Introduction to Thin Film Processing

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) 1

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 P e given by the Clausius-Clapeyron equation: dp e /dt = P e H v /RT 2 where H v = latent heat of evaporation (or sublimation) R = gas constant Assuming that H v is independent of T gives P e exp(- H v /RT) Strong exponential (Arrhenius) T dependence! 2

Evaporation Flux Evaporation flux J related to P e : J = αn a (P e - P h )(2πMRT) -1/2 α = evaporation coefficient (~1) P h = hydrostatic pressure (= 0 in vacuum) N a = Avogadro's number M = molecular weight J = 3.513 x 10 22 P e /(MT) 1/2 (molec. cm -2 s -1 ) P e in Torr and M in AMU Insert P e 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 J o Fraction dj/j o falling on area da at distance r from source given by dj/j o = da/4πr 2 Substrate area da s at angle θ to flux Projected area da = da s cosθ, so dj/da s = J o cosθ/4πr 2 3

Film Thickness Distribution: Surface Source Source flux distribution Typical dependence: cosφ φ = emission angle dj/da s = J o cosθ cosφ/πr 2 Film accumulation velocity: R = (dj/da s )/N (e.g. cm/s) N = atomic density (atoms/cm 3 ) Distribution Calculation Point source with substrate plane at distance h R = J o cosθ/4nπr 2 = J o h/4nπr 3 = J o h/4nπ(h 2 + l 2 ) 3/2 Surface source with substrate plane at distance h: Example: source and substrate planes parallel R = J o cosθ cosφ / Nπr 2 = J o (h/r) (h/r) / Nπr 2 = J o h 2 / Nπ(h 2 + l 2 ) 4

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: N 2, CO, CO 2, 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 CaF 2, 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 5

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 6

Multiple Source Evaporation QuickTime and a TIFF (LZW) decompressor are needed to see this picture. 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 SPUTTERING Physical process resulting from the impingement of an energetic particle on a surface Only one of many "ionsurface 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 7

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 E i Ion mass M Ion impingement angle θ I 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 U o 8

Calculated Sputtering Yield Y(E i,θ i ) = (K it /U o )S n (E i /E it )f(θ i ) E it and K it are scaling constants E it =(1/32.5)(1+M i /M t )Z i Z t (Z i 2/3 +Z t 2/3 ) 1/2 kev K it (1/3)(Z i Z t ) 5/6 Latter valid for Z t /Z i ~ 1/16-5 S n = reduced nuclear stopping cross section S n (ε)=0.5[ln(1+ε)/{ε+(ε/383) 3/8 }], whereε= E i /E it f(θ i ): ion incidence angle dependence f(θ i ) = cos -n θ i, 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 U o will normally be greater for the compound Rate dependence in reactive sputtering 9

Multi-Component Targets Atoms with lower U o 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 X A and X B J A /J B = Y A X A /Y B X B 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 Ar atoms Produces more Ar + ions! Multiple sources and rotation used 10

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 and charge-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 11

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 12

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 SiH 4 (g) Si (s) + 2H 2 (g) Occurs at 800 o C T 1350 o C Compounds obtained by combining gases, e.g. (C 2 H 5 ) 3 Ga (g) + PH 3 (g) GaP (s) + vapor products Hydrolysis, e.g. 2AlCl 3 (g) + H 2 O (g) Al 2 O 3 (s) + 6HCl (g) Hydrogen reduction (of halide compounds) e.g. BCl 3 (g) + 3/2H 2 (g) B (s) + 3HCl (g) SiCl 4 (g) + 2H 2 (g) Si(s) + 4HCl(g) 13

Thermodynamic Considerations Reaction equilibrium ( G 0 = -RT ln K) predicts partial pressures at equilibrium G 0 = reaction free energy change R = gas constant Example: SiH 4 (g) = Si (s) + 2H 2 (g) K = p H22 (a Si )/p SiH4 a = activity = 1 for a pure solid p H22 /p SiH4 = exp(- G 0 /RT) G 0 is negative and >> RT, so p H2 >> p SiH4 For typical atmospheric-pressure reactor, then p H2 1 atm, and (equilibrium) Growth should proceed for pressure exceeding this value p SiH4 = exp( G 0 /RT) (atm) Growth-Rate-Limiting Steps Rate-limiting step may change with parameters: T, flow rate, substrate orientation, partial pressure, etc. Basic mechanisms: Chemical reaction rate r k exp(- E a /RT), E a = activation barrier (e.g. chemisorption, surface diffusion, desorption) Mass transfer processes (gas diffusion) Rate T m where m 1.5-2 Gas flow supply 14

Temperature Dependence Assume A(g) C(s) + B(g) Assume an open flow system Reactant A is at initial partial pressure P A 0 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 r DA = k A (P A0 - P A* ) k A = gas diffusion coefficient for A P A* = partial pressure at substrate surface P A0 = partial pressure in source flow 2. Surface reaction (first-order reversible) r s = k f P A* - k r P B * k f and k r are forward and reverse first-order reaction rate coefficients P B* = partial pressure of B at substrate surface 3. Diffusion rate of B away from surface r DB = k B (P B * - P B0 ) 15

Rate Calculation 2 Note: source gas does not contain B initially, so P B0 = 0 Steady state: the three rates are equal r DA = r s = r DB = r (= growth rate) Combining the above rate eqns gives r = P A0 /(1/k A + 1/k f + k r /k B k f ) First-order processes: k f /k r = K (equilibrium constant) Assume gas diffusion coefficients are equal k g = k A = k B, such that r = P A0 /[1/k f + (1/k g )(1 + 1/K)] Van t Hoff expression yields: K ~ cexp(- H/RT) Predicted T Dependence Including above dependences in full expression: r = P A 0 / {Aexp( E a /RT) + BT -3/2 [1+Cexp( H/RT)]} Calculated dependence of r on T A, B, and C chosen so k f, k g, and K ~1 at 750C Kinetics, diffusion, and equilibrium constant play roughly equal roles E a = 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 16

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 17

Down-Flow Reactor QuickTime and a TIFF (LZW) decompressor are needed to see this picture. 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 HYBRID TECHNIQUES Combine selected advantages of three basic techniques Examples: Chemical beam epitaxy Reactive sputtering Plasma CVD 18