Linköping Studies in Science and Technology Dissertation No. 948

Plasma Characterization &

Thin Film Growth and Analysis in

Highly Ionized Magnetron Sputtering

Jones Alami

Plasma and Coatings Physics Division Department of Physics and Measurement Technology

Linköping University, SE-581 83 Linköping Sweden

Linköping 2005

ISBN 91-85299-40-5 ISSN 0345-7524

Printed by UniTryck, Sweden 2005

To Father and Mother

In an instant illumination can be achieved. It is as easy as turning on a light.

The problem is finding the switch in the dark.

Abstract

The present thesis addresses two research areas related to film growth in a

highly ionized magnetron sputtering system: plasma characterization, and thin film

growth and analysis. The deposition technique used is called high power pulsed

magnetron sputtering (HPPMS). Characteristic for this technique are high energy

pulses (a few Joules) of length 50-100 µs that are applied to the target (cathode) with

a duty time of less than 1 % of the total pulse time. This results in a high electron

density in the discharge (>1×1019 m-3) and leads to an increase of the ionization

fraction of the sputtered material reaching up to 70 % for Cu.

In this work the spatial and temporal evolution of the plasma parameters,

including the electron energy distribution function (EEDF), the electron density and

the electron temperature are determined using electrostatic Langmuir probes. Electron

temperature measurements reveal a low effective temperature of 2-3 eV. The degree

of ionization in the HPPMS discharge is explained in light of the self-sputtering yield

of the target material. A simple model is therefore provided in order to compare the

sputtering yield in HPPMS and that in dc magnetron sputtering (dcMS) for the same

average power.

Thin Ta films are grown using HPPMS and dcMS and their properties are

studied. It is shown that enhanced microstructure and morphology of the deposited

films is achieved by HPPMS. The Ta films are also deposited at a number of substrate

inclination angles ranging from 0o (i.e., facing the target surface) up to 180 o (i.e.,

facing away from the target). Deposition rate measurements performed at all

inclination angles for both techniques, reveal that growth made using HPPMS resulted

in an improved film thickness at higher inclination. Furthermore, the high ionization

of the Ta atoms in HPPMS discharge is found to allow for phase tailoring of the

deposited films at all inclination angles by applying a bias voltage to the substrate.

Finally, highly ionized magnetron sputtering of a compound MAX-phase material

(Ti3SiC2) is performed, demonstrating that the HPPMS discharge could also be used

to tailor the composition of the growing Ti-Si-C films.

Preface

The present thesis is the summary of the work I conducted as part of a PhD

study in the Thin Film Physics and Plasma & Coatings Physics divisions at Linköping

University in Sweden, between November 2001 and June 2005. I consider myself

lucky to have had the opportunity to be a member of the research groups, not only for

providing a great research environment but also for contributing to my development

as a human being. As a PhD student, I learned that with good planning, a passion for

knowledge, and a conscious attitude, research becomes simply a way of life. I would

like therefore to thank the people that made it such an inspiring and rich milieu,

especially: Ulf Helmersson for introducing me to the fields of plasma physics and thin

films, and for always being available for discussions and new ideas. The

administration at IFM for making the Physics Department just a lovely work place.

Jon Tomas Gudmundsson, my unofficial plasma supervisor, for a great colaboration

and nice visits to Iceland. Lars Hultman for your inspiring leadership skills. Inger

Eriksson for always being so helpful and friendly. Karl-Olof Brolin for your

creativity and for making the difficult tasks easy. Jon Andersson and Erik Wallin for

the many discussions about science and life, and for always being positive. Hans

Högberg for an effective collaboration and for a much appreciated help when I needed

it most. Per Eklund for a great collaboration and for your enthusiasm. Jochen

Schneider for the research strategy talk. Denis Music for your inspirational ideas. Leif

Samuelsson for your help with the probe station and understanding what I actually

meant when we discussed “electronics”. All of you who helped me by reading and

commenting the manuscript, Ulf, Hans, Jon Tomas, Erik W., and Per E.. Everyone

from The Plasma & Coatings Physics, and the Thin Film Physics groups for the

excellent atmosphere each one helps to create within our groups. My dear friends

Dejan, Erik A., Tarik, Driss, Mahiar, Oskar… for your support and friendship.

Finally, the people I admire the most. Helene, my dearest, for your support and love,

and my wonderful Family for your passionate and truthful engagement in my

happiness.

List of papers included in the thesis

Paper I:

Evolution of the electron energy distribution and plasma parameters in a pulsed

magnetron discharge, J. T. Gudmundsson, J. Alami, and U. Helmersson.

App. Phys. Lett. 78 (2001) 3427.

Paper II:

Spatial and temporal behavior of the plasma parameters in a pulsed magnetron

discharge, J. T. Gudmundsson, J. Alami, and U. Helmersson.

Surf. Coat. Technol. 161 (2002) 249-256.

Paper III:

Plasma dynamics in a highly ionized pulsed magnetron discharge, J. Alami, J. T.

Gudmundsson, J. Böhlmark, and U. Helmersson

Submitted to Plasma Sources Science and Technology.

Paper IV:

Ion-accoustic solitary waves in a high power pulsed magnetron sputtering

discharge, K. B. Gylfason, J. Alami, U. Helmersson, and J. T. Gudmundsson.

Submitted to Journal of physics D: Applied physics.

Paper V:

Ion-assisted Physical Vapor Deposition for enhanced film properties on non-flat

surfaces, J. Alami, P.O.Å. Persson, D. Music, J. T. Gudmundsson, J. Böhlmark, and

U. Helmersson

J. Vac. Sci. Technol. A 23 (2005) 278.

Paper VI:

Structural analysis and growth of beta- and bcc-Ta on inclined surfaces by

highly ionized magnetron sputtering, J. Alami, P. Eklund, J.M. Andersson, M.

Lattemann, E. Wallin, J. Bohlmark, P. Persson, and U. Helmersson

Manuscript in final preparation.

Paper VII:

Ionized magnetron sputtering of Ti-Si-C MAX-phase thin films from a Ti3SiC2

compound target, J. Alami, P. Eklund, J. Emmerlich, U. Jansson, O. Wilhelmsson,

H. Högberg, L. Hultman, and U. Helmersson

Manuscript in final preparation.

I took a major part in the planning of all the studies, carried out all the plasma

diagnostic work and all the thin film growth, performed or took part in the thin film

characterization and analysis, and wrote papers III, V, VI, and VII. U. Helmersson

contributed to all papers in terms of planning and discussions. J. T. Gudmundsson was

the main contributor to the scientific discussions and data analysis related to the

plasma physics (Papers I-IV) and wrote papers I and II. K. B. Gylfason contributed

with data analysis and writing of paper IV. H. Högber and P. Eklund were the major

figures for MAX-phase related issues. Other co-authors contributed with film

characterization and scientific discussions.

Additional publications (not included in the thesis)

1 High power impulse magnetron sputtering discharges and thin film growth: A

brief review, U. Hemlersson, M. Lattemann, J. Alami, J. Bohlmark, A. P Ehiasarian,

and J. T. Gudmundsson. Society of Vacuum Coaters 48th Annual Technical

Conference Proceedings (2005).

2 Ionization of sputtered metals in High Power Pulsed Magnetron Sputtering,

J. Bohlmark, J. Alami, A. P. Ehiasarian, C. Christou, and U. Helmersson. J. Vac. Sci.

Technol. A, 23(1), 18-22 (2005).

3 Spatial electron density distribution in high power pulsed magnetron

sputtering, J. Bohlmark, J. T. Gudmundsson, J. Alami, M. Latteman, and U.

Helmersson. Submitted for publication.

4 Influence of pressure and power on the composition and time evolution of

plasmas in high power impulse magnetron sputtering, P. Ehiasarian, R. New, P. E.

Hovsepian, J. Böhlmark, J. Alami, and U. Helmersson. Society of Vacuum Coaters

47th Annual Technical Conference Proceedings, 28-32 (2004).

5 Measurement of the magnetic field change in a pulsed high current magnetron

discharge, J. Bohlmark, U. Helmersson, M. VanZeeland, I. Axnäs, J. Alami and N.

Brenning. Plasma Sources Sci. Technol., 13, 654-661 (2004).

6 Plasma Parameters During High Power Pulsed Magnetron Sputtering, J.

Alami, J. T. Gudmundsson, J. Böhlmark, K. B. Gylfason and U. Helmersson.

Proceedings of The 16th international symposium on plasma chemistry, ISPC 16,

Taormina, Italy, 2003.

7 Low temperature deposition of alpha -Al/sub 2/O/sub 3/ thin films by

sputtering using a Cr/sub 2/O/sub 3/ template, P. Jin, G. Xu, M. Tazawa, K.

