DIRECT SUPPORT BY SPOT WELDING AND CHEMICAL ETCHING OF COMPOUND SEMICONDUCTOR WHISKER

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Submitted on 1 Jan 1987

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DIRECT SUPPORT BY SPOT WELDING AND CHEMICAL ETCHING OF COMPOUND

SEMICONDUCTOR WHISKER

T. Inoue, M. Nakada

To cite this version:

T. Inoue, M. Nakada. DIRECT SUPPORT BY SPOT WELDING AND CHEMICAL ETCHING

OF COMPOUND SEMICONDUCTOR WHISKER. Journal de Physique Colloques, 1987, 48 (C6),

pp.C6-595-C6-599. �10.1051/jphyscol:1987697�. �jpa-00226905�

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DIRECT SUPPORT BY SPOT WELDING AND CHEMICAL ETCHING OF COMPOUND SEMICONDUCTOR WHISKER

T. Inoue and M. Nakada

Faculty of Engineering, Osaka Electro-Communication University, 18-8, Hatsu-cho, Neyagawa, Osaka 572, Japan

A b s t r a c t

When e l e c t r o n and i o n beams o f angstrom o r d e r u s i n g f i n e e m i t t e r t i p s w i t h s m a l l r a d i u s can be o b t a i n e d e a s i l y , we have some m e r i t s i n t h e f i e l d o f t h e e l e c t r o n microscope w i t h h i g h r e s o l u t i o n , t h e m i c r o - f a b r i c a t i o n t e c h n o l o g y and t h e scanning t u n n e l i n g microscope. For t h i s purpose, we have been exp- erimented t h e p r o p e r t i e s o f a coapound semiconductor w h i s k e r s (Sic, CaAs and Gap) w i t h v a r i o u s a n g l e a s e m i t t e r a a t e r i a l s . T h i s t i n e , we t r i e d t o use S i c whiskers which have v e r y s m a l l d i a m e t e r ( 0 . 2 - 1 p m 4 ) . They c a n ' t be seen d i r e c t l y w i t h eye b u t can be seen o n l y w i t h t h e r e f l e c t i o n o f t h e l i g h t . These whiskers a r e P - S i c t y p e w i t h Z i n c - b l e n d e s t r u c t u r e . Mounting o f S i c whisker t o h a i r p i n l o o p i s n o t easy because o f t h e i r s m a l l diameter and t h e h i g h w e l t i n g p o i n t . I n t h i s a r t i c l e , t h e d i r e c t mounting method w i t h s p o t w e l d i n g and e l e c t r o chemical e t c h i n g t o o b t a i n f i n e t i p a r e presented.

E l e c t r o chemical' e t c h i n g o f t h e whisker was performed by u s i n g e t c h i n g s o l u t i o n f i l m s t r e c h e d i n s i d e o f p l a t i n u m r i n g . T h o s e e n i t t e r s o b t a i n e d by t h i s method had a r a d i u s o f below 300 angstrom and t h e i r s u r f a c e s were e x t r e m l y smooth. For t h e o t h e r compound semiconductor (CaAs and Cap), we a r e a l s o o b t a i n e d u s e f u l e m i t t e r by t h e above t r e a t m e n t .

l n t r o d u c t i on

S i c has as many k i n d s o f c r y s t a l s t r u c t u r e as many ways o f a r r a n g i n g S i atomes and C atomes i n o r d e r . A t p r e s e n t , more t h a n 130 k i n d s o f S i c c r y s t a l s t r u c t u r e a r e known and more than 44 k i n d s [ I ] o f them have been r e p o r t e d i n t h e papers. P r a c t i c a l l y , t y p i c a l s t r u c t u r e s a r e 3C, 15R, 6H, 4H and few o t h e r s . A c c o r d i n g t o P a u l i n g t h e o r y 123, 3C s t r u c t u r e i s t h e most u n s t a b l e one because o f i t s l e a s t b i n d i n g energy and 2H s t r u c t u r e i s most s t a b l e .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987697

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C6-596 JOURNAL DE PHYSIQUE

and R, rhombohedron and each f i g u r e b e f o r e those n o t a t i o n s show r e p e a t number o f S i atom and C atom i n a p e r i o d .

