cvd reactor

8/6/2019 CVD Reactor

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Lec. 4 Mon., Feb. 14, 2005 1

Chemical Vapor Deposition (CVD)Processes: gift ofSiO2 - Expose Si to steam => uniform insulating layer

clean and simpleor metal film growth : high vacuum, single element clean and simple

CVD is the single most widely used deposition method in IC manufacture

Contrast with CVD: toxic, corrosive gas flowing through valves,Tup to 1000C, multiple, simultaneous chemical reactions,

gas dynamics, dead layers

(Dr. EstisZmaart?)

whose idea was it?

Lec. 4 Mon., Feb. 14, 2005 2

CVDreactors

Controlmodule

Fourreactionchambers(similar to thosefor Si oxidation)

Control T,gas mixture,pressure,flow rate


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Lec. 4 Mon., Feb. 14, 2005 3

CVD is film growth from vapor/gas phasevia chemical reactions in gas and on substrate:

Do not want Si to nucleate abovesubstrate (homogeneous nucleation),buton substrate surface (heterogeneous nucleation).

Pyrolysis: thermaldecomposition

at substrate

filmSusceptor

ReactorTwall

Tsub> Twall

Transportof precursors acrossdead layerto

substrate

More details

e.g. SiH4 (g) Si (s) + 2H2 (g)

Chemical reaction:Decomposed species

bond to substrate

Removal ofby-products

Lec. 4 Mon., Feb. 14, 2005 4

J2~ k

iC

i

Adsorption3

BulktransportReactantmolecule

Carrier gas(Maintain hi p,

slow reaction)

1

Transportacross bndrylayer2

Decomposition4Desorption6

Diffusion of (g)by-product

7

Bulk transportof by-product

8

CVD Processes

Reaction with film5

Surface diffusion

J1"D

g#C


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Lec. 4 Mon., Feb. 14, 2005 5

Gas transport

J1"D

g#C

Transport acrossboundary layer2

Hi vel, low P

Low vel, hi P

Knudsen NK"

L


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Lec. 4 Mon., Feb. 14, 2005 7

Several processes in seriesBoundarylayer

J2= k

sC

s

Simplify CVD to 2 steps:

BA

ABJ2

Sticking coefficientAB,0 AB 1

AB bouncesoff surface Goodadhesion

J1=

Dg

"#C

Reaction rate constant, ks

no sold-state diffusion here,

reaction occurs at surface.

Lets analyze, solve forJ2

Lec. 4 Mon., Feb. 14, 2005 8

In steady state: J1 = J2,hg ( Cg - Cs ) = ksCs J2 = ksCs =

hgks

hg + ksCgCs =

hg

hg + ksCg

,

Boundarylayer

J2= k

sC

s

BA

AB

J2J

1= hg Cg "Cs( )

Two main CVDprocess:

J1=

Dg

"#C

Electrical analogy: J1 = J2,R = R1+R2G = 1/R= G1G2 /(G1+G2)Two processes in series; slowest one limits film growth


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Lec. 4 Mon., Feb. 14, 2005 9

J2= ksCs =

hgks

hg + ksCg

Film growth rate

" v = J#

area # t

$

%&

'

()

1

Nf#

vol

$

%&

'

()

,

Slower process controls growth

v =hgks

hg + ks

Cg

Nf=

Cg Nf1

hg+

1

ks

Boundarylayer

J2= k

sC

s

BA

AB

J2J1 = hg Cg "Cs( )

Two main CVDprocess:

J1= Dg

"#C

v (thickness/time)

Lec. 4 Mon., Feb. 14, 2005 10

Examine these 2 limits of growth:hg limited or ks limited

Transport limited growth, hg


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Lec. 4 Mon., Feb. 14, 2005 11

Transport limited growth :

v =hgCg

Nf "

3DCg

2LNf Re=

3#vxCg Re

4LNf

Most CVD is done in this limitwhere gas dynamics,reactor design are important.

