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Introduction to Thin Film

Processing

Deposition Methods

� Many diverse techniques available

� Typically based on three different methods for providing a flux of atomic or molecular material

• Evaporation

• Sputtering

• Chemical vapor deposition (CVD)

� First two: physical vapor deposition (PVD)

• solid or molten source

• vacuum environment

• absence of chemical reactions (usually)

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EVAPORATION

� First report: Faraday 1857

� Observed thin films from metal wires

resistively heated in an inert gas

� Development of vacuum pumps and

resistively-heated sources led to early

evaporated thin film technology

� Early applications: mirrors, beam splitters

Vapor Pressure

� Rate of evaporation (or sublimation) obtained from equilibrium vapor pressure

� Equilibrium vapor pressure Pe given by the Clausius-Clapeyron equation:� dPe/dT = Pe∆Hv/RT

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• where ∆Hv = latent heat of evaporation (or sublimation)

• R = gas constant

� Assuming that ∆Hv is independent of T gives � Pe ∝ exp(-∆Hv/RT)

� Strong exponential (Arrhenius) T dependence!

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Evaporation Flux

� Evaporation flux J related to Pe:

� J = αNa(Pe - Ph)(2πMRT)-1/2

• α = evaporation coefficient (~1)

• Ph = hydrostatic pressure (= 0 in vacuum)

• Na = Avogadro's number

• M = molecular weight

� J = 3.513 x 1022 Pe/(MT)1/2 (molec. cm-2 s-1 )

• Pe in Torr and M in AMU

� Insert Pe to give evaporation rate

Film Thickness Distribution:Point Source

� Flux arriving at substrate determined by source/chamber geometry

� Assume a point source -evaporated flux equal in all directions

� Total flux Jo� Fraction dJ/Jo falling on area dA at distance r from source given by

� dJ/Jo = dA/4πr2

� Substrate area dAs at angle θ to flux

� Projected area dA = dAscosθ, so

� dJ/dAs = Jocosθ/4πr2

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Film Thickness Distribution:

Surface Source

� Source flux distribution

� Typical dependence: cosφ

• φ = emission angle

� dJ/dAs = Jo cosθ cosφ/πr2

� Film accumulation velocity:

� R = (dJ/dAs)/N (e.g. cm/s)

� N = atomic density (atoms/cm3)

Distribution Calculation

� Point source with substrate

plane at distance h

� R = Jocosθ/4Nπr2 = Joh/4Nπr

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= Joh/4Nπ(h2 + l2)3/2

� Surface source with

substrate plane at distance h:

� Example: source and substrate

planes parallel

� R = Jo cosθ cosφ / Nπr2

= Jo (h/r) (h/r) / Nπr2

= Jo h2 / Nπ(h2 + l2)

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Vacuum Requirements

� Chamber pressure criteria:

� Minimize scattering� Base pressures <10-4 Torr yield mean free path >45 cm

� Background impurity incorporation� Depends on incorporation probability of impurity and growth rate

• Typical background species: N2, CO, CO2, hydrocarbons

• UHV systems generally preferred for high purity films

• Residual gas impingement rates

• Increasing growth rate dilutes impurities

Multicomponent Evaporation

� Time-varying film composition

� Compounds:� Most evaporate dissociatively and non-congruently

• E.g. III-V compounds, such as GaAs

� Non-dissociative evaporation• CaF2, AlN, SiO

� Dissociative but congruent (equal rates) • Some II-VI compounds (e.g.) CdTe,

� Alloys:� Ideal (Raoultian) Solutions

• Evaporated flux equals source composition

� More common: deviations from ideality

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Evaporation sources

� Resistive heating

� Refractory metal (W, Mo, Ta,

Nb) filaments or boats

� Indirect heating of quartz,

graphite, cBN, etc.

� Laser evaporation

� Pulsed-laser deposition (PLD)

� Arc evaporation

� Electron-bombardment

heating

� Effusion (Knudsen) cells

Effusion Cells

� Commonly used in molecular

beam epitaxy (MBE)

� Highly-controlled evaporation

process

� Ideal Knudsen cell: small opening

� Pressure inside crucible close to

equilibrium value

• Flux depends only on cell

temperature and aperture size

� Practical effusion cell: Large

opening

� Needed in practice for high

growth rates

• Flux distribution varies with fill

level

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Multiple Source Evaporation

� Example: Molecular Beam Epitaxy (MBE)

� Multi-element compounds and alloys

� Individual evaporation sources for each element

� Source-substrate distance - trade-offs:� Deposition rate

� Compositional uniformity

• Greatly improved by substrate rotation

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SPUTTERING

� Physical process resulting from the impingement of an energetic particle on a surface

