Sputtering behavior of graphite and molybdenum at low bombarding energies

1431

Journal of Nuclear Materials 122 & 123 (1984) 1431-1436 North-Holland, Amsterdam

SPUTTERING

BEHAVIOR OF GRAPHITE AND MOLYBDENUM

AT LOW BOMBARDING

ENERGIES

E. HECHTL Physik-Department,

Technische

Universitat

MUnchen,

D-8046 Garching/MUnchen,

Fed. Rep. of Germany

J. BOHDANSKY Max-Planck-Institut of Germany

fir Plasmaphysik,

EURATOM Association,

D-8046 Garching/MLnchen,

Fed. Rep,

Sputtering yields for the fusion-relevant materials pyrolytic graphite and molybdenum are reported for incident Ot and noble gas ions in the energy range of 100 eV to 10 keV. The graphite targets were cut in two different crystallographic orientations and bombarded with noble gases to establish the mass dependence of the sputtering yield. Oxygen bombardment was done with graphite and MO targets at room temperature and at 750'~ to investigate chemical effects. Compared to sputterinq with Ne, 0 sputtering results in increased yields for graphite targets and decreased yields for MO targets.

1. INTRODUCTION

The beam retardation

The main impurity in the plasma of presentday tokamaks

originatesfrom

sputtering

wall and limiter materials'. to the main components oxygen

is a major

In addition

as well as physical

oxygen sputtering

is always

on target temperature. data on sputtering different

sputtering',

Therefore,

to depend

in this work,

elsewhere3.

at increased

target

a target heater was added which

floats at the high potential The heater, a tungsten these measurements

of the

target.

filament, was used in

solely as radiation

the other sides of the filament, radiation

shield of molybdenum

The temperature

are reported.

neon sputtering

For the irradiations

source.

The back of the sample sees the filament.

with oxygen for two

target temperatures

For comparison,

can result in

data are expected

for the targets is described

temperature,

of these materials,

impurity which

present. Since oxygen bombardment chemical

of first

system with the turntable

data are also

sheet is used.

of the target was measured

a micro-pyrometer4 In addition

On

a double

with an accuracy

an infrared thermometer

with

of l°C.

151 was

given. In this case only physical sputtering

aimed at the samples

occurs and these data approxima.te the expected

which was switched

amount of physical

of measurement

(see Fig.1). The thermometer

neon and oxygen have similar masses.

was calibrated

for each target with the pyro-

2. EXPERIMENTAL

meter

sputtering

for oxygen since

using a gold mirror

into position for the time

meter. The accuracy of the infrared The irradiations differentially

were performed

in a

isotope separator.

beam retardation

(to get the desired

the

ions are deflected

cylindrical

condenser

high energetic

sputtering

pumped chamber which is linked

to a Harwell-type

neutral

is 5'C. The base pressure

operation

chamber

thermo-

in the

is 1~10~~ mbar. During

the pressure

rises to a few times

Before

10m7mbar. The samples had a size of 10x20 mn2,

energy),

the irradiated

in an electrostatic

to separate

them from

particles

in the beam.

0022-3115/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

area was approximately

1 cm2,

and the current density on the samples was about 0.1 mA/cm*

1432

E. Hechtl, J. Bohdansky /Sputtering

behavior ofgraphite

and molybdenum

FIGURE 1 Beam retardation system with exploded view of the target heater. 1, grounded electrode of the immersion lens; 2, sheet envelope for the high voltage electrode of the immersion lens; 3, high voltage electrode of the immersion lens; 4, turntable for the targets; 5, one of six mounted targets; 6, cover sheet in plane of target being irradiated; 7, gold mirror for temperature reading with the infrared thermometer.

The carbon samples were made of pyrolytic graphite

(Union Carbide,

USA).

graphite

is an anisotropic

Pyrolytic

material,

the samples were cut in two different graphic orientations:perpendicular respectively

to show mirror

therefore

than 1~9.

from the mass change according

and parallel

y=-o_

100 pg in each

accuracy

yields

better

are calculated

to equation

(1)

N am (I) I

MzN where No is Avogadro's

time was chosen to produce

a mass loss of approximately

The sputtering

sheet polished

finish.

The irradiation

by a Mettler ME 22

with an absolute

crystallo-

to the net plane. The molybdenum

samples were made of molybdenum

sample which was determined microbalance

mass increase

number,

of the target,M2

4 m the measured the target atomic

mass in g/mol (Ml being the ion mass),

and N

1433

E. Hechtl, J. Bohdansky / Sputtering behavior of graphite and molybdenum

the number of incoming projectiles. tion does not take into account the

implanted

ions which

large dose measurements, by sputtering

This equa-

The uncertainty

the mass of

is justified

in the yield data is esti-

mated for molybdenum

in these

at 5% and for graphite

at 10%.

where the mass loss

is much higher than any mass

gain due to the implantation

of bombarding

ions.

3. RESULTS AND DISCUSSION The results are summarized plotted

in Table 1 and

in Figs. 2 to 4.

TABLE 1 Sputtering yield data for graphite and molybdenum bombarded by oxygen and noble gas ions. (~1 perpendicular, ~11 parallel to net plane). =:=================================================================================================== E(eV)

He + CII

He + Cl

Ne + CII

Ne + CI

Ar + CII Ar +CI

Kr -f CII

Kr+CI

Xe+UI

Xe+CI

100 150

0.10

0.042

0.081

0.058

0.056

0.053

300 600

0.10

0.095

0.26

0.18

0.34

0.305

0.21

0.18

0.20

0.11

0.089

0.81

0.56

0.94

0.78

0.96

0.77

0.71

0.56

1.11

0.66

1.29

0.92

1.26

1.01

1.38

1.24

1000 3000 10000

===============~=========================================~=====~~====================================== (750 C) E(eV

0 + CII

(750 C) 0 XI

0.84

0.91

300 600

0

+Mo

0

-+ vo

0.21

100 150

(750 C) Ne+Mo

1.08

0.86

1000

0.25

0.012

0.054

0.36

0.037

0.19

0.52

0.106

0.60

0.21

3000

1.245

1.15

0.84

0.41

10000

1.06

1.17

0.95

0.54

0.43

0.87

E. Hechtl, J. Bohdansky / Sputtering behavior of graphite and molybdenum

1434

3.1 Sputtering

of graphite

masses

the graphite

the ion mass dependence

sputtering

barding energy.

materials

of

energies

of 150 eV,

data the targets were kept at room temperature.

the bombardment no significant

which occurs

dependence

and7.

cut

cut perpendicular.

a pair depends

separating

between the corres-

the two curves of

on the projectile

mass. The

widest gap occurs where the bombarding

in

mass

nearly equals the target mass. This seems to

of a target by noble gases, temperature

references6

to the net plane shows higher values

ponding yields)

argon, krypton, and xenon were used, For these

sputtering,

in

The gap (i.e. the difference

ions, the noble gases helium, neon,

In pure physical

parallel

than that for samples

600 eV, 3 keV, and 10 keV were applied. As bombarding

as discussed

In all cases, the yield for samples

yield at a fixed bom-

Bombarding

energy is increased.

This tendency was found also for other target

Fig. 2 shows four pairs of curves, each pair representing

as the bombarding

is to be

F .._

be most pronounced

t

for an energy of 150 eV.

I

o+ (75OOC 1

E

3 9 w F

$e+

2oNe+

'

1o-2 0

"

LO&+ ‘I

I ' " 50

ION

‘1 aLKr+ ' '

ATOMIC

132 Xe+

I

1

,

100

150

MASS

FIGURE 2 Sputtering yields of graphite of two different orientations as a function of projectile mass and for different projectile energies. The measured values are marked with open-symbols for graphite cut parallel and with full symbols-for graphite cut perpendicular to the net plane. expected.

In the plot, full symbols represent

data of graphite plane whereas graphite

cut perpendicular

open symbols represent data of

cut parallel

individual

to the net

to the net plane. Each

curve shows a maximum which tends

to flatten and to shift to higher bombarding

ION ENERGY

(eV)

FIGURE 3 Sputtering yields of pyrolytic graphite versus ion enerqv. The projectile ions are O+ and Ne+ respecti;ely. The surface of the target is cut parallel to the net plane (open symbols) and perpendicular to the net plane (full symbols) respectively. Ouring O+ bombardment the targets were kept at 750°C. In Fig. 3 the sputtering is plottedversus

yield of graphite

the energy of the bombarding

ions. Again, full symbols refer to graphite

1435

E. Hechtl, J. Bohdansky / Sputtering behavior of graphite and molybdenum

cut perpendicular

to the net plane and open

symbols refer to parallel upper curve represents graphite

bombarded

temperature

orientation.

The

the yield data of

with O+-ions

at

a target

of 75O'C. The data were taken

using graphite

targets of both orientations.

The data points in this case are very close, consequently

they are represented

by one

single curve. This curve is flat and the yield values are around one. The data agree with the sputtering

yield values obtained

when the graphite

ature'. These findings our explanation

are in agreement with

given in reference2:

In Ot-bombardment product

of graphite,

carbon monoxide

coming O+-ion

the product molecule

atom and

leaves the target. This

chemical

effect dominates by Ot-ions

the sputtering

already

of

at room temperature

no change with increasing

FIGURE 4 Sputtering yields of molybdenum as a function of ion enerav. The oroiectiles are O+ and Net respectively. During the oxygen bombardment the target was kept at room temperature (lower curve) and at 750°C (intermediate curve).

target

is observed.

For comparison

with physical

two lower curves are plotted senting

sputtering

by Net-ions.

orientation.

the

in Fig.3 repre-

sputtering

a chemical

by Ot-ions,

an oxide layer is formed with a

lower MO sputtering

bombarded

metal. Therefore,

the yield

temperature

yield

than pure molybdenum

the Ot yield curve at room

is much lower than the Net yield

curve. It is likely that different

on the crystallographic

Without

O+-sputtering,

sputtering

data of graphite

In physical

clearly depends

reaction

formed in Ot bombardment

in

we would expect the yield curves

be responsible

of molybdenum

In Fig.4 we compare the sputtering bombarded

temperature tering yield Net-ions

yields

of

with Ot- ions at room

and at 75O'C. In addition curve of molybdenum

the sput-

bombarded

by

Moo3 liquefies

near the melting

point it begins to sublimate. 3.2 Sputtering

oxides are

with the most stable

being Mo03. At normal pressure, at 791'C. At temperatures

to be close to the Ne+-curves.

molybdenum

(eV)

is formed. The in-

graphite

temperature

ION ENERGY

the volatile

binds to a graphite

and therefore

previously,

target was at room temper-

for the yield

This fact might curve taken at

75O'C. The yields

show an increase compared

room temperature.

This can be explained

partial removal of the protecting

to

as a

oxide layer

by sublimation.

is shown. Though Ne+ and Ot have compar-

able masses,

the yield data of Ot-ions at both

temperatures

are considerably

yield data of Net-ions. to chemical

reactions

smaller

than the

Again, we attribute

between Ot-ions

denum. When the molybdenum

this

and molyb-

target is bombarded

ACKNOWLEDGEMENTS The authors are very indebted to R. Obermaier and W. Ottenberger assistance.

for valuable

technical

1436

E. Hechtl. J. Bohdansky /Sputtering:

REFERENCES 1. R. Behrisch, 1047.

J.Nucl.Mater.

85 & 86 (1979)

2. E.Hechtl, J.Bohdansky, 103 & 104 (1981) 333.

J.Roth, J.Nucl. Mater.

3. E. Hechtl, Nucl.Instr.

Meth. 186 (1981)453.

4. Micro Pyrometer by Pyro-Werke Hebbelstr. 5, West Germany.

GmbH Hannover,

5. Infrared Thermometer by E2 Technology, Carpinteria, California, USA. 6. H.L. Bay, J. Bohdansky, 41 (1979)77.

E. Hechtl,

Rad. Eff.

7. E. Hechtl, J. Bohdansky, J.Roth, Proc. Symp. on Sputtering April 28-30, 1980, Perchtoldsdorf, Vienna, Austria.

behavior of graphite and molybdenum