cvd and pvd thin film techniques

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Vapor Deposition Pattern Transfer: Additive techniques- Physical and Chemical Vapor Deposition RAJEEV R PILLAI

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AN OUTLINE OF DIFFERENT THIN FILM TECHNIQUES

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Page 1: CVD AND PVD THIN FILM TECHNIQUES

Vapor Deposition Pattern Transfer:

Additive techniques-Physical and Chemical Vapor

Deposition

RAJEEV R PILLAI

Page 2: CVD AND PVD THIN FILM TECHNIQUES

Content

Physical vapor deposition (PVD)– Thermal evaporation

– Sputtering– Evaporation and sputtering compared

– MBE– Laser sputtering– Ion Plating– Cluster-Beam

Chemical vapor deposition (CVD) – Reaction mechanisms

– Step coverage– CVD overview

Epitaxy Electrochemical

Deposition

Page 3: CVD AND PVD THIN FILM TECHNIQUES

Physical vapor deposition (PVD)

The physical vapor deposition technique is based on the formation of vapor of the material to be deposited as a thin film. The material in solid form is either heated until evaporation (thermal evaporation) or sputtered by ions (sputtering). In the last case, ions are generated by a plasma discharge usually within an inert gas (argon). It is also possible to bombard the sample with an ion beam from an external ion source. This allows to vary the energy and intensity of ions reaching the target surface.

Page 4: CVD AND PVD THIN FILM TECHNIQUES

Physical vapor deposition (PVD): thermal evaporation

Heat Sources Advantages DisadvantagesResistance No radiation Contaminatione-beam Low contamination RadiationRF No radiation ContaminationLaser No radiation, low

contaminationExpensive

N = No exp- ekT

6

The number of molecules leaving a unit area of evaporant per second

Page 5: CVD AND PVD THIN FILM TECHNIQUES

Physical vapor deposition (PVD): thermal evaporation

Si

Resist

d

Evaporant container with orifice diameter DD

Arbitrary surface element

1-exp (+d/)

Kn = /D > 1

A ~ cos cos /d2

N (molecules/unit area/unit time) =3. 513. 1022Pv(T)/ (MT)1/2

The cosine law

This is the relation between vapor pressure ofthe evaporant and the evaporation rate. If a high vacuum is established, most molecules/atoms will reachthe substrate without intervening collisions. Atoms andmolecules flow through the orifice in a single straight track,or we have free molecular flow :

The fraction of particles scattered by collisions with atoms of residual gas is proportional to:

The source-to-wafer distance must be smaler than the mean free path (e.g, 25 to 70 cm)

Page 6: CVD AND PVD THIN FILM TECHNIQUES

Physical vapor deposition (PVD): thermal evaporation

t2

t1

Substrate

t1

t2

= cos 1

cos 2

3

Surface feature

Source

Source

Shadow

t1/t2=cos/cos

= (RT/2M)1/2 /PT

From kinetic theory the mean free path relates to the total pressure as:

Since the thickness of the deposited film, t, is proportionalTo the cos , the ratio of the film thickness shown in the Figure on the right with = 0° is given as:

Page 7: CVD AND PVD THIN FILM TECHNIQUES

Physical vapor deposition (PVD): sputtering

W= kV iPTd

-V working voltage- i discharge current- d, anode-cathode distance- PT, gas pressure- k proportionality constant

Momentum transfer

Page 8: CVD AND PVD THIN FILM TECHNIQUES

Evaporation and sputtering:comparison

Evaporation SputteringRate Thousand atomic layers per second

(e.g. 0.5 µm/min for Al)One atomic layer per second

Choice of materials Limited Almost unlimited

Purity Better (no gas inclusions, very highvacuum)

Possibility of incorporatingimpurities (low-medium vacuumrange)

Substrate heating Very low Unless magnetron is used substrateheating can be substantial

Surface damage Very low, with e-beam x-raydamage is possible

Ionic bombardment damage

In-s itu cleaning Not an option Easily done with a sputter etch

Alloy compositions ,s tochiometry

Little or no control Alloy composition can be tightlycontrolled

X-ray damage Only with e-beam evaporation Radiation and particle damage ispossible

