EE143 F2010 Lecture 12 Vacuum Basicsee143/fa10/lectures/Lec_12.pdf · Professor N Cheung, U.C....

Professor N Cheung, U.C. Berkeley

Lecture 12EE143 F2010

1

Vacuum Basics

1. Units– 1 atmosphere = 760 torr = 1.013x105 Pa– 1 bar = 105 Pa = 750 torr– 1 torr = 1 mm Hg– 1 mtorr = 1 micron Hg– 1Pa = 7.5 mtorr = 1 newton/m2

– 1 torr = 133.3 Pa

2. Ideal Gas Law: PV = NkT– k = 1.38E-23 Joules/molecule –K

= 1.37E-22 atm cm3/K– N = # of molecules– T = absolute temperature in K– [Note] At T = 300 K ; kT = 3.1E-20 torr-liter

1



2

For mixture of non-reactive gases in a common vessel,each gas exerts its pressure independent of others.

Ptotal = P1 + P2 + … + PN (Total P = Sum of partial pressure)Ntotal = N1 + N2 + … + NNP1V = N1kTP2V = N2kT...................PNV = NNkT

3. Dalton’s Law of Partial Pressure



3

4. Average Molecular Velocity

v = (8kT/ m)1/2

where m = molecular weight of gas molecule

5. Mean Free Path ofmolecular collision

=1

2 d2o n

where n = molecular density = N/V,do = molecular diameter

[Note] For air at 300 °K, =6.6

P( in Pa) =0.05

P( in torr)with in mm

Assumes Maxwell-Boltzman Velocity Distribution



4

6. Impingement Rate,

=n v

4 = # of molecules striking unit surface /unit time.

= 3.51022 × PmT

in #/cm2-secwith P in torr, m in amu

[Note] For air at 300 °K ; (in #/cm2 -sec) =3.81020 P

Example Calculation : Contamination from Residual Vacuum

For a residual vacuum of 10-6 torr, = 4 1014/cm2-secIf each striking molecule sticks to the surface, the equivalentdeposition rate of the residual gas is ~ 1/3 of a monolayer of solid persecond.



5

Pressure (Torr)

Tim

e to

form

a m

onol

ayer

(sec

)

Impi

ngm

ent R

ate

(Mol

ecul

es/c

m2 s)

Mea

n fr

ee P

ath

(mm

)

At 25oC

M

I

P

1 meter!

m/min

Vacuum Basics (Cont.)

ResidualVacuum

PlasmaProcessing

CVD



6

Thin Film Deposition

substratefilm

Applications:Metalization (e.g. Al, TiN, W, silicide)Poly-Sidielectric layers; surface passivation.

EvaporationSputtering

Reactive Sputtering

Chemical Vapor DepositionLow Pressure CVD

Plasma Enhanced CVD

Physical Methods Chemical Methods



7

(1) Evaporation Deposition

Al film

wafer

Al vapor

Al

hot heatingboat (e.g. W)

I

electronsource

crucible iswater cooled

Al vapor

PmkT2

P P eHkT

0

Vapor PressureEvaporation Rate (max)

m=molecular weight of vapor molecule

Log P

1/T

e

Vacuum Torr 10 -5



8

Vapor Pressure versus Temperature



9

Basic Properties of Plasma

• The bulk of plasma contains equal concentrations of ionsand electrons.

• Electric potential is constant inside bulk of plasma. Thevoltage drop is mostly across the sheath regions.

• Plasma used in IC processing is a “weak” plasma,containing mostly neutral atoms/molecules. Degree ofionization is 10-3 to 10-6.



10Source: www.icknowledge.com



11

DiffusionFlux

DriftFlux

E 0

E ~ 0



12

Al

(2) Sputtering Deposition

Al AlAr+

Deposited Al film

Al target

Ar plasma

waferheatsubstrate to~ 300oC (optiona12l)

NegativeBias ( kV)

I

Gas Pressure 1-10 m TorrDeposition rate =

sputtering yield

ion current

constant I S

Ar+

Example:DC plasma

12



13Source: www.icknowledge.com



14



15

S # of ejected target atoms

incoming ion.