Yoshimura, D. Music, J. Alami, and U. Helmersson. J. Vac. Sci. Technol. 20 (2002)

2134.

8 X-ray reciprocal space mapping studies of strain relaxation in thin SiGe layers,

W.-X. Ni, K. Lyutovich, J. Alami, C. Tengstedt, M. Bauer, and E. Kasper. J. Crystal

Growth 227-228 (2001) 756.

9 Ionized-PVD by pulsed sputtering of Ta for metallization of high-aspect-ratio

structures in VLSI, U. Helmersson, Z. S. Kahn, and J. Alami. Proceedings of the

Third International Euroconference on Advaced Semiconductor Devices and

Microsystems (IEEE Catalogue number: 00EX386), Somolenice Castle, Slovakia,

2000.

1. INTRODUCTION....................................................................................................3

2. PART I. PLASMA CHARACTERIZATION .......................................................5

2. 1. PLASMA DEFINITION ...........................................................................................5

2. 2. PLASMA GENERATION .........................................................................................7

2.3. PLASMA MODELING AND THE DISTRIBUTION FUNCTION .......................................9

2. 3. 1. The distribution function and the plasma parameters ...............................9

2. 4. HPPMS DISCHARGE DIAGNOSTICS ...................................................................11

2. 4. 1. Measurement setup ..................................................................................11

2. 4. 2. Plasma measurements using a Langmuir cylindrical probe....................12

4. 2. 2. Filtering and smoothing...........................................................................14

2. 4. 3. Ion current determination ........................................................................16

2. 4. 4. Direct display of plasma parameters using a triple probe setup.............17

2. 5. SOLITARY WAVES .............................................................................................20

3. PART II. THIN FILM GROWTH AND CHARACTERIZATION .................23

3. 1. GROWTH MODELS .............................................................................................23

3. 2. DEPOSITION OF THIN FILMS BY MAGNETRON SPUTTERING.................................26

3. 2. 1. Magnetron sputtering...............................................................................26

3. 2. 2. The ionization process .............................................................................27

3. 3. EFFECTS OF IONIZED DEPOSITION ON FILM STRUCTURE.....................................32

3. 3. 1. Film densification using HPPMS.............................................................32

3. 3. 2. Phase formation.......................................................................................34

3. 3. 3. Enhanced adhesion by HPPMS ...............................................................35

3. 4. SPUTTERING OF A MAX-PHASE COMPOUND TARGET BY HPPMS.....................37

3. 5. THIN FILM CHARACTERIZATION ........................................................................39

3. 5. 1. Transmission electron microscopy (TEM)...............................................39

3. 5. 2. Scanning electron microscopy (SEM)......................................................39

3. 5. 3. Atomic force microscopy (AFM)..............................................................40

3. 5. 4. X-ray diffraction (XRD) ...........................................................................40

3. 5. 5. X-ray photoelectron spectroscopy (XPS).................................................41

3. 5. 6. The four-point electrical probe................................................................41

4 SUMMARY .............................................................................................................43

5 OUTLOOK..............................................................................................................47

REFERENCES...........................................................................................................49

1. Introduction

Thin films are material layers with thickness ranging from a few nm to a few

µm. They are used today in a wide range of applications, such as coatings for cutting

tools, optical and decorative coatings, solar cells applications, and as diffusion layers

in integrated circuits. The film growth can be carried out by a number of deposition

techniques, including chemical vapor deposition (CVD) [1,2] where chemical

reactions precede deposition, and the chemical bonding state of the deposit is different

from the precursor gases. Other deposition techniques are sputtering and evaporation.

They are called physical vapor deposition (PVD) processes, because they produce a

vapor of the material to be deposited by physical means, i.e., by heating (evaporation)

or by energetic ion bombardment (sputtering) [3-6]. Because of the presence of a

plasma, sputtering exhibits added benefits in the deposition process for many thin film

technologies [7]. PVD was first discovered in 1852, and was developed as a thin film

deposition technique by Langmuir in the 1920s. It has better step coverage than

evaporation as the adatoms have higher surface diffusion energy. The sputtering

process is also much better at producing layers of compound materials and alloys [8]

and in controlling the microstructure and phase of the deposit. A widely used

sputtering technique is dc magnetron sputtering where a magnetic field is

superimposed on the electric field. This leads to electron confinement near the target

resulting in higher ionization of the sputtering gas, and consequently, higher

deposition rates are obtained [9-11]. Although dcMS is a successful magnetron

sputtering technique, it suffers from fundamental problems such as low target

utilization, target poisoning and poor deposition rates for dielectric materials. In order

to alleviate some of these problems, alternative techniques, such as radio frequency

magnetron sputtering (rfMS) [11-17], additional ionization by rf coils [18] or

microwaves [19], variable or unbalanced magnetic configuration [20] or increased

magnetic confinement by a multipolar magnetic setup [21] has been demonstrated.

Pulsing the applied voltage is another alternative magnetron sputtering technique,

where sputtering is achieved principally by asymmetric bipolar or by unipolar pulsing

of the target power. In bipolar pulsing, the power is applied to the target at a

frequency of 10 – 200 kHz [17, 22, 23]. This technique has become established as one

of the main sputtering techniques for deposition of oxide and nitride films [10,11,24].

3

In unipolar pulsing, the power supply operates at low (or zero) power level and pulses

to a high voltage for a short period each cycle [25, 26].

In many applications, increasing the ion fraction of the sputtered material

gives an advantage in film deposition, as it allows for directional deposition and

control of the charged sputtered species bombardment-energy [27, 28]. This is

difficult in conventional deposition techniques, as the plasma density is too low to

cause significant ionization of the sputtered material, and most of the ions are ions of

the sputtering gas [29]. A way to increase the electron density in the discharge is by

using an electron cyclotron resonance plasma as a plasma electron source [30,31],

employing an inductively coupled plasma [18], or adding a hollow cathode electron

source in the target vicinity [32]. Increasing the power to the sputter source and

therewith increasing the electron density can also promote ionization through

electron-impact processes. While this is usually not possible in dcMS due to target

overheating and eventual damage, applying high energetic pulses to the target at a low

duty time is a solution.

High Power Pulsed Magnetron Sputtering (HPPMS), also known as high

power impulse magnetron sputtering (HIPIMS), has been demonstrated to give high

plasma (electron) densities without overheating the target, when high power pulses

with a duty time lower than 1 % of the total pulse time were applied to the target [33-

37]. With peak power densities typically of several kW cm−2, plasma densities 2 to 3

orders of magnitude higher than those measured in dc magnetron discharges can be

achieved [paper I-III]. This gives ionization fractions of the sputtered material ranging

from 4.5 % for C [38] to 70 % for Cu [36].

In this work two areas related to HPPMS are addressed. First, a study of the

plasma dynamics and energetics is presented. This is the content of papers I to IV,

where the spatial and temporal evolution of the discharge are described, and the

temporal behavior of the plasma parameters, such as the electron energy distribution

function (EEDF), the electron density, and the electron temperature are determined.

Second, the role of ion deposition and ion bombardment in the HPPMS technique is

investigated by depositing thin films and characterizing their properties. As a case

study, a “simple” material system (Ta) is investigated in papers V and VI, and a

complex ternary material system (Ti-Si-C) in paper VII. The benefits of using

HPPMS for deposition on inclined surfaces are also discussed and demonstrated using

experiments together with the theoretical background and analysis.

4

2. Part I. Plasma characterization As stated in chapter 1, different growth techniques are used for thin film

deposition, where the deposition process can be a CVD or a PVD process. Sputtering

is today the primary alternative in PVD. In sputtering, a low-pressure discharge

(plasma) is sustained between the target and the chamber walls. When an electric field

is applied, positively charged gas ions (e.g. Ar, Kr, N2) bombard the target surface

after being accelerated by the potential drop across the sheath between the target and

the plasma. In the bombarding process, ions, atoms and electrons are ejected from the

target surface. The emitted electrons are accelerated by the same potential drop and

gain therefore considerable energy, enough to ionize the sputtering gas atoms.

2. 1. Plasma definition

By some estimates, 99% of the observable universe is in the plasma state, also

known as the fourth state of matter [39]. The use of the term "plasma" for an ionized

gas started in 1927 with Irving Langmuir (1881-1957). Plasma is defined as a quasi-

neutral gas of charged and neutral particles that exhibit a collective behavior [40]. The

charged particles must satisfy two important criteria for the gas to qualify as a plasma;

first, sufficiently high charge density, so that the long range Coulomb forces are a

significant factor in determining the statistical properties of the particles. Second, low

enough density, so that the Coulomb force of a near neighbor particle is much less

than the cumulative long range Coulomb force exerted by the many distant particles.

