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Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose...

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Chemical Vapor Deposition (CVD) Processes: gift of SiO 2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single element… CVD: toxic, corrosive gas flowing through valves, T … Contrast with up to 1000°C, multiple simultaneous reactions, gas dynamics, dead layers… whose idea was it? All layers above poly-Si made by CVD, except gate oxide and aluminum Mon., Sept. 15, 2003 1
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Page 1: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Chemical Vapor Deposition (CVD)

Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer…or metal film growth : high vacuum, single element…

CVD: toxic, corrosive gas flowing through valves, T

… Contrast with up to 1000°C, multiple simultaneous reactions,

gas dynamics, dead layers… whose idea was it?

All layers above poly-Si made by CVD, except gate oxide and aluminum

Mon., Sept. 15, 2003 1

Page 2: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

CVD

reactors

Controlmodule

Fourreactionchambers (similar to those for Si oxidation)

Control T,gas mixture,

pressure,flow rate

Mon., Sept. 15, 2003 2

Page 3: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

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

homogeneous nucleation),

e.g. SiH4 (g) Æ Si (s) + 2H2 (g)

Do not want Si to nucleate above substrate (but on substrate surface (heterogeneous nucleation).

Twall

Reactor

Transportof precursors

acrossdead layer to substrate

Pyrolysis: thermal

Susceptorfilm

T sub> TwallChemical reaction:Decomposed speciesbond to substrate

decomposition at substrate More details…

by-productsRemoval of

Mon., Sept. 15, 2003 3

Page 4: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

CVD Processes

8

1 Bulk Bulk transporttransport of byproduct

Reactantmolecule 7 Diffusion of

TransportCarrier gas

2across bndry 4

(g) byproduct (Maintain hi p, layer Decompositionslow reaction) 6 Desorption

3 Adsorption 5J1 µDgDC Reaction with film

J2 ~ kiCi

Surface diffusionMon., Sept. 15, 2003

4

Page 5: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Gas transport

J1 µDgDC

Transportacrossboundarylayer

2

Knudsen NK ≡ lL

<1L

Viscous flow

Dgas ªlvx

2

Mon., Sept. 15, 2003 5

Page 6: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Revisit gas J1 = hg(Cg - Cs)dC D (Cg - Cs)J1 = D =dx d(x)dynamics:

Boundary layer Layer thickness, d(x)lv x (unlike solid)And we saw gas diffusivity D =2 z u

gas vel: u0boundary layer

Cg d (x)d (x) u = 0s Cs

wafer waferx x = L

hxFluid dynamics: d(x) = r = mass density, h = viscosityru0

L1 h 2 L Reynolds #: Re = ru0L

d = Ú d(x)dx =23

Lru0L

≡3 Re ease of gas flow h

L 0D 3 D

Æ ReSo: hg =d 2 L

Mon., Sept. 15, 2003 6

Page 7: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Several processes in series

Simplify CVD to 2 steps: Boundary

AB layerDgJ1 =d

DC J2 BA

J2 = k C s s Reaction rate constant, k

Sticking coefficient gAB, s

…as in oxidation, but no0 ≤ gAB ≤ 1 sold-state diffusion here,

reaction occurs at surface. AB bounces Good off surface adhesion

Let’s analyze, solve for J2…

Mon., Sept. 15, 2003 7

Page 8: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

J1 = J2,

hg ( Cg - Cs ksCsJ2 = ksCs =

hgks

hg + ks

CgCs =hg

hg + ks

Cg,

Boundarylayer

J2 = ksCs

BA

AB

J2

J1 = hg Cg - Cs( ) process:

J1 =Dg

dDC

In steady state:

) =

Two main CVD

J1 = J2,

1+R2

1G2 /(G1+G2)

Electrical analogy:

R = R

G = 1/R= G

Two processes in series; slowest one limits film growth

Mon., Sept. 15, 2003 8

Page 9: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Boundarylayer

J2 = ksCs

BA

AB

J2

J1 = hg Cg - Cs( )

Two main CVDprocess:

J1 =Dg

dDC

J2 = ksCs =hgks

hg + ks

Cg

≡ v = J #area - t

Ê

ËÁ

ˆ

¯˜

1

N #vol

Ê

ËÁ

ˆ

¯˜

, v =hgks

hg + ks

Cg

N f

=Cg N f

1hg

+1ks

Film growth rate

Slower process controls growth

Mon., Sept. 15, 2003 9

Page 10: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Boundarylayer

J2 = ksCs

BA

AB

J2

J1 = hg Cg - Cs( )

Two main CVDprocess:

