1 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)
18
Embed
Introduction to Thin Film Processingtam.northwestern.edu/summerinstitute/_links/_courses/2007...Effusion (Knudsen) cells Effusion Cells Commonly used in molecular beam epitaxy(MBE)
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
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)
2
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
2
• 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!
3
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
4
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
3
= 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)
5
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
� 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)
16
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
17
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