More uniform ug, Cg

uniform film growth rate , v

Remedy for boundary layer

Susceptor, 3o -10o

Reaction limited growth :

v =ksCg

Nf=

Cg

Nf k0e

"#G

kT

G = free energy change in reaction(G Hfor gas

becasuegas reaction no S)

BA

J2

Choice of reactants andtemperature are critical

Lec. 4 Mon., Feb. 14, 2005 12

CVD FILM GROWTHGAS TRANSPORT-LIMITED REACTION-RATE LIMITED

v =3"vxCg

4NfLRe

v =ksC

g

Nf

=

Cg

Nf

k0e"#G

kT

G =free energy change in reaction

G =H - TS

(G Hfor gasnoSfor

gas reaction)

v ~ e"#H

kT

"=kBT

2#d2Pg,

vx=

2kBT

"m,

v "T12

u0

Re ~ u0

Cg

Pg=

1

kBT


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Lec. 4 Mon., Feb. 14, 2005 13

Most CVD is transport-limited. Slow, layer-by-layergrowth, epitaxy. Requireshigh T, low pressure, low gasviscosity. Chamber design,gas dynamics control

process.To reduce nucleation ofproducts in gas phase, uselow partial pressure (LPCVD).

v "T12 u

0

Transportlimited high Tlow T

gas" vel ,u0

ln (v)

1 / T

Arrhenius-like

H

ln (v)

T1000K 400K

T1/2

Rate:v ~ e"#H

kT

Reactionlimited

Lec. 4 Mon., Feb. 14, 2005 14

We sawCVD is film growth from vapor/gas phase via chemical reactionsin gas and at substrate: e.g. SiH4 (g) Si (s) + 2H2 (g)

Pyrolysis:thermal decomposition

at substrate

filmSusceptor

ReactorTwall

Tsub> Twall

Transportof precursors acrossdead layer to

substrate

Removal ofby-products

Review CVD

Chemical reaction:Decomposed species

bond to substrate


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Lec. 4 Mon., Feb. 14, 2005 15

Transport-limited CVD.

Chamber design, gas dynamics

control film growth.

Non uniform film growth.

Slow, layer-by-layer growth,

epitaxy, require high T,

low pressure, /L = NK>> 1.That puts you in the

Reaction-limited regime

v "T1/ 2

u0

Gastransportlimited

Rate:v ~ e"#H

kT

high T

low T

u0

ln (v)Reactionlimited

1 / T

Arrhenius-like

H

ln (v)

T1000K 400K

T1/2

Lec. 4 Mon., Feb. 14, 2005 16

Some CVD reactionsSilane pyrolysis (heat induced reaction)

SiH4 (g) Si (s) + 2H2 (g) ( 650C)

This poor Si at 1 atm, so use low pressure

(Si-tetrachloride actually much more complex than this;

8 different compounds are formed, detected by RGA)

Silane oxidation (450C)SiH4 (g) + O2(g) SiO2 (s) + 2H2 (g)

Crystalline

Poly Si

etch

PSiCl4

PH

2

vSi - tetrachloridereductionSiCl4 (g) + 2H2 (g)

Si (s) + 4HCl (g) (1200C)

(by LPCVD for gate oxide)

rate

SiCl4 H2

c1c2


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Lec. 4 Mon., Feb. 14, 2005 17

Phosphine2PH3 (g)2P (s) + 3H2 (g)

Or As4 (g) + As2 (g) + 6 GaCl (g) + 3 H2 (g) 6 GaAs (s) + 6 HCl g75 0C" #""85 0C

" ###

Si-nitride compound formation3 SiCl2H2 (g) + 4NH3 (g) Si3N4 (s) + 6H2(g) + 6HCl (g) (750 C)

Some CVD reactions (cont.)

Least abundantelement on surfacelimits growth velocity

Trimethyl Ga (TMG) reduction(CH3)3 Ga + H2 Ga (s) + 3CH4

2AsH3 2As (s) + 3H2rsene

GaAs growth

Doping DiboraneB2H6 (g)2B (s) + 3H2 (g)

Lec. 4 Mon., Feb. 14, 2005 18

How can you select process parameters to getdesired product and growth characteristics?Consider: SiH4 (g) SiH2 (g) + H2 (g) Three unknown pressures

K "

PH 2

#PSiH2

PSiH

4

=K0e

$%G

kT

3) Equilibrium constant, K(cf. Law of mass action)

And similarly foreach reaction.

