1Department of Solid State Sciences

Title

Discharge voltage variations when switching from pure metallic to poisoned sputtering

R. De Gryse, D. Depla, G. Buyle, J. Haemers

AIMCAL

2005

MYRTLE

BEACH

2

Chemisorption: experiment

AIMCAL

2005

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360

340

320

300

280

260

targ

et v

olta

ge (

V)

5 6 7 8 9

12 3 4 5 6

oxygen flow (sccm)

Constant current: 200 mATarget: AluminiumConstant Ar pressure: 2x10-3 mbar

5 L/s28 L/s42 L/s228 L/s469 L/s

Situating the problemPoisoning : drop in target voltage

: exceptions!

3

Reactive sputtering: different oxides (I)

AIMCAL

2005

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600

500

400

300

200

100

0

Dis

cha

rge

vo

ltage

(V

)

Ag Al Au Ce Cr Cu Mg Nb Pt Re Ta Ti Y

target material

time

sta

tus

sta

tus

voltage

sta

tus

argon

oxygen

magnetron

Dt

Experimental conditions : 0.3 A, 0.3 Pa, waiting time : 5s, target diameter : 50 mm

VAr

VO2

Vox,Ar

How behave metals ?

4

Reactive sputtering: different oxides (II)

AIMCAL

2005

MYRTLE

BEACH

The full line shows the discharge voltage behaviour for oxidized targets sputtered in pure Ar, i.e. each curve shows the transition from VoxAr

to VAr, or stated differently the sputter removal of the formed oxide layer on the target surface.

Experimental conditions: constant current 0.3 A, argon pressure 0.3 Pa. The dotted line shows the discharge voltage behaviour during the sputtering (in pure Ar) of a target exposed to oxygen (total exposure approximately 1.4x105 L)

410

400

390

0.001 0.1 10

350

300

0.001 0.1 10

490

480

470

460

0.001 0.1 10

250

200

150

0.001 0.1 10

380

370

360

350

0.001 0.1 10

420

400

0.001 0.1 10

300

200

100

0.001 0.1 10

450

400

350

0.001 0.1 10

500

480

460

440

0.001 0.1 10

440

420

400

380

0.001 0.1 10

400

350

0.001 0.1 10

400

380

360

340

0.001 0.1 10

250

200

150

0.001 0.1 10

time (s)

dis

ch

arg

e v

olta

ge

(V

)

Ag Al

Au Ce

Cr Cu

Mg Nb

Pt Re

Ta Ti

Y plasma exposure

oxygen exposure (1.4x105 L)

5

Target voltage simulation : pumping speed

AIMCAL

2005

MYRTLE

BEACH

360

340

320

300

280

260

targ

et v

olt

age

(V)

6543210flow O2 (sccm)

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

qtc

-qti

6543210

5 L/s 28 L/s 42 L/s 224 L/s 469 L/s

360

340

320

300

280

260

targ

et v

olt

age

(V)

6543210flow O2 (sccm)

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

qtc

-qti

6543210

5 L/s 28 L/s 42 L/s 224 L/s 469 L/s

simulation

experiment

Chemisorption

Shallow implantationOxide layer

6

Chemisorption vs. implantation

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2005

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PAr= 2x10-3 mbar SO2 = 0.1

PO2= 5x10-4 mbar SO = 1

PO= 5x10-5 mbar

2 2A

Pz 2 . 6 3 x 1 0

M T=

A. Chemisorption

Impingement rate ZA (O2) = 1.34x1017 cm-2s-1

ZA (O) = 1.9x1016 cm-2s-1

Condensing rate CA (O2) = 1.34x1016 cm-2s-1

CA (O) = 1.9x1016 cm-2s-1

å CA (O) = 3.2x1016 cm-2s-1

B. Shallow implantation of ions (subplantation)

Discharge current density = 25 mA.cm-2 or 1.56x1017 cm-2s-1

mole fraction oxygen (O) : f = 0.2 3.12x1016 cm-2s-1

Ar+ O

O2+

O2O+

7

Overview : two effects

AIMCAL

2005

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Neutral SpeciesChemisorption at

target surface

Ionic Species 2( R , R )+ +Shallow implantation

Compound growth

Dissolved reaction gas

POISONING: 2 EFFECTS

o o( R , R )2

8

AIMCAL

2005

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336

334

332

330

abso

lute

tar

get

volt

age

(V)

0.60.50.40.30.20.10.0mole fraction nitrogen

on addition on removal

No hysteresis

Constant Ar flow

Constant power (50 W)

Reactive sputtering of Ag in Ar/N2

Why maximum ?