I t i s known that 3C strut- Table 1. C r y s t a l l o g r a p h i c Feature o f P - S i c Whisker

t u r e i s a c u b i c c r y s t a l c a l l e d P

P - S i c whisker

I n t h i s experiment we used f i - S i c whisker which has z i n c - b l e n d e s t r - u c t u r e ( 3C s t r u c t u r e ) o u t o f SIC which has many c r y s t a l s s t r u c t u r e .

The P - S i c whisker has many good - S i c a'nd o t h e r s such as 15R,6H,4H a r e cab fed a - S i c , where C r e p r e - s e n t s c u b i c s t r u c t u r e , H , hexagonal

qua1 i t i e s such t h a t i t s hardness i s 13 (diamond=15),its me- l t i n g p o i n t i s h i g h a t 2830°C, and a l s o i t i s r e s i s t i v e f o r r a d i a t i o n and chemi- c a l s . Moreover i t s energy band gap i s narrowest i n S i c c r - y s t a l s as Eg =2.40eV and t h e e i e c t r i c a l r e s i s t i v i t y i s about 0.01 Q * c m which i s

lower t h a n o t h e r S i c c r y s t a l s ( Table 1 ) C3,41. The diameter o f t h e used whiskers was i n most cases between 0.2 and 1.0

f i m , i n a few cases

C r y s t a l ' i n e structure . L a t t i c e constant

Energy gap at- 300'K

3C (Cubic s t r u c t u r e ) 4.3596 (A)

2.402 (eV) Density

E l e c t r o n aobi l i t y a t 300'K E l e c t r o n a f f i n i t y

I

^Lennth

I

-10 (inn)

I

3.21 ( g / c a S ) 1000 (cae/V.sec) 4 (eV)

R e s i s t i v i t y Hardness Diameter

Fig. 1 SEM image o f 6-Sic whiskers

0.01 ( Q - c m ) 13

0.2-1.0 ( u )

i t was about 5 f i m and t h e i r l e n g t h was about Pt Tube 10 mm. These w h i s k e r s looked l i k e c o t t o n

f i b e r s and some s p e c i a l l y prepared sharp twee- z e r s were u t i l i z e d t o t a k e one o u t o f those whiskers. F i g . 1 shows SEM image o f P - S i C whiskers. From t h i s image we can see t h a t t h e r e i s a v a r i e t y o f w h i s k e r s such as c u r v i n g end, and h a v i n g many necks and t h i c k

ones and t h i n ones. Out o f t h i s whisker, we took whisker i n which

ones,ones whose t h i c k n e s s i s d i f f e r e n t a t each tip

were l o n g e r and u n i f o r m i n t h i c k n e s s and used them as f i e l d e m i t t e r s .

Spot welded SIC whisker on Hai r p i n Loop

Formerly, S i c w h i s k e r s have been ensured by i n s e r t i n g t h e whisker i n a p l a t i n u m ( P t ) tube which was s p o t

welded t o a t u n g s t e n h a i r p i n loop. Fig. 3 Spot welding arrangement

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S i c w h i s k e r i s i n s e r t e d i n t o t h e t u b e and p r e s s e d and f i x e d ( F i g . 2).

However, t h i s method i s n o t s u i t a b l e f o r t h e c l e a n s u r f a c e s t u d y due t o o u t gas f r o m t h i s P t t u b e .

Recause h e a t i n n t e m o e r a t u r e o f t h e

Welder N e e d l e

W Double Hairpin LOA

- . . . ..

-

en1 t t e r i s r e s t r i c t e d below t h e m e l - I

'

^-. v I

t i n g p o i n t ( 1 7 7 2 " C ) o f P t and t h e r m a l

1

c l e a n s u r f a c e c a n n o t g e t enough.

T h e r e f o r e , i n t h i s work,we t r i e d t o sic W h i s k e r / \Cu P l a t e

s p o t w e l d [ 5 1 a S i c w h i s k e r on t o a Fig. 4 Double e m i t t e r f o r anisotropy h a i r o i n l o o p . As t h e r e s u l t t h e f i e -

l d e n i t t e r s u r f a c e can be a c h i e v e d s i m p l y by h e a t i n g i t t o a b o u t 2400°C.

