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Section 5: Thin Film DepositionPart 2: Chemical Methods

Jaeger Chapter 6

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Chemical Vapor Deposition (CVD)

film

substrate

chemical reaction

source

More conformal deposition vs. PVD

step

t

t

Shown hereis 100% conformaldeposition

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ZYX OHCSiOHOCSi

ateorthosilicenetetraethylTEOSc

HSiOOSiH

HOPOPH

SiOOPglasssilicatephosphoPSGb

HSiOOSiH

SiOa

2452

2224

2522

252

2224

2

.:)(

2

62534

.:)(

2

)(

gas 350oC-500oC

solid

350oC-500oC

gas

LPCVD Examples

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HFWHWF

Wf

HSiSiH

SiPolye

HNSiNHSiH

NSid

Co

63

)(

2

)(

123

)(

26

2600

4

24334

43

LPCVD Examples

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CVD Mechanisms

reactant

1

2

5

43

stagnant gas layer

surfacediffusion

substrate1 = Diffusion of reactant to surface2 = Absorption of reactant to surface3 = Chemical reaction4 = Desorption of gas by-products5 = Outdiffusion of by-product gas

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CVD Deposition Rate [Grove Model]

F

F

=

=

3

1

kTE

os

G

ekk

hD

F1F3

Si

film

= thickness of stagnant layer

31 FF

D [ CG - CS] /

kS CS

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G

sG

C

kh

F

11

13

timewithcontantN

F

dt

dx 3

N = atomic densityof deposited film

Film growth rate = F3 / N

Note: This result is exactly the same as the Deal-Grove modelor thermal oxidation with oxide thickness =0

Grove model of CVD (cont’d)

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Deposition Rate versus Temp

[log scale] Rate

0

R T3

2gas transport limited

surface-reaction limited

1/T

low T

high T

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Growth Rate Dependence on Flow Velocity

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(1) More conformal deposition, if T is uniform

(2) Inter-wafer and intra-wafer thickness uniformity less sensitive to gas flow patterns. (i.e. wafer placement).

Wafer topography

LPCVD Features

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Comments about LPCVD

(2) Mass depletion problem

inout

more less

(1) Sensitivity to gas flow patternFurnace tube

wafers

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• Ionized chemical species allows a lower process temperature to be used.• Film properties (e.g. mechanical stress) can be tailored by controllable ion bombardment with substrate bias voltage.

Plasma Enhanced CVD

Atomic Layer Deposition

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• The process involves two self-limiting half reactions that are repeated in cycles• Unlike CVD, in ALD pulses of precursors are introduced in each cycle• ALD is highly conformal and enables excellent thickness uniformity and control down to nm-scale