These criteria are satisfied in a range of plasmas as shown in Fig.1 (a), where the

densities corresponding to the different plasmas are presented. The HPPMS plasma

covers a large electron density range, with consequences related to the ionization

fraction in the plasma and to the film deposition and structure.

5

Fig. 1. Different plasmas and their corresponding electron densities are shown. The

box representing the HPPMS plasma is shaded.

Plasmas, unlike ordinary gases, support a wide variety of wave modes because

of the charged particles [40,41]. One example of these wavelike disturbances is the

ion acoustic wave, which is an electrostatic wave occurring in a field free plasma.

Another is the magnetosonic wave, which is an acoustic wave in which the

compressions and rarefactions are produced not by motions along the electric field E

but by E×B drifts across E [40], where B represents the magnetic field. Finally, the

electromagnetic wave, which can typically be described by an electric field of the

form ([ txiEtx )]ω−= kE exp),( 0 , where k is called the wave vector, x is the

propagation direction, and ω is the oscillation frequency. E0 and ω are in general

complex valued. If the wave is electromagnetic in nature then the magnetic field

obeys a similar relation also. The (angular) frequency (ω) and the wave vector (k) are

functionally related to one another, at least in linear theories of plasma waves, through

a dispersion relationship ω(k). Knowledge of the dispersion characteristics of the

propagating waves is certainly necessary for an understanding of the plasma state.

Plasma, however, is a non-linear medium and unless the waves are truly of small

amplitude, non-linear effects must be taken into account. Such non-linear effects

conspire to produce wave-like disturbances that do not conform to the forms given

above, and interesting phenomena such as solitary waves (solitons) are observed as a

result in plasmas. This will be discussed in more detail in section 2.5.

6

2. 2. Plasma generation

In order to start a discharge, some charged particles are needed to start the

process of ionization and excitation of gas particles. This is provided to a large extent

by weak background radiation from radioactive minerals where some ionized

molecules and free electrons exist, and to a less extent from cosmic rays. By applying

an electric field these electrons gain enough energy for further ionization and

generation of secondary electrons, which in turn contributes to creating a so-called

Townsend discharge with a relatively low, but avalanching, current (Fig. 2).

Fig. 2. A schematic of the Townsend discharge created by applying an electric field

between two plates.

If the current increases enough, exceeding the breakdown field for the gas, a glow

discharge is obtained. Figure 3 shows a photo of an Ar discharge for a sputtering

system with a Ta sputter source (target). In order to sustain the plasma, regeneration

of electrons is necessary [42]. This is often the most important means of maintaining

the plasma as electrons lose their energy to the gas in ionizing processes, and are

eventually lost to the surrounding chamber walls. Electrons also lose energy by elastic

collisions, transforming energy to neutral species thereby putting them in an excited

state, i.e., not all collisions result in ionization and secondary electrons generation

[41]. As a result, a range of kinetic energies is obtained for the electron population,

often described mathematically by a Maxwell-Boltzmann, a Bi-Maxwellian, or a

Druyvesteyn distribution. More on distributions is presented in section 2.3.

7

Fig. 3. A photograph showing the HPPMS discharge in a magnetron sputtering

system with a cylindrical target.

In many magnetron-sputtering applications, low-pressure gas discharges are

used with only about 1% or less of the gas ionized. Nevertheless, the excited neutral

atoms, molecules, and radicals gain energy by collisions, and have typically a

temperature close to room temperature (~1 meV), compared with the warmer ions

(tens of meV) and hotter electrons (a few eV). The electron density in such plasmas is

in the range 1014-1017 m-3, the lower values being for dc glow and dc magnetron

discharge, and the higher values more typical for electron cyclotron resonance and

inductively coupled discharges.

8

2.3. Plasma modeling and the distribution function

Plasmas are commonly modeled as fluids, and each species is described with

its fluid equation [41]. The fluid approximation is sufficiently accurate to describe the

majority of observed phenomena. When this is not viable, the accumulated behavior

of an ensemble of charged particles is described using a statistical approach, in the

form of the velocity distribution function. The distribution function is given as

f(r,v,t)d3rd3v, i.e., the number of particles inside a six-dimensional phase space

volume d3rd3v at (r,v), at time t [41]. Knowing the distribution function, fundamental

features of a large ensemble can be quantified. Knowing the cross sections for the

relevant interactions in the plasma, measurements of the EEDF can be used to

calculate all the rates [43]. By integrating over velocity, the average or (macroscopic)

quantities associated with the plasma can be defined:

∫= uurr dtftn ),,(),( plasma number density (1)

∫= uururu dtfn

t ),,(1),( plasma bulk fluid velocity (2)

∫ −= ruur fn

mt ()(2

),( 2ε uu dt),, plasma mean kinetic energy (3)

where m is the particle mass. This procedure of integrating over velocity space is

referred to as taking velocity moments, with each one yielding a physically significant

quantity. Knowledge of the electron energy distribution function, thus, permits us to

determine important parameters. In modeling work, the electron distribution is often

assumed to be Maxwellian. However, the electron energy distribution in a dc

magnetron discharge has been shown to be bi-Maxwellian like [44] or Druyvesteyn

like [41,45]. A Druyvesteyn like electron energy distribution has significantly fewer

high-energy electrons than the Maxwellian for the same overall mean energy;

resulting in less excitation and ionization.

2. 3. 1. The distribution function and the plasma parameters

As we know at this stage, the electron energy (speed) distribution function

(EEDF) should be determined in order to study the plasma energetics. Fortunately,

9

one can measure the EEDF using a Langmuir probe. The use of a Langmuir probe to

determine the EEDF was shown by Druyvesteyn in 1930, but it is not until the

introduction of digital filtering in the 1990’s that it was put in practice in a significant

way. Progress in analog and digital electronics has made it feasible to perform EEDF

measurements in any modern plasma experiment where a classical Langmuir probe

can be used. The EEDF, ge(V) can be calculated if the current-voltage relation in the

plasma is determined, in which case it is given by the Druyvesteyn formula as [41]:

2

221

222)(

dVId

meV

AemVg e

pre

= (4)

where Apr is the probe area, Ie is the electron current and V=Φp-Vb is the potential

difference between the plasma potential, defined as the potential at which 02

2

=dV

Id e ,

and the probe potential. With ε=eV, the electron density ne is determined as:

∫∞

=0

)( εε dgn ee (5)

If the plasma is in equilibrium, the EEDF is Maxwellian it simplifies the task,

as the Maxwellian distribution can be described by a single electron temperature,

which makes all the calculations simpler. For non-equilibrium plasmas, such as low-

pressure discharge plasmas, the electrons are not in equilibrium with other plasma

components, nor with the electron ensemble itself [46]. This means that the EEDF is

non-Maxwellian, and as a result, the plasma modelling is much more complicated

[45]. Consequently, a single electron temperature cannot fully describe the EEDF and

it is more appropriate to describe the energetics of the electrons by a mean electron

energy. At low pressures, the EEDF is generally non-Maxwellian and the electron

temperature is thought of as an effective electron temperature, Teff, representing the

mean electron energy determined from the EEDF [41] according to:

( ) εεεε dgn e

e∫∞

=0

1 and k ε32

=effT (6)

10

2. 4. HPPMS discharge diagnostics

2. 4. 1. Measurement setup

Figure 4 shows a schematic of the plasma measurement setup used in this work.

A standard planar weakly unbalanced magnetron is operated with a metal target of 15

cm in diameter. The sputtering target is located inside a stainless steel chamber of a

radius of 22 cm and a height of 75 cm. The chamber wall serves as an anode for the

discharge. The cathode is driven by a pulsed power supply from Chemfilt R&D AB

that delivers voltages of up to 2500 V and currents of up to 1200 A, at a repetition

frequency of 50 Hz. The pulse length is in the range of 50 – 100 µs. Measurements

are made with average powers comparable with those used in dcMS, i.e., the thermal

load of the target is kept low. The target current and voltage are monitored by a

Tektronix TDS 520 C (500 MHz, 1 Gs/s) oscilloscope using a Tektronix P6015 high

voltage probe (1000 × attenuation), a Tektronix CT-04 high current transformer (20

kA peak current and bandwidth 20 MHz), and a TCP202 current probe (15A ac/dc

and bandwidth 50 MHz).

Fig. 4. Experimental setup: A Langmuir probe is placed at different positions in the

space in front of the target surface. The probe voltage and current are measured

using an oscilloscope module and a PC. The data is stored in the PC, which functions

also as the probe’s power-supply control-unit, for later analysis.

11

Figure 5 shows typical voltage and current time-traces in a HPPMS system,

run with a Ta target and an Ar gas pressure held at 20 mTorr, as reported in paper III.

