Chemical Vapor Deposition (CVD) презентация

Chemical vapor deposition (CVD) systems
Atmospheric cold-wall system used for deposition of epitaxial silicon.
(SiCl4 + 2H2 → Si + 4HCl)
Low pressure hot-wall system used for deposition of polycrystalline and amorphous films, such as poly-silicon and silicon dioxide.
Figure 9-4

Steps involved in a CVD process
Transport of reactants to the deposition region.
Transport of reactants from the main gas stream through the boundary layer to the wafer surface.
Adsorption of reactants on the wafer surface.
Surface reactions, including: chemical decomposition or reaction, surface migration to attachment sites (kinks and ledges); site incorporation; and other surface reactions (emission and redeposition for example).
Desorption of byproducts.
Transport of byproducts through boundary layer.
Transport of byproducts away from the deposition region.
Steps 2-5 are most important for growth rate.
Steps 3-5 are closely related and can be grouped together as “surface reaction” processes.
Reaction rate may be limited by:
Gas transport to/from surface.
Surface chemical reaction rate that depends strongly on temperature.
Figure 9-5

F1 = diffusion flux of reactant species to the wafer through the boundary layer (step 2) = mass transfer flux
(1)
F2 = flux of reactant consumed by the surface reaction (steps 3-5) = surface reaction flux,
where hG is the mass transfer coefficient (in cm/sec).
(2)
where kS is the surface reaction rate (in cm/sec).
In steady state: F = F1 = F2
(3)
Equating Equations (1) and (2) leads to
(4)
The growth rate of the film is now given by
(5)
where N is the number of atoms per unit volume in the film and Y is the mole fraction (partial pressure/total pressure) of the incorporating species, CT is total concentration of all molecules in the gas phase .
Derivation of film growth rate
(similar to/simpler than Deal-Grove model for thermal oxidation)
Figure 9-6

(a). If kS << hG, then we have the
surface reaction controlled case:
(6)
(b) If hG << kS, then we have the mass transfer,
or gas phase diffusion, controlled case:
(7)
ks increases with temperature.
(Arrhenius with EA depending on the particular reaction, e.g. 1.6 eV for single crystal silicon deposition).
hG ≈ constant
(diffusion through boundary layer is insensitive to temperature)
(5)
Derivation of film growth rate (continued)

CVD film growth rate
Actually hG is not constant (depends on T)
Figure 9-8 Growth or deposition rate for silicon by APCVD. The partial pressure of the reactant gas is 0.8Torr (1atm=760Torr!!).
H2 is used as the carrier or diluent gas for the solid curves.
For SiH4, using N2 carrier gas increases the growth rate, because the carrier gas H2 is a reaction product of SiH4 decomposition, thus slowing down the reaction.
Deposition rate vs. gas glow rate
Figure 9-8

Chemical Vapor Deposition (CVD) growth rate
kS limited deposition is VERY temperature sensitive.
hG limited deposition is VERY geometry (boundary layer) sensitive.
Si epitaxial deposition is often done at high T to get high quality single crystal growth. It is then hG controlled, and horizontal reactor configuration is needed for uniform film thickness across the wafer.
When a high film quality is less critical (e.g. SiO2 for inter-connect dielectric), deposition is done in reaction rate controlled regime (lower temperature). Then one can greatly increase the throughput by stacking wafers vertically (for research, usually 25 wafers per run; 100-200 for industry).

Gas moves with the constant velocity U.
Boundary layer (caused by friction ) increases along the susceptor, so mass transfer coefficient hG decreases.
Source gas also depletes (consumed by chemical reaction) along the reactor.
Both decrease growth rate along the chamber.
To compensate for this, one can:
Use tilted susceptor.
Use temperature gradient 5-25°C.
Gas injectors along the tube.
Use moving belt.
Other factors affecting growth rate: thickness of boundary layer and source gas depletion
DG: diffusivity
μ: gas viscosity
ρ: gas density
U: flow velocity
X: gas flow direction
Should be ∂C/∂y, since diffuse along y-direction

Mass transport in gas
Two flow Regimes
Molecular flow (diffusion in gas, particle transfer).
Viscous flow (laminar & turbulent flow, moment transfer).
Laminar flow is desired.
In CVD growth rate model, it was assumed that mass transport across the stagnant layer proceeds by diffusion.
Mass transport depends on:
δ→ tube radius when x is large
Transport of reactants:
Flow along x-direction.
Diffusion along y-direction. (anyway, no flow along y-direction)

Low Pressure Chemical Vapor Deposition (LPCVD)
Atmospheric pressure systems (APCVD) have major drawbacks:
• At high T, a horizontal configuration must be used (few wafers at a time).
• At low T, the deposition rate goes down and throughput is again low.
The fundamental reason (I think) for the low throughput of APCVD is that only a small percentage of the gas is reactant gases, with the rest carrier/diluent gas.
Obviously, the solution is to operate at low pressure – LPCVD.
But
(8)
In the mass transfer limited regime,
This is not one expects: lower pressure means less reactants, so lower rate. But for APCVD, the reactant gas is only a small portion of the total gas.