Basic steps of thin film growth1. Thermal accommodation2 Adsorption (physisorption) of2. Adsorption (physisorption) of

atoms/molecules3. surface diffusion4. formation of molecule-molecule and

substrate-molecule bondings ( h i ti )(chemisorption)

5. nucleation: aggregation of single atoms/moleculesatoms/molecules

6. structure and microstructure formation (amorphous- polycrystalline -single-( p p y y gcrystalline, defects, roughness, etc.)

7. changes within the bulk of the film, e.g. diff i i th tdiffusion, grain growth etc.

Adsorption Processdesorption

reflectionvapormolecule

z

J

γ1−δ

x

substrate

Ji

δ ζ, αcSc

L

surface

T

η

physisorption chemisorption incorporation utilization

LTssubstrate temp.

a

Ji=impinging flux γ = accommodation coefficient δ=trapping probabilityζ= chemisorption reaction probability αc=condensation coefficient Sc = sticking coefficientη= utilization fractionη= utilization fraction

Ref: D. L. Smith, Thin-Film deposition Principles & Practice, 1995, McGrawHill, Boston

Thermal Accommodation• Impinging atoms must lose enough energy thermally to stay on surface

Reflected Er,Tr

Incident Ev,Tv

• assume Equivalence between energy and temperature; E = kT. Substrate; Ts

•Thermal accommodation coefficient•αT=0 → Er=Ev → elastic collision (no energy loss) rvrv TTEE −

=−

=α

tcoefficienion accommodat Thermal

•αT=1 → Er>Ev → all excess energy loss

•Examine energy transfer to lattice:

svsvT TTEE −

=−

=α

one dimensional model from B.McCarrol and G. Ehrlich, J. Chem. Phys. 38, 523 (1963).

C id h i f t t d b impinge• Consider a chain of atoms connected by springs

•if rebound is strong enough - atom

ko ko kimpinge

reboundescapes•if not - atom is trapped - oscillates and loses energy to lattice

ebou d

Thermal Accommodation

i iko ko kimpinge

reboundrebound

•atom is trapped if Ev < 25 Edesorb •Edesorb ~ 1-4 eV•Ev < 25 - 100 eV or Tv < 2500 - 10,000 K

trapped•most deposition processes have E < 10 eVmost deposition processes have Ev < 10 eV•Most atoms are trapped

•Thermal accommodation is very f t d 10 14 dfast; around 10-14 seconds

Adsorption Processmolecule arrives from the vapor phase:attractive force at distance of a few atomic diameters from the substrate

l l l d W l fnon-polar molecules: van-der-Waals forcespolar molecules: stronger forcestransfer of kinetic energy to the substrate, adsorptionprec rsor adsorption eak bonding as a prec rsor to strong bondingprecursor adsorption: weak bonding as a precursor to strong bondingSiH4 (g) → ... → SiH4 (p) → Si (c) + 2H2 (g) ; p=physisorption; (c) chemisorption

alloy films: 2 components in the vapor phasealloy films: 2 components in the vapor phaseZn (g) + Zn (c) → Zn2 (p)Zn (g) + Se (c) → ZnSe (c)

H- passivated surface: Si (g) + H (c) → Si (p)chemisorption only on non-passivated sites Si (g) + Si (c) → Si (c)chemisorption only on non passivated sites Si (g) Si (c) → Si (c)

stronger bonding at surface steps

metal atoms on non-metallic substrates:metal-metal bondings stronger than metal-substrate bondings

Precursor adsorption model2Y(g)

b a

cY2(g) = gas-phase molecule

40

≈∆fH of Y2(g)

c Y(g) = gas-phase atomEa = activation energyEp=0: enthalpy in the vapor phase,no kinetic energy

f f fEp, kJ/mol

Y2(g)

Eda

E

Ea Era

a

ΔfH: enthalpy of formation of Y2Ed: desorption barrier (physisorbed)Er: reaction barrier (p) → (c)Ea: reaction barrier vapor → (c)E th l i h i b d t t