For the applied target negative voltage VT, a decrease of a few hundred volts is seen

as the plasma ignites, and a maximum current of hundreds of Amperes is achieved 20-

40 µs after the pulse initiation. This corresponds to peak powers of hundreds of kW,

and peak power densities of a few kWcm-2, calculated over the target surface.

Fig. 5. Time evolution of the target voltage (VT) and current (IT) for a Ta target and

an Ar gas pressure of 20 mTorr.

2. 4. 2. Plasma measurements using a Langmuir cylindrical probe

Electrostatic probe diagnostic is the primary technique for obtaining

information concerning the plasma state in low-pressure gas discharges. In its

classical application, known as the Langmuir technique, the probe method allows us

to find the basic plasma parameters such as the electron temperature, the plasma and

floating potentials, the plasma density, and the electron energy distribution function

(EEDF) [47]. The floating potential is defined as the potential at which the current

drawn by the probe is nil, i.e., equal electron and ion currents. The plasma potential is

defined as the potential at which ions cease to be attracted to the probe [41].

A Langmuir probe in its most basic form is simply a thin wire, typically made

of tungsten, that is inserted into a plasma. The probe is biased at a variable voltage,

and the collected current provides information on various plasma properties. Fig. 6

shows a schematic of a simple Langmuir probe.

12

Fig. 5. A schematic of the Langmuir cylindrical probe

2. 4. 2. 1. Measuring the I-V curve

The relation between the applied Langmuir probe voltage and the collected

current can be presented by a curve commonly known as probe I-V characteristic.

This is quite a straightforward measurement in equilibrium plasmas, where plasma

conditions do not change with time. However, in time dependent plasmas, as is the

case of the HPPMS plasma, time-dependent electron currents are measured for a

number of voltages, spanning over the voltage range of interest. In order to make the

measurements more efficient and less time consuming, we built a probe station

specific for the HPPMS discharge. The Langmuir probe is coupled via a coaxial cable

to part 2 of the station. The different parts of the probe station that are presented in

Fig. 6 are:

Part 1: A power supply. It supplies the probe with a voltage of ± 20 V

Part 2: It contains voltage-dividers for measurement of the probe voltage and the

probe current; it also contains a transformer for powering the oscilloscope module

(part 3).

Part 3: The oscilloscope module with a 12-bit resolution.

Part 4: A trigger signal transformer. It transforms the target voltage trigger signal into

a suitable trig signal to the oscilloscope (± 5V).

The probe station is controlled using PScope, which is a software that we

developed and tested successfully at Linköping University. PScope saves the

measured data in the computer, in a form that is suitable for further analysis in the

Matlab software package.

A typical measurement setup starts by measuring the probe current, as the

voltage drop across a resistance Rs=10 Ω. The time resolved probe current is recorded

for a fixed probe bias voltage, for 1000 microseconds from the pulse initiation. This is

repeated in the voltage range -10 V to +15 V with a step voltage of 0.1 V. Thus, each

13

data set is a 1000 × 500 matrix with the 1st column showing time, the 2nd, 4th, 6th,…

columns representing the probe voltages, and the 3rd, 5th, 7th,… representing the probe

currents. For each time value the I-V curve is then constructed and analyzed using a

software specially developed for the purpose [48] and revised for analyzing the

HPPMS discharge. The measured data is noisy, and the first step in the analysis is

therefore filtering and smoothing of the data.

Fig. 6. A photo of the probe station with the different parts numbered according to the

text.

4. 2. 2. Filtering and smoothing

The plasma is inherently noisy. In order to calculate the EEDF, the second

derivative of the measured curve is needed, which amplifies the noise. Therefore the

data is filtered before proceeding further. This can be done by using a Blackman filter.

The Blackman filter window is given by [49]:

Mn

MnnwB

ππ 4cos08.02cos5.042.0)( +−= , n= 0, 1, 2, …, M (7)

where M is the size of the window and controls the degree of smoothing. The

Blackman window minimizes the side-lobe level while having a steep roll-off on the

side lobes (Fig. 7). This is a standard moving average window and the filtering is easy

to implement in the Matlab software package [48].

14

Fig. 7. The Blackman window.

After filtering (Fig 8 (a), Fig. 8(b)), the derivative is found by applying the

central difference approximation (Fig. 8 (c), Fig. 8(d)). It should be noted that there is

always a compromise between accurately producing the EEDF maximum, and the

high electron energy tail. Larger windows flatten the maximum while achieving good

smoothing for higher energies, while small windows retain the shape of the maximum

with less smoothing for higher energies. The effect of these distortions can be

corrected for however, by fitting the measured EEDF to the function

)exp( xR bag εε −= where a, b and x are constants. The Maxwellian and the

Druyvesteyn electron energy distributions are special cases with x=1 and x=2

respectively. The value x is determined by performing a least-squares analysis of

)/)(ln( εεRg versus εx for various x. To find the best fit, the maximum of the EEDF

and the corresponding energy are found. The equation is fitted to the measured EEDF

(Fig. 8 (e)) from the electron energy at which the EEDF has the maximum until it has

fallen two orders of magnitude. The best fit to the equation is then interpolated to zero

electron energy, before the electron density (Fig. 8 (f)) and effective electron

temperature are calculated.

15

Fig. 8. The I-V curve before (a) and after (b) filtering, measured 200 µs after the

pulse start for a discharge with a Ta target and an Ar sputtering gas held at 20

mTorr. The corresponding first (c) and second (d) derivatives, and the calculated

EEDF (e) and electron density (f) are shown.

2. 4. 3. Ion current determination

For ion current measurements, flat probes are commonly used. This is because

by using this measurement setup, the collecting area is essentially independent of the

sheath thickness [41]. A schematic of a flat probe is shown in Fig. 9, where a metal

disk, in this case of 2 cm in diameter, is equipped with a guard ring that is kept at the

16

probe potential. However, only the current on the surface of the probe is collected. To

keep the probe potential constant during operation of the cathode, a power supply

with sufficient capacitance on the output circuit should be utilized. Principally, the

theoretical approach for treating ion current measurements, using a flat probe, are the

same as that of the wire probe described in the preceding section.

Fig. 9. A schematic of the flat probe with the probe and the guard ring kept at the

same potential.

2. 4. 4. Direct display of plasma parameters using a triple probe setup

In some applications, measuring a time-dependent electron current that varies

intensely in magnitude could cause problems related to the used measurement

electronics. This is a typical problem in measuring the electron temperature in

HPPMS, where the noise amplification resulting from the electron saturation current

differentiation and the following integrations has been shown to give big

uncertainties. To avoid this problem, the triple probe [50,51] with direct temperature

display is utilized. The triple probe theory assumes a Maxwellian electron energy

distribution. It consists of three Langmuir probes with identical geometrical

configurations and dimensions. Figure 10 shows a schematic of such a probe.

17

Fig. 10. A schematic of the triple probe circuit with potentials and currents of each

probe shown.

The three probes P1, P2 and P3 are placed 2 mm away from each other, forming an

equal-sided triangle. This is a large enough separation to adequately shield them from

one another. Negative potentials Vd2 and Vd3 are applied to P2 and P3 with reference to

P1, so that the probes are individually biased to three different potentials. The current

density on each probe is the sum of the ion saturation current density and the electron

current density so that:

-I1=-SJeexp(-φV1)+SJi(V1) (8)

I2=-SJeexp(-φV2)+SJi(V2) (9)

I3=-SJeexp(-φV3)+SJi(V3) (10)

φ=e/kTe (11)

Je=nee(kTe/2pme)1/2 (12)

Because the electron currents in all three probes are much higher that the

corresponding ion currents, the assumption Ji(V1)= Ji(V2)= Ji(V3)=Ji is made. This

gives:

)exp(1)exp(1

3

2

31

21

d

d

VV

IIII

φφ

−−−−

=++ (13)

18

This serves then to determine Te directly by drawing

curves of the probe-ratio (I1+I2)/(I1+I3) vs Te for a number of Vd3 and a fixed Vd2.

It is also possible to have direct display of the electron temperature, using an

oscilloscope for example. In that case one of the three probe is set to be floating (Fig.

11). With I2 =0, the currents flowing through P1 and P2 obey I1 =I3. Equation (13) will

therefore modify to give:

)exp(1)exp(1

21

3

2

31

21

d

d

VV

IIII

φφ

−−−−

==++

(14)

This means that, having Vd3 fixed, the electron temperature can be determined solely

by knowing Vd2, using equation (11).

Fig. 11. A schematic of the triple probe circuit with one of the probes kept at the

floating potential.

19

2. 5. Solitary waves

As mentioned in section 2.1, a number of waves can exist in a plasma, ranging

from electromagnetic to magnetosonic and ion acoustic waves. For a comprehensive

theoretical description of these waves, I refer to the books in references 40 and 41. I

will however provide in this section a short description of solitary waves that are also

the subject of paper IV.