Examine these 2 limits of growth, h or k limited…g s

Transport limited growth, Reaction limited growth,

k << h :

v =Cg N f

1hg

+1ks

s gh << k :g s

gh C 3DC kTg x g3lv C Re v =ksCg =

Ck0e

-DG

v = g g Æ Re = N f 2LN f 4LN f

N f N f

ease of gas flow

Mon., Sept. 15, 2003 10

Page 11: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Transport limited growth : Reaction limited growth :

h C 3DC 3lv C Re k C C -DG

g x g g kTv = g g Æ Re = v = s g = k0eN f 2LN f 4LN f N f N f

Most CVD is done in this limit where gas dynamics, ∆G = free energy change in reaction

reactor design are important. (∆G @ ∆H for gas

becasue gas reaction no ∆S)

3o -10o

BA

J2

Remedy for boundary layer

Susceptor,

More uniform ug, C fi Choice of reactants and g

uniform film growth rate , v temperature are critical

Mon., Sept. 15, 2003 11

Page 12: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

CVD FILM GROWTH

GAS TRANSPORT-LIMITED REACTION-RATE LIMITED 3lv C

DGv = x g k C C -g kT4N f L Nv = s g = k0e

f N f

2kBTv = , ∆G = free energy change in reaction x pmkBT

Re

l = ,2pd 2Pg (∆G @ ∆H for gas Æ no ∆S for

gas reaction)Re ~ u01

=kBT

v ~ e- DHkT

v µT 12 u0

g

Pg

C

Mon., Sept. 15, 2003 12

Page 13: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Transport ln (v) high Tlimited 1

2v µT u0low T

Reactionlimited

gas - vel , u0- DH

Rate: v ~ e kT

Most CVD is transport- limited. Slow, layer-by-layer ln (v)

growth, epitaxy. Requires T 1/2high T, low pressure, low gas

viscosity. Chamber design, fi DHgas dynamics control process. Arrhenius-likeTo reduce nucleation of 1 / Tproducts in gas phase, use T

1000K 400Klow partial pressure (LPCVD).

Mon., Sept. 15, 2003 13

Page 14: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Review CVD We saw…

CVD is film growth from vapor/gas phase via chemical reactions

Mon., Sept. 15, 2003 14

4 (g) Æ 2 (g)

Pyrolysis:

at substrate

filmSusceptor

Reactor

Twall

T sub> Twall

Transport

across

substrate

by-products

:

bond to substrate

in gas and at substrate: e.g. SiH Si (s) + 2H

thermal decomposition

of precursors

dead layer to

Removal of

Chemical reactionDecomposed species

Page 15: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

ln (v

v µT u01/ 2

Transport-limited CVD.Chamber design, gas dynamics

control film growth. Non uniform film growth. ln (v)

Slow, layer-by-layer growth, epitaxy, require high T,

low pressure, l/L = NK >> 1.

That puts you in the

limited

Rate: v ~ e- DHkT

high T

low T

u0

)

Reactionlimited

Arrhenius-likeH

T 1/2

Gas transport

fi D

Reaction-limited regime 1 / TT

1000K 400K

Mon., Sept. 15, 2003 15

Page 16: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Some CVD reactions

Silane pyrolysis

(heat induced reaction)

SiH4 (g) Æ Si (s) + 2H2 (g) ( 650°C)

This fi poor Si at 1 atm, so use low pressure

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

(by LPCVD for gate oxide)

vSi - tetrachloride reduction

SiCl4 (g) + 2H2 (g) Æ

Si (s) + 4HCl (g) (1200°C) CrystallinePSiCl4

PH 2

Poly Si

(Si-tetra…actually much more complex than this; etch

8 different compounds are formed, detected by RGA)

Mon., Sept. 15, 2003 16

Page 17: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Some CVD reactions (cont.)Doping

Phosphine Diborane

2PH3 (g) Æ2P (s) + 3H2 (g) B2H6 (g) Æ2B (s) + 3H2 (g)

GaAs growthTrimethyl Ga (TMG) reduction

(CH3)3 Ga + H2 Æ Ga (s) + 3CH4 Least abundant element on surface

Arsene 2AsH3 Æ 2As (s) + 3H2 limits growth velocity

750°Cæ Æææ 6 GaAs (s) + 6 HCl gOr As4 (g) + As2 (g) + 6 GaCl (g) + 3 H2 (g)¨ æææ850°C

Si-nitride compound formation 3 SiCl2H2 (g) + 4NH3 (g) Æ Si3N4 (s) + 6H2(g) + 6HCl (g) (750° C)

Mon., Sept. 15, 2003 17

Page 18: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

How can you select process parameters to get desired product and growth characteristics?

