AN ATOM-PROBE STUDY OF MOLYBDENUM-CARBON REACTION

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AN ATOM-PROBE STUDY OF

MOLYBDENUM-CARBON REACTION

Y. Ishikawa, K. Takahashi, T. Yoshimura

To cite this version:

Y. Ishikawa, K. Takahashi, T. Yoshimura. AN ATOM-PROBE STUDY OF MOLYBDENUM- CARBON REACTION. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-365-C2-370.

�10.1051/jphyscol:1986256�. �jpa-00225690�

JOURNAL DE PHYSIQUE

Colloque C 2 , supplBrnent au n03, Tome 47, m a r s 1986 page cz-365

AN ATOM-PROBE STUDY OF MOLYBDENUM-CARBON REACTION

Y. ISHIKAWA, K. TAKAHASHI and T. YOSHIMURA

M e c h a n i c a l E n g i n e e r i n g R e s e a r c h L a b o r a t o r y , H i t a c h i Ltd..

K a n d a t s u , Tsuchiura-shi 300, J a p a n

Abstract

-

^An energy focusing TOF atom-probe was constructed, which is capable of detecting 50 ions per a t m i c layer a t intermediate evaporatian voltage. The mass resolutionAm/m is 11800 a t half peak height for the MO

(110) plane and each of the detected MO ions is c l e a r l y discriminated corresponding t o each MO isotope.

This atom probe was utilized t o study the reaction between molybdenum and carbon. Carbon was deposited onto a ^MO t i p and t h e t i p was heated up t o 8 0 0 ' ~ i n various atmospheres. Migration and precipitation of carbon into the MO (110) plane were monitored by the FIM and analyzed a t h i c layer by layer by the atom probe. Carbon atms were found t o migrate m the clean MO (110) plane by heating a t 6 0 0 ' ~ i n a vacuum of 10-7 Pa and t o precipitate into MO.

This migration was restrained by the presence of nitrogen or oxygen a t a pressure of 10-4 Pa a t 600'~. An atmosphere of nitrogen appears t o exhibit more pronounced e f f e c t on preventing carbon migration than oxygen.

I

-

INTRODUCTION

The main consideration in selecting m t e r i a l s for use in an ultrahigh vacuum system is that they o f f e r the desired degree of vacuum i n those system. This a b i l i t y is primarily governed by t h e vapor pressure and outgassing characteristics of the materials t o be used.

Molybdenm is used mainly as b o l t s and nuts, heating c o i l s and radiation shields in UHV equipments because of its hard and heat r e s i s t a n t nature in addition t o its vacuum related properties. The outgassing r a t e of molybdenum can be greatly reduced by baking a t elevated temperatures. Though ccmplete outgassing of molybdenum can be achieved above 1 6 7 0 ' ~ / l / , r e c r y s t a l l i z a t i o n usually l i m i t s its baking temperature to 900°C a t maximum. This leaves a considerable r a t e of outgassing t o molybdenum p a r t s i n p r a c t i c a l use above 500°C. Primary outgassing canponents are CO /2/.

In UHV thin film process carbon contamination is a g r e a t problem. Carbon containing gases such a s CO,

q ,

C&, o r C2Hq a r e a source of carbon that can be dissolved in the bulk or stay on the surface and may degrade the quality of thin film grown.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986256

C 2 - 3 6 6 JOURNAL DE PHYSIQUE

This led us to initiate a study of the reaction between C and MO in order to find a means to get rid of C contamination from MO surface.

The TOF atom-probe is considered as a most powerful tool in providing ccmpositional depth profiles of C segregation m MO.

With this in mind, we have undertaken an atom-probe study of the response of carbon contaminated MO surfaces to heat treatment in various atmospheres.

I1

-

EXPERIMENTAL METHODS

The schematic drawing of the energy-focusing atom-probe is shorn in Fig.1. The instrunent consists of three parts, a storagelpreparation chamber, an FIM chamber and a TOF mass spectrometer in the Poschenrieder configuration /3/.