The discovery of solitary waves dates back to 1834, when John Scott Russell,

a Scottish scientist was observing a boat being drawn along 'rapidly' by a pair of

horses. When the boat suddenly stopped, Russell noticed that a bow wave continued

forward "at great velocity, assuming the form of a large solitary elevation, a well-

defined heap of water which continued its course along the channel apparently

without change of form or diminution of speed". The photo in Fig. 12 shows a solitary

wave generated in the Scott Russell Aqueduct on the Union Canal near Heriot-Watt

University.

Fig. 12. Photo: courtesy to D B Duncan and J C Eilbeck, Heriot-Watt University,

Edinburgh.

The observations of Russel appeared to contradict the nonlinear shallow-water

wave theory of Airy (1845), which predicted that a wave with elevations of finite

amplitude could not propagate without change of form, i.e., the top of the wave would

move faster than the base resulting in a steepning of the wavefront (Fig 13) [52].

20

Fig. 13. A breaking wave.

This issue was resolved by Joseph Boussinesq (1871) and independently by Lord

Reyleigh (1876), who showed that the steeping of the wave can be balanced by the

dispersion (Fig. 14), where different frequency components of the wave travel with

different speeds, leading to a wave of permanent form [52]. For a long time, however,

the solitary wave was considered to be a rather unimportant curiosity in the

mathematical structure of nonlinear wave theory. It was not until the 1950's that

studies conducted by Fermi et al. [53] showed how important Russell's discovery had

been [54,55]. Today's most advanced fiber-optic communications use stable pulses of

light identical to Russell's waves, now called solitons (from solitary waves), to carry a

large amount of information over thousands of kilometers of fibers.

Fig. 14. A dispersive wave.

Ion acoustic solitary waves can be described as constant-velocity waves where

the group velocity is equal to the phase velocity. Thus ion-acoustic solitary waves are

stationary pulses or wave packets that propagate in non-linear dispersive media. They

were first observed in a double-plasma device by Ikezi et al. [56]. Since the initial

21

observation, these waves have been the subject of numerous experiments, including

multiple soliton production in a double-plasma device [57,58] and excitation of

cylindrical [59] and spherical [60] ion-acoustic solitons.

Hershkowitz and Romesser [59] summarized the distinguishing characteristics

of the one-dimensional solitary wave. These characteristics are common to one-

dimensional solitary waves in planar, cylindrical and spherical geometries and

provide a reference for comparison with experimental data. They are:

1. Arbitrary positive (compressive) density perturbations evolve into a superposition

of spatially separated solitons.

2. The number and amplitude of solitons is determined by the solutions of the time-

independent Schrödinger equation.

3.The soliton velocity is given by:

)/(3/11 0nncu

S

δ+= (15)

where δn/n0 is the maximum density perturbation and cS is the ion acoustic velocity.

4. The square of the normalized spatial width, D2, of a soliton is proportional to

(δn/n0)-1, which implies D2δn/n0 = constant.

5. Solitons retain their identity upon collision with other solitons.

In addition to characteristic 2, it has been shown that to the continuous spectrum of

unbound states corresponds a linear oscillatory tail [61]. The energy of a solitary wave

is proportional to (δn/n0)2D. For non-planar expanding solitons the energy will decay

due to geometry as 1/r and 1/r2, for cylindrical and spherical geometries, respectively.

When combining this geometrical decay with characteristic 4, we find that the solitary

wave’s amplitude will decay as:

ηrnn

∝∂∂

0

(16)

where r is the radius of the soliton and η=-2/3 for a cylindrical geometry but η=-4/3

for a spherical geometry [ 60,62].

22

3. Part II. Thin film growth and characterization

3. 1. Growth models

Thin films are formed on a substrate by a process of nucleation and growth.

Nucleation depends on the interaction between the adatoms and the substrate and is

affected by the surface mobility of the species, which depends on the substrate

temperature, the deposition rate, and the energy of the arriving atoms. The shape of

the nuclei depends on the relative interfacial free energies between the film and the

substrate γfs, the film and the vapor γfv, and the substrate and the vapor γsv [63]. The

interfacial energies balance each other at the interface so that γsv= γfs+ γfvcosθ, where

θ is the contact angle, as seen in Fig. 15.

Fig. 15. Schematic of a nucleus and the corresponding free energies of the substrate,

the film, and the vapor.

During the early stages of thin film growth, one of three main growth modes

dominates [1]. If γsv≥ γfv+ γfs and θ=0, i.e., the atoms or molecules in the growing film

are more strongly bound to the substrate than to each other, the growth is two-

dimensional Frank-van der Merwe (FM) or layer-by layer growth, where a continuous

monolayer is formed prior to the deposition of a subsequent layer. If γsv< γfv+ γfs and

θ>0, the atoms or molecules in the growing film are more strongly bound to each

other than to the substrate, and the film grows three-dimensionally. This is called

Volmer-Weber (VW) or island growth, where the deposited atoms tend to aggregate

into islands with thickness of several monolayers. Finally, the mixed Stranski-

Krastanov (SK) mode, which starts in a monolayer growth mode for a few

23

monolayers before shifting to island growth. The intermediate SK mode is not

completely understood, but factors such as the film-substrate, lattice mismatch, or the

strain energy accumulated in the growing film, may be the cause of such growth [28].

For the three growth modes, similar mechanisms control depositions and

agglomeration of the incoming atoms in the initial and stationary growth phases.

Fig. 16. Microstructure zone diagram for metals deposited by magnetron sputtering.

T is the substrate temperature and Tm is the coating material melting point [64].

During deposition, the surface diffusion, the bulk diffusion, and the adsorption

scale directly with the melting temperature of the deposit (metal). This can result in 4

different film structures (Fig. 16):

zone 1 exhibits amorphous or fine grained polycrystalline films, with a high density of

crystallographic defects. The absence of surface diffusion at low Ts yields columnar

structures and results in porous and rough films.

Zone 2 exhibits a dense columnar structure with smoother surfaces as a result of

surface diffusion controlled growth. The columnar grains can have highly faceted

surfaces due to surface energy anisotropy. The grain size generally increases with

increasing Ts.

Zone T is the transition zone between zones 1 and 2 [64] that is observed

when the gas pressure is decreased (Fig. 16). This is because a decrease of the gas

pressure results in an increase in the energetic bombardment of the films.

24

The influence of ion bombardment on the film properties of metals can be reconciled

with the accepted models of thin-film growth. However, energetic condensing

particles or bombardment promote the zone T structure [64,65]. Evaporated atoms

usually have energies in the vicinity of 0.1 eV, while sputtered atoms usually have

energies of a few eV. Condensing atoms from these different sources will produce

films of different densities.

25

3. 2. Deposition of thin films by magnetron sputtering

3. 2. 1. Magnetron sputtering

Figure 17 shows a typical magnetron set up where a permanent magnet is

placed at the back of the cathode target, which generates magnetic field lines that

enter and leave through the cathode plate, as shown in Fig. 18.

Fig. 17. A schematic of the magnetron with two concentric magnets with opposite

polarities.

A simulation of the magnetic field is shown in Fig. 18. This was made for a

magnetron sputtering cathode with a circular target (similar to the setup used in the

work presented here) using the magnetic field shareware package Finite Element

Method Magnetics (FEMM) [66]. The simulation results show the magnetic field lines

that link the outer ring-magnet and the center pole. At a distance d ~ 6 cm from the

target, the magnetic field strength is nil along the target axis, and at d > 6 cm, the

magnetic field lines lead away from the target. The simulation also shows the

magnetic field strength, B, in the form of shaded gray zones.

26

Fig. 18. Magnetic filed lines and flux simulated using FEMM.

The high B value near the surface of the target results in a trapping of the

electrons in this area. This is because of the modification of the electron trajectory, as

electrons within the dual (electric + magnetic) field environment experience the well-

known Lorentz force. This is given in the equation,

)( BvEF ×+−== qdt

mdv (17)

where q, m, and v¸ are the electron charge, mass, and speed, respectively. The

magnetic field, prolongs therewith the electron residence time in the plasma close to

the target, which allows for operating the sputtering discharge at higher current

densities, lower voltages, and lower pressures than can be obtained in a conventional

glow discharge. and thus enhance the probability of ionizing (and exciting) collisions.

3. 2. 2. The ionization process

In conventional magnetron sputtering techniques, the extra energy needed in

some process conditions is usually provided by heating the entire substrate. This leads

to restrictions with respect to choice of substrate material. Instead of using thermal

energy as enhancement energy, various techniques, such as ion beam deposition [28],

have made use of the kinetic energy of the accelerated ions. In conventional dcMS,

the ion content of the discharge is usually low (~1 %). The plasma electron density is

low (~ 1×1016 m-3), and the main mechanism for ionization is penning ionization [41].