-40

Edb

Erbb

Ec: enthalpy in chemisorbed state

1kJ/mol ~ 1eV/atom

-400

precursor physisorption

d t d ti di tl i t

600 dissociative chemisorption

advantageous: condensation directly into chemisorbed statehigh kinetic energy and molecule dissociationin the vapor phase required (sputtering, PLD)

Ref: D. L. Smith, Thin-Film deposition Principles & Practice, 1995, McGrawHill, Boston

-600 dissociative chemisorption

Adsorption

Rate of chemisorption Rr = rate constant kr × ML concentration nS0 ×coverage θRate of desorption Rd = rate constant kd × ML concentration nS0 ×coverage θ

ns0: number density of surface atoms in a ML

R t th ll ti t d (A h i l ) ⎟⎞

⎜⎛ ERates are thermally activated (Arrhenius laws)

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

s

iii RT

Evk exp0

dri RRJ +=−⋅⋅ )1( θδ (conservation of mass)

# of physisorbed species that h i b d bcan chemisorb or desorb

ζδθδ

θ ⎥⎥⎤

⎢⎢⎡

0/JJkR

nJ Si ζθδ

θ ⋅=

⎥⎥⎥⎥

⎦⎢⎢⎢⎢

⎣⎟⎟⎠

⎞⎜⎜⎝

⎛ −−⋅+

=⋅⋅=⇒++

= 1

0

00

0

0

exp1/

J

RTEE

vv

JnkRkknJ

S

dr

r

diSrr

drSi

Si

( ζ : Chemisorption coefficient)

Adsorption

assumption: ki independent of surface site (no surface steps etc.)TS low enough to avoid thermal decompositionTS low enough to avoid thermal decomposition

(Er-Ed) > 0: activation energy for chemisorption, Rr ↑ if TS ↑(e.g. CVD, decomposition of SiH4 - can also be induced by ( g , p 4 ynucleation at nucleation sites like steps or non-passivated surface atoms)

(Er-Ed) < 0: Rr ↓ if TS ↑, desorption rate increases stronger than reaction rate(e. g. CVD at too high TS)

nucleation is problematic if precursor-precursor bonding is stronger thanprecursor-substrate bonding ⇒ island growth, inhomogeneous coveragee.g. Zn/Cd on glass or NaCl

high Ea : metal atoms stay physisorbed, desorb or nucleate to islands

Diffusion

extremely important for thin film formation• allows adsorbed species to form clusters (homogeneous nucleation)• allows adsorbed species to find heterogeneous nucleation sites (steps etc )• allows adsorbed species to find heterogeneous nucleation sites (steps etc.)• adsorbed atoms move in potential energy "landscape"generated by substrate or thin film surface atoms: diffusion, hopping

DiffusionES < Ed , Ec : only partial breaking of bonds

⎟⎟⎞

⎜⎜⎛

−⋅=Evk expMolecular hopping rate: (influence of substrate temperature TS)⎟⎟

⎠⎜⎜⎝ ⋅

−⋅=s

ss TRvk exp0Molecular hopping rate: (influence of substrate temperature, TS)

(v0s=1013…1016 Hz: attempt frequency)Diff i d lk di dDiffusion: random walk, not directed.

Equal hopping probabilities for forward and backward motion

Diff i l th kDiffusion length,

(r: rms change in distance per hopping event, N0: number of hops, a: lattice constant, t: diffusion time)

tkaNaNr s ⋅⋅=⋅≈⋅=ΛΛ 00:

)

⎪⎪⎪⎫

== −

meVESv

s

s

2010 113

0

⎪⎪

⎪⎪⎪

⎬==

KTmeVEs

1000200

nmm

5300

=Λ=Λ μ (physisorbed)

(chemisorbed)

⎪⎪⎪

⎭==

nmast

3.01 Strong influence of bonding conditions!

Diffusiondiffusing molecules may desorb or be buriedaverage time between adsorption and burial by incident molecules: tb=n0/Jin0: adsorption site density (#cm-2), Ji: incident flux (#cm-2s-1)

desorption from chemisorbed state after

maximum in Λ close to re-evaporation temperaturemaximum in Λ close to re evaporation temperaturebest film quality (smoother, less defects, more homogeneous)