REACTION GAS ^IMAGING GAS

Fig.1 The schematic drawing of the energy-focusing TOF atom-probe

Specimens are loaded via a fast-entry airlock into the storage/preparation chamber here up to five specimens can be kept and where various treatuents such as heating, deposition, or gas adsorption can be carried out. A specimen can then be transferred to the FIM chamber facilities. The FIM chamber consists of a gimbal assembly

supporting a specimen and chevron-screen assembly which can be screw-driven to vary the magnification of an FIM image and an area of the tip to be analyzed by the atom- probe. The minimum and maximum distances betmen the tip and the screen are 50 and 150 mn, respectively, giving an acceptance angle of from 3.8Oto 1.4'. A specimen tip and thermocouple wires are spot welded to a U-shaped loop of a 0.3 w dim. MO wire. The Loop and the thermocouple wires are inserted into the four Ni coils held in a sapphire rod. The gimbal assembly is similar to the m e described by Nishikawa et a1 141. The total flight path is 2.5 m long and the deflected space is 200 an*.

Either high voltage pulses of a few nanosecond width or nanosecond laser pulses, can be applied to the specimen. High voltage pulses are supplied by a pulser with a two stage mercury-wetted reed relay 151.

Pulsing and data acquisition are fully computer controlled. The mass spectrun can be displayed in real time.

A vacuum of 10-8 Pa is routinely obtained for the FIM and the TOF mass spectrometer chamber and 10-7 Pa for the storage/preparation chamber. The large acceptance angle gives the detection of approximately 50 ions per atomic layer at intermediate evaporation voltage for the MO (110) plane and the mass resolutiondm/m was 11800 at half peak height. Each of the detected MO ions was clearly discriminated

MO ISOTOPES

31 32 33 34

MASS-TO-CHARGE RATIO m/n

Fig.2 An atom-probe mass spectrum of detected Mo3" ions.

corresponding t o each MO isotope as shown i n Fig.2.

The specimen MO t i p s were prepared from a cold drawn MO wire of 0.3 mn d i m . mile polishing t h e wire to make a t i p , a g r a p h i t e lubricant used f o r wire drawing was l e f t a t the t i p shank a s a carbon source. The specimen could be heated r e s i s t i v e l y and i t s temperature was measured by a c h r c m e l - a l e 1 thermcouple which was spot welded t o the t i p shank. The asetched specimens were heated a t 600°C f o r 60 min in vacuum (10-7 Pa) t o contaminate the t i p apex with carbon migrated from t h e t i p shank. The t i p was then f i e l d evaporated by superimposing pulse voltage m dc voltage a t a r a t i o of 1 t o 3 a t 4 0 ' ~ to take an atan-probe mass spectrum. Then the t i p s were heated t o 600 and 800°C f o r 60 min i n vacuum (10-7 Pa), oxygen o r nitrogen (both a t 10-4 Pa).

111

-

RESULTS AND DISCUSSION

Figure 3 is an example of an atan-probe spectrum obtained from a MO surface

contaminated with carbon a t 6 0 0 ' ~ f o r 60 min i n vacuum (10-7 Pa). The main peaks can be i d e n t i f i e d as ~ 2 + , C', M04+, b3+, &+and MO helides. The concentration of C and MO for the above data is shown as a ladder diagram i n Fig.4. The number of C ions versus t h a t of MO ions detected is plotted i n the order of detection. The f i r s t layer was enriched with C and the next ten layers showed a rather constant composition of Mo3C2 and then a composition of MozC, an only s t a b l e MO carbide phase a t roan temperature, appeared.

Continuous f i e l d evaporation of these C contaminated layers produced a clean MO (110) surface surrounded by a random s t r u c t u r e l e s s image of unreconstructed MO-C surface a s seen in Fig.5. An atom-probe analysis of the bright spot area present in the

surrounding surface yielded a MO-C concentration p r o f i l e similar t o Fig.4. ^{f i e C} concentration i n the MO-C layers decreased gradually with depth.