Hysteresis Ag/N2

9

AIMCAL

2005

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Target Poisoning

VD

VD

VD ?

Chemisorption of metal species

Dissolved gas

Compound growth

Shallow implantationof charged species

Overview : compound growth ?

10

AIMCAL

2005

MYRTLE

BEACHi

Thornton relation

0D

0 i

WV =

e e g

VD : Discharge or target voltage

W0 : effective ionisation energy

ei : ion collection efficiency (for magnetron : almost 1)

e0 : fraction of maximum possible number of ions VD /W0 produced by an electron

(for magnetron : almost 1)

g : the effective secondary electron emission coefficient

I S E Em Eg = g × ×

gISEE: Ion induced secondary electron emission

m : multiplication factor

E : Effective gas ionisation probability

Thornton equation

11

AIMCAL

2005

MYRTLE

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oD

i e

WV

E ( p ) I S E E m=

g × × e × e

1 i eD I S E E

oV E ( p ) m

w- e × e

= × × × g

1D I S E EV A B- = g +

Relationship discharge voltage ISEE

12

AIMCAL

2005

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Experimental result

4.0

3.5

3.0

2.5

2.0inve

rse o

f th

e d

isch

arg

e v

olta

ge (

10

31

/V)

0.200.150.100.050.00

ISEE coefficient

AlAgAuCeCrCuMgNbPtReTaTiY

13

AIMCAL

2005

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BEACH

288.80.383162.20.178274.8Y

428.60.08391.90.114335.6Ti

431.90.058435.30.115321.1Ta

616.80.0394820.084379.7Re

588.30.022531.40.048461.0Pt

480.90.045468.50.130308.1Nb

270.10.414156.30.136330.0Mg

402.90.072411.10.082388.3Cu

428.30.094379.20.091359.6Cr

3660.285207.70.184277.9Ce

653.90.037487.70.057471.2Au

305.80.2259.10.091376.4Al

549.20.069410.30.110394.9Ag

VO2 (V)ISEE oxidized. target

VOxAr (V)ISEE metalVAr (V)Target material

Some numbers

14

AIMCAL

2005

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30 msec before steady state is reached

Total ion flux: 1016 ions/cm2

à Sputtering of the oxide layer on the target

à Preferential sputtering of oxygen ?

à Formation of suboxides ?

Wittmaack(*): suboxide formation results in a

negligible yield enhancement

s

b

( M / O )R

( M / O )=

Reduction factor

M,O are the mole fractions of metal and oxygen at surface (s) and in the bulk (b)

(*)K. Wittmaack , Surf. Sci. 419 (1999) 249-264

VAr

VO2

Vox,Ar

Reduction factor : definition

time

sta

tus

sta

tus

voltage

sta

tus

argon

oxygen

magnetron

Dt30 msec

15

AIMCAL

2005

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The influence of the reduction on the inverse of the ISEE coefficient. The reduction was calculated using SRIM. The noble elements (Ag, Au, Cu and Pt) have not been included as the SRIM calculation of their sputter yield was not in agreement with the experimental results

Influence of reduction

25

20

15

10

5

0

1/IS

EE

1.21.00.80.60.4

reduction R

Al

Ce

CrMg

Nb

Re

Ta

TiY

s

b

( M / O )R

( M / O )=

1 1.41.2 1.81.6

16

Preferential sputtering oxygen(*)Surface reductionSuboxide formation

AIMCAL

2005

MYRTLE

BEACH

Target Poisoning

Chemisorption of neutral species

Shallow implantationof charged species

Compound growth(oxides, nitrides…

Dissolved gas

Congruent sputtering(Al, Mg, Ce, Y)

(*)exact mechanism not known yet

VD

VD

VD

VD

Conclusions

17

Acknowledgements

AIMCAL

2005

MYRTLE

BEACH

Acknowledgements

The authors are indebted to the

for financial support

company

Department of Solid State Sciences

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