DiffusionES < Ed , Ec : only partial breaking of bonds

⎟⎟⎞

⎜⎜⎛

−⋅=Evk expMolecular hopping rate: (influence of substrate temperature TS)⎟⎟

⎠⎜⎜⎝ ⋅

−⋅=s

ss TRvk exp0Molecular hopping rate: (influence of substrate temperature, TS)

(v0s=1013…1016 Hz: attempt frequency)Diff i d lk di dDiffusion: random walk, not directed.

Equal hopping probabilities for forward and backward motion

Diff i l th kDiffusion length,

(r: rms change in distance per hopping event, N0: number of hops, a: lattice constant, t: diffusion time)

tkaNaNr s ⋅⋅=⋅≈⋅=ΛΛ 00:

)

⎪⎪⎪⎫

== −

meVESv

s

s

2010 113

0

⎪⎪

⎪⎪⎪

⎬==

KTmeVEs

1000200

nmm

5300

=Λ=Λ μ (physisorbed)

(chemisorbed)

⎪⎪⎪

⎭==

nmast

3.01 Strong influence of bonding conditions!

Nucleation

surface energy per unit area, γ: energy per unit area needed to create or increase a surface(non constant number of surface atoms) unit: Jm-2(non- constant number of surface atoms) unit: Jm 2

surface stress: force per unit length needed to increase a surface(constant number of surface atoms, solids only) unit: Nm-1, includes strain contributioncontribution

Nucleation

γγΔ

⋅Δ⋅⋅=Δ⋅⋅=Δ 22WF

bxAW

Force acts tangentiallyγ⋅=⋅Δ

Δ= 2

bxW

bF Force acts tangentially

Tends to decrease surface area

Surface energy exists because bonds are broken to create/increase the surface

(surface stress: bonds are elastically strained)(surface stress: bonds are elastically strained)

Strong driving force: minimization of surface energy (spherical soap bubble)

Fundamental to thin film growth:

Surface energy can be minimized by surface diffusionSurface energy can be minimized by surface diffusion

min→⋅ AγChemical compositionChemical composition crystallographic orientation atomic reconstruction

Surface totpgraphy

Nucleationγ usually is anisotropic, i.e. differently oriented surfaces have different γ(differences in metals are of the order of % - larger in covalent or ionic systems)

fcc – crystal (Au, Al): 111 surfaces have lowest surface energy atoms in closed –fcc crystal (Au, Al): 111 surfaces have lowest surface energy atoms in closed packed (111) lattice planes have most in-plane bonding partners and smallest interplanar bonding

bcc (Cr Fe): 110bcc (Cr,Fe): 110

hcp (Zn, Mg): 0001

diamond (Si Ge): 111 polar/ionic bondingdiamond (Si, Ge): 111 polar/ionic bonding,

Zinc blende (GaAs, ZnSe): 110 planes with lowest γ have

CaF2: 110 same number of cationsCaF2: 110 same number of cations

NaCl: 100 and anions

surface reconstruction: atomic positions and surface bonds are different from thosesurface reconstruction: atomic positions and surface bonds are different from those in the bulk in order to decrease γ (≤ 50 %!) – can increase Er (PS CS)

surface passivation: addition of a ML of an element, dangling bonds react to terminated bonds prevents reconstruction often more effective thanterminated bonds – prevents reconstruction, often more effective than reconstruction.

