HIGH TEMPERATURE SEQUENCE IMAGES OF FIELD ION MICROSCOPY

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HIGH TEMPERATURE SEQUENCE IMAGES OF FIELD ION MICROSCOPY

M. Doyama, K. Ishimoto, T. Nishida, M. Obara, Y. Suzuki, S. Tanigawa

To cite this version:

M. Doyama, K. Ishimoto, T. Nishida, M. Obara, Y. Suzuki, et al.. HIGH TEMPERATURE SE-

QUENCE IMAGES OF FIELD ION MICROSCOPY. Journal de Physique Colloques, 1987, 48 (C6),

pp.C6-53-C6-58. �10.1051/jphyscol:1987609�. �jpa-00226812�

JOURNAL DE PHYSIQUE

Colloque C6, supplCment au nO1l, Tome 48, novembre 1987

HIGH TEMPERATURE SEQUENCE IMAGES.OF FIELD ION MICROSCOPY

M. ~ o ~ a m a * , K. Ishirnoto, T. ~ishida*: M. 0bara**: Y. Suzuki and S. ~anigawa' Department of Metallurgy and Materials Science, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyoku, Tokyo 113 Japan

ABSTRACT

Atomist i c sequence images of high temperature f i e l d ion microscopy are presented f o r copper and aluminum specimens. A possible mechanism i s proposed. The images are formed by the specimen s e l f atoms. These s e l f atoms are ionized when they c l imb up the atomic steps on the surface.

1. I ntroduction

To obtain f i e l d ion microscopic images of the atoms on the ssurface OF metals, the specimen i s cooled t o ² temperature be1 ow l i qu i d n i trogen temperataure.

lshimoto e t a l l ) have found that atomistic images can he obtained even when the the t i p of the specimen i s heated t o a high temperterature.

Later Polanshutz and Krautzz) obtained simi l a r results. The image a t high temperatures i s dynamic and changes slowly as the time proceeds. I n t h i s paper, the dynam i cal behavior o f the high temperature images i s reported.

2. Experimental Procedures

The f i e l d ion microscope used i n t h i s experiment i s s i m i l a r t o the one reported i n the previous paper'). A channel p l a t e mu1 t i p l i e r was used t o obtain the images. The images are photographed by a camera or recorded by a TV camera and video tape recorder.

Present Address:

Department o f l ron and Steel Engineering, Faculty of Engineering, Nagoya University, Furocho, Chi kusaku, Nagoya 464 JAPAN

** Hitachi Central Research Laboratory, Kokubunjishi, Tokyo 185 JAPAN

***

,

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987609

C6-54 J O U R N A L D E PHYSIQUE

To obtain the atomistic images a t high temperatures, the f o l lowing procedure was normally used.

1) A metal l ic specimen t i p was mounted i n the f i e l d ion microscope.

2) The chamber was baked and evacuated t o the order of 10-9 Torr.

3) The specimen was f i e l d evaporated a t a voltage s l i g h t l y higher than the usual best imaging voltage.

4) The temperature o f the t i p was gradually increased by passing a current through a heating loop whi l e a' voltage s l i g h t l y lower than the usual best i mag i ng vo I tage was app l i ed

.

5) The specimen t i p was heated t o a temperature between Tm/2 and Tm/3, where Tm i s the melting temperature of the specimen in'Kelvin while a high voltage was applied.

6) After obtaining the usual f i e l d ion images a t l iguid nitrogen temperature, the temperature of the specimen was raised. The images were blurred, weakened and out of focused a t medium temperatures above l iqu i d nitrogen temperature.

7) Imaging gas was not essential t o obtain atomistic images a t high tempratures.

3. Resu l ts

Figures I (a)-(f) and Figs 2 (a)-(f) are the sequence of the f i e l d ion micrographs of a copper (99.99% pure) specimen heated a t about 500°C. Figures 3 (a)-(f) are the sequence of the f i e l d ion micrographs of an aluminum specimen a t about 400QC.The time interval of the each picture was a few seconds. The pattern changes slowly. For exampleat the r i g h t belowof Figs. 1 a n d 2 c i r c l e s a r e observed. These c i r c l e s shrink from time t o time.The inner c i r c l e shrinks faster than .the outer circles. The rate of shrinkage was larger when the temperature of the t i p was higher o r when the c i r c l e s are smal ler. The r a t e i s larger when the applied voltage i s higher. Figs 1 and 2 i s f o r the same specimen.

