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Chemical Vapor Deposition


Dr. Philip D. RackAssistant Professor

Department of Materials Science and Engineering

The University of TennesseeTel (865) 974-5344

Fax (8654) [email protected]



Chemical Vapor Deposition - a technique for depositing thin film of materials on wafers or other substrates. Source gases are introduced into a reaction chamber and energy is applied through heat, plasma generation, or other techniques that result in the decomposition of the source gas and reaction the chemicals to form a film.


Important Properties Pertaining to CVD

Grain SizeCrystallographic structureSurface roughnessConformality, planarityDensityComposition (stoichiometric)Stress in the filmAdhesionElectrical

ResistivityDielectric Strength


Degree of Crystallinity

Amorphous - no recognizable long range order to the positioning of atoms within the material

Polycrystalline (poly) - intermediate case, crystalline subsections that are disjoint relative to each other

Crystalline (epitaxial) - atoms are arranged in an orderly three-dimensional array


5-Step CVD Process

GAS Flow5

adsorbateor

adatomSubstrate

e-weak

1

2 3Reactants Products

4 Stagnant layer

1 Diffusion across stagnant layer2 Adsorption on surface3 Surface reaction, migration film formation4 Desorption of Products5 Diffusion of products back into gas stream


Growth Kinetics

originalsilicon surfaceStagnant Layer

F1

Cs

Cg

MainGasFlow

Gas Stream

FilmGrowth Wafer

F2Cs - Surface reactant concentrationCg - Gas stream reactant concentration

The reactant gas diffuses through the stagnant layer, dissociates, and deposition occursFor example: SiH2Cl2 → SiCl2 + H2 → Si + 2HCl

Adapted from page 58 VLSI Technology by S.M. Sze ©1983 McGraw Hill

H H

H

H Cl

H

AsH3

As

Si

SiCl2 HCl AsH3 H2


Growth Kinetics - Stagnant Film Model

originalsilicon surface

F1

Cs

Cg

MainGasFlow

s

Diffusion flow

Stagnant Layer(linear gradient)Gas Stream

EpiGrowth Wafer

F2Cs - Surface reactant concentrationD = Diffusivity

Cg - Gas stream reactant concentration

surfacereaction

flow

F1 = D ( Cg - Cs )s

Diffusivity times the concentration gradient

Ds = hg = gas phase mass transfer coefficient

F1 = hg (Cg - Cs ) F2 = Ks Cs For steady state Epitaxial Growth

F1 = F2Therefore Cs = hg Cg

Ks + hgF1 > F2 reaction limitedF1 < F2 mass transport limited


Gas Transport -- Boundary Layer Theory

Fluid Mechanics of Gas FlowsGas moving down a tube of diameter d in the x direction at a velocity vA diffusion boundary layer s(x) is formed whose thickness increases as the gas moves down the tube

21

5)(

=

vxx

ρηδ

η = the gas viscosityρ = the gas mass density

v = gas stream velocity

δ(x)diffusion boundary layer

wafers

reactor chamber

d v MAIN GAS FLOW

xGraphite Susceptor


Surface Reaction

The surface reaction can be modeled by a thermally activated process at a rate R, where;R = Ro e[-Ea/kT] Arrhenius relationshipRo is the frequency factor, Ea is the activation energy in eVT is the temperature in °K

Polycrystalline or Amorphous( LPCVD Range ) (EPI Range)

Mass transport limited

Surface reactionlimited

Slope ~ Ea

Log (deposition

rate)

1/Temp (°K)-1

Very sensitive to Temperature variations

Relatively insensitive toTemperature variations

Cold wall chamber can be used

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