Nucleationthin film nucleation: interplay of 3 surface energies per unit area

γs: substrate free surface

∑γf : film free surface

γI : substrate/film interface

relative magnitudes of these quantities strongly influence nucleation

∑ → minjj Aγ

relative magnitudes of these quantities strongly influence nucleation

(provided that nucleation is not kinetically limited and can approach equilibrium)

Smith 5 8 or Ohring 5 2

layer-by-layer growth (Frank-van der Merwe)sfi γγγ <+

Smith 5.8 or Ohring 5.2

island growth (Volmer-Weber)

minimization of total surface energy:fsi

fsi

γγγ

γγγ

+>

+≈

low-γ facets of islands

f ML l b l th t i l d th

fsi γγγ

few ML layer-by-layer, then crossover to island growth

(not only a γ-effect, see Ch,7 - Epitaxy) (Stranski-Krastanov)

Nucleation

3D- nucleation (islands) is usually undesirablemitigation strategy: change one or more of the γj such that γi + γf > γs

γ is lower for materials with same type of bonding (metallic/covalent/ionic)- γi is lower for materials with same type of bonding (metallic/covalent/ionic)- γi is lower in case of chemical reactivity

Au on glass 3D- nucleationCr on glass 2D- nucleation O-Si ⇒ Si-Cr/O-Cr bondingsAu on Cr 2D- nucleation strong metallic bondingAu on Cr 2D nucleation, strong metallic bonding--------------------------------------------------------------------------------------------------

Au / Cr / glass layer-by-layer, wettingu / C / g ass aye by aye , ett gCr is an intermediate ’glue’ layer; 3-10nm sufficient(continuous layer)Ti: similar good bonding materialg g

Nucleationalternative methods to prevent island growth:

ion beam irradiation of the substrate surface(breaks bonds, enhances reactivity, destroys islands i.e. disturbs equilibrium - ion beam irradiation is often very effective)

apply a surfactant ⇒ reduces γf more than γS(water on glass: drops - soapy water on glass: layers)

γi + γf > γs

↓↓γi + γf < γs

Classical Nucleation

heterogeneous nucleation• takes place at "active" surface sites (steps, defects, contamination); low local γi

t h th it b diff i di tl f th h• atoms reach these sites by diffusion or directly from the vapor phasehomogeneous nucleation• at random positions

if s fficient high n mber of atoms meet thro gh diff sion to form a stable n cle s• if sufficient high number of atoms meet through diffusion to form a stable nucleus• surface energy → critical radius for nucleation

Nucleation

formation of a nucleus:

1 ) Gibbi’s free enthalpy of the nucleus ΔGV decreases if JC/JV > 11.) Gibbi s free enthalpy of the nucleus, ΔGV, decreases if JC/JV > 1

(JC: condensing molecular flux, JV: evaporating molecular flux)

molVmolCVV V

rappRT

VVG

33ln)( ⋅⎟⎟

⎠

⎞⎜⎜⎝

⎛⋅−=−−=Δ μμ :

Vpp

supersaturation

2.) surface energy balance:

curved surface of the nucleus

interfacei

fS

ra

raG

γ

γ2

2

21

+

=Δ

substrate surfaceSra γ22−

GGG Δ+ΔΔ SV GGG Δ+Δ=Δ

Nucleation onNucleation on nonwetting substrate

Ji

J Af,γfJv lhfGibb

;lnlnJJRT

ppRT vc

v

c

vcv ααμμ ===−

Ji

rγs

f,γfv

)(

nucleusper changeenergy free Gibbs

AVVG ffmc

cv γμμ +−−=Δ

A γ

γs

23

34

4.ln rV

rppRT f

mcv

πγπ

+−=

μ = chemical potential of condensateAi,γi

Ji= incoming particle flux

μc = chemical potential of condensateμv = chemical potential of vaporpv = saturation vapor pressurep = vapor pressureJv=evaporating particle flux

A = area (i:interface, f:film, s:substrate)γ = surface energy

p = vapor pressureJc = condensing molecular fluxJv = evaporating molecular fluxV = molar volume of the condensateVmc molar volume of the condensate