In order to increase the ionization fraction, the plasma density in the region between

the source and the substrate must increase so that the probability for electron-impact

27

ionization increases [41]. The higher ionization fraction is beneficial for the film

morphology as particle bombardment of the growing film transfers kinetic energy to

the adatoms of the growing surface [67], which can be used to lower the process

temperature and modify the microstructure of the film. In order to do this, the power

applied to the cathode is increased. However, since most of the power applied to the

cathode is transformed into heat, the technique is limited to relatively low power

densities before the target cooling is insufficient. High plasma density, without

overheating the target, can be achieved by HPPMS, where high power pulses are

applied to the target with a low duty cycle.

3. 2. 2. 1. Self-sputtering in HPPMS

In the HPPMS process (section 2.4), the sputtering takes place during the

pulse-on time, with a duty factor of less than 1 %. Fig. 5 is redrawn here for

convenience as Fig. 19, where the target voltage and current time-traces are shown. It

is observed that the negative target voltage VT drops from 100V to ~ 800 V as the

target current IT starts to build up, and that a peak power of 0.4 MW is achieved 20 µs

after the pulse start. This is followed by a plasma decay as the voltage and the current

decrease and eventually die out ~ 80 µs after the pulse initiation. During the pulse-on

time, the composition of the discharge is dominated by the sputtered metal ions, as

has been shown by optical emission spectroscopy (OES) [68]. This has not been fully

explained at this point, nevertheless, it was suggested that during the pulse-on time,

self-sputtering contributes to the sputtering process, i.e., sputtering is performed by

the target metal ions that are attracted back to the target by the potential drop between

the plasma and the target. [69,70].

28

Fig. 19. Time evolution of the target voltage (VT) and current (IT) for a Ta target and

an Ar gas pressure of 20 mTorr.

The effect of self-sputtering is examined, using a simple model, by comparing

the sputtering yields obtained with HPPMS and dcMS, for the same average power

(Fig. 20). The ratio between the HPPMS and the dcMS sputter yields (R=SHPPMS/SDC)

could be estimated by modeling the yields as follows:

SHPPMS = IAr×γAr + IMe×γMe – IMe (18)

SDC = I’Ar×γAr (19)

Where IAr and IMe are the Ar and metal ion currents in the HPPMS system and I’Ar is

the Ar ion current in the dcMS system. γAr and γMe, are the sputter yield for Ar and

metal respectively. If we write the total ion currents as:

IAr+IMe=IiH (20)

I’Ar≈Ii

D (21)

Where Ii

H and IiD are the total ion currents in HPPMS and dcMS respectively, then,

( ) ( )Ar

Hi

MeMeArArDi

Hi

ArDi

MeMeArAr

DC

HPPMS

γI1γIγI

II

γI1γIγI

SS

×−+×

×=×

−+×==R

29

−+−=

Ar

MeHi

MeHi

MeDi

Hi

γ1γ

II

II1

II (22)

The current ratios Di

Hi

II and H

i

Me

II could be determined by measuring the currents in the

system. However, it is seen from equation 22 that R not only depends on the applied

average power, i.e., current ratios, but also on the target material and the sputter gas.

Fig. 20. A schematic of the sputtering process during HPPMS.

Weight gain differences between a floating and a positively biased substrate

showed that, for a number of target materials, including Al, Zr, Ti, Ta, and Cr, the

degree of ionization of the sputtered metal was high. Figure 21 shows the results

obtained using a Ta target. Other works showed that the ionization fraction was 70 %

using a Cu target [36], roughly 40 % using a TiAl-target [68] and 4.5 % using a C

target [38].

30

Fig. 21. Ion deposition measurements. Experimental setup (a), and the ion fraction in

the deposited film for Ta as a function of the pulse energy, measured as the weight

gain difference between a floating and a biased substrate (b).

In order to put this in the context of self-sputtering, Fig. 22 shows measured

relative deposition rate (HPPMS with respect to dcMS) for some metals as a function

of the self-sputtering yield, SS, divided by the Ar yield, SAr. It is seen that materials

with high self-sputtering yields resulted in increased deposition rate ratios.

Fig. 22. Deposition rate ratio, of HPPMS by dcMS, versus the self-sputtering yield

ratio SS/SAr [70].

31

3. 3. Effects of ionized deposition on film structure

3. 3. 1. Film densification using HPPMS

In general, ion bombardment is accompanied with surface defects such as

surface vacancies [28,67]. The resulting film growth strongly depends on the

individual film-substrate combination and the process parameters such as the substrate

temperature, the ion energy, and the ion-to-neutral ratio [71]. It is therefore beneficial

to acquire means to control the bombarding energy of individual ions in order to

enhance the deposited film’s properties. In this regard, the advantages of using

HPPMS are many. Dekoven et al. [67] found that ultra thin carbon films grown by

HPPMS had significantly higher densities (2.7 g/cm3) than films grown by

conventional dcMS (< 2.0 g/cm3), i.e., the film density is closely related to the ion

bombardment and ion deposition during the process. This is also supported by paper

VII where Ti-Si-C films were deposited by HPPMS. As is seen in the SEM

micrographs in Fig. 23, the HPPMS film exhibited an enhanced morphology

compared to the dcMS grown film.

Fig. 23. Thin Ti-Si-C films grown by dcMS (a), and HPPMS (b) at 14 cm from the

target surface, using an average target power of 150 W, a substrate bias voltage of -

10 V, and an Ar gas pressure of 5 mTorr.

Film densification was also investigated for deposition on tilted substrates. In

general, deposition on complex-shaped substrates entails new challenges to the

deposition process. When the substrate is tilted with respect to the deposition flux

(oblique incidence), interesting effects such as shadowing can take place, which can

32

result in changes in the optical, magnetic and/or electrical properties of the deposited

films [72,73]. The oblique incidence effect was discovered in 1959 and many papers

have been written on the subject [72,73]. We investigate the oblique incidence effect

in the HPPMS system by growing thin films on non-flat surfaces, mimicked by

substrates at different angles with respect to the target surface, and compare their

morphologies and properties with dcMS grown films. Scanning electron micrographs

of Ta films deposited using a bias voltage of –50 V are shown in Fig. 24. They show

that in the dcMS case, films deposited at a 90o inclination angle with respect to the

incidence flux exhibit a columnar morphology, with the columns tilted from the

substrate normal toward the deposition flux incidence direction. The films are porous

and rough. The HPPMS films, on the other hand, show a dense and smooth surface,

and the columns are along the substrate normal. This indicates that a large part of the

depositing species are ions, as is discussed in more detail in paper V. The ion

bombardment is known to influence the morphology and the density of the growing

film. However, the use of magnetron sputtering deposition for the purpose of

depositing dense flat films on inclined surfaces is a feature that is unique to HPPMS

at this point.

Fig. 24. SEM micrographs of Ta films deposited by dcMS (a) and HPPMS (b) at 10

cm from the target surface, with an average power of 300 W, a bias voltage of –50 V,

and an Ar gas pressure of 5 mTorr.

33

Using HPPMS, densification of films grown facing away from the target (at a

180º angle with respect to the target surface) was possible by applying a bias to the

substrate. Fig. 25 shows TEM micrographs of Ta films deposited by HPPMS, facing

away from the target, at floating potential and at a –30 V bias voltage.

Fig. 25. TEM micrographs showing Ta films grown at a 180o angle with respect to the

target surface by HPPMS with the substrate floating (a) and biased with a voltage of -

30 V (b). The average target power was 150 W and the Ar gas pressure 5 mTorr.

3. 3. 2. Phase formation

The formation of high compressive stress might promote the formation of

dense phases of a material [28,74]. For example, at sufficiently high deposition

energy, diamond can be grown as the thermodynamically stable phase rather than

graphite. [28]. McKenzie et al. [75] showed that at a high pressure, corresponding to

compressive stresses, sp3 bonding could be promoted in a growing C film by ion

bombardment. More clearly, the influence of the process parameters on the phase

formation of coatings becomes evident for films with different crystalline phases. As

an example, Ta thin films usually have a metastable tetragonal structure (β-Ta), when

they are deposited on Si substrates. The formation of Ta metastable phases has been

reported in a large number of sputtering experiments [76-82]. The β-phase is

sometimes formed alongside the stable α-phase (bcc-Ta). β-Ta is harder, more brittle,

and less ductile than the bcc-Ta. It is known to contain a high concentration of

impurities and defects, and to have a resistivity of ~170–210 µΩcm, much higher than

34

the ~15–60 µΩcm of the stable bcc phase. [79,80,82]. Using HPPMS, β-Ta and bcc-

Ta films could be grown for different bias voltages. This is shown in paper VI to be

directly related to the ion energy and the ion-to-neutral ratio in the discharge.

