Metal Deposition

8/6/2019 Metal Deposition

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Filament evaporation

metals are raised to their melting point by

resistive heating under vacuum

metal pellets are placed on a filament that

acts as a resistor

multiple filaments can be mounted inside

the system and selected with a switch


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Evaporation Apparatus

S.A.Campbell, 1996


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Filament Evaporation

the metal pellets give off a vapor and the

atoms travel in a straight line away from thesource until they strike the sample

the start and stop of the deposition is

controlled by a mechanical shutter


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Ideal Gas Law

PV = nRT

P = pressureV = volume

n = number of moles

R = gas constant

T = temperature

n = PV/RT


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Mean Free Path

distance a molecule travels in a vacuum in astraight line, before its motion is randomized

by a collision with another object


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Mean Free Path

M

RT

8=average velocity

time to collision

mean free path

22

1nd

tcoll =

22

1

ndtMFP coll

==

R = gas constant n = number of moles

M = molecular weight d = molecular diameter


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Mean Free Path

n ~ P number of moles proportional to pressure

22

1

ndMFP

=

MFP ~ 1/n ~ 1/P

as P decreases MFP increases


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T

eeP ~

Vapor Pressure

S.A.Campbell, 1996


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Evaporation Rate

eevap PkT

Mr

2=

revap = evaporation rate

M = atomic mass

k = Boltzmans constant

T = temperaturePe = vapor pressure


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Deposition Rate

deposition rate depends on the location andorientation of the wafer in the chamber


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Deposition Rate

cos2d

r

revap

dep

=

rdep = deposition rate (thickness/sec)revap = evaporation rate (mass/sec)

= solid angle over which source emits (unit less steradians)

d = source to substrate distance

= material density

= inclination of substrate away from direction to source

d


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Uniformity

d1 d2

d1 < d2

rdep (d1) > rdep (d2)

d1 d2

d1 d2

rdep (d1) rdep (d2)

x

x

Case 1: d x Case 2: d >> x


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Filament evaporation

can try to increase temperature to

compensate for low dep rate at far distance

too high of a evaporation rate can result in

condensation of the material into droplets.

droplets on wafer cause poor surface

morphology


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Deposition & Uniformity

better uniformity = lower deposition rate

its a tradeoff!


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multiple wafer deposition will result in

wafers with different thicknesses

d1

d2d2 > d1


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Planetary

a rotating planetary can help improveuniformity

d1 d2

d2 = d1


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Quartz monitor

used to monitor deposition rate

quartz oscillates at a resonant frequency and

is exposed to evaporated material

as material deposits on quartz, the resonant

frequency changes due to additional mass

on the crystal


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Quartz monitor

Quartz monitor in evaporation chamber likely a

relative reference (due to position dependency) and

must be calibrated with profilometry

evaporated materials have different atomic mass,

and thus rates are material dependent

when enough material has been deposited on the

quartz, it must be changed because it no longer

shows a clear resonant frequency


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Step Coverage early days all metal deposition done by

evaporation

lateral dimensions decreased more rapidlythan vertical making step coverage more

critical

S.A.Campbell, 1996


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Step Coverage

cos

2d

rr

evap

dep

=recall that

= 90, cos= 0


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Step Coverage

1. evaporation sources are small and thesource to wafer distance (d) is large,

therefore the arrival angle () is limited

2. also, MFP are long for evaporation(molecule motion not randomized)

d

arriving material is highly

collimated for two reasons


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Step coverage

planetaries can be used so that the sourceappears hemispherical and thereby improvestep coverage

heating of substrates (~60% of meltingtemperature) during deposition can improve

step coverage by promoting movement ofmolecules over the surface after impact


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Improving Step Coverage

W.S.Ruska, 1987


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One persons trash....

poor step coverage can be used to advantage

in lift off process

film is deposited on top of patterned

photoresist layer, layer on top of resist is

easily lifted

substrate

resist


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since the filament must be as hot as the

material being evaporated, contaminationfrom the filament itself evaporating can be a

concern at high temperatures


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E-beam evaporation

typically used where high temperature is

needed and filament evaporation is a

concern

thermal emission of electrons from a heated

tungsten filament


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E-beam evaporation

magnetic field and rastering used to steer

and focus the electron beam into a crucible

containing material

beam bends 270 to minimize Tungsten

deposition from evaporation off of filament

different materials can be selected by arotating crucible selector


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E-beam evaporation

scan plates

W.S.Ruska, 1987


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X-rays

x-rays can be generated by an e-beam

system due to highly excited electrons in the

material being evaporated decaying back tocore levels

radiation can damage CMOS and Silicon

based devices


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Alloys

many modern silicon technologies require

alloys to form reliable contacts and metal

lines (e.g. Al w/ 1% Si, or Al w/ 0.5% Cu).

