements of Chemical Reaction Engineering, 3rd

ed.; Prentice Hall:

Upper Saddle River, NJ, 1999.

[76] C. Rau, W. Kulisch, Thin Solid Films, 1994, 249, 28- 37.

[77] Y.-H. Kim, S.-K. Lee, and H. J. Kim, Journal of Vacuum Science and Technology

A, 2000, 18, 1216-1219.

[78] D. Hegemann, C. Oehr, and A. Fischer, Journal of Vacuum Science and

Technology A, 2005, 23, 5-11.

[79] Y. H. Zhao, M. H. Abraham, and A. M. Zissimos, Journal of Organic Chemistry,

2003, 68, 7368-7373.

[80] Y. Itami, B. Marciniec, and M. Kubicki, Organometallics, 2003, 22, 3717-3722.

[81] V. E. Shklover, N. G. Bokii, Y. T. Struchkov, K. A. Andrianov, B. G. Zavin, and

V. S. Svistunov, Journal of Structural Chemistry, 1974, 15 (5), 841-849.

[82] C. Nataro, W. M. Cleaver, C. W. Allen, Journal of Inorganic and Organometallic

Polymers, 2009, 19, 566-569.

[83] W. S. O’Shaughnessy, S. K. Murthy, D. J. Edell, and K. K. Gleason,

Biomacromolecules 2007, 8, 2564-2570.

[84] W. S. O’Shaughnessy, Functional Thin Film Polymers for Biopassivation of

Neuroprostethic Implants, PhD thesis, Massachusetts Institute of Technology,

2007.

[85] D. Hegemann, H. Brunner and C. Oehr, Surface and Coatings Technology, 2003,

174-175, 253-260.

[86] Creatore, F. Palumbo, R. d’Agostino, P. Fayet, Surface and Coatings Technology,

2001, 142-144, 163-168.

[87] A. Grüniger, A. Bieder, A. Sonnenfeld, P. R. von Rohr, U. Müller, R. Hauert,

Surface & Coatings Technology, 2006, 200, 4564 – 4571.

[88] S. Zanini, C. Riccardi, M. Orlandi, E. Grimoldi, Vacuum, 2008, 82, 290–293.

[89] A. Grill and D. A. Neumayer, Journal of Applied Physics, 2003, 94, 6697-6707.

[90] S. K. Murthy, B. D. Olsen, K. K. Gleason, Journal of Applied Polymer Science,

2004, 91, 2176–2185. [91] J. A. Bittencourt, Fundamentals of plasma physics, 3

rd ed.; Springer, New York,

2004.

[92] M. A. Lieberman and A. J. Lichtenberg, Principles of plasma discharges and

materials processing, 2nd

ed; Wiley-Interscience, Hoboken, 2005.

[93] R. d’Agostino, Plasma deposition, treatment, and etching of polymers, Academic

Press, Inc., San Diego, California, 1990.

[94] N. Mutsukura, K. Kobayashi, and Y. Machi, Journal of Applied Physics, 1990, 68

(6), 2657-2660.

[95] H. Kobayashi, A. T. Bell, and M. Shen, Macromolecules, 1974, 7, (3), 277-283.

73

[96] H. Kobayashi, M. Shen, and A.T. Bell, Journal of Macromolecular Science, 1974,

A8, 1354.

[97] H. Yasuda, Plasma polymerization, Academic Press, Inc., Orlando, Florida 1985.

[98] S. M. Rossnagel, J. J. Cuomo, W. D. Westwood, Handbook of plasma processing

technology, Noyes Publications, New Jersey, 1990.

[99] A. Grüniger, A. Bieder, A. Sonnenfeld, P. Rudolf von Rohr, U. Müller, R. Hauert,

Surface & Coatings Technology,2006, 200, 4564 – 4571.

74

75

Acknowledgements

In order to bring a master thesis to a good end, a lot of effort and support are required.

Therefore I would like to thank the people that contributed to this work and supported me

during the past year as a graduate student in the group of PMP.