The characteristic features of the atomistic images a t high temperattrres are as follows:

1) Rings corresponding t o low index planes are observed.

2) These low index planes are connected by dark zone l i nes f o r high temperature f i e l d ion microscopy, instead of bright zone lines i n normal f i e l d ion micro- scopy.

3) The atomistic images i n the region surrounded by dark zone lines or higher index planes are brighter.

4) The numer of rings between two low index planes i s much less and the number of imaging spots on a c i r c l e i s much less than those f o r normal f i e l d ion microscopy. This implyes that the magnification i s about ten times larger than that f o r usual f i e l d ion microscopy. The higher magnification was obtained for ametal having the lower melting point. Themagnificationfor aluminumwas higher than that f o r tungsten.

5) The atomistic image spots appear t o be vibrating.

6) The motion of some atomistic images looks l i k e that due t o the thermal a c t i - vat i on process.

7) Some spots twinkle l i ke stars. Some bright spots gush from a certain part of the region, mostly higher index planes, and move t o a low index plane.

8) The atomistic image pattern changes slowly.

9) The atomistic images lasted f o r as long as several minutes a t about 1000°C for tungsten and more than one hour f o r copper and aluminum, when the tempera- ture and voltage were kept i n an appropriate range.

10) Superposed images are of ten observed.

Figure 1. (a)-(f) Sequence of high temper a t about 500°C.

(f)

-ature f i e l d ion micrographs f o r copper

C6-56 JOURNAL DE _PHYSIQUE

Figure 2. (a)-(f) Sequence o f h i g h temperature f i e l d ion micrographs f o r copper a t about 500°C. The specimen i s the same o f Fig. 1.

Figure 3. (a)-(f) Sequence of high temperature f i e l d ion micrographs f o r aluminum.

C6-5 8 JOURNAL DE PHYSIQUE

4. D i scuss i ons

1) The atomistic images a t high temperatures are not formed by imaging gas such as he1 ium but specimen s e l f atom ions. The he1 ium gas ions are too l ight t o form a clear atomistic images a t high temperatures. These are not made by the residual gas, because the residual gas pressure changes from 10-8 t o 10-5 Torr but the intensity was not changed. The residual momenta of imaging atoms transverse t o the f l ight d i r e c t i o n b l u r the imaging spots. No appreciable difference i n the brightness of the atomic images f o r copper and gold was found.

2) This i s d i f f e r e n t from the images made by normal f i e l d evaporation4). The patterns do not change so rapidly as normal f i e l d evaporation microscopy.

3) Magnification i s much larger than that f o r normal f i e l d ion microscopy. The small radius of curvature a t the t i p i s expected.

Fo l low i ng poss i bt e mechanism i s proposed. Specimen sei f ions d i f f use from shank to the t i p due t o the high e l e c t r i c f i e l d gradient and high temperature.

Whi I e the s e l f ions c l imb up an edge o f low index planes (steps), these ions are easi 1 y ionized, because the d i f f u s i ng atoms have larger number of bonds w i t h atoms on the surface compared with the climbing atoms. These s e l f ions form the atomistic images. The c i r c l a s images due t o the surface steps shrink from time t o time. The atomistic images l a s t hours. This could be used as a sharp ion soirrce or electron source.

5. Summary

High temperature f i e l d ion images are obtained without imaging gas when a channel p l ate i s used and the app l i ed vo l tage was slow 1 y i ncreased. This image i s f a i l y stable. The image changes slowly and l a s t hours i f the applied voltage i s appropriate.

References

I ) K. Ishimoto, H. K. Park, T. Nishida and M. Doyama: Surface Sci.,41 (1974) 10

2) V. Pol anschuts and E. Krautz: Z. Metal I kde, 65 (1974) 623.

3) M.Doyama,T.Nishida,M.Obara and S.Tanigawa: jap.J.Appl.Phys.,l7 (1978) 811.

4) R. J. Walko and E. W. Muller: Phys. Status s o l i d i (a), 9 (1972) Kg.