Surface Energy Balance and critical radius

sifs rararaG

=

++=Δ

nucleusof surface curved

23

22

21 γγγ

++

substrate interface

SV GGG Δ+Δ=Δ

22

321

3

321

)ln(27

)(4*)(

)ln(3

)(2*

⎟⎟⎞

⎜⎜⎛

−+=Δ

−

−+−= sifsif

pRTa

aaarGp

VRTa

aaar

γγγγγγ

Growth of nuclei with r>r* to lower total enthalpy more stable nuclei

33 )ln(27 ⎟⎟

⎠⎜⎜⎝ vmol

vmol pVapV

Growth of nuclei with r r to lower total enthalpy more stable nucleiNuclei with r< r* spontaneously disintegrateCritical radius r* and nucleation barrier decrease with increasing supersaturation

Contact Angle

sin

)cos1(22

2

1

θπ

θπ

⋅=

−⋅=

a

afor a spherical nucleus

3/)coscos32( 33

2

θθπ +−⋅=a

balance of surface forcesSmith 5.12

balance of surface forces (acting tangentially)

θ+ θγγγ cosfiS +=

4coscos32

ln3

16)(

33 θθπγ +−⋅

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛=Δ ∗ f

pp

VRT

rG

⎟⎠

⎜⎝

⎟⎠

⎜⎝ Vmol pV

θ = 0: ΔG(r*) = 0 ideal wetting, layer-by-layer growth, no nucleation barrier nucleation even if p < p (oxidation of metals at very low oxygen partial pressure)nucleation even if p < pV (oxidation of metals at very low oxygen partial pressure)

θ = 180°: ΔG(r*) = max – corresponds to bulk homogeneous nucleation

Precursor adsorption model2Y(g)

b a

cY2(g) = gas-phase molecule

40

≈∆fH of Y2(g)

c Y(g) = gas-phase atomEa = activation energyEp=0: enthalpy in the vapor phase,no kinetic energy

f f fEp, kJ/mol

Y2(g)

Eda

E

Ea Era

a

ΔfH: enthalpy of formation of Y2Ed: desorption barrier (physisorbed)Er: reaction barrier (p) → (c)Ea: reaction barrier vapor → (c)E th l i h i b d t t

-40

Edb

Erbb

Ec: enthalpy in chemisorbed state

1kJ/mol ~ 1eV/atom

-400

precursor physisorption

d t d ti di tl i t

600 dissociative chemisorption

advantageous: condensation directly into chemisorbed statehigh kinetic energy and molecule dissociationin the vapor phase required (sputtering, PLD)

Ref: D. L. Smith, Thin-Film deposition Principles & Practice, 1995, McGrawHill, Boston

-600 dissociative chemisorption

Adsorption

assumption: ki independent of surface site (no surface steps etc.)TS low enough to avoid thermal decompositionTS low enough to avoid thermal decomposition

(Er-Ed) > 0: activation energy for chemisorption, Rr ↑ if TS ↑(e.g. CVD, decomposition of SiH4 - can also be induced by ( g , p 4 ynucleation at nucleation sites like steps or non-passivated surface atoms)

(Er-Ed) < 0: Rr ↓ if TS ↑, desorption rate increases stronger than reaction rate(e. g. CVD at too high TS)

nucleation is problematic if precursor-precursor bonding is stronger thanprecursor-substrate bonding ⇒ island growth, inhomogeneous coveragee.g. Zn/Cd on glass or NaCl

high Ea : metal atoms stay physisorbed, desorb or nucleate to islands

2D-NucleationΘ = 0: no nucleation barrier?

Sufficient surface diffusion: adsorbed atoms form “2D-gas” at the surfaceReplace surface energy by step energy (bonding partners are missing)

l ih

2

2

*)( *

eon terrac nucleation shomogeneou

⎞⎛=Δ=

RTrGnRTr ββ

V

s

mol

2

V

s

mol)ln()ln(⎟⎟⎠

⎞⎜⎜⎝

⎛nn

VRTan

nVRTa

2D nucleation (2)

Spiral growth of thin films (only if surface diffusion is strong):No homogeneous nucleation necessary, always steps present

2D nucleation (3)

Nucleation ratenucleation rate = d/dt (surface density of stable nuclei)early stages: nuclei don’t grow through direct impingement of gas phase atomsmore important: rate at which adsorbed atoms attach a critical nucleuspadsorbed atoms remain until desorption for (adatom lifetime)