First of all I would like to thank my graduation supervisor, Richard van de Sanden, for

providing useful comments on my thesis. I would also like to thank Adriana Creatore, as

my direct supervisor, for her guidance through the project, the many fruitful discussions

and all the time and effort she has put in reading every new part of the thesis, providing

new insights into the project and making me a better physicist. And of course, many

thanks to my daily supervisor Gianfranco Aresta for his enthusiastic, energetic and

mainly scientific input and cooperation during my graduation project, in his own (in the

PMP group well known) Italian way.

Besides, I would like to thank Vincent Vandalon and Bart Macco for performing AFM

measurements. The PMP technicians, Janneke Zeebregts, Ries van de Sande, Joris

Meulendijks and Herman de Jong, also deserve many thanks for the effort they have put

in repairing and adjusting the set-up when needed. In addition I would also like to thank

Ad and Wim Kemper for their work on the grid heating system.

Many thanks also go to Peter van de Weijer and Piet Bouten from Holst Centre for

producing and processing the samples required for the calcium test performance of the

various multilayer diffusion barrier systems, as well as for the discussions about the

outcomes.

Furthermore I would like to thank the people from the student room for the useful

discussions on work related topics, but on the other hand also for the necessary

distraction from the serious work during the day, leading to a nice work climate and a

pleasant past year.

Besides the support from the PMP group, I would like to have special thanks for my

parents for supporting me and showing a lot of patience not only during my graduation

project but also throughout my whole education of the past years.

76

77

Appendix A

Table A-1: V3D3 monomer and p(V3D3) polymer vibrational modes assigned on the basis of the

references reported in literature. Wavenumber

polymer (cm-1)

Wavenumber

monomer (cm-1)

Literature (cm-1) Group Vibrational mode

3057

3058 3075-3090; 3060 =CH2 in vinyl νa [23, 76]

3018

3017 2995-3020 =CH2 in vinyl νs [23, 76]

2974 2976

2950-3050 =CH- in vinyl νa [29]

2962

2966 2965-2969; 2960 -CH3 sp3 νa [23, 76]

2935 2953

2950-3050 =CH- in vinyl νs [29]

2910

- 2916-2936 -CH2 sp3 νa [24, 29, 76, 89]

2878

2905 2856-2960; 2862-

2882; 2906

-CH3 sp3 νs [24, 29, 89]

2861

- 2855 -CH2 sp3 νs [76]

-

1926

1910 =CH2 in vinyl ωs [76]

1597 1597

1600; 1594 C=C in vinyl ν [29, 69, 76]

1460

- 1460 -CH2 sp3 δa [76]

1408 1408

1400 -CH2 in Si-(CH2)x-

Si and in vinyl

δ [29, 76]

1273

1275 1260 =CH- vinyl in

(CH2=CHSiMe2)O

2

ρ [72]

1260

1261 1255-1280; 1242 Si-CH3 δs [23, 24, 72, 76]

1002

995

1016

1010-1020; 1000-

1100

Si-O-Si in (SiO)3

1002 cm-1 (SiO)3

short polym.

chains

995 cm-1 (SiO)3

long polym. chains

νa [24, 69]

970 - 990 Si-(CH2)x-Si ω [29]

-

963 950; 960 =CH2 in vinyl ω [29, 69, 76]

798

804 805 Si-CH3 ρ [76, 90]

617

621 610 Si-CH=CH2 δ [24, 29, 69, 76]

ν, δ, ρ, ω denote stretching, bending, rocking, and wagging vibrational modes respectively, a and s indicate

asymmetric and symmetric vibrations.

78

79

Appendix B

Theoretical background of plasma physics

In general the plasma is considered as the fourth state of matter and can be defined as an

ionized gas consisting of neutrals, ions and free electrons, which undergo a collective

behavior due to long-range Coulomb forces [91, 92]

. Since there is a balance between ions

and electrons the plasma as a whole is quasi-neutral. For the generation of the plasma

sufficient kinetic (thermal) energy, i.e. heating, is required for the atoms and molecules to

dissociate and ionize them. For the gas molecules to be dissociated, the involved kinetic

energy must overcome the binding potential energy. In order to produce the plasma, an

external electric field (e.g. by a radio frequency power source as applied for the PECVD

process in this thesis, addressed in Appendix C) is applied across the already present

ionized gas molecules/atoms (ionization degree of 10-11

-10-12

) accelerating the lighter free

electrons to sufficiently high energy in order to ionize other atoms and molecules due to

an increased collision probability (ionization avalanche) reaching an ionization degree of