� Only one of many "ion-surface interaction" effects

� Time scale: 1 - 5 x 10-13 s after impact � After this energies less than threshold for displacement, ~10eV

� Remaining energy dissipated as heat

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Sputter yield Y

� Definition:

� (# sputtered atoms) / (# impinging ions)

� Dependences

� Crystallographic orientation of surface

• Usually given for polycrystalline or

amorphous materials, where

crystallographic effects average

� Ion energy Ei� Ion mass M

� Ion impingement angle θI� Atomic number Z

Sputtering Theory

� Linear collision cascade theory � Reproduces general features of yield data

� Momentum transferred from incident ion to target atoms via binary collisions

� Fast recoils in turn displace other atoms

• Increasing number of lower energy recoils

� Modeled as isotropic "collision cascade"

� Yield calculation

• Count recoils crossing the surface plane

• To escape: energy perpendicular to surface > surface binding energy Uo

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Calculated Sputtering Yield

Y(Ei,θi) = (Kit/Uo)Sn(Ei/Eit)f(θi)

� Eit and Kit are scaling constants

� Eit=(1/32.5)(1+Mi/Mt)ZiZt(Zi2/3+Zt

2/3)1/2 keV

� Kit ≈ (1/3)(ZiZt)5/6

• Latter valid for Zt/Zi ~ 1/16 - 5

� Sn = reduced nuclear stopping cross section

� Sn(ε)=0.5[ln(1+ε)/{ε+(ε/383)3/8}], where ε = Ei/Eit

� f(θi): ion incidence angle dependence� f(θi) = cos

-nθi, with n ≈ (5 ± 2)/3

Other Yield Effects

� Reactive ion species: ion-target compound formed� Compound volatile, increase in Y

• Reactive ion etching

� Compound involatile, Y decreases since Uo will normally be greater for the compound• Rate dependence in reactive sputtering

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Multi-Component Targets

� Atoms with lower Uo preferentially removed

� Lower mass atoms receive more energy, are preferentially sputtered

� Surface segregation at high target temperature

� Thus, initial flux deviates from target composition � Binary target with mole fractions XA and XB� JA/JB = YAXA/YBXB� Surface composition changes

� At steady state:� Sputtered flux composition equals the target composition

� Technologically important

Glow Discharge Sputtering

� Technologically simple method� Vacuum chamber backfilled with inert gas (e.g. Ar, mTorr range)

� Negative voltage (~1kV) applied to target

� Ar+ ions accelerated to target

• Sputtered atoms ejected, deposit on substrate

• Secondary electrons accelerated away from target

� Secondary electrons impact Aratoms

� Produces more Ar+ ions!

� Multiple sources and rotation used

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Magnetron Sputtering

� Most common sputtering technique

� Magnetic field used to increase secondary electron path lengths

� Increased ionization, increased rates

� Applied voltage 200-500V

� Pressure typically 1-10 mTorr

• Minimal gas scattering andcharge-exchange

� Drawback: non-uniform target erosion

Radio-frequency sputtering

� Used for sputtering of insulating

targets

� RF voltage applied between target

and ground

� Blocking capacitor in circuit

� Induced DC voltage:

� Initially, more electrons reach target

than ions, inducing voltage

• High electron current upon positive

voltage excursion

� Steady state: no net target current in

RF cycle

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Chemical Vapor Deposition

Chemical reactions

between vapor and

substrate surface

provide source of

material for film

deposition

Steps in the CVD process

� Transport of reactants to the growth region

� Transfer of reactants to the crystal surface

� Adsorption of reactants

� Surface processes; including reaction, surface diffusion, and site incorporation

� Desorption of products

� Transfer of products to main gas stream

� Transport of products away from growth region

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Basic criteria for CVD reactions

� Reactive species must be transported at appropriate partial pressure to the substrate surface

� Substrate temperature must be high enough to initiate a heterogeneous reaction

� One product of the reaction must be the film material

� All other reaction products must be sufficiently volatile to be removed into the gas stream

Typical Reactions

� Pyrolysis (thermal decomposition)� Example: silane pyrolysis

SiH4 (g) → Si (s) + 2H2 (g)

• Occurs at 800oC ≤ T ≤ 1350oC

� Compounds obtained by combining gases, e.g.