Changes in sourcematerial

Easy Expensive

Decomposition ofmaterial

High Low

Scaling-up Difficult Good

Uniformity Difficult Easy over large areas

Capital Equipment Low cost More expensive

Number ofdepositions

Only one deposition per charge Many depositions can be carriedout per target

Thickness control Not easy to control Several controls possible

Adhesion Often poor Excellent

Shadowing effect Large Small

Film properties (e. g.grain s ize and s tepcoverage)

Difficult to control Control by bias, pressure,substrate heat

Page 9: CVD AND PVD THIN FILM TECHNIQUES

Physical vapor deposition (PVD): MBE, Laser Ablation

-

MBE– Epitaxy: homo-epitaxy hetero-epitaxy

– Very slow: 1µm/hr– Very low pressure: 10-11 Torr

Laser sputter deposition– Complex compounds (e.g. HTSC, biocompatible ceramics)

Page 10: CVD AND PVD THIN FILM TECHNIQUES

Physical vapor deposition (PVD): Ion cluster plating

Ionized cluster: it is possible to ionize atom clusters that are being evaporated leading to a higher energy and a film with better properties (adherence, density, etc.). – From 100 mbar (heater

cell) to 10-5 to 10-7 mbar (vacuum)--sudden cooling

– Deposits nanoparticles Combines evaporation with a

plasma» faster than sputtering» complex compositions» good adhesion

Page 11: CVD AND PVD THIN FILM TECHNIQUES

Gas cluster ions consist of many atoms or molecules weakly bound to each other and sharing a common electrical charge. As in the case of monomer ions, beams of cluster ions can propagate under vacuum and the energies of the ions can be controlled using acceleration voltages. A cluster ion has much larger mass and momentum with lower energy per atom than a monomer ion carrying the same total energy. Upon impact on solid surfaces, cluster ions depart all their energy to an extremely shallow region of the surface. Cluster plating material is forced sideways and produces highly smooth surfaces.

Also individual atoms can be ionized and lead to ion plating (see figure on the right, example coating : very hard TiN)

Physical vapor deposition (PVD):Ion cluster plating and ion plating

Page 12: CVD AND PVD THIN FILM TECHNIQUES

Chemical vapor deposition (CVD): reaction mechanisms

Mass transport of the reactant in the bulk

Gas-phase reactions (homogeneous)

Mass transport to the surface

Adsorption on the surface Surface reactions

(heterogeneous) Surface migration Incorporation of film

constituents, island formation

Desorption of by-products Mass transport of by-

produccts in bulk

CVD: Diffusive-convective transport of depositing species to a substrate with many intermolecular collisions-driven by a concentration gradient

SiH4SiH4

Si

Page 13: CVD AND PVD THIN FILM TECHNIQUES

Chemical vapor deposition (CVD): reaction mechanisms

Fl = Dc

x

(x) x

U

1

2

1

L(x)dX

2

30

L

L

UL

1

2

ReL UL

= 2L

3 ReL

Energy sources for deposition:– Thermal– Plasma– Laser– Photons

Deposition rate or film growth rate(Fick’s first law)

(gas viscosity , gas density, gas stream velocity U)

(Dimensionless Reynolds number)

Laminar flow

L

(x)

dx

(U)

(Boundary layer thickness)

Fl = Dc

2L3 ReL (by substitution in Fick’s first law and x=)

Page 14: CVD AND PVD THIN FILM TECHNIQUES

Mass flow controlled regime (square root of gas velocity)(e.g. AP CVD~ 100-10 kPa) : FASTER

Thermally activated regime: rate limiting step is surface reaction (e.g. LP CVD ~ 100 Pa----D is very large) : SLOWER

Chemical vapor deposition (CVD): reaction mechanisms

Fl = Dc

2L3 ReL

R = Ro e - Ea

kT

Page 15: CVD AND PVD THIN FILM TECHNIQUES

Chemical vapor deposition (CVD): step coverage

Fl = Dc

2L3 ReL

R = Ro e - Ea

kT

Step coverage, two factors are important– Mean free path and

surface migration i.e. P and T

– Mean free path:

w

z

is angle of arrival

kT

21

2 PTa2

Fld arctan

w

z

Page 16: CVD AND PVD THIN FILM TECHNIQUES

Chemical vapor deposition (CVD) : overview

CVD (thermal)– APCVD (atmospheric)