difficult to produce well controlled alloys

by evaporation, due to differences in vapor

pressure between the two materials


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Alloys

in both filament and e-beam evaporation,

multi layer films of different materials can

easily be created by sequential processing

two ways to attempt alloys 1) single alloy

evaporation, 2) simultaneous evaporation of

two different materials


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Alloys

S.A.Campbell, 1996


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Sputtering

better step coverage

less radiation than e-beam

better at producing alloys

easier to deposit metals with high melting

points


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Sputter deposition parallel plate system similar to etcher

gas is ionized and accelerated towards target

material material is sputtered off the target

if bombardment energy roughly 4 times

bond energy of solid, atoms will be knockedlose (relatively easy, most bond energiesseveral eV)


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Sputter Deposition

plasma is generated using inert gas which

does not react with source material (want a

pure deposition!)

wafer must be placed close to target to

collect ejected material (short MFP due to

high pressure)


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DC Plasma

to serve as an electrode, material must be

conductive, so DC sputtering is not possible

with insulating materials

insulating materials must use RF plasma


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Plasma

a plasma is initiated by applying large voltage

across a gap containing a low pressure gas

governed by Paschens law

( ) bLPLP

Vbd

+

log~

Vbd = voltage dropP = pressure

L = gap between electrodes

b = constant


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Plasmadark space

plasma glow

W.S.Ruska, 1987


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Plasma

sputter rate depends on ion flux to the target

2

2/3

1~d

V

mJ

bd

ion

ion

Jion = ion flux

mion = mass of ionVbd = voltage drop across dark space

d = dark space thickness


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Ion bombardment

< 10 eV

Ion bounce off or

adsorbs to surface

10 eV to ~1000eV

Ions sputter atoms off

target material

> ~1000eVIons implanted into

target

S.A.Campbell, 1996


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Sputter Yield

Ions bounce off

surface or adsorb

onto surface

Ions begin

to implantinto target

S.A.Campbell, 1996


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DC Plasma

voltage drop can be concentrated on one electrode

by making it relatively small

the large electrode is the entire chamber (A2), whilethe small electrode is the target (A1)

4

1

2

2

1

=

A

A

V

V


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Step Coverage

plasma deposition has short mean free pathdue to higher pressure (morerandomization), therefore materialapproaches from more angles providing

better step coverage


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Step coverage as aspect ratios increase > 0.5, it is difficult

for even sputtering to achieve good step

coverage

for high aspect ratios, CVD process is used

for better fill, such as W contacts in a

modern IC fab


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Magnetic Field sputter yield can be increased by applying a

magnetic field giving electrons a spiral path

increasing probability of creating an ion

ion density increases from 0.0001% to 0.03%

to avoid excessive heating the target is

cooled


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Magnetron

Hall Effect

W.S.Ruska, 1987


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Circular Magnetron

S. M. Rossnagel, 1999


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Uniformity

deposition rate under circular magnetron has aGaussian profile as a function of lateral distancefrom the target

to improve uniformity, the wafer can be rotated, aswell as moved in an orbital motion

substrate

target

planetary


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Heart shaped magnetron

moving parts now

outside of chamber,

less particles

more uniform

wear on targetmeans less waste

S. M. Rossnagel, 1999


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Film morphology at low temperature and ion energy, films can be

amorphous and highly porous. films of this type

can oxidize readily and be resistive desire small grains in moderate temperature and

ion energy range

high temperature and energy can cause large grain

size. films can be rough and appear hazy


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Stress

stress dependent on

substrate temperature

deposition rate

film thickness

background chamber ambient

2

2

31 R

TE

t

=

= stress

= wafer bow

t = thickness of film

E = Youngs modulus

= Poisson ratio

T = thickness of wafer

R = radius of wafer

S.A.Campbell, 1996


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Pro/Con Summary

1. better step coverage

2. alloys

3. high temp materials

4. sputter pre-clean

Sputter

Deposition

1. radiation

2. alloys difficult

3. poor step coverage

1. good for liftoff

2. high temp materials

E-beam

evaporation

1. limited source material

2. alloys difficult

3. high temp materials difficult

4. poor step coverage

1. simple apparatus

2. good for liftoff

Filament

evaporation

ConProMethod

Metal Deposition

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