IN SITU SCANNING ELECTRON MICROSCOPY OF THE MORPHOLOGICAL EVOLUTION OF METAL TIPS

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IN SITU SCANNING ELECTRON MICROSCOPY OF THE MORPHOLOGICAL EVOLUTION OF METAL

TIPS

M. Drechsler, S. Ramdani

To cite this version:

M. Drechsler, S. Ramdani. IN SITU SCANNING ELECTRON MICROSCOPY OF THE MOR- PHOLOGICAL EVOLUTION OF METAL TIPS. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-171-C7-175. �10.1051/jphyscol:1986731�. �jpa-00225924�

IN SITU SCANNING ELECTRON MICROSCOPY OF THE MORPHOLOGICAL EVOLUTION OF METAL TIPS

M. DRECHSLER a n d S. RAMDANI

CRMC2-CNRS, U n i v e r s i t . 6 d ' A i x - M a r s e i l l e 11, Campus d e Luminy, C a s e 9 1 3 , F-13288 M a r s e i l l e Cedex 0 9 , F r a n c e

R6sum6 - On d 6 c r i t u n e t e c h n i q u e p o u r G t u d i e r i n s i t u 1 ' G v o l u t i o n m o r p h o l o g i q u e d e s p o i n t e s c r i s t a l l i n e s j u s q u ' s 1500 K p a r m i c r o s c o - p i e G l e c t r o n i q u e 2 b a l a y a g e e t v i d e o . La t e c h n i q u e e s t u t i l i s e e p o u r d e s m e s u r e s p r e l i m i n a i r e s d e l ' a u t o d i f f u s i o n d e s u r f a c e d e l ' o r p a r 1'Gmoussement d e s p o i n t e s .

A b s t r a c t - A t e c h n i q u e i s d e s c r i b e d t o s t u d y i n - s i t u t h e m o r p h o l o g i - c a l e v o l u t i o n o f t i p c r y s t a l s up t o 1500 K b y s c a n n i n g e l e c t r o n mi- c r o s c o p y a n d v i d e o . The t e c h n i q u e i s u s e d f o r p r e l i m i n a r y m e a s u r e - m e n t s o f t h e s u r f a c e s e l f - d i f f u s i o n o f g o l d b y t i p b l u n t i n g .

1. INTRODUCTION

M e t a l s t i p s a s u s e d i n f i e l d e m i s s i o n a r e p a r t i c u l a r s u i t e d t o s t u d y m o r p h o l o g i c a l e v o l u t i o n s f o r e x a m p l e t h o s e by a c a p i l l a r i t y i n d u c e d s u r f a c e s e l f - d i f f u s i o n m a t t e r t r a n s p o r t . A m e a s u r e m e n t o f t h e t i p r a - d i u s i n c r e a s e o f a n a n n e a l e d t i p by s c a n n i n g e l e c t r o n m i c r o s c o p ' ~ . ~ ( S E M ) /1//2//3//4// o r by m e a s u r i n g f i e l d e l e c t r o n e m i s s i o n c h a r a c t e r i s t i c s /5//6// e n a b l e s a d e t e r m i n a t i o n o f s u r f a c e s e l f - d i f f u s i o n c o e f f i c i e n t s

(D ) . The c h a n g e o f t h e m o r p h o l o g y of a t i p h a s b e e n v i s u a l i z e d s o f a r onTy a t room t e m p e r a t u r e , t h a t i s a f t e r a n i n t e r r u p t i o n o f t h e a n n e a - l i n g a n d o f t e n o n l y a f t e r a t r a n s p o r t o f t h e t i p f r o m a n u l t r a h i g h vacuum s y s t e m t o t h e SEM. Our a i m was t o i n t r o d u c e a n e x p e r i m e ! , c a l t e c h n i q u e t o v i s u a l i z e C i r e c t l y t h e m o r p h o l o g i c a l e v o l u t i o n o f h e a t e d t i p s . S u c h a t e c h n i q u e may e n a b l e : ( 1 ) On l o n g t e r m s i m p l i f i e d a n d more p r e c i s e m e a s u r e m e n t s o f D , ^{( 2 ) A} d i r e c t v i s u a l i z a t i o n a n d s t u d y o f o t h e r t y p e o f m o r p h o l o g i c a l e v 8 l u t i o n s ( a s t h e c r y s t a l g r a i n r o t a t i o n d e s c r i b e d i n t h i s volume / 7 / ) . ( 3 ) The u s e o f a v i d e o t e c h n i q u e t o re- g i s t e r m o r p h o l o g i c a l e v o l u t i o n s . I n t h i s p a p e r we d e s c r i b e t h e t e c h n i - q u e we u s e d s o f a r a s w e l l a s p r e l i m i n a r y m e a s u r e m e n t s o f t h e s u r f a c e s e l f - d i f f u s i o n o f g o l d .

2 . EXPERIMENTS

The s c a n n i n g e l e c t r o n m i c r o s c o p e u s e d is a J e o l 35C. F i r s t l y i t was n e c e s s a r y t o d e t e r m i n e t h e t e m p e r a t u r e u p t o w h i c h t h e h e a t e d c r y s t a l i s d i r e c t l y v i s u a l i z e d b y t h e SEM. E x p e r i m e n t s w i t h a t u n g s t e n l o o p showed t h a t a r e a s o n a b l y good i m a g e was o b t a i n e d u p t o ^1- 1500 K ( c o n - s i d e r a b l y h i g h e r t h a n i n d i c a t e d by J e o l ) .

The f i r s t t i p m a t e r i a l u s e d was Zn. B u t we d i t n o t s u c c e e d t o a v o i d a s l i g h t h i g h t e m p e r a t u r e c o r r o s i o n o f t h e Zn.