1) Total pressure = partial Ps Ptot= P

SiH4+ P

H2+ P

SiH2

still have 1 unknown P

PSiH2

+ PSiH4

4PSiH

4

+ 2PSiH

2

+ 2PH

2

= const2) Conservation ofratio =>Si

H

These 3 equations provide a starting place for growth parameters.(Many equations for real systems; done on computer)

Do a run, analyze results, tweak process.

still have 2 unknown Ps

rate

SiCl4 H2

c1c2


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Lec. 4 Mon., Feb. 14, 2005 19

Where does come from?K " PH 2 #PSiH2PSiH

4

=K0e$%G

kT

Consider mass action for electrons and holes:

Intrinsic semiconductor

ni2= nipi

More free electrons=> more recombination,

fewer holes (Eg same)

Conduction

band

Valence

band

EF

ni

pi

K PSiH

4

= PH

2

" PSiH

2

Kindicates a biasat equilibriumin the reactiontoward the products(different molecular species)

EFDonor

levels

n

p

N-type semiconductor

n

p

ni2= np

ecombinationprobability setby energy gap andnumber of each species

Consider mass action for class groups

Lec. 4 Mon., Feb. 14, 2005 20

When a system undergoes a process in which

only heat energy, dQ, andpVwork occur at constant T,

then dG =Vdp

dG = NkBTdp

por RT

dp

pdQ = TdS Vdp

dGA

B

" = RT d ln(p)# GB $GA = RTln pB /pA( )A

B

"

exp "dG

RT

#$%

&'()

pB

pA

Define this ratiopB/pA, as K

K "PH 2#P

SiH2

PSiH4

=K0e$%G

kTFor multiple reagents:

Entropy of mixing, , leads to similar result

dSmix

= "RNiln(N

i)


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Lec. 4 Mon., Feb. 14, 2005 21

ExerciseAssume reaction: AB A + B Ptot = 1 atm, T= 1000 K,

K= 1.8 109 Torr exp ( - 2 eV / kB T) = 0.153

Assume PA PB find PAB

PAPB

PAB

Solution: K= = 0.153

Small value of K, 0.153 Torr, implies that at equilibrium,

the product of the right-hand side partial pressuresIs but 15% of the reactant (left-hand-side) partial pressure;

the reaction may not produce much in equilibrium. What if you change T?

and Ptot = PA + PB + PAB , PA PB. 760Torr= 2PA + PAB

PA2 = 0.153PAB = 0.153 (760 - 2 PA), quadratic PA = 10.8 Torr = PB ,

so PAB = 738 Torr

PA

2+ 0.306P

A" 0.153# 760 = 0

Lec. 4 Mon., Feb. 14, 2005 22

Atmospheric Pressure CVD: APCVD(little used today, but illustrative)High P, small => slow mass transport, large reaction rates;

film growth limited by mass transfer, boundary layer;

(quality of APCVD Si from silane is poor, better for dielectrics).

Example:

SiH4 + 2O2 SiO2 + 2H2O T= 240 - 450C

Done in N2 ambient (low partial pressure of active gas, reduces film growth rate)add 4 - 12% PH3 to make silica flow, planarize.

low T,

reaction ratelimited

ln v Transport ltd APCVD

1/T


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Lec. 4 Mon., Feb. 14, 2005 23

Low Pressure CVD (LPCVD) for dielectrics and semiconductorsEquilibrium not achieved at low P where

(molecular flow, few collisions).

"

L=K

n>1

"=kBT

2#d2P

lower P => higher Dg, hg; this improves transportreduces boundary layer,extends reaction-controlled regime (which is where you operate)

ln v

Transportlimited Reaction limited

1/T

hg at 760 Torr

ks term

LPCVDhg at 1 Torr

Lec. 4 Mon., Feb. 14, 2005 24

Low Pressure CVD (LPCVD) for dielectrics and semiconductors Ifhot-wall reactor uniform Tdistribution but

surface of reactor gets coated.

So system must be dedicated to

1 species to avoid

contamination.

Ifcold-wall reactor

Reduce reaction rate,

reduce deposition on

surfaces. For epi Si.

All poly-Si is done by hot-walled LPCVD;good for low pin-hole SiO2, conformality


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Lec. 4 Mon., Feb. 14, 2005 27

cvd reactor

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