3/1/2016
Chemical Vapor Deposition
CHE323/CHE384
Chemical Processes for Micro- and Nanofabrication
www.lithoguru.com/scientist/CHE323
• CVD is used to deposit most dielectrics
and silicon, but also for some metals
• Performed in furnaces, like oxidation and
diffusion, and in newer single-wafer
reactors
• Chemical reactions produce the desired
species being deposited
Lecture 24
Chemical Vapor Deposition,
part 1
Chris A. Mack
Adjunct Associate Professor
Reading:
Chapter 13, Fabrication Engineering at the Micro- and Nanoscale, 4th edition, Campbell
© Chris Mack, 2013
1
Chemical Vapor Deposition
– Reactions occur in the gas phase
– Solid is produced, that then lands on the wafer
– Poor uniformity, many particles
• Solid surface (heterogeneous) reactions
– Reaction occurs on the wafer surface
– Solid formed is the deposited film
– This is what we try to achieve
3
J1 = flux of reactant to wafer surface
Dg = diffusivity of reactant in gas
Cg = bulk reactant concentration =
Cs = surface gas concentration
 = boundary layer thickness
hg = Dg/ = mass transfer coefficient
© Chris Mack, 2013
gas
• Transport of reactants to wafer surface
(diffusion)
• Adsorption of reactants on surface
• Reaction
– Assumed to be rate-limiting step
• Desorption of by-products
• Transport of by-products into gas stream
© Chris Mack, 2013
Cg
2
Simplified CVD Mechanism
• Gas-phase (homogeneous) reactions
CVD Mechanism, Step 1
© Chris Mack, 2013
© Chris Mack, 2013
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CVD Mechanism, Step 2
Si
Cg
Cs
gas
Si
Cs
J2 = flux of reactant as it reacts
ks = reaction rate constant
CS = reactant concentration at wafer surface
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© Chris Mack, 2013
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3/1/2016
Steady State
• Assume that the surface reaction is the ratelimiting step
• Steady state is reached where all fluxes are equal
• Solve for the unknown concentration (CS) and
eliminate from equation
© Chris Mack, 2013
Simplified Rate Equation
,
≪
,
1
© Chris Mack, 2013
8
• In general, Ea(reaction) >> Ea(gas diffusion)
1
∝
,
 varies slowly with T
• At high temperatures, kS >> hg
– Diffusion-limited regime
• At low temperatures, kS << hg
Note: Pg must be kept low to prevent homogeneous reaction
© Chris Mack, 2013
9
Deposition Rate vs. Temp
– Reaction-limited regime
© Chris Mack, 2013
10
Lecture 24: What have we learned?
• Why is heterogeneous CVD preferred over
homogeneous CVD?
• What are the two steps in our very
simplified mechanism for CVD?
• Explain diffusion-controlled vs. reactioncontrolled regimes for CVD?
Diffusioncontrolled
Reactioncontrolled
© Chris Mack, 2013
• For N = # atoms/cm3 in deposited film,
Effect of Temperature
• Two regimes,
≫
• Steady State reaction rate
• Overall deposition rate
7
• Deposition rate
Simplified Rate Equation
11
© Chris Mack, 2013
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