Then g o l d was u s e d w h i c h was f o u n d t o b e h a v e c o r r o s i o n f r e e . C o n i c a l g o l d t i p s w e r e p r e p a r e d f r o m a 0 . 1 2 5 mrn d i a m e t e r ( 9 9 . 9 9 % ) g o l d w i r e by e l e c t r o l y t i c e t c h i n g i n HCL ( 5 0 % ) + HN03(50 O). S u c h a t i p i s

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986731

C7-172 JOURNAL DE PHYSIQUE

welded on a tungsten heating loop (fig. 1). The temperature of the loop is determined by a resistivity measurement. At the beginning, we had difficulties to determine correct temperatures. Therefore tem- peratures were controlled by a micro-pyrometer. Moreover by micro-

1 - Gold tip at 1200 K spotwelded on a tungs- ten loop. The diameter of the (bright) tungsten wire is 0.1 mm. This is an opti- cal (not SEM) micrograph of the tip taken through a vacuum window of the SEM.

photography we obtained direct images of the heated tip in the SEM (fig. 1). From such micrographs it was estimated : (1) the tempera- ture difference between the tip basis and the tip end (heated by heat conduction). It is smaller than 20' (for 1200 K), (2) the tip end heating by electron impact of the microscope beam, which is also less than 20°. The 1200 K tungsten surface appears in fig. 1 brigh- ter than the 1200 K gold surface because the spectral emmissivity of tungsten is about 2 times greater than that of gold.

For the video registation a simple VHS system was used (Brandt/Hita- chi). The reason to use video was at the beginning to show morpholo- gical tip evolutions in lectures (as at the 33rd IFES, Berlin, July 1986). In the course of our video experience we became more and more aware that video is first of all an important help to study morpho- logical evolutions because : (1) It is a comfortable and cheap man- ner ofpregistration. Compared to photographic registration the image quality can be controled immediatly. (2) Only by video, details of kinetic processes are registered. (3) The time is automatically re- gistered by a video chronometer. Thus time measurements become com- fortable and more precise and allow quantification of such kinetic processes.

3. SEM in situ study of a surface self-diffusion (Au)

The essential aim of this preliminary surface self-diffusion study is not as usual to obtain data for a metal (gold) which has not yet been studied by a tip evolution method. Rather, the first aim is to test the in-situ method to study such diffusion as well as the use of video for this purpose. As no ultra-high vacuum SEM was availa- ble we had to do the test with a usual SEM (Jeol 35C). On gne hand this is inconvenient because of the limited vacuum (-2.10 Torr).

On the other hand it may be interesting to begin experiments which may permit to study problems as : How vary surface phenomena as sur-

face self-diffusion and evaporation if they occur not in ultra-high vacuum but in the presence of a contamination typical for electron microscopy.

The coefficient of surface self-diffusion (D ) is determined by a tip blunting experiment using the equation 78//1//2//3//4/ :

a metal tip of 10' by capilla- rity induced surface self-dif- fusion matter fluxes (using results of /8/ and / 9 / ) .

Fig. 2b - Photographic super- position of 4 video SEM phases of the morphological evolution of a gold tip. T = 1200 K; t =

0 ; = 17 min ; t3 = 55 min 1 ;

t4 =t?67 min.

Measured are the temperature T. The time intervals (t -t ) and the tip radii R1 and R2 (fig. 2). Approximately known are2thk constant A /8/, the surface free energy,2yrthe atomic volume Q and v the n h e r of diffusing atoms per cm

.

4. DISCUSSION

The experimental tip shape evolutions are in sufficient agreement with the calculated evolution (fig. 2) so that equation 1 can be u- sed to determine approximate values of D . Typical data thus obtained are presented in fig. 3.

At present is to early to compare the Ds values of fig.3 in a quzctitative manner with the Ds values obtaines for gold by other

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C7-174 JOURNAL DE PHYSIQUE

methods as the sinosoidal profile decay (see /10//11/), because in all cases of such gold studies the presence and the influence of ad- sorbates is not sufficiently studied. Qualitatively it can be said that the order of magnitude of the D values of Fig. 3 agrees with those found by other methods /10//11$, but the corresponding activa- tion energy we found is considerably greater. This is not surprising because the surface layer on the gold tip is probably different from that on the former specimens. It is well known that the operation of a SEM leads to the formation of a carbon layer and there is no rea- son to assume that such a layer is not present during this tip blun- ting. In the case of tungsten it is known that a carbon layer leads to an increase of the activation energy of D by a factor three /12/.

This may be analogous to the activation enerZy increase we found (fig. 3).

Besides the normal tip evolution (fig. 2 and 3) exist sometimes lon- ger periods in which the tip blunting (surface self-diffusion) is blocked, perhaps caused by a particular close packed carbon layer.

In other periods free evaporation (formation of limit radii/i3/) seems to be blocked. More detailed experiments using the described in-situ SEM technique may perhaps enable to obtain more information on such interesting surface phenomena.

5. CONCLUSIONS

- Morphological evolutions on heated crystals (tips) can be direc- tly visualized by a scanning electron microscope for temperatures up to % 1500 K.

- Such evolutions can be registered by video. It is illustrative to see the shape changes of the micro-crystals, but first of all it is interesting to measure morphology kinetics as a function of tempe- rature and time.

- The described technique can be used to determine surface self-dif- fusion coefficients (D ) by measuring tip radius increases.Thus pre- liminary D data of go?d have been obtained. S

- Under the conditions of the SEM experiments ( Q Torr, carbon layer) the free evaporation and the surface self-diffusion of the gold are sometimes blocked.

ACKNOWLEDGEMENTS

The authors acknowledge the valuable technical assistence of Mr. S.

Nitsche and Mr. F. Quentric.

REFERENCES

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