CUMULATIVE NUMBER OF C IONS DETECTED

S g.. g a.. cn

0

NUMBER OF DETECTED IONS

Fig.5 An FIM image of the clean Fig.7 An FIM image of the clean Mo (110) plane surrounded by the MO (110) surface contaminated unreconstructed MO-C surface with carbon at-600 "C for 1 h.

CUMULATIVE NUMBER OF MO

IONS

DETECTED a

g

^150.-

3

I

=

^100.-

U

e

50.-

5

S

Fig.6 The effect of heating atmosphere on C and MO concentration profiles.

IN VACUUM

IN OXYGEN IN NITROGEN

The e f f e c t of atmospheres during heating a t 6 0 0 ' ~ on the C contamination of the clean MO (110) surface is shown i n Fig.6. In vacum carbon readily migrated and covered the (110) surface, showing a s t r u c t u r e l e s s image of Fig.7. The depth of the

s t r u c t u r e l e s s layers is estimated t o be about 10 a t a n i c layers. An hexagonal close- packed Mo2C s t r u c t u r e is known as an only s t a b l e molybdenum carbide existing a t room temperature and t o p r e c i p i t a t e with a good l a t t i c e matching with bcc MO a t the Mo2C -MO interface 161. However no ordered s t r u c t u r e was observed on the MO (110) plane during the FIM observation. The atan- robe spectrum showed that ~ 2 t a n d ~ o 3 f were

S

m j o r species detected, followed by MO

+,

b 4 + and

cf.

3 0 - F < : I

o 0 50 100 150 200 250 300 350 400 450

In oxygen the migration of C over the (110) plane occurred, though the penetration depth of C i n t o the MO (110) lane appeared t o be small. During the atom-probe

analysis a large nmber of Mof+ and ions were detected in addition t o ~ o F i o n s . Above 500°c, oxygen is chemisorbed and oxides a r e formed on MO surfaces 171. These chemisorbed oxygen and oxides may a c t as a barrier for surface migration of C atan t o sane extent. However the reaction of C with oxygen may reduce the chemisorbed oxygen and oxides and carbon migrates into the MO (110) plane.

JOURNAL DE PHYSIQUE

Nitrogen was more effective in preventing the C migration and only a few C ions were detected. In this case the atom-probe s ectrum showed the presence of a small number of MO nitride ions such as M O N ~ ~ and MO&+ in addition to two major species, ~ 0 2 t and

MOH

ions. It has been known /8/ that nitrides are formed on MO above 400'~ and stable up to 700°C. These nitrides appeared to act as an effective barrier to the surface migration of carbon to the MO (110) plane.

A completely carbon covered MO (110) surface was heated to 800'~ for 60 min in vacuum.

During the heating H2, CO and C02 were evolved. Contrast to the heating at 600°c, the atom-probe analysis showed the effective removal of MO-C layers. This effect may be attributed to the reaction of carbon with the adsorbed oxygen and release of its gaseous products.

IV

-

CONCLUSION

A high performance atom-probe was constructed and used to study the reaction between MO and C. Although the present work is preliminary and the explanation is tentative, it is certain that carbon is migrated to the clean MO (110) plane and form an

awrphous MO-C layer. Migration of C and formation of a MO-C layer on the MO (110) plane was observed at 600aC. Oxygen appears to suppress the C migration because the reaction of C with MO oxide and oxygen redxes the number of migrating C atom.

Nitrogen is more effective for impeding the C migration because MO nitrides serve as a barrier for C migration.

Acknowledgement

The authors wish to express their thanks to Professor 0. Nishikawa for his valuable advices during construction and operation of the atom-probe. Thanks are also due to Dr. E. Nomura for his designing and building the electronics system of this atan-probe.

REFERENCES

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52 (1981) 810,

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2

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