10-5

-10-6

. Basically the electric field transfers energy to the free electrons and ions,

although this is faster for light electrons compared to relatively heavy ions. To sustain the

plasma, the external electric field is required, as otherwise, once the ionizing source is

removed, recombination of the charged particles would take place until equilibrium is

reached. The criterion of charge neutrality can be locally disturbed due to the thermal

particle energy, while an electrostatic potential energy tends to restore the neutrality. The

length scales at which the disturbances occur are in the order of the so-called Debye

length (λD). Within this length scale the charged particles shield themselves from

electrostatic fields. The Debye length has been shown to be dependent on the temperature

(T) and the electron density (ne).

2/1

2

0

en

kT

e

D

(B.1)

where 0 is the vacuum permittivity, k the Boltzmann constant and e the electron charge.

The presence of a surface, e.g. an electrode, in the plasma gives rise to the formation of a

plasma sheath, in which the condition of charge neutrality is not satisfied as this region is

energetic electron deficient [93]

. The size of the plasma sheath is proportional to the

electric field and the electron mobility and therefore increases with increasing applied

power and decreasing pressure [94]

. Typically the thickness of such a sheath is in the order

of Debye lengths. Since the electrodes become negative due to the high mobility of

electrons bombarding the surface, a potential difference between the plasma and the

surface, usually referred to as sheath potential, affecting the ion bombardment, is created.

Due to the Debye shielding as mentioned before, the existence of a plasma should fulfill

some criterions: first, the characteristic dimension of the plasma (L) should be larger than

the Debye length (L >> λD), second, ne · λD3 >> 1 meaning that the average distance

between the electrons must be very small compared to λD resulting in a high electron

density inside the Debye sphere (being a sphere with radius λD screening electrostatic

field outside the sphere), sometimes a third criterion is considered being macroscopic

charge neutrality ( ie nn ). The fourth criterion follows from the local disturbances of

the charge neutrality giving rise to internal space charge fields which cause collective

80

particle motions in order to restore the neutrality. Since these electrons move beyond the

equilibrium position due to their inertia, an electric field is created in opposite direction.

Therefore the electrons collectively oscillate and this motion is characterized by the

plasma frequency ωpe,

2/1

0

2

e

epe

m

en (B.2)

Since collisions between electrons and neutrals tend to damp the oscillation, the electron-

neutral collision frequency (νen) must be smaller than the plasma frequency,

enpe (B.3)

with νpe = ωpe / 2π.

81

Appendix C

Principle of RF PECVD

The RF PECVD deposition technique is based on the generation of a plasma in between

two electrodes (Fig. C-1). For the deposition of thin films by means of RF PECVD, the

reaction gasses are injected in between the electrodes where the plasma is produced by

applying an alternating current (AC) source with a radiofrequency (RF) source at 13.56

MHz. As explained in Chapter 3, a matching network is included in the system in order to

preserve the input power and avoid reflected power in order to sustain the plasma.

Grounded bottom

electrode (substrate)

RF power supplyMatching

network

Top electrode

Gas injection

Deposition chamber

Plasma

To pump To pump

Grounded bottom

electrode (substrate)

RF power supplyMatching

network

Top electrode

Gas injection

Deposition chamber

Plasma

To pump To pump

Fig. C-1: Plasma generated between two electrodes in a parallel-plate reactor for the RF

PECVD process.

