pvd method

7/30/2019 PVD Method

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6.152J/3.155J 1

PHYSICAL VAPOR DEPOSITION (PVD)PVD II: Evaporation

We saw CVD Gas phase reactants:pg 1 mTorr to 1 atm.

Good step coverage, T> 350 K

We saw sputtering Noble (+ reactive gas)p 10 mTorr; ionized particles

Industrial process, high rate, reasonable step coverage

Extensively used in electrical, optical, magnetic devices.

Now see evaporation: Source material heated,peq.vap. =~ 10-3 Torr, pg < 10

-6 Torr

Generally no chemical reaction (except in reactive deposn),

= 10s of meters, Knudsen numberNK>> 1

Poor step coverage, alloy fractionation: pvapor

Historical (optical, electrical)

Campbell, Ch. 12 is more extensive than Plummer on evaporation


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Standard vacuum chambers

pi 10-6 Torr (1.3 10-4N / m2)

Mostly H2O, hydrocarbons, N2 He byresidual gas analysis (RGA = mass spec.)


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p < 10-8 Torr demands:

Stainless steel chamber

Bakeable to 150oC

Turbo, ion, cryo pumps

Ultra-high vacuum chambers


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-6 -4 -2 0 2 4

Log[P(N/m2)]

25

23

Log[n (#/m3)]

21

19

17

15

= 10 0.01 cm

p = 10-10 10-8 10-6 10-4 10-2 100 Torr

1 Atm=0.1 MPa

760 mm

14 lb/in2

Generally /L < 1

Films less pure

Epitaxy is rare

Evaporation

Ballistic, molecular flow, /L >> 1

High purity films

Epitaxy

Sputtering

CVD

Knudson number 1


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Atomic flux on surface due to residual gas

J atomsarea t

= nvx

2= p

2kBT2kBTm

= p2mkBT

= J

Given 10-6 Torr of water vapor @ room temp, find flux

p =106 Torr 1atm760 T

105Pa

atm, kBT RT( )= 0.025eV = 4 1021J

p =1.3104

m

2mH 2O =

18

NA

= 31026kg

What is atomic density in 1 monolayer (ML) of Si?

N= 5 x 1022 cm-3 => 1.3 x 1015 cm-2.

So at 10-6 Torr, 1 ML of residual gas hits surface every 3 seconds!

Epitaxy requires slow deposition, surface mobility,So you must keep pressure low to maintain pure film

J= 4.8 1014atoms/molecules

cm2sec


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Now add evaporation source

Equilibrium vapor pressure:

H= heat of vaporization

pv = p0 exp H

kBT

Strong Tdependence

e-

+

_

e-beam

B field

Resistive heater

I

RF-induction

heater

Work

function

e-

Heat of

vaporization

solid

V(x)

free


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Vapor pressure of elements employed in

semiconductor materials. Dots

correspond to melting points

Rely on tables, attached:pvapor> > pvac,

Elemental metals easy to evaporate, but

alloys

compounds

Differentialpvapor

so use 2 crucibles

or deposit multilayers

and diffuse

Oxides, nitrides deposit in oxygen(or other) partialp


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(note W, Mo

can be used

as crucibles.)


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The source also flux

J=

pvap

2msourcekBTsource

For Al at 1000 K, pvap = 10-7 Torr (from figure)

m = species to be evaporated, = 27 amu for Al

JAl 2 x 1013 Al/cm2-s just above crucible

Mass flow out of crucible ~J Ac m ( mass / t)

We expressed flux of residual gas:

J 5 1014 moleculescm2s(Chamberp = 10-6 TorrJ=

p

2mkB

T

Net flow out of crucible ~J Ac (# / t)Ac

Therefore heat Al to T> 800 C

but thats not all

Note: 3 different temperatures: Tsource Tevaporant >> Tsubstrate > Tchamber = Tresid gas RT.

System not in thermal equilibrium; only thermal interaction among them is by radiation

and/or conduction through solid connects (weak contact). No convection when NK


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How much evaporant strikes substrate? At 10-6 Torr, trajectories are uninterrupted.

While a point source deposits uniformly on a sphere about it, a planar source does not:

Geometric factor =

c

2R 2 cos1 cos2cos

1 =cos

2 = 2r

Deposition rate = JmAc

4r2

m

area t

or J

Ac

4r2

#

area t

substrate

1

2R

R1

2

r

r

substrate

Convenientgeometry

Geometric factor = c

4r2

Film growth rate = JmAc

4r2

1

f

thick

t

cos1J


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6.152J/3.155J 11

vox =H pg N

1

h

+tox

D

+1

ks

oxide

In PVD growth, strike balance

R =deposition rate

Surface diffusion rate

R > 1 stochastic growth, rough

R < 1 layer by layer, smooth (can heat substrate)

Film growth rate

for evaporation

= JmAc

4r2

1

f

thick

t

v =pvap

2msourcekBTsource

m

m

Ac

4r2=

pvap

m

Ac

4r2msource

2kBTsource

Cf. CVDvf =

CgN

1

ng+

1

k


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Exercise

Deposit Al (2.7 g/cm3) at r= 40 cm from 5 cm diam.

crucible heated to 1100C (cfTmelt) pAl vap 10-3 Torr,

pH2O =106 Torr

Compare arrival rate of Al and H2O at substrateand calculate film growth

rateJH2O =

106 760( )105

2 0.025eV e( ) 18 NA( )= 4.8 1018

molecules

m2s

JAl =10

3

105

760( )2 1373kB( ) 27 NA( )

Ac

4r2

=1.76 1018 atoms

m2s

Ac = 5

2

2

Leave shutter closed so initial Al deposition can getter O2 and H2O.

Also, better done at lower orhigher deposition rate.pH2O

(this is not good)

v = pvapm

Ac4r2

msource2kBTsource

1011 m /s slow!


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Step coverage is poor in evaporation (ballistic) - Shadow effects

Heat substrate to increase

surface diffusion.

DS =D0S exp Ea

Surf

kT

Ea

S better step coverage

W


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Other methods of heating charge.

Resistive heater

I

RF-induction

heater e-

+

_

e-beam

B field

Can you suggest other methods?

Laser: Pulsed Laser Deposition (PLD), laser ablation

Ion beam deposition (IB)D:

keep substrate chamber at low P, bring in ion beam

through differentially pumped path.

Source,

target

Substrate,

film

Ion beam

Ion beamCan also use ion beam on film to add energy

(ion beam assisted deposition, IBAD)


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Surface energy in a growing film depends on the number of bonds

the adsorbed atom forms with the substrate (or number unsatisfied).

This depends on the crystallography of the surface faceand on the type of site occupied (face, edge, corner, crevice).

Macroscopically, a curved surface has higher surface energy

(more dangling bonds) than a flat surface.


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Microstructure types observed in sputtered films

with increasing substrate temperature

normalized to melting temperature of deposited species.

Quenched growth Thermally activated growth

< a < a Ts/Tm > 0.3 Ts/Tm > 0.5


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Interfaces between dissimilar materials

Four characteristic equilibrium, binary phase diagrams, above,

and the types of interface structures they may lead to, below (non-equilibrium).

The first-column figures would apply to Ga-As, the second to Si-Ge. Third case,

B diffuses into A causing swelling; A is forced by swelling into B as a second phase.


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Schematic stress strain curveshowing plastic deformation

beyond the yield point.

Upon thermal cycling,a film deposited under conditions

that leave it in tensile stress

may evolve through compression

then even greater tension .

pvd method

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