0.1 < S < 30

Al

Al

Al

Ar

Sputtering Yield S



16

Sputtering Yield of bombarding ion atomic number

For reference only



17

The Sputtering Yield with incidence angle



18

Ar+

Aflux

Bflux

AxBy

targetFilm has same composition oftarget at steady state.

Because SA SB, Target surface will acquire a compositionAx’By’ at steady state.

Sputtering of Compound Targets



19

Target

AxBy

Deposited Filmon substrate

AxBy

AxBy

Ax’By

’

Ax’By

’

t=t1

t=t2Difference betweent2 and t1 must becomposition depositedon substrate

Bulk targetcomposition

Proof



20

Reactive Sputtering

• Sputter a Ti targetwith a nitrogenplasma

Ti Target

N2 plasma

Ti, N2+

TiN

Example: Formation of TiN

Substrate

Note: TiN is a metallic compoundwith a golden color. It is used frequentlyin ICs as a metallic conductor. Whensandwiched between two thin films, it canalso be used as a “diffusion barrier” toblock thin-film reactions



21

Radio Frequency Plasma Generation

For reference only

•More efficient plasma generation•Needed if substrate is electricallyinsulating



22



23

Thickness Uniformity with various PVD sources

(i) Point-like Source

(ii) Plane-like Surface SourceF

surface

•Flux F leaving source surface isindependent of (isotropic source)Example: Laser vaporization of a wire tip

F

•Flux F leaving surface cosExample: E-beam evaporation

Emission source

surface

Emission source



24

Let F = emission flux from source

Receiving flux F’ at a distance r from source = F/ r2

If the receiving surface is making an angle with respectto the F’ vector

Thickness deposited F’ cos = F cos / r2

Film Thickness Deposition on Wafer

F

F’

source

waferr



25

wafer-l +lx =0

R0R

point-source(emission flux has no dependence)

Thickness t at x=0

cos / Ro2

= 1 / Ro2

Note : =0at position x=0

For this special geometry, =

Example 1 : Flat Wafer directly on top of Point Source



26

23

2

01

0

3

coscos

3

22

0

0

0

220

220

Rl

lR

Rxt

lxt

R

RlRlR

x=0 x=+l

Thickness t at position x= +l cos / R2



27N Cheung EE243 S2010 Lec 1727

Thickness Variation of PVD

(n=0)

(n=1)

*For this particular geometry, =



28

Example 2 : Surafce Source & Spherical Receiving Surface

2cos21

coscos21

cos'waferonThickness

cos21'fluxReceiving

cosflux FEmission

r

r

Fr

F

Since cos = (r/2)/R

Thickness = constant !(1/r2) (r2/4R2)

waferR

R

r

plane-source

F ’



29

Step Coverage Issues



30

Step Coverage Problem with PVD

• Both evaporation and sputtering have directional fluxes.

wafer

step

filmFlux

film

“geometricalshadowing”



31

stepfilm

“self-shadowing”

t=t1

t=0 t=t1

shadowingdistance

t=0



32



33

Sputtering Target

Methods to Minimize Step Coverage Problems

• Rotate + Tilt substrate during deposition• Elevate substrate temperature (enhance

surface diffusion)• Use large-area deposition source



34

Dip Photoresist in Chlorobenzene to slow down developingrate of surface layer .

Surface layer treated by chlorobenzene

Photoresist

Directional Deposition Flux

film

Patterning of deposited layer using directional deposition.

Lift-off Technique



35

Sputtering Target

Profile due to onesmall-area source

Superposition of all small-area sources

Sputtering Target

Profile due to onesmall-area source

Superposition of all small-area sources

•Better lateral thickness uniformityArea of sputtering target can be made much larger than that of an evaporating source.A larger area can be considered as a superposition of many small-area sources.By adding the flux from all the sources, a large area source will provide better lateral filmdeposition uniformity on wafer.

•For multi-component thin films, sputtering gives better compositioncontrol using compound targets. Evaporation depends on vapor pressure ofvarious vapor components and is difficult to control.

Advantages of Sputtering over Evaporation

EE143 F2010 Lecture 12 Vacuum Basicsee143/fa10/lectures/Lec_12.pdf · Professor N Cheung, U.C....

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