The delivered energy contained in the applied electric field is transferred to free electrons

which are colliding in the plasma leading to ionization of atoms and molecules. By

supplying the RF power to the top, target electrode of a parallel plate system with the

bottom electrode (which functions as substrate in this work) grounded, free electrons are

colliding with the incoming monomer molecules which consequently generates more

electrons, ions, radicals and molecules in excited states. This leads to the fragmentation

of the monomer and the deposition through various chemical pathways as the dissociation

energy of the monomer molecule can be overcome by the free electrons which are

accelerated by the electric field. The formation of ions and free radicals takes place

simultaneously, but the concentration of free radicals in the plasma is usually higher up to

five or six orders of magnitude compared to ions as estimated by Bell [95-97]

making the

plasma process very reactive. Although the bottom electrode is grounded, ion

bombardment is still an important issue as it is affected by the plasma sheath size. The

electrode surface will develop a negative bias with respect to the glow discharge, and this

gives rise to the ion bombardment. The choice of a symmetric or asymmetric couple of

electrodes will affect the sheath potential, meaning that the sheath thickness is increased

for the bottom electrode if this is smaller than the powered top electrode [98]

. For a

detailed study of the mechanism of the plasma sheath formation one can refer to literature [91, 92]

, as this is outside the scope of this work. The film properties are therefore

dependent on the choice of the deposition parameters: plasma pressure, oxygen-to-

monomer flow rate ratio, argon flow rate, power, and substrate temperature. By

82

controlling the deposition parameters the organic or inorganic character of the layer can

be tuned as the plasma chemistry and plasma-surface reactions are affected.

RF PECVD vs. other plasma deposition techniques

In the case of pulsed PECVD (PPECVD) the monomer fragmentation can be minimized

since the plasma is pulsed for well defined time steps (with continuous gas flows) instead

of continuous as in the RF PECVD process. Due to the lower fragmentation degree (due

to shorter plasma time), monomer structural retention is much more likely, especially

when the duty cycle, i.e. the plasma-on time, is small [46]

although it is reported that the

cross-linking degree becomes less. The repetition of well-defined units is also improved

for the pulsed plasma process. Additionally, the growing layer is only subjected to less

energetic ions and therefore damaging of the growing layer is limited and only possible

during the plasma-on time [27]

. During the off-time only long-lived neutral species

participate in the process and propagate via the conventional polymerization mechanisms,

since no plasma is present to create additional radicals. Another approach of the PECVD

process is the use of remote PECVD in which the gases are injected into the plasma zone

(glow region which is for example created in a quartz tube) where the molecular

fragmentation takes place. The substrate on the other hand is not exposed to the plasma as

it is placed outside the plasma zone (in the so-called after-glow region), so diffusion and

convection are driving the process as long-lived, reactive species/radicals from the carrier

gas are created in the gas-phase, and subsequently mix with the monomer to produce the

precursors required to start the polymerization process on the substrate. The substrate is

not subjected to ion bombardment in this case and the substrate temperature can be

maintained at a reduced temperature. Another example of remote plasma deposition is the

expanding thermal plasma (ETP) technique based on an expanding plasma corresponding

with a pressure gradient between the plasma source (a cascaded arc at high pressure) and

the downstream region (low pressure). The remote character is related to the independent

control of precursor chemistry and pressure of the downstream region from the plasma

source due to the pressure difference. Fragmentation also takes place in the plasma

region, while the substrate is located at the end of the expanding plasma where the

created radicals can reach the surface and react accordingly.

Independent of the deposition technique the layer density and smoothness of the layer’s

surface can be adapted by applying an external substrate bias voltage which delivers ion

bombardment during the film growth [24, 44, 99]

.

83

Appendix D

Detailed explanation of the grid heating system

The grid heating system used during the i-CVD process is controlled by a temperature

control box. This box provides the possibility to vary the grid temperature from room

temperature up to 773 K. The functioning of the grid heating system is described in more

detail below, based on the schematic shown in Fig. D-1.

Fig. D-1: Schematic representation of the grid heating circuit used to resistively heat the grid to

the desired temperature during the i-CVD process. The system is based on the measure

The relation between the temperature and resistance of a conductor is given by,

refref TTRR 1 (D.1)

where R is the conductor resistance at temperature T, Rref is the resistance at a reference

temperature Tref, α is the temperature coefficient of resistance for the conductor material,

T (K) is the conductor temperature, and Tref (K) is the reference temperature. The

temperature coefficient of the tungsten wire has been determined by calibrating the

heating system (explained in Appendix E) and was found to be α = 0.00404 K-1

. This is a

measure of the increase in resistance upon an increase in temperature of one degree

Kelvin.