– LPCVD (<10 Pa)– VLPCVD (<1.3 Pa)

PE CVD (plasma enhanced)

Photon-assisted CVD Laser-assisted CVD MOCVD

Tensile stress causes concave bending of a thin substrate

Compressive stress causes convex bending of a thin substate

Deposited film

Deposited film

Page 17: CVD AND PVD THIN FILM TECHNIQUES

The LCVD method is able to fabricate continuous thin rods and fibres by pulling the substrate away from the stationary laser focus at the linear growth speed of the material while keeping the laser focus on the rod tip, as shown in the Figure . LCVD was first demonstrated for carbon and silicon rods. However, fibres were grown from hundreds of substrates including silicon, carbon, boron, oxides, nitrides, carbides, borides, and metals such as aluminium. The LCVD process can operate at low and high chamber pressures. The growth rate is normally less than 100 µm/s at low chamber pressure (<<1 bar). At high chamber pressure (>1 bar), high growth rate (>1.1 mm/s) has been achieved for small-diameter (< 20 µm) amorphous boron fibres.

Chemical vapor deposition (CVD) : L-CVD

Page 18: CVD AND PVD THIN FILM TECHNIQUES

Epitaxy

VPE:– MBE (PVD) (see above)– MOCVD (CVD) i.e.organo-

metallic CVD(e.g. trimethyl aluminum to deposit Al) (see above)

Liquid phase epitaxy Solid epitaxy:

recrystallization of amorphous material (e.g. poly-Si)

Liquid phase epitaxy

Page 19: CVD AND PVD THIN FILM TECHNIQUES

Epitaxy

Selective epitaxy Epi-layer thickness:

– IR– Capacitance,Voltage

– Profilometry– Tapered groove– Angle-lap and stain

– Weighing

Selective epitaxy

Page 20: CVD AND PVD THIN FILM TECHNIQUES

Electrochemical deposition: electroless

Electroless metal displacement

Electroless sustainable oxidation of a reductant– Metal salt (e.g.NiCl2)– Reductant

(e.g.hypophosphite)– Stabilizer:bath is

thermodynamically unstable needs catalytic poison (e.g. thiourea)

– Complexing agent : prevent too much free metal

– Buffer: keep the pH range narrow

– Accelerators: increase deposition rate without causing bath instability (e.g. pyridine)

Deposition on insulators (e.g. plastics): seed surface with SnCl2/HCl

1. Zn(s) + Cu 2+(aq) ------> Zn 2+(aq) + Cu(s)

2. Reduction (cathode reaction) : Ni+2 + 2e- —> Ni

Oxidation (anode reaction): H2PO 2- + H2O—> H2PO3

- +2H+ +2e- ------------------------------------------

Ni+2 + H2PO2- + H2O —> Ni + H2PO3

- + 2H+

e.g. electroless Cu: 40 µmhr-1

Cu

Page 21: CVD AND PVD THIN FILM TECHNIQUES

Electrochemical deposition: electroless

Evan’s diagram: electroless deposition is the combined result of two independent electrode reactions (anodic and cathodic partial reactions)

Mixed potential (EM): reactions belong to different systems

ideposition = ia = ic and I=A x i

deposition Total amount deposited: m max=

I t M/Fz (t is deposition time, Molecular weight, F is the Faraday constant, z is the charge on the ion)

CMOS compatible: no leads required

Evan’s diagram

F= 96,500 coulombs=1, 6 10 -19 (electron charge) x 6. 02 10 23 (Avogadro’s number)

+

-

Page 22: CVD AND PVD THIN FILM TECHNIQUES

Electrochemical deposition :electrodeposition-thermodynamics Electrolytic cell

– Au cathode (inert surface for Ni deposition)

– Graphite anode (not attacked by Cl2)