3. 3. 3. Enhanced adhesion by HPPMS

Of fundamental importance to the performance of a thin film is its adherence

to the substrate. Adhesion is determined by two main factors, one being the strength

of the interface between film and substrate, the other one being the stress state of the

film. Hirsh and Varga [83] found that ion bombardment increased the adhesion of

IBAD (ion-beam assisted deposition) Ge films to glass substrates, as a result of

reduced stress and formation of an intermediate layer between the film and the

substrate, which increases the bonding forces [28]. Münz et al. [84] utilized ionized

metal from an arc source for etching and implantation of a substrate with the aim to

increase coating adhesion. The technique is named arc bond sputtering (ABS). An arc

source gives a very high degree of metal ionization, but also produces macroscopic

particles [85]. HPPMS is therefore an attractive alternative to ABS. HPPMS was also

used to improve the adhesion of thin CrN films [86-88]. Its main advantage compared

to arc deposition is that macroscopic droplets can be avoided. It was showen that the

microstructure of the coatings grown by HPPMS was dense with, neither voids that

are usually associated with conventional magnetron sputtered films, nor large-scale

growth defects, usually observed in arc evaporated films, as shown in Fig. 26.

Furthermore, the HPPMS-based sample pretreatment stage contributed to the strong

adhesion of CrN coatings, with high sliding wear resistance. The films also showed

better corrosion protection behavior than the corresponding ABS coatings due to the

lack of macro-particles and the voids created close to them.

35

(a)) ( axcdsaaa a)a

Fig. 26. TEM cross sectional micrograph of the CrN-HPPMS coating [ref. Harry]

36

3. 4. Sputtering of a MAX-phase compound target by HPPMS

Mn+1AXn-phases (n= 1-3) are materials with exceptional structural, electrical,

and mechanical properties [89]. They are ternary material-systems, where M is a

transition metal, A is a group-A element, and X is C and/or N.

I II III IV V VI VII VIII

H M Early Transition Metal He

Li Be A A-Group Element III-V B C N O F Ne

Na Mg X Carbon and/or Nitrogen Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La- Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Ac- Unq Unp Unh Uns Uno Une Uun Uuu Uub Uut Uuq Uup Uuh Uus Uuo

211 M2A1X1: Ti2AlN, Zr2SnC,…, ~ 50 Compounds

312 M3A1X2: Ti3SiC2, Ti3AlC2, Ti3GeC2

413 M4A1X3: Ti4AlN3, Ti4SiC3, Ti4GeC3

Fig. 27. The periodic table of elements showing the different groups that form the

MAX-phase materials, with different shades. The box under shows the synthesized

MAX-phase materials

Although they were discovered in the 1960’s [90,91], the interest in the Max-

phase materials was only awakened in the later years [92], due mainly to the difficulty

in synthesis. The MAX-phase materials combine metallic electrical properties and

ceramic mechanical properties. Their crystal structure consists of a hexagonal unit cell

belonging to the space group D46h-P63 / mmc. Fig. 28 illustrates the unit cells for

Ti2SiC, Ti3SiC2, and Ti4SiC3 stoichiometric MAX-phases, where the edge-sharing

M6X (Ti6C or Ti6N) octahedra is interleaved with layers of pure A-elements, thus

forming a nanolaminated material [90]. Ti-Si-C is one of the materials system that has

been studied the most. Films were grown by different methods [93-96] and their

properties were studied. In this work Ti-Si-C thin film growth by HPPMS was

pioneered. This is presented in paper VII, where the films were deposited on MgO

37

substrates at different substrate inclination angles with respect to the sputter source

from a compound Ti3SiC2 target, for a range of deposition conditions. This resulted in

different film compositions as studied by XRD and XPS, indicating the possibility to

tailor the film composition simply by inclining the substrate.

Fig. 28. Unit cells for Ti2SiC1, Ti3SiC2, and Ti4SiC3.

Results from these experiments (paper VII) and Ta deposition experiments (papers V

and VI) show that in HPPMS three parameters related to ions in the plasma could be

controlled, allowing for better control and potentially tailoring of the film structure.

These are: the ion energy, which could be determined by choosing the power to the

target and the bias voltage to the substrate, the ion to neutral flux, which was showed

to depend on the deposition angle with respect to the target surface, and the metal flux

in the plasma. Thus, work on the Ti-Si-C system shows that HPPMS can be used in

more complex materials system, but a better understanding of the deposition

dynamics and the gas phase scattering in the system is needed.

38

3. 5. Thin film characterization

In this chapter, the main tools used for characterization of the deposited films

in the present work are shortly described. Different textbook references are given

where appropriate for detailed descriptions of these techniques.

3. 5. 1. Transmission electron microscopy (TEM)

This is a powerful tool with an optimal image resolution of ~1 Å for an

accelerating voltage of 200 kV. Figure 29 presents a comparison between a number of

microscopic tools. Materials for TEM must be specially prepared to thicknesses that

allow electrons to transmit through the sample. This is a time consuming task,

especially for cross-sectional images. TEM images provide information on the sample

microstructure and exhibit contrasts caused by defects and grain boundaries, for

example. There are two mechanisms responsible for the contrasts observed in a TEM

image, a diffraction contrast used for the formation of bright field and dark field

images, which reveal areas that are diffracting the electron beam differently due to

changes in orientation or presence of crystallographic defects, and a phase contrast

that uses the phase difference of the transmitted electrons to form the image [99,100].

Fig. 29. The range of spatial resolutions for the eye and some microscopic techniques.

3. 5. 2. Scanning electron microscopy (SEM)

SEM is a versatile and widely used tool as it allows the study of both

morphology and composition of materials. A monochromatic electron beam is

focused onto a fine probe that is scanned over a rectangular area of the sample. As the

electrons collide with the surface, a number of interactions occur resulting in the

39

emission of electrons and photons from the surface. The intensity of the secondary

electrons is point-to-point mapped onto a screen. The electron beam in the cathode ray

tube illuminating the screen is synchronized with the electron beam scanning the

sample. High-resolution images of the morphology or topography of a specimen at

very high magnifications can be obtained. Compositional analysis of a material may

also be obtained by monitoring secondary X-rays produced by the electron-specimen

interaction. Thus detailed maps of elemental distribution can be produced from multi-

phase materials. Characterization of fine particulate matter in terms of size, shape, and

distribution as well as statistical analyses of these parameters, may be performed.

More details are found in reference [100, 101,102]

3. 5. 3. Atomic force microscopy (AFM)

AFM, also known as scanning force microscopy, is a technique with minimal sample

preparation requirements. It is based on detection of force interaction between a

sample surface and a sharp tip mounted on a flexible cantilever, and produces, in such

a way, topographic images of a surface with atomic resolution in all three dimensions.

When the tip is brought to a close proximity (a few Å) of a sample’s surface,

repulsive Van der Waals forces between the atoms of the tip and those of the sample

cause the cantilever to deflect. The magnitude of the deflection depends on the

distance between the tip and the sample. Normaly the lateral AFM resolution of an

AFM is about 1 nm. However, with a highly sharp tip and a flat sample, even atomic

resolution could be obtained [100].

3. 5. 4. X-ray diffraction (XRD)

XRD is a powerful non-destructive method that is used to measure structural

properties, such as residual stress, grain size, epitaxial relations, texture, defect

structure, and crystal structure. The X-rays have wavelengths in the order of a few

angstroms, the same as typical interatomic distances in crystalline solids. When

certain geometric requirements are met, X-rays scattered from a crystalline solid can

constructively interfere, producing a diffracted beam. The condition for constructive

interference is given by the Bragg’s law:

nλ=2dsinθ,

40

where n is an integer, λ is the wavelength, d is the interatomic spacing, and θ is the

diffraction angle. The diffracted beam intensity will depend on several factors such as

the chemical composition of the film and the local arrangement of the atoms. Detailed

descriptions of the technique are found in reference. [89, 103]

3. 5. 5. X-ray photoelectron spectroscopy (XPS)

XPS provides elemental analysis and information about chemical bonding

properties. An incoming photon of known energy hω causes ejection of a core

electron, thus leaving the atom in an excited state. The kinetic energy EK of this core

electron is equal to the excitation energy minus the binding energy and the work

function φ. This is described by the following equation:

EB=hω-EK-φ.

XPS provides chemical bond information through the chemical shifts, i.e. an

element experiences a slight net negative charge when bonded to an electropositive

element. The opposite happens when the bond is to an electronegative element [100].