As h a i r p i n l o o p m a t e r i a l was used t u n g s t e n w i r e (0.15 mm@,0.05 m m @ ) and t a n t a l u m w i r e ( 0.15 m m @ , 0.075mm@). Here t h e p r o b l e m i s t h a t i t c a n n o t be o b s e r v e d w i t h eye because S i c w h i s k e r i s s o t h i n ( a b o u t 0 . 2 - l y m @ ) .

We seldom o b s e r v e d even

w i t h t h e r e f l e c t e d l i g h t ^Table^2. Spot weld c o n d i t i o n

from t h e r e w h i s k e r s . The ^Specimen

I

Diameter(mm)

I

Vol tape range(V)

I

^Optimuw^{V O I}t a g e ( ~ )

a u t h o r c a r r i e d o u t t h e

1

^0.05

1

30-40

1

35

a t w i n eyes m i c r o s c o p e o f m a g n i f i c a t i o n x100. Second, a h a i r p i n l o o p was c r o s s e d on t h e w h i s k e r a t i t s m i d d l e p o i n t ( F i g . 3). T h i r d , w e l d e r e l e c t r o d e s p o t w e l d a s f o l l o w s .

F i r s t , we p u t a S i c w h i s k e r on s p o t w e l d s t a g e w i t h t h e s p e c i a l

i s a p p l i e d o n t h i s c r o s s p o i n t .

A t t h i s t i m e t h e w e l d e r e l e c t r o d e s c o n t a c t p l a n e must be b r u s h e d up smooth s u r f a c e . I t becomes

S i c w h i s k e r and t h e l o o p by t h i s method.

T a b l e 2 shows e x - p e r i m e n t a l v a l u e s o f welding' v o l t a g e .

I n f i g . 4 i s shown a d o u b l e e m i t t e r p r e p - a r a t i o n method t h a t was d e v i s e d a l s o t o s t u d y t h e bond a n i s - o t o r o p y . T h i s i s done by r e p e a t i ng t h e s i n g l e e m i t t e r p r o c e d u r e m e n t i o n e d above. The w e l d e r was a N a t i o n a l H i - Max YC-121. I n F i g .

5a we show t h e SEM image o f a welded t i p made by t h e above p r e p a r a t i o n and t w e e z e r s bv o b s e r v a t i o n

w

- T,

Fig. 5 a ) SEM image o f spot welded S I C WhlSKer t l p b ) Enlarged image o f a )

i n Fig.Sb, a h i g h m a g n i f i e d p i c t u r e o f t h i s welded p a r t .

0 . 1 5

. ^0.075

0.15

E l e c t r o - c h e m i c a l e t c h i n g of SIC w h i s k e r

As m e n t i o n e d above,SiC w h i s k e r s used were between 0.2-1 y m i n d i a m e t e r .

65-75 45-55 60-70

70 50 65

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C6-598 JOURNAL DE PHYSIQUE

A c c o r d i n g l y , we may c o n s i d e r t h a t t h e y can be used as e m i t t - e r s w i t h o u t any chemical t r e a t m - ent. However t h e i r apex were

i r r e g u l a r shaped. F i g . 6a shows such a non etched S i c whisker.

I t was o n l y t r e a t e d by r e s i s - t i v e l y h e a t i n g i n t h e vacuum.

T h i s whisker t i p e m i t s w i t h v o l t a g e s below 1000 V (Fig. 6b).

But i t s t i p apex does n o t o b t a i n h e m i s p h e r i c a l s u r f a c e .

T h e r e f o r e t even f i n e whisker should be etched t o have hemis- p h e r i c a l t i p apex. The s i n g l e h a i r p i n l o o p assembly s p o t welded i n method d e s c r i b e d above i s h o l d on X - Y moving s t a g e o f an o p t i c a l microscope and e l e c t r o chemical e t c h i n g o f t h e whisker

i s performed by e t c h i n g s o l u t i o n f i l m s t r e c h e d i n s i d e of p l a t i n u m w i r e r i n g (Fig. 7a). The e t c h i n g s o l u t i o n c o n s i s t o f 5% HF and 61

% H N O s a t a r a t i o o f 1

:

5 . We can a l s o e t c h t h e e m i t t e r even o n l y by chemical e t c h i n g u s i n g t h i s s o l u t i o n w i t h o u t any vo I tage

.

However i t does

n o t p r o v i d e a smooth and g l o s s y Fig. 6 a) End o f nonetched S I C whisker s e r f a c e . A c c o r d i n g l y , e l e c t r o

chemical e t c h i n g i s r e q u i r e d t o b) F i e l d emission image of a ) o b t a i n smooth s u r f a c e s .

