Introduction to Thin Film...
Transcript of Introduction to Thin Film...
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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