Thin Film Deposition: Chemical Vapor Deposition

Thin Film Deposition: Chemical Vapor Deposition

Winnie Yu

2OutlineOutline

Thin film deposition CVD vs. PVD Advantages/disadvantages of CVD CVD materials Standard properties of chemical vapor deposition Physics of CVD Types of CVD Summary

3Thin Film DepositionThin Film Deposition

Physical Vapor Deposition(PVD)

Chemical Vapor Deposition(CVD)

Target

Substrate

Gas

Gas

Substrate

Gas

Substrate

4PVD vs. CVDPVD vs. CVD

Non-conformal deposition

Highly directional deposition

Conformal deposition

Multidirectional deposition

PVD CVD

5Advantages of CVDAdvantages of CVD

Uniform distribution over large areas

No compositional gradients across substrate

No need to break vacuum for source changes

More selective area deposition because of higher activation energy for reaction with foreign substances.

6Disadvantages of CVDDisadvantages of CVD

Mostly involve safety and contamination

Hydrides and carbonyls are poisonous (especially arsine)

Metalorganics are pyrophoric (ignite in contact with air)

High cost for compounds with sufficient purity

7What is CVD used for?What is CVD used for?

Typically used to deposit dielectrics SiO2, Si3N4

Other materials can also be deposited via CVD Most common are W and TiN for semiconductors Al, B, C, Co, Fe, Mo, Ni, Nb, Ta

8CVD ReactorCVD Reactor

9CVD ReactorCVD Reactor

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Physics of CVDPhysics of CVD

adsorption

reaction byproduct

substrate

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Physics of CVDPhysics of CVD

film composition & structure

gas supply

convection

gas-phasediffusion

adsorption

surface reaction

deposition

transport

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Physics of CVD: ConvectionPhysics of CVD: Convection

Movement of gas through the reactor after injection

Gas flow pattern Determines gas residence time and heating Influences gas-phase reaction

Depends on mass, momentum, energy conservation, ideal gas law

Depends on reactor geometry

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Physics of CVD: Reactor geometryPhysics of CVD: Reactor geometry

asymmetricreactor

tube reactor

Asymmetric reactor distributes reactant most uniformly Tube batch reactor processes the most wafers

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Physics of CVD: ConvectionPhysics of CVD: Convection

Mean fluid velocity Low : laminar flow (gas can react before leaving reactor) High : turbulent flow

Typical = 4 cm/s

Degree of turbulence characterized by Reynolds number

Re > 1200 yields turbulent flow

Re Lu=

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Physics of CVD: DiffusionPhysics of CVD: Diffusion

Convection Diffusion Adsorption

Diffusion through boundary layer

Diffusion is due to the concentration gradient between the surface and the reservoir

boundarylayer

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Physics of CVD: ReactionPhysics of CVD: Reaction

Reaction typically begins in the gas phase due to heating

Reaction products are more reactive with substrate than with source gases

It is necessary to control both gas-phase and surface reactions

Gas

Gas

Substrate

reactions

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Reactions used in CVDReactions used in CVD

Pyrolysis SiH4(g) Si(s)+2HCl(g)

Oxidation SiH4(g) + 2O2SiO2(s) + 2H2O(g)

Hydrolysis 2AlCl3(g) + 3H2O(g) Al2O3(s) +6HCl(g)

Reduction WF6(g) + 3H2(g) W(s) + 6HF(g)

Displacement Ga(CH3)3(g) + AsH3(g) GaAs(s) + 2CH4(g)

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Physics of CVD: Deposition RatePhysics of CVD: Deposition Rate

ks = surface reaction rate hg = mass transfer coefficient If ks

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Types of CVDTypes of CVD

Low-Pressure CVD

Plasma-Enhanced CVD

High Density Plasma CVD

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Low Pressure CVDLow Pressure CVD

CVD in 0.25 2 Torr range (1 atm = 760 Torr)

Operation in the surface reaction limited regime without lowering temperature

Allows for stacking of wafers and higher throughput

Typically performed in 300 900C range

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Plasma-Enhanced CVDPlasma-Enhanced CVD

LPCVD with plasma source

Ionized gas in plasma supplies energy to reactant gas

Lowers deposition temperature to 200 350 C

Good for deposition on multilayer films sensitive to temperature

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High Density Plasma CVDHigh Density Plasma CVD

PECVD with very high density plasma

RF bias on substrate

Lower deposition temperature of 20 150C

Much lower pressure needed: 1 10 mTorr

Better quality films, with less voids

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Summary Summary

CVD vs. PVD CVD is more conformal and allows for batch processing Higher risks and costs due to gaseous materials

CVD is typically used to deposit dielectric materials, but can be used for metals

CVD process steps are very complicated combination of chemical reactions and gas kinetics

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References References

D. Smith, Thin Film Deposition: Principles and Practice

D. Dobkin, Principles of Chemical Vapor Deposition: Whats going on inside the reactor

J. Plummer, Silicon VLSI Technology: Fundamentals, Practice and Modeling

S. Wolf, Silicon Processing for the VLSI Era: Volume 1, Process Technology