Repetition: Evaporation of Alloys

Evaporation of an alloy corresponds to a fractional destillation. The reason for this is the unhindered material transport within the source.

100

10

1

0.1

1

k=10

k=2

k=1 n/n

log(R /R )A

A B

A

00 0

0

B

B

A B

Alloy composition: A:B=1:1A is the more volatile material (p > p )

particle number for t = 0total number of evaporated particles

n = n + nn = n + n

Repetition: Sputtering

Solid source, i. e. arbitrarysource geometry

Low deposition temperature

Wide parameter field

High deposition rates can bereached

Good coating adhesion

Coating composition =source composition

Interesting film propertiesSource (water cooled)

+

+

+

+ +

Substrate

-600V

Ground

+Deposition materialWorking gas, neutral or reactive

Elementary Processes: Characteristics:

Repetition: Gas Discharge

Experimental set-up:

I/V characteristic:I: Ohmic behaviorII: Saturation regionIII: collisional ionization/

Townsend-dischargeIV: normal glowV: anormal glow

secondary electronemission

-

-

+

+

d = 0.1 - 1 m

p = 0.1 - 10 Pa

U = 1 - 5 kV

U

Iself sustainedgas discharge

non-self sus-tained gas discharge

I

IVV

I II II

Repetition: RF-Sputtering

An excess electron current is generated by the higher electron mobility. It leads to a negative net voltage at the target, idependent wether the target is conductive or not.

Repetition: Magnetron-Sputtering

Repetition: I/V Characteristics

)ln( UkIR

Empirical correlation:

R = Erosion rateI = Discharge currentU = Discharge voltage

Magnetron discharges work at significantly lower gas pressures!

Sputter Yield I

Yn

n

<n> = mean number of particles emitted per impingementn+ = number of impinging ions

Y is dependent on several parameters of the ions and of the target material.

Sputter Yield II

Dependence on :

Target materialIon energy

Sputter Yield III

Dependence on:

Ion impingement angleIon mass

Sputtering Regimes: Single Knock On

+

Ion energy small,and/or ion mass small

110: YM

E eV YEU

100

:

U0 = Surface binding energy

Sputtering Regimes: Linear Collision Cascade I

Ion energy: 0.1 - 10 keVCollision potentials: E+ 0.1 - 1 keV: Born-MayerE+ 1 - 10 keV: Thomas-Fermi

02

4UE

MMMMY

t

t

Mt = Mass of target atoms

+

Ejection volumeapprox. 1 nm3

Sputtering Regimes: Linear Collision Cascade IIPerpendicular impingement:

Sputtering Regimes: Linear Collision Cascade IIIOblique impingement:

Sputtering Regimes: Thermal Spike

+

Ion energy > 10 keV

YUk TB

exp 0

i. e. an evaporation-characteristic of the ejection volume

Linear Collision Cascade: Global Characteristics

Y = 0,5 - 4

+

Ejection volumeapprox. 1 nm3

Sputtering Regimes: Simulation

www.srim.org

Stopping Range of Ions in Matter

Energy Distribution of Ejected Particles

The energy distribution of sputtered particles is significantly different from that of thermally evaporated ones.

Linear Collision Cascade: Energy Distribution

E-2

E

n(E)dE

U /20 EmaxE

dE

UEEdEEn 3

0

)(

Emax = maximum energy, E Emax

E = mean emission energy

Linear Collision Cascade: Angular Distribution

n<1n=1

n>1

n n( ) cos n 1 E < 1 keVn 1 E > 1 keV

Sputtering of Alloys: Different Y

In the case of the homogenous distribution of the constituents the vapor composition is (after a transient regime) identical to the target composition.

Sputtering of Alloys: Cone Formation I

If a low yield material is present in the form of macroscopic preciptates, cones can be formed on the target surface.

Sputtering of Alloys: Cone Formation II

The terminating surfaces of the cones are often low index crystal planes or have an inclination corresponding to surfaces with maximum sputter yield.

Sputtering of Single Crystals: Channelling

Ions may penetrate a single crystal more or less deep in dependence on their impingement direction.

Sputtering of Single Crystals: Wehner-Spots

Focusing of the impulse along densly packed crystallographic directions:

Y = maximum along these directions! If a hemispherical collector is placed above the target, one can detect the so-called "Wehner Spots".

Reactive Processes IIn the case of reactive sputtering processes compounds of the sputtered material and the reactive gas are formed at the target and the substrate.

Gas flows of thereactive gas, qi :

pct0 qqqq

Berg-model

q0 ... Total flowqt ... Flow to targetqc ... Flow to wallqp ... Flow to pump

q

p

Reaktivegas (N )

(Ti)-target

(RF)-voltage

Pump

Recipient wall/substrate

t

N

2

c

0

p

q

q

q

Reactive Processes IIBalance of areal coverages and particle flows:

1 ... Reacted surface target2 ... Reacted surface WallF1,3 ... Flows of reaction productF2,4 ... Flows of metal particles

J ... Flow of workong gasF ... Flow of reactive gas

Result: system of numerically solvable balance equations

J J F

F

F F F F

ATarget

Substrate/Wall

Q

Q

1- Q

1- Q

A

1

1

2 2

1

2 3 4

t

c

Reactive Processes: Example TiN IErosion rate at the target in dependence on the N2 -flow:

Hysteresis at the transition from the metallic to the nitridic mode.

0

D

C B

A

0

2

4

6

8

10metallic

unstable

nitridic

12

14

0.2 0.4 0.6N -flow [sccm]

Ero

sion

rate

[a. u

.]

2

0.8 1 1.2 1.4

Reactive Processes: Example TiN IIPressure in the chamber in dependence on the N2 - flow:

At first all N2 is consumed; the unstable operating point A would be the optimum working condition

0

C

B

00.2

0.2

0.1

0.4 0.6N -flow [sccm]

Tota

l pre

ssur

e [P

a]

2

0.8 1 1.2 1.4D A

no plasma

metallic

nitridic

TiN: Experimental DataThe hysteresis in the relation betweem N2 -flow and total pressure is well visible.

0,4 0,6 0,8 1,0 1,2 1,4 1,6

0,22

0,22

0,23

0,23

0,24

0,24

0,25

Tota

l pre

ssur

e [P

a]

B

A

C

D

increase N -flowdecrease N -flowtheory

2

2

N -flow [sccm]2

Reactive Processes: Large PlantsSputtering plant for the reactive deposition of solar cell materials.

1.5 m

Reactive sputtering processes have recently been accepted as suitable methods for the deposition of oxidic, nitridic and carbidic materials.