From the above equation (D.1) the reference resistance Rref has been determined as well

by assuming a reference temperature Tref = 273 K, and this resulted in a value of 5.79952

Ω. Since both R(273 K) and α are known, eq. D.1 allows for the determination of the

resistance value required to achieve a certain grid temperature.

The grid system in this set-up is resistively heated which means that the resistance value

can be set as input value, and the temperature control box will adapt the power supply

until the measured resistance value matches the manually set value. Based on the

schematic representation in Fig. D-1, the mathematical background can be described as

follows:

The voltage over the grid is measured and thereafter amplified with an amplification

factor k1. This results in a signal A,

84

VkA 1 (D.2)

Besides the voltage, the current I in the circuit, is measured through a measure junction

(with a factor k2) and then amplified by a variable amplifier k3, resulting in a signal B,

IkkB 32 (D.3)

The variable amplification factor k3 is determined by the set resistance value and

according to eq. D.1 it is also related to the grid temperature. Once the resistance value is

manually set, the factor k3 remains constant.

The signals A and B are consequently supplied to a log ratio amplifier in order to obtain a

ratio between the voltage over and the current through the grid which in turn can be

related to the grid resistance Rgrid. Actually a difference of two logarithmic signals is

determined, but this can be rewritten as a logarithm of the ratio. The output signal Vu of

the log ratio amplifier is,

)log(32

14

Ikk

VkkVu (D.4)

with k4 a factor related to the log ratio amplifier.

The aim of the temperature control box is to adapt the power supply (positive or

negative) until the measured resistance value matches with the set value. To control this

matching a PI-controller (Proportional-Integral controller) is included which compares

the measured to the set value. As long as there is a difference, the power supply will be

adapted. If the values match, the output signal Vu is equal to zero, which means that the

set grid temperature is reached. The supplied power will remain constant, because the PI-

controller measures a zero signal. In this case the relation between the output signal Vu

and the amplification factors kx (x = 1, 2, 3) is given by,

10log032

1

32

1 Ikk

Vk

Ikk

VkVu (D.5)

Because the grid resistance Rgrid is determined by the ratio between V and I,

1

32

k

kk

I

VRgrid (D.6)

the temperature control box will adapt the power supply until condition D.5 is fulfilled, or

in other words until the measured resistance value (D.6) matches with the set value. So,

basically it is a repetitive loop in which the power supply is varied every time until the

matching is achieved.

85

Appendix E

Calibration of the grid heating system

The calibration of the grid heating system has been performed by placing the grid with

wires inside an oven filled with nitrogen in order to ensure thermal equilibrium. A

thermocouple has been placed in close proximity of the wires but without making

contact. The temperature of the oven has been subsequently increased from room

temperature to 529 K and resistance values have been acquired from time to time for

various temperatures. By plotting the resistance R as a function of the temperature

difference T-Tref, both Rref and α can be obtained by performing a linear fit (Fig. E-1)

according to equation D.1 of Appendix D. As reference temperature 273 K is chosen, and

thus the constant resistance R(273 K) is corresponding with the intercept while α can be

derived by dividing the slope of the curve by R(273 K), resulting in R(273 K) = 5.79834

Ω and α = 0.00404 K-1.

0 50 100 150 200 250

6

7

8

9

10

11

12

R (

)

T (K)

Equation y = a + b*x

Weight No Weightin

Residual Sum of Squares

0.01029

Pearson's r 0.9999

Adj. R-Square 0.99978

Value Standard Erro

R Intercept 5.7983 0.01921

R Slope 0.0234 9.58019E-5

Fig. E-1: Calibration of the grid heating system by measuring the resistance for various

temperatures. Both R(273 K) and α can be determined or derived from the linear fit.

The calibration only has to be performed once, but when replacing the wire the resistance

has to be measured again at room temperature and compared to previous values. By using

a scaling factor, given by the ratio between the measured resistance value of the new wire

and the one obtained from the calibration, both at room temperature, the new resistance

values and corresponding temperatures are obtained. These are the resistance values that

are set as input values in order to achieve a certain wire temperature.