Chemical Vapor Deposition. Chemical Vapor Deposition

Transcription

1 Chemical Vapor Deposition Deposition of a non-volatile solid on a substrate through the reaction of two or more volatile compounds in the gas phase. Two Categories Chemical Vapor Deposition Sources react on substrate. Chemical Vapor Transport Gases used as carriers over liquid or solid sources and transported to react on substrate. Hydride CVD Deposition of InGaAs or InP 1

2 Systems CVD Discussion Subjects Chemical Reactions (Thermodynamics) Overall guidelines for process selection / Bubblers Vapor Transport (Fluids) Pressures greater than 10 Torr Knudsen number > 110, Viscous flow Kinetics Growth rates Diffusion/ Reaction Rates Why CVD? For film deposition: Requires DG V < 0 driving force for nucleation P V > P S Nuclei Size and Deposition Rate depends on DG V Control DG V PVD CVD Chemical Reaction DG 2

3 CVD System Basic Parts: Gas Sources Gas Distribution Reactor Exhaust Controls and Monitoring MOCVD System Exhaust Quartz Tube UPC Reactor MFC s Filters Bubbler H 2 NH 3 RF Coil TMGa/Al Susceptor Gas Sources Pump Scrubber 3

4 Reactor Construction Vertical Horizontal Hot-Wall Cold-Wall Pressure Atmospheric Low Pressure (<10 Torr) Heating induction radiant resistice Temperature High Low Substrates Multiple (Vertical/Horizontal) Rotation Why CVD? For film deposition: Requires DG V < 0 driving force for nucleation P V > P S Nuclei Size and Deposition Rate depends on DG V Control DG V PVD CVD Chemical Reaction DG 4

5 Pyrolysis Ex: Reduction Ex: Oxidation Ex: Compound Formation Ex: Reaction Types (1) SiH 4 ( g) Æ Si(s) + 2H 2 g SiCl 4 ( g) + 2H 2 g SiH 4 ( g) + O 2 g SiCl 4 ( g) + CH 4 g ( ) ( ) Æ Si( s) + 4HCl( g) ( ) Æ SiO 2 ( s) + 2H 2 ( g) ( ) Æ SiC( s) + 4HCl( g) Disproportionation Ex: 2GeI 2 ( g) Reaction Types (2) 300 C,600 C æ æ æ Æ Ge s ( ) + GeI 4 ( g) Substrate Region Position GeI 4 + Ge 2GeI 2 Ge + 2I 2 Æ GeI 4 GeI 4 + Ge Æ 2GeI 2 Source Region Temperature ( C) 5

6 Reversible Transfer Reaction Types (3) Ex: As 4 ( g) + As 2 ( g) + 6GaCl( g) + 3H 2 ( g) 750C,850C æ æ æ Æ 6GaAs s ( ) + 6HCl( g) Typical in Hydride Vapor Phase Transport Chemical Reaction All of the chemical reactions can be written in the simplified form ( ) + bb( g) Æ cc( s) + dd( g) aa g Homogeneous reaction: all reactions in the gas phase Heterogeneous reaction: mediated by surface of susbtrate Many reactions are reversible Thermodynamical constraints DG 0 = -RT lnk 6

7 Conditions for Equilibrium Equilibrium between all species present in the reactor. Equations to consider Equilibrium constants Total Pressure Input Molar Ratios Reduction of Chlorosilanes (1) Free energy of formation for several species in the Si-H-Cl system. 7

8 Reduction of Chlorosilanes (2) Deposition Epitaxial Si Cl/H=0.1 Poly-Si Composition of gas phase versus reactor temperature. Total pressure = 1 atm, Cl/H = 0.01 Gas Transport Gas transport is the process by which volatile species flow from one part of a reactor to another. Deposited film uniformity Deposition growth rates Utilization of process gasses. CVD processes, Total Pressures > 20 Torr Viscous flow regime. 8

9 Gas Velocity The velocity of a molecule can be decomposed in the following way: v = v d + v thermal In average (no composition gradients) v = v d fi v thermal = 0 Drift Total Mass Flux: J = rv d + j diff u = fluid velocity field Diffusive/Thermal With respect To drift velocity Hydrodynamic Basic Equations r t + ru ( ) = 0 Ê Á Ë t + u ˆ u = F m - 1 r Ê Á P - m Ë 3 u ˆ + m r 2 u Ê Á Ë t + u ˆ q = - 1 c V Continuity Equation Navier-Stokes Equation ( u)q + K 2 q rc V Heat Conduction Equation where q = k B T 9

10 Navier-Stokes Equation Under steady-state conditions and no external forces: ( u )u = - 1 r Ê Á P - m Ë 3 u ˆ + m r 2 u Using a few vector identities and taking the curl: - [ u ( u) ] = m r 2 ( u) Inertial effects (mass) Viscosity u r = 0 No sources u r Steady-State and no sources is the vorticity Approx. Non-Compressible (density const) Reynolds Number Re = ru 0 L m For an ideal gas: Re ª T >2000 inertial effects <2000 viscosity effects Useful equations for flow in a tube: Re = 4M Q prtm D u 0 = Q PA Gas velocity 10

11 Axial Flow through a Pipe Viscous Regime 2 ( u) = 0 r 0 u r u = u( r)ˆ z Use cylindrical coordinates to solve equation ( r,j,z) Typical values for Reynolds number Viscous Regime ( ) = u max 1- r 2 Á u r Ê Ë Velocity Profile r 0 2 ˆ where u max = - r 2 0 P 4m z Boundary Layer d 11

12 Boundary Layer At boundary layer flow nearly stagnant. Transport by Diffusion through layer of thickness d by concentrations gradient d For parabolic profile d = 0.7 r 0 Diffusion Viscosity Effects Tube Wall Growth Kinetics Transport reactants through the boundary layer Adsorption of reactants at the susbtrate. Surface diffusion, chemical reactions, and incorporation into the lattice. Transport of products away from the substrate through the boundary layer. 12

13 Diffusion through boundary layer Mass flux is given by: J = C( r)u( r) - D C( r) bulk viscous flow diffusion In the boundary layer approximation u = 0 J = -D C( r) Pure Diffusion Flux Diffusion Kinetic Theory predicts: Usually represented by: D ~ T 3 2 P D = D 0 P 0 P Ê T ˆ Á Ë T 0 n where n = 1.8 Flow through the stagnant boundary layer is: J i = - D P i - P i0 ( ) drt 13

14 Chemical Reactions: Reaction Rates Reactants proceed to products via formation of an activated complex R Æ A Æ P Energy E 1 * * E -1 dn dt = kn First-Order Kinetics DH Reaction Coordinate Growth Rate Mass flux J gs = h g ( C g - C s ) d C g Gas Film C s Reaction Consumption J S = k s C s J gs J s 14

15 Temperature Dependence Growth Rate Temperature Dependence G = k h s g C g k s + h g N 0 k s ~ e -E RT strong dependence h g ~ D d ~ T weak dependence Low Temperature Surface Reaction Control G = k SC g N 0 High Temperature Mass Diffusion Control G = h gc g N 0 Growth Rate: Si 15

16 Growth Rate: MOCVD Growth of III-V compounds & alloys (nitrides, arsenides and phosphides). Provided sufficient amount of V source. For Al and Ga (Al x Ga 1-x As,P,N): G = K( T) [ P Ga V Ga + 2P Al V Al ], 2P x = Al V Al P Ga V Ga + 2P Al V Al where P i are the partial pressures and V i are the volumetric flow rates. 16