3. 5. 6. The four-point electrical probe

The four-point probe measurement setup is shown in Fig. 30. Four wires (or

probes) are attached to a sample surface. A constant current is made to flow the length

of the sample through probes labeled 1 and 4 in the figure. This can be done using a

current source or a power supply as shown. The sample’s resistance to the flow of

electrical current causes a potential drop as the current flows along the sample, for

example between the two wires (or probes) labeled 2 and 3 in the figure. The

resistance of the sample between probes 2 and 3 is the ratio of the voltage drop to the

value of the output current of the power supply [1].

41

Fig. 30. A schematic of a four point probe.

42

4 Summary One of the main benefits with magnetron sputtering is that it makes it possible

to grow thin films on a range of substrate materials, as the substrate temperature can

be kept low. The HPPMS technique differentiates itself from other magnetron

sputtering techniques in that the sputtered material ionizes to a high degree. Although

at its infancy, the technique shows the potential of ionized magnetron sputtering and

opens new possibilities for the thin film processing. In the present work a light has

been shed on the plasma dynamics in the HPPMS system by determining the plasma

parameters and studying the plasma dynamics. Furthermore, the technique has been

used to grow metal and compound films and their properties have been characterized.

This work is a continuation of efforts made by ex-colleagues and in the realm of

efforts made by others working on similar techniques [104-108] to demonstrate the

possibilities entailed by highly ionized magnetron sputtering. Hereby, the

contributions to the field are presented as the papers included in the present thesis.

Paper I

We present a study of the electron energy distribution function (EEDF) in the

HPPMS discharge. We measure the temporal behavior of the electron energy

distribution function in the pulsed magnetron, using electrostatic Langmuir probe

measurements. We show that towards the end of the pulse, two energy groups of

electrons are present, i.e., a high-energy and a low-energy electron group. With the

disappearance of the high-energy group, the electron density peaks, and the EEDF is

Maxwellian-like. Eventually, the plasma becomes more Druyvesteyn-like with a

lower electron density and a higher average electron energy.

In this paper, we measured an electron density of 1018 m-3. However, the

measurements were made at a single position in the chamber (10 cm away from the

target) and without varying the Ar gas pressure. The next step was therefore to study

the plasma evolution at different process conditions.

Paper II

We demonstrate how the plasma evolves with a changing gas pressure at

various distances from the target. We measure the temporal behavior of the EEDF for

43

the different discharge conditions, and show that the peak electron density along the

axis of the target decreases with increasing gas pressure. The effective electron

temperature peaks at approximately 2.3–3.4 eV, at the same time from the pulse start,

independent of position in the discharge. This indicates that high-energy electrons

escape the high-density confinement next to the cathode at the end of the pulse.

Analyzing the discharge for different sputtering gas pressure, we find that the plasma

temporal behavior varies with the pressure. At higher pressures (>5 mTorr) a second

stationary peak is apparent hundred of microseconds after the pulse end. This is

treated in paper III.

Paper III

We study the spatial as well as the temporal behavior of the plasma

parameters. We show that a high plasma density (> 1×1019 m-3) was achieved for

reasonably high powers (a few hundreds kW), and that elevated density values are

sustained for hundreds of microseconds after the pulse to the target was off. We also

paid more attention to the second peak exhibited in the density (or saturation current)

curves. A number of experiments on the electron and ion saturation currents show that

the second peak is a wave reflection of the walls of the chamber. We study this wave

behavior and show that it is mainly caused by ion acoustic waves.

Parallel to this, we study the wave behavior of the first peak, which makes

paper 4.

Paper IV

Paper IV consists of a study of the wave behavior of the first peak in the

electron (and ion) saturation current. We determine the waves to be ion-acoustic

solitary waves, without the presence of an initial quiescent background plasma. We

investigate how the velocity of the wave depends on pulse energy and gas pressure.

The velocity decreases with increasing pressure and was almost independent of the

pulse energy. The amplitude of the wave increases with increasing pulse energy, and

the waves do not conform to solutions given by the KdV solitons model. However,

the amplitude decay of the solitary waves at 5 mTorr fits reasonably with a model of

spherically expanding KdV solitons. These findings indicate that the simple KdV

model is not sufficient to explain the solitary waves observed in our system.

44

Paper V

We describe the deposition of thin Ta films using the HPPMS technique. We

show that by using a highly ionized flux, dense films with smooth surfaces can be

obtained at surfaces placed normal to the sputtering source. We compare the

properties of these films with thin Ta films grown by dc magnetron sputtering under

similar conditions, and show that the superior qualities of the films grown by HPPMS

are achievable by controlling energy and directionality of the sputtered species.

The control of the deposition ionized flux energy can be further used to tailor

the phase of the deposited films in materials where the phase formation is energy

dependent. This issue is investigated by growing Ta films at different inclination

angles and different flux energies. This is done in paper VI.

Paper VI

We show that by using highly ionized magnetron sputtering, control of the Ta-

phase formation can be achieved by applying a bias voltage to the substrate. We show

the existence of a voltage window for deposition of bcc-Ta. The bias voltage needed

for the bcc-Ta to form is decreased from 70 to 30 V when the applied target voltage

increass from 740 to 1000 V. The bcc-phase formation depends also on the inclination

angle of the substrate with respect to the target surface. For high inclination angles of

135 and 180o, a mostly bcc-film is achieved for a low bias voltage of 10 V, while for

lower inclination angles, a higher bias voltage is needed. We attribute this to the high

ion-to-neutral ratio in the deposition flux, as this increases with increasing the

inclination angle.

HPPMS deposition of high quality metal films on inclined surfaces is

therewith demonstrated. Furthermore, increase of the deposition rate at inclined

surfaces, and tailoring of the phase of the deposited film is achieved. However, only

single element (metal) materials are used so far. In order to further investigate the

range of depositions possible with HPPMS, a ternary compound (Ti-Si-C) is used in

paper VII.

45

Paper VII

Using a ternary compound target Ti3SiC2, we deposit Ti-Si-C films on MgO

substrates. We show that Ti-Si-C films grown at an angle with respect to the

sputtering source have different properties than films grown facing the target. This is

in agreement with the XPS measurements, which show that the bonding

configurations also change with inclination angle. Furthermore, it is shown that the

composition of films deposited by sputtering from a Ti3SiC2 target can be controlled

by an appropriate choice of substrate inclination angle and bias voltage. At a 90o

inclination angle and a 5 mTorr Ar gas pressure, the films show X-ray diffraction

peaks that could possibly be attributed to the MAX-phase Ti4SiC3. The results of this

study the potential of using HPPMS in deposition of complex materials at low

temperatures. However, further studies must be made in order to better understand the

energetics of the sputtered species in the system.

46

5 Outlook In my work as a PhD student, I answered some of the questions related to

plasma behavior and the role of ion deposition for film properties when the highly

ionized magnetron sputtering system is used. The thesis also raised many new

questions, the answers to some of which hold the key for an interesting development

for this type of magnetron sputtering. I state in particular the following issues:

Plasma dynamics in a system with a compound target

The gas phase scattering in a discharge is an important factor that affects the

ion flux at the substrate. Using a compound target as the sputter source, the different

elements collision cross-sections need to be quantified in order to determine the ion

flux properties. In HPPMS, this is further complicated as the plasma state varies with

time. On the other hand, the present work showed that HPPMS could be used to grow

dense and smooth Ti-Si-C films at different substrate-to-target geometries. Further

studies should be therefore made in order to understand the evolution of the ionized

flux.

Self-sputtering in HPPMS

They are many indications [68,86,106,109] that for some materials, self-

sputtering during the duty time of the pulse takes place in the ionized magnetron

sputtering process. The effects of this contribute directly to the degree of ionization of

the sputtered material. It would be beneficial to study the relationship between the

target material, the self-sputtering process, and the ion flux evolution and intensity in

order to better tailor the deposited film’s properties.

Magnetic field configuration

Experiments made with different magnetic field setups (not presented in this

work) showed that the magnetic field strength and configuration play a big role in the

current drawn through the target. By understanding the full effect of this, magnetrons

can be designed for specific applications. Furthermore, magnetic coils can be used to

guide the plasma charged species into parts of a substrate that are normally not

reached by the flux. This study would demand a close collaboration between

47

simulations and experimental work in order to optimize the system for the actual

purposes.

Reactive sputtering by HPPMS

Initial trials for reactive sputtering HPPMS showed promising results [88].

However, no substantial work has been done in this regard, especially for oxides. This

is a very interesting area with a potential for enhancement of thin oxide films

properties.

Deposition on complex-shaped substrates

Some of the work presented in this thesis dealt by depositing thin films on

inclined surfaces. It showed that much higher deposition rates with enhanced film

properties were achieved on surfaces facing away from the target surface using

HPPMS than by conventional growing methods. Moreover, the arriving flux to these

surfaces had a high ion fraction. The homogeneity of the films grown at the different

substrate-to-target geometries can be enhanced by mapping out the relation target-

power target-material magnetic-setup.

48

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