Supplementary slides

Plasma Processing Principles 2

Sputtering :

A mechanism in which atoms aredislodged from the surface of amaterial by collision with high energyparticles

Sputtering technology is most widelyused for depositing metallic filmsincluding Al and Ti

PHYSICS OF SPUTTERING

Argon Gas

Target

Substrate

Vacuum Chamber

DCPower Supply

+

_

Argon Gas

Target

Substrate

Vacuum Chamber

DCPower Supply

+

_

Plasma Processing Principles 3

(1) Reflection or neutralization of ions

(2) Ejection of electrons called secondary electrons

(3) Ion implantation to dope the devices in controlled amount of depth profile of impurities

(4) Structural rearrangement, called radiation damage such as vacancies, interstitials, and lattice defects

(5) Ejection of atoms, called sputtering,followed by series of collisions in atarget due to ion impact

PHYSICS OF SPUTTERING

Plasma Processing Principles 4

ttiiii umumvm cos

22222

2

1)sin(

2

1

2

1ttiiiii umvumvm

22

222

2

cos)(

4)cos

2(

2/1

2/1

it

ti

it

ii

ii

t

ii

tt

mm

mm

mm

vm

vm

m

vm

um

2)(

4

it

ti

mm

mm

: Energy transfer function

a maximum value, 1, when ti mm

,

PHYSICS OF SPUTTERING (Energy transfer in a collision )

Plasma Processing Principles 5

Sputtering yield (S) is defined asthe number of target atomsejected per an incident ion.

The sputtering yield isproportional to energy of theincident ion (E) but inverselyproportional to the surfacebinding energy (U0).

That is

02)(

4

U

E

mm

mmS

it

ti

PHYSICS OF SPUTTERING (Sputtering Yield)

Plasma Processing Principles 6

Applications of Sputtering

(1) Deposition of metal films

(2) Cleaning the surface

(3) etch-back for planarization

Limitations of Sputtering

(1) Rates of the sputter etching and deposition are slow, normally below 100nm/min, compared to RIE and CVD

(2) Bad step coverage compared to CVD

(3) Bad etching profiles

PHYSICS OF SPUTTERING

Plasma Processing Principles 7

Consider a system in which, one of theelectrodes that is often to be thechamber wall is electrically groundedand the other electrode is RF-powerapplied through a blocking capacitor.

In the DC discharge, the time-independent current flows from theanode through the plasma to thecathode. On the other hand, the netcurrent or the time-averaged RFcurrent must be zero

PARALLEL PLATE RF DISCHARGES

Why RF? Insulating films can be processed as well as conducting films.

Plasma Processing Principles 8

V

tj

dA

C o

For continuous plasma, RF frequency higher than about 100kHz is required.

(Ex)-1 mm thick SiO2-1000V DC Bias, 1 mA/cm2 ion current

This requires 130kHz RF for continuous plasma

At low frequency, plasma is short-lived (ON and OFF).

PARALLEL PLATE RF DISCHARGES

Plasma Processing Principles 9

RF generator voltage (Va)

At low frequency, target

voltage (Vb) changes with

ion bombardment and

electron current

>> Steady state with zero

net current

PARALLEL PLATE RF DISCHARGES