Fig. 7 a) E l e c t r o l y t i c etching apparatus on microscope b) Enlarged etching segment o f a)

B i a s v o l t a g e i s between AC 1-2 V ( F i g 7 - b ) . E s p e c i a l l y , we used 2V.

J u s t a f t e r the b i a s v o l t a g e was a p p l i e d , t h e whisker was p u l l e d o u t o f r i n g . BY r e p e a t i n g t h i s process r e p r o d u c i b l e S i C e m i t t e r t i p s were o b t a i

t h e ned

.

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were below.300angstrom and t i p s u r f a c e was e x t r e m e l y smooth.

A f t e r e t c h i n g t h e specimen were washed i n e t h y l a l c o h o l o r d i s t i l l e d water f i l l e d i n t o t h e p l a t i n u m r i n g o f Fig.7b.

These processes were c a r r i e d o u t by o b s e r v a t i o n a o p t i c a l microscope o f 150x magnitude.

F i g . 8 shows t h e e t c h i n g method o f a double h a i r p i n

l o o p e m i t t e r C61 f o r bond a n i - s o t r o p i c s t u d y . T h i s e t c h i n g method i s s i m i l a r t o t h a t o f s i n g l e h a i r p i n loop e m i t t e r s .

Only d i f f e r e n c e i s t h a t t h e

Loop Holder

U

Liquid Electrode b i a s v o l t a g e was a p p l i e d betw-

een r i n g and i t s 1 o w - Fig. 8 Etching diagram f o r double e m i t t e r e r t u n g s t e n h a i r p i n l o o p and

was t u r n e d o f f j u s t a f t e r f i n i s h i n g t h e e t c h i n g .

T h i s process was accomplished by o b s e r v a t i o n lOOx t w i n eyes microscope.

A p a i r o f e m i t t e r s o b t a i n e d by t h e above-process: one i s o r i e n t e d t o < I l l >

d i r e c t i o n and t h e o t h e r i s o r i e n t e d t o < l l l > d i r e c t i o n . These e m i t t e r s used f o r c h a r a c t e r i s t i c o b s e r v a t i o n o f s u r f a c e s t r u c t u r e . I n P - S i c whisker which has z i n c - b l e n d e t y p e c r y s t a l s t r u c t u r e , such o r i e n t a t i o n as S i t a k e s - i t s t o p l a y e r i s u s u a l l y c a l l e d < I l l > and C t a k e s i t s t o p l a y e r i s c a l l e d < I l l > .

C o n c l u s i o n

As a r e s u l t o f t h i s r e p o r t , we a r e now a b l e t o s p o t weld SiC w h i s k e r s on h a i r p i n loops d i r e c t l y . As a r e s u l t , t h e h e a t i n g temperature c o u l d be e l e v a t e d up t o 2830°C. The o u t g a s i n g o f e m i t t e r t i p can be achieved e a s i l y by d i r e c t h e a t i n g . T h i s e m i t t e r c l e a n i n g t e c h n i c s y i e l d s s t a b l e r e p r o d u c i - b i l i t y under u l t r a h i g h vacuum c o n d i t i o n s . The e m i t t e r t i p s u r f a c e t h a t was o b t a i n e d by e l e c t r o - c h e m i c a l e t c h i n g was g l o s s y and v e r y smooth.

E m i t t e r t i p which has a r a d i u s o f below 300 angstrom can be o b t a i n e d e a s i l y . References

1) W. F. Knippenberg: P h i l l i p s Res. Rep. 18 (1963) 161.

2) L.Pauling: Nature o f t h e Chewical Bond 3rd. C o r n e l l Univ. Press (1960).

3) R. C. M a r s h a l l , Tr. J. W. Tanst and C. E. Ryan: S i l i c o n Carbide.

U n i v e r s i t y o f South C a r o l i n a Press (1973).

4) J. I. Pankor: Electro-Luminescence. S p r i n g e r V e r l a g (1977).

5 ) T. lnoue and M. Nakada: Growth and S u r f a c e P r o p e r t i e s o f LaBe C r y s t a l s , J. Vac. S c i . Technol. 21 (1982).

6) S. Tamaki and T. Inoue: Remolding o