• (C2H5)3Ga (g) + PH3 (g) → GaP (s) + vapor products

� Hydrolysis, e.g.� 2AlCl3 (g) + H2O (g) → Al2O3 (s) + 6HCl (g)

� Hydrogen reduction (of halide compounds) e.g.� BCl3 (g) + 3/2H2 (g) → B (s) + 3HCl (g)

� SiCl4(g) + 2H2(g) → Si(s) + 4HCl(g)

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Thermodynamic Considerations

� Reaction equilibrium (∆G0 = -RT ln K) predicts partial pressures at equilibrium� ∆G0 = reaction free energy change

� R = gas constant

Example: SiH4 (g) = Si (s) + 2H2 (g)� K = pH2

2(aSi)/pSiH4• a = activity = 1 for a pure solid

� pH22/pSiH4 = exp(-∆G

0/RT)

� ∆G0 is negative and >> RT, so pH2 >> pSiH4� For typical atmospheric-pressure reactor, then pH2 ≈1 atm, and

� pSiH4 = exp(∆G0/RT) (atm) (equilibrium)

• Growth should proceed for pressure exceeding this value

Growth-Rate-Limiting Steps

� Rate-limiting step may change with parameters: � T, flow rate, substrate orientation, partial pressure, etc.

� Basic mechanisms:� Chemical reaction rate

rk ∝ exp(-∆Ea/RT),

• ∆Ea = activation barrier (e.g. chemisorption, surface diffusion, desorption)

� Mass transfer processes (gas diffusion)

• Rate ∝ Tm where m ≈ 1.5 - 2

� Gas flow supply

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Temperature Dependence

� Assume A(g) ⇔ C(s) + B(g)

� Assume an open flow system

� Reactant A is at initial partial pressure PA0

� A and an inert gas, at a total pressure of one atmosphere, flow to the growth region

� Assume three steps occur in series:� Diffusion of A to the surface

� Reaction of A at the surface to deposit C and form product B

� Diffusion of B away from the surface

Rate Calculation 1

1. Diffusion rate of A to surface rDA = kA(PA

0 - PA*)

� kA = gas diffusion coefficient for A

� PA* = partial pressure at substrate surface

� PA0 = partial pressure in source flow

2. Surface reaction (first-order reversible)rs = kfPA

* - krPB*

� kf and kr are forward and reverse first-order reaction rate coefficients

� PB* = partial pressure of B at substrate surface

3. Diffusion rate of B away from surfacerDB = kB(PB* - PB

0)

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Rate Calculation 2

� Note: source gas does not contain B initially, so PB0 = 0

� Steady state: the three rates are equal� rDA = rs = rDB = r (= growth rate)

� Combining the above rate eqns givesr = PA

0/(1/kA + 1/kf + kr/kBkf)

� First-order processes: kf/kr = K (equilibrium constant)

� Assume gas diffusion coefficients are equal

� kg = kA = kB, such that

� r = PA0/[1/kf + (1/kg)(1 + 1/K)]

� Van’t Hoff expression yields: K ~ cexp(-∆H/RT)

Predicted T Dependence

� Including above dependences in full expression:

� r = PA0 / {Aexp(∆Ea/RT) + BT

-3/2[1+Cexp(∆H/RT)]}

� Calculated dependence of r on T

� A, B, and C chosen so kf, kg, and K ~1 at 750C

• Kinetics, diffusion, and equilibrium constant play

roughly equal roles

� ∆Ea = 50 kcal/mole, ∆H = 0 or -38 kcal/mole

� T < 750C: kinetically controlled

� T > 750C:

• ∆H = 0, diffusion limits r (weak T dependence)

• ∆H = -38 kcal/mol, r decreases with increasing T

• Diffusion limit modified by thermodynamic term

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Comparison With Experiment

� Data for GaAs CVD� Exothermic reaction

� Agrees well with above prediction

� Dependence on substrate crystallographic orientation� Surface reaction rate (and activation energy) depends on details of molecular interaction with surface

Other CVD Considerations

� Mass flow rates

� Reactor geometry� Hot wall versus cold wall

� Gas flow dynamics and deposition uniformity

� Deposition uniformity versus source-gas utilization

� Reactor pressure� Decreased pressure below atmospheric - typically ~ 1 Torr

� Increases deposition rate and uniformity by increasing gas diffusivity

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Down-Flow Reactor

� Commonly used for

semiconductor growth

� “Shower-head” gas feed

� Rotating substrate improves

uniformity

� Process designed with help of

software tools

� Finite-element flow models

� E.g. Fluent, FEMLAB

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HYBRID TECHNIQUES

� Combine selected advantages of three

basic techniques

� Examples:

� Chemical beam epitaxy

� Reactive sputtering

� Plasma CVD