⎟⎠⎞

⎜⎝⎛= −

RTEc

cC exp10ντ

if atoms aggregate during tc they stay on the substrate

E i hi h i i hi h d i f l iEc is highest at steps, contaminations, etc. - higher density of nucleinucleation rate:

ilib i t ti f iti l l i

ω** ANN =′

: equilibrium concentration of critical nuclei,nS: total nucleation site density

: attachment area of the critical nucleus

kTGsenN /** Δ−=

2*)(4* rA π= : attachment area of the critical nucleus

: rate at which adatoms impinge onto

*)(4* rA π=

MRTNPP ASV

παω

2)( −

=MRTπ2

αc=condensation coefficient

Nucleation rate

ω = jump rate × surface density of adatoms = jump rate × vapor impingement rate

⎟⎞

⎜⎛⋅⎟

⎞⎜⎛−= − E

vpNEv cAS expexp 1

00ω

assume

⎟⎠

⎜⎝

⎟⎠

⎜⎝ RT

vMRTRT

v cs exp2

exp 00 πω

:0cos vv ≈⎞⎛ Δ ∗GEEN )(

complex expression, but exponential dominates:

⎟⎟⎠

⎞⎜⎜⎝

⎛ Δ−−⋅⋅⋅=′ ∗

RTrGEE

nMRT

pNrCN sc

sA )(

exp2π

p p p

nucleation strongly depends on ΔG(r*), thus can be influenced by T, p

∂ ∗r r* increases with T because supersaturation decreases

)(

0

Δ∂

>∂∂

∗

p

rG

Tr r increases with TS because supersaturation decreases

late film coalescence (@ high average thickness)

density of stable nuclei increases slower with increasing

)(

0)(

Δ∂∂

>∂

Δ∂

∗∗

p

rGr

TrG density of stable nuclei increases slower with increasing

T late film coalescence (@ high average thickness)

hi h d iti t ll * f t i f0)(,0 <∂

Δ∂<

∂∂

TT prG

pr higher deposition rate smaller r , faster increase of

density of nuclei

Cluster Coalescence

kinetic theories of nucleation :

number density of stable nucleinumber density of stable nuclei

decreases after a certain time

coalescence of nuclei

driving force: minimization of surface energy

a. cluster migration & rotation, coalescence results from random collisions of clusters

EC related to ES, s = 1…3⎟⎠⎞

⎜⎝⎛−⋅∝

RTE

rrD C

S exp1)(⎠⎝ RTr

Cluster Coalescence

chemical potential, μI, of a spherical nucleus consisting of i atoms (Ω:atomic volume):

⎞⎛

⋅+=∞p

pRTr i

i μμ ln)( 0 vapor pressure pi

⎟⎠⎞

⎜⎝⎛ Ω

⋅= ∞ rRTprpi

γ2exp)(

b mass transport f ( 0)b. mass transport by “evaporation”

growth of large nucleus at the

c. convex surface (r > 0):

atoms evaporate

concave surface (r < 0):expense of the small one

co ca e su ace ( 0)

atoms condense

Cluster Coalescence

Zone Model

Structure Zone modelValid for many deposition techniquesValid for metals semiconductorsValid for metals semiconductors and insulators

Superposition of physical processes which establish structural zones

Z1 zone

Evaporation:TS/TM < 0.3 – surface diffusion can be neglected (Λ < a)highly disordered columnar small diameter (~ 10 nm) crystalsg y ( ) ytapers columns, dome tops, voided boundariespromoted by substrate roughness and oblique deposition

Sputtering:TS/TM < 0.1 (0.15 Pa) … TS/TM < 0.1 (0.15 Pa) kinetic energy of depositing atoms compensates low thermal mobility

High disorder density, hard

ZT (transition) zone

Sputtering:0.1 < TS/TM<0.4 (0.15 Pa) … 0.4 < TS/TM<0.5 (4 Pa) S M ( ) S M ( )fibrous grains, dense grain boundaries