Consider: SiH4 (g) Æ SiH2 (g) + H2 (g) TThree unknown pressures

1) Total pressure = Â partial Ps Ptot = PSiH4+ PH2

+ PSiH2

…still have 22 unknown Ps

Si2) Conservation of ratio => H

PSiH2+ PSiH4

4PSiH4+ 2PSiH 2

+ 2PH 2

= const

…still have 11 unknown P

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

K ≡PH 2

⋅PSiH2

PSiH4

= K0e-

DGkT

= ∆H for gas

And similarly for each reaction.

These equations provide a starting place for growth parameters. (Many eqs. for real systems; done on computer) Do a run, analyze results,

tweak process. Mon., Sept. 15, 2003 18

Page 19: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

-Where does K ≡

PH 2⋅PSiH2 = K0e

D

kTG

come from? PSiH4 Consider “mass action” for class groups…

Consider “mass action” for electrons and holes:

Intrinsic semiconductor N-type semiconductor

Conduction nin

band EFDonor levelsEF

pi pValence band

nRecombination 2 2nprobability set ni = ni pi i = npby energy gap and More free electronsnumber of each species p => more recombination,

fewer holes (Eg same)PH2

⋅ PSiH2= K PSiH4 K indicates a bbias at equilibrium in the reaction

toward the products(different molecular species)

Mon., Sept. 15, 2003 19

Page 20: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

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

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

Assume PA ≈ PB find PAB

Solution: K = PAPB and at 1000 Kelvin, K = 0.153 Torr,PAB

and Ptot = PA + PB + PAB , PA ≈ PB. \ 760 T = 2PA + PAB

PA2 = 0.153 PAB = 0.153 (760 - 2 PA) Æ PA = PB = 10.9 Torr, PAB = 738 Torr

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

the product of the right-hand side partial pressures

Is but 15% of the reactant (left-hand-side) partial pressure;

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

Mon., Sept. 15, 2003 20

Page 21: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Atmospheric Pressure CVD: APCVD (little used today, but illustrative)

High P, small l => 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 Æ SiO2 + 2H2O T = 240 - 450°C

Done in N2 ambient (llow partial pressure of active gas, reduces reaction rate)

Mon., Sept. 15, 2003

add 4 - 12% PH3

low T,

limited

ln v Transprt ltd APCVD

T

to make silica flow, planarize.

reaction rate

1/21

Page 22: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Low Pressure CVD (LPCVD) for dielectrics and semiconductors

Equilibrium not achieved at low P where l= K >1nL(molecular flow, few collisions).

l =kBT2pd 2P

lower P => higher Dg, hg improves transport

reduces boundary layer,

Mon., Sept. 15, 2003 22

F.9.13

LPCVDhg

ln v

Transportlimited Reaction limited

T

hg

ks term extends reaction-limited regime

at 1 Torr

1/

at 760 Torr

Page 23: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Low Pressure CVD (LPCVD) for dielectrics and semiconductors

Hot wall reactor fi uniform T distribution but surface of reactor gets coated. So system must be dedicated to 1 species to avoid contamination.

Cold wall reactor Reduce reaction rate,

deposition on surfaces. For epi Si.

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

Mon., Sept. 15, 2003

23

Page 24: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

Low Pressure CVD (LPCVD) for dielectrics and semiconductors In such non-equilibrium, large l cases,

growth rate is reaction limited,

Low P LPCVD kinetically controlled,reaction-rate limited.

Silane pyrolysis SiH4 (g) Æ Si (s) + 2H2 (g) ( 575 - 650°C)

10 - 100 nm/min

(Atm. P APCVD equilibrium, transport ltd.)

LPCVD+ requires no carrier gas + fewer gas-phase reactions, fewer particulates + eliminates boundary layer problem + lower P => higher Dg, extends reaction-limited regime + good conformal growth (unlike sputtering or other PVD methods

which are directional) - strong temperature dependence to growth rate + easier to control T with hot-wall furnace

Mon., Sept. 15, 2003 24

Page 25: Chemical Vapor Deposition (CVD)...Chemical Vapor Deposition (CVD) Processes: gift of SiO2 - Expose Si to steam => uniform insulating layer… or metal film growth : high vacuum, single

R.F. Plasma-enhanced CVD (PECVD) for dielectric

MOS metallization: avoid contact interaction betw. Al & Si, SiO2, T < 450°C

At low T, surface diffusion is slow,

must supply kinetic energy for surface diffusion.

Plasma provides that energy…and enhances step coverage.

What is a plasma? Ionized noble gas, accelerated by AC (RF) or DC voltage, collides with active species in gas and at surface, importing Ekin

Metal CVD

Step coverage is important for electric contacts.

oxideWF6 + 3H2 Æ W + 6 HF oxide

DG ≈ 70 kJ / mole (0.73 eV/atom) semi

below 400°C

Mon., Sept. 15, 2003 25


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