Two electrode cells (anode, cathode, working and reference or counter electrode) e.g. for potentiometric measurements (voltage measurements)

Three electrode cells (working, reference and counter electrode) e.g. for amperometric measurements (current measurements)

Page 23: CVD AND PVD THIN FILM TECHNIQUES

Electrochemical deposition :electrodeposition-thermodynamics (E)

E E 0 RT

zFln a

Mz

² G=² G2-² G1 ² G=-(E2-E1)zF=-EcellzF

² G=² G0-RT ln aMz+=² G0-RT ln CMz+z+

² G= - EzF

E2 > E1 : - battery

E2 < E1 : + E ext > E cell to afford deposition

(Nernst equation)

1. Free energy change for ion in the solution to atom in the metal (cathodic reaction): or also

2. The electrical work, w, performed in electrodepositionat constant pressure and constant temperature: and since V =0

G G m(free energy pure metal) - Ge(free energy of ion in the electrolyte)

G = - w + PV

3. Substituting Equation (2) in (1) one gets

(1)

(2)

4. Repeat (1) and (2) for anodic reaction:

or

Page 24: CVD AND PVD THIN FILM TECHNIQUES

Electrochemical deposition :electrodeposition-thermodynamics ()

A thermodynamic possible reaction may not occur if the kinetics are not favorable

Kinetics express themselves through all types of overpotentials

E -E o = anodic and - is cathodic)

Page 25: CVD AND PVD THIN FILM TECHNIQUES

² G* = ² G#+

kc

kT

he

G #_

RT

k

k

ckT

heF

RT

i

k

z F k

c z FkT

heF

RT

i

k

zF kc

z F

kT

he

(1 )FRT

Electrochemical deposition :electrodeposition-kinetics-activation control

Understanding of polarization curves: consider a positive ion transported from solution to the electrode

Successful ion jump frequency is given by the Boltzmann distribution theory (h is Planck constant):

(without field)

(with field)

Page 26: CVD AND PVD THIN FILM TECHNIQUES

Electrochemical deposition :electrodeposition-kinetics-activation control

ie i

k

c zFkT

he

(1 )FeRT i

i

c zFkT

he

FeRT

e

i i

i

iie (e(1 )FRT e

FRT )

a blog(i)

(Butler-Volmer)

(Tafel law)

At equilibrium the exchange current density is given by:

The reaction polarization is then given by:

The measurable current density is then given by:

For large enough overpotential:

Page 27: CVD AND PVD THIN FILM TECHNIQUES

Electrochemical deposition :electrodeposition-kinetics-diffusion control

dCdX

Cx

0 Cx0

c RTnF

lnCx=0

C0

i nFD0C

0 Cx0

I l nFAD0C

0

i il (1 enFcRT )

From activation control to diffusion control:

Concentration difference leads to another overpotential i.e. concentration polarization:

Using Faraday’s law we may write also:

At a certain potential C x=0=0 and then:

Cx=0

C0

1- i

i lwe get :

Page 28: CVD AND PVD THIN FILM TECHNIQUES

Electrochemical deposition :electrodeposition-non-linear diffusion effects

D0t 1

2

I l nFAC0 D0

t

1

2

I l nFAC0 D0

t

1

2 + AnFD0

C0

r

Nonlinear diffusion and the advantages of using micro-electrodes:

An electrode with a size comparable to the thickness of the diffusion layer

The Cottrell equation is the current-vs.-time on an electrode after a potential step:

For micro-electrodes it needs correction :

I l nFAD0C

0

Page 29: CVD AND PVD THIN FILM TECHNIQUES

Electrochemical deposition :electrodeposition-non-linear diffusion effects

I l,m rnFD0C0 (disc)

I l,m 2rnFD0C0 (hemisphere)

I l,m 4rnFD0C0 (sphere )

I l,m AnFD0C

0

r L

The diffusion limited currents for some different electrode shapes are given as (at longer times after bias application and for small electrodes):

If the electrodes are recessed another correction term must be introduced:

Page 30: CVD AND PVD THIN FILM TECHNIQUES

THANKS