High disorder density, hard, high strength, low ductility

Z2 zone

Evaporation: 0.3 < TS/TM<0.5 Sputtering: 0.4 < TS/TM<0.7surface diffusion increasingly importantsurface diffusion increasingly importantMany materials: sharp transition Z1→Z2 @ TS/TM = 0.3

)/exp(,),exp( MMc TTDTEED −∝∝−∝

correlation Ec & TM: bonding strengthcolumns, dense grain boundaries (voids are filled by surface

)/exp(,),exp( sMsMcsB

s TTDTETk

D

g ( ydiffusion)Less defects than in Z1,ZTTransition temperature Z1→Z2 increase with deposition rate, Ji

⎞⎛ E1⎟⎠⎞

⎜⎝⎛−∝Λ

RTE

Js

i 2exp1

Z2 zone

Column tops often facetted – hard, low ductility

Wid l ( ll f t )Wide columns (small surface curvature)Widen at the expense of narrow columns (large surface curvature)

Lateral gro th ntil col mn diameter Φ>> ΛLateral growth until column diameter Φ>> Λ

⎟⎞

⎜⎛ T

⎟⎟⎠

⎞⎜⎜⎝

⎛−∝Φ

s

M

TTexp

⎟⎞

⎜⎛ 11

⎟⎟⎠

⎞⎜⎜⎝

⎛−Ω=Δ

21

11rr

γμ

Diffusion sufficiently high to establish mechanical equilibrium at the interface (thermal grooving)

Z3 zone

Evaporation:TS/TM>0.5 - significant surface and bulk diffusionTS/TM 0.5 significant surface and bulk diffusion recrystallization/grain growth, Oswald ripening during thin film growthlarge equiaxed grain, grain size → film thicknessrelatively smooth surfaces, grain boundary groovingy , g y g g

Sputtering: 0.6 < TS/TM<1.0S M

Low distortion density, soft

Zone vs Substrate TemperatureZone 3Zone 3

Zone 2

Zone1

Ts/TM

Evaporation

s/ M

0 0.5 0.9

SputteringSputtering

Zone 2Zone1

Zone T

Zone 3

Zone 2

Zone example

Kinetically Restricted Growth

low TS/TM: surface diffusion can be neglectedS M g

atoms stick immediately to the surfaced t b i d b f h iand get buried before a hopping process occurs

ballistic depositionballistic depositionstatistic roughening due to fluctuations in the deposition rate

& shadowing:g

columnar growth, void formation

In or Out

in most cases: Z1 undesiredoptical applications: absorption diffuse scatteringoptical applications: absorption, diffuse scatteringisolators: defect induced conductivitysemiconductors: trapping sites…

If high Ts cannot be applied: ion beam assisted deposition (IBAD) ZT

Z1 porosity is of advantage forZ1 porosity is of advantage forgas detectors (adsorption of gas changes property, e.g. electrical

resistance)catalytic applications, e.g. fuel cellscatalytic applications, e.g. fuel cellscoatings that are subject to large T-changes(missing lateral stability prevents delamination due to differential

thermal expansion)p )

Summary

high deposition rate & low substrate temperature:⇒ fine- grained polycrystalline or amorphous film, coalescenceat small average thickness relatively smoothat small average thickness, relatively smooth

low deposition rate & high substrate temperature:⇒ coarse- grained polycrystalline (or single- crystalline film),⇒ g p y y ( g y ),coalescence at high average thickness, relatively rough

Summary

models used here ("capillarity theory") give a simple picture and correct tendenciesbut: results are not exactbut: results are not exactcalculations often result in too small r*, even if correct parameters (γj,...) are usedvalidity of macroscopic concepts (like γj) is questionableeverything is based on the assumption of a system in thermodynamical equilibriumeverything is based on the assumption of a system in thermodynamical equilibriumbut: most preparation processes are subject to kinetic constraintsKinetic nucleation theories can be found in the books by Smith and Ohring