Preliminary investigations of the surface chemistry of titanium gettering

1151

Journal of Nuclear Materials 122 & 123 (1984) 1151-1155 North-Holland, Amsterdam

PRELIMINARY

INVESTIGATIONS

J.R. ELLIOTT:

OF THE SURFACE

R.S. WILLIAMS:

CHEMISTRY

L. KELLER=and

Department of Chemistry*, Tokamak U.C.L.A., Los Angeles, California

OF TITANIUM

GETTERING

R.J. TAYLOR=

Fusion Laboratory=, 90024+

Department

of Mechanics

and Structures=,

The concentration of adsorbed oxygen on titanium foils and thin films deposited on stainless steel has been measured as a function of adsorbant temperature and surface carbon concentration using The oxygen gettering efficiency of foils is dependent on the surface Auger electron spectroscopy. carbon concentration, but saturated surfaces can be successfully regenerated by heating the foils However, carbon diffuses into titanium films from to dissolve oxygen and carbon into the bulk. heated steel substrates and poisons the titanium surface with respect to oxygen adsorption. the walls

1. INTRODUCTION Experiments Auger electron

involving

the in situ analysis

spectroscopy

(AES) of the evapora-

tion of titanium

onto 304 stainless

strates followed

by exposures

steel sub-

to various gases

have been used to model the conditions the Macrotor

and hot wall Microtor'

the base pressure analysis

it was possible

present

tokamaks.

of the ultra-high

vacuum

system was in the low lo-"

gases present

kamak systems with partial

pressures

torr range in a controlled

manner.

in the 10m8 pre-

vious studies of adsorbed

in arcing of the

tion or sputtering

on pure foils and on deposited sults are obtained the base pressure

that occurs

thin films.

for the evaporation

Re-

of Ti at

of the UHV system and in the

presence of H2 and CO at pressures

similar to

those found in the tokamak systems. Cycles of H2 discharge deposition

cleaning

are used in tokamaks

and titanium

i.e., being able to restore the gettering

As seen in the Gibb's Free Energy of for-

(fig. 1), metals which

are known to be good oxygen getters form oxides that are stable up to temperatu;? melting

points ofmostmaterials.

sible getter material

tium into the walls and adsorption

tion properties

ium surfaces

DE-AM03-76SF-00010,

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

B.V

appears

Mod.

017.

and measures

the

These oxides by hy-

of finding a reverto be remote.

This study focuses on the chemistry

oxygen under different

by USDOE Contract

exceeding

are also stable with respect to reduction

loclean vacuum and to reduce high Z impurities. 12 The uptake of impurity gases as well as tri-

'This work supported

capaci-

(AGO) data plotted as a function of temper-

ature for various metals

a

of gases onto

feature

of such a process,

drogen, so that the possiblity

to maintain

on the sur-

desirable

ty of the coating by heating and pumping on the system.

surface chemistry

Another

would be the reversibility

re-

due to thermal desorp-

by ions impacting

face from the plasma.

mation

be-

reactor is one in which there is trap-

lease of high Z impurities

shows that there are significant tween the practical

The ideal surface for the first wall of

ping of all gases except hydrogen and minimal

gases on the surfaces 2-9 of Ti foils have been performed, but this work differences

Other effects

the wall coating which results

a tokamak

of Ti

effects.

into the

and flaking of

As

in the to-

Several

plasma with deleterious

plasma.

(UHV)

in reactor design,

of gas uptake are embrittlement

in

torr range,

to study the interaction

surfaces with background

by

is of importance

since these species can be released

of titan-

their ability to adsorb

circumstances.

The adsorp-

of pure titanium polycrystalline

J.R. Elliott et al. / The surface chemistry of titaruutn gettering

sorption

studies were all Matheson

used without

atures were measured

-40

research grade

further purification.

Sample temper-

by Pt-PtlO%Rh

that were spot welded

thermocouples

to the back of the samples.

Initial studies of Ti surfaces were done on a foil of .125 mm thickness

-80

foil was cleaned

and 99.99% purity.

The

in vacuum by Ar ion bombardment

= 2x10-5 torr at energies

up to 3 keV. at 'Ar The samples were heatgd resistively by passing

-120

currents

of up to 20 A through them which allowed

-160

sample temperatures

-200

304 stainless

up to 75OOC.

In the experiments

with titanium

deposited

on

steel, the substrate was prepared

by electropolishing in a solution of Cr03 in ace14 tic anhydride, washing with water and rinsing

-240

in methanol system.

-280

I

I

I

remove atmospheric

1200

1800

Ti was evaporated

I

600

0

before being installed

Sputtering

in the UHV

with 1 keV Art was used to gases adosrbed

on the surface.

onto the substrate

by resistive

heating of a filament made from an alloy of 75%

Temperature FIGURE 1 Gibb's free energy vs. temperature oxides

(K> for various

Ta and 25% Ti supplied

by Uion Carbide.

thickness was measured

by a quartz crystal

ness monitor

at the same distance

tions believed were modeled stainless

and compared

to be present

steel substrates.

which

of Ti on 304

The adsorption

of

oxygen on Ti surfaces was studied as a function of the temperature

of the foil or film and the

presence of other species adsorbed

in the gas phase or

on the surface.

3. RESULTS AND DISCUSSION Initial Auger spectra of the titanium fore any cleaning tamination

by Ar ion sputtering the sample at 75O'C.

formed in a standard pressure out.

at 3 keV for 3 hcurs with After high temperature

here were per-

Varian UHV system with a gas -10 torr range after bake-

Auger spectra were collected

analyzer

with a Physi-

model lo-2346 cylinderical

mirror

(CMA) using a 3 keV incident electron

beam energy and a modulation V peak to peak on the CMA.

diffused

sput-

to the surface

from the bulk of the foil and were removed

presented

in the low 10

cal Electronics

which was removed

tering, Auger spectra showed traces of S, P and

PROCEDURE

The surface analyses

foil be-

showed carbon and oxygen con-

from the atmosphere

Cl, which had apparently 2. EXPERIMENTAL

rates

range.

to the condi-

in Macrotor,

by vacuum deposition

thick-

from the fila-

ment as the sazple, with typical deposition in the 1 to 6 A per minute

foils were determined

Film

amplitude

of 2 to 4

Gases for surface ad-

milder

sputtering

by

(1 keV for 15 min. with no an-

After these two sputter cleanings,

nealing). therewereno

impurities

detected

by AES.

This

surface was stable to oxygen and carbon contamination from the ambient

gases in the UHV chamber

for several hours.

Surface concentrations

estimated

to the following

accordinq

were

equation

1153

bation,

where C is the surface x, H is the absolute sensitivity

is averaged electrons

concentration

of species

peak height and S is the

The surface concentration

The clean, ion bombarded

reached saturation to 30 Langmuir

coverage

of oxygen

sec.), which resulted tration of 30%. resulted

in a surface oxygen concenof oxygen at 200°C

it was possible

approximately

at room temperature

k

(b)

0.4,+-

lil

to

the adsorb-

120 fi of Ti at

oxygen

produced

co

steel

a surface

introduced

into the

This surface was then heated to 375'C to

determine

if there was any oxygen desorption

the surface or dissolution measured

spectra of the film before and after heating give

into the bulk of the foil.4

which readily adsorbed

by a decrease

Auger signal.

bon was present tration),

in the intensity

of the

change was noted in

A high concentration

to render the film com-

to 02 adsorption.

'

Ti

FIGURE 2 Adsorption of Oxygen by a Ti Film (a) before and (b) after heating

However,

it was

The subsequent

exposure

not show any increase centration,

of the film to 5L O2 did

in the surface oxygen con-

indicating

that the surface carbon

noted that the presence of carbon limited the

and oxygen contamination

saturation

of oxygen that could ad-

with respect to further O2 adsorption.

After this experiment,

bon originates

concentration

sorb on the Ti surface. the substrate sputtering

surface was recovered

constitutes

passivated

a thin film of 90 A of Ti was

also at 6 !/min without

incorporation

the surface The car-

from the steel substrate where it

0.08% by weight.

Since both carbon and oxygen form binary compounds with Ti, a co-adsorption

0

Subsequently, deposited,

by Art ion

at 2.5 kV for 3 hours with PAr = 1.5 x

10-5 torr.

670' 5L02

of car-

in the film (20% surface concen-

but not enough

pletely passive

from

into the Ti film as

No significant

the Auger spectrum.

Auger

the results shown in figure 2b.

remove the adsorbed oxygen by heating

Evaporationoof

in 35

heated to 67O'C.

of the surface with about

the rate of 6 A/min onto a clean stainless

system.

onto the steel substrate

min. and was immediately

at 25'C on exposure

In both cases,

to diffuse

substrate

Ti was deposited

A 94 ! thin film of

surface

the foil to over 400°C, which enabled ed oxygen

of a

in which the Ti film was heated to oxygen.

(1 Langmuir = loo6 torr

Adsorption

in saturation

15% oxygen. completely

C,

over the escape depth of the Auger

and does not imply a percentage

monolayer.

next experiment, before exposure

of the peak to peak Auger signal as

given in ref. 15.

OXY-

gen adsorptionwasfurtherillustrated in the

I1

L

The poisoning effect of carbon for

experiment

performed with CO and O2 to elucidate tition for available

was

the compe-

Ti atoms by both adsorbates.

of carbon into the film as shown in figure 2a.

The substrate

This film was able to adsorb more oxygen at sat-

layer of Ti evaporated

uration due to the smaller

face was then exposed to a total of 5L CO and

initial carbon concen-

for these adsorptions

was a 70 "A

onto the steel.

The sur-

2OL O2 as shown in fig. 3.

the molecules

that hit the surface stick and are

dissociated

to form binary Ti compounds. At -8 298'K and PC0 = 2x10 torr, roughly a monolayer

of CO can adsorb onto a Ti surface if the sticking

probability

in 200 sec.,

is unity.

The exper-

iment is shown in figure 4, where the deposition rate of Ti is 1.2 !/min

(one monolayer

in 2-3

min., or about equal to the CO collision

rate).

Chemisorption

to the

is seen in this experiment

extent that only 4% of the surface concentration of oxygen can be adsorbed surface. observed

The remainder

and removed

from the

of the carbon and oxygen

in the Auger spectra was most probably

as mixed oxide and carbide of Ti. Ti

5L o*

13L

151

3OL

FIGURE 3 Adsorption of Oxygen by a Titanium =2x10torr at pco

500°C

film deposited

The surface was heated at the end of the exposures to determine

if there was significant

of one of the adsorbates

migration

away from the surface.

As the sticking coefficient of CO is close to 8,16 unity , a 5L exposure to CO should passivate It it is bound on the surface with-

the surface.

out dissociation

and diffusion

This data indicates sufficient

that a monolayer

to saturate

can still be adsorbed film.

Again,

of CO is not

by active Ti sites on the

heating allows carbon to accumulate from origins

This experiment

indicates

just the surface atoms contribute

in the sub-

that more than to the getter-

ing ability of the Ti films. A measurement gettering

collision

of the rate of Ti removal from

can be obtained

the substrate

by evaporating

Ti onto

at a rate that is less than the

rate of a reactive gas such as CO.

calculation

Ti lLC0

5L 5L02 1OL

the film and that oxygen

in the film, presumably strate.

into the film.

A

of the rate of reaction of surface

Ti atoms with an active gas species the kinetic theory of gases assuming

is given by that all of

FIGURE 4 Coadsorption of Oxygen and CO on a Titanium film. Oxygen is adsorbed when the surface has a monolayer coverage of CO.

An important question with respect to the tokamak operation titanium

is what effect hydrogen

deposition.

where Ti was deposited hydrogen of 10S6 torr.

An experiment

has on

was performed

at a partial pressure

of

Under these conditions,

the rate of Ti deposition

is much less than the

rate of hydrogen

striking

the surface, which

should encourage

the formation

dride.

of titanium hy-

There was no effect of the hydrogen on

1155

J.R. Elliott et al. j The surface chemistry of titanium gettering

the oxygen adsorption ability of the Ti film,

REFERENCES

4. CONCLUSION

1. L. Keller et al, Symposium on Energy Removal and Particle Control in Toroidal Fusion Devices, Princeton, July 26-29, 1983.

We have shown that stainless steel substrates

2. H.E. Bishop, J.C. Riviere and J.P. Coad, Surface Sci. 24 (1971) 1.

for Ti deposition are not inert as compared to pure Ti, and can have a deleterious the gettering

The carbon dissolved higher chemical

in stainless

potential

Ti film.

steel is at a

than when combined

with Ti, thus the reaction ed.

effect on

capacity of an evaporated

forming TiC is favor-

steel, may diffuse

to

We have also shown that carbon originating

gettering oxygen.

8. E. Bertel, R. Stockbauer and T.E. Madey, J. Vat. Sci. Technol. Al (1983) 1075.

the

ability of the Ti film with respect to Gases containing

Solid State Cotinn.10 (1972) 933.

7. J.S. Solomon and W.L. Baun, Surface Sci. 51 (1975) 228.

sorption.

gases in the system effects

Sur-

6. T. Smith, Surface Sci. 38 (1973) 292.

the Ti film surface and poison it to oxygen ad-

from background

Bignolas, M. Bujor and J. Bardolle, 4. J.B. face Sci. 108 (1981) L453. 5. D.E. Eastman,

Carbon, which is present at a concentration

of .08% in 304 stainless

3. N.R. Armstrong and R.K. Quinn, Surface Sci. 67 (1977) 451.

carbon can passivate

9. P.H. Dawson, Surface Sci. 65 (1977) 41.

In the case of CO, this is observed -8 torr. These exwith a partial pressure of 10

10. L. Oren and R.J. Taylor, Nucl. Fusion 176 (1977) 1143.

periments

11. L. Keller et al., J. Nucl. Mater. 493.

Ti surfaces.

duction,

demonstrate

that TiO is stable to re-

that TiC is stable to further oxidation,

and that these species are passive on the surface. For Ti to be used as a getter, continuously dissolving

to

12. H.F. Dylla, 4th Intl. Conf. on Plasma Surface Interactions in Controlled Fusion Devices (1980).

provide a fresh surface, either by

carbon and oxygen

heating or by continuously substrate

it is necessary

111 (1982)

surfaces.

intopurebulk

evaporating

Ti by

13. C.H.P. Lupis, Chemical Thermodynamics erials (Elsevier, 1983).

of Mat-

Ti onto 14. R.O. Adams, J. Vat. Sci. Technol. Al (1983)lZ. 15. L.E. Davis et al., Handbook of Auger Electron Spectroscopy (Physical Electronics Industries, 1976). 16. S.R. Morrison, The Chemical Physics of Surfaces (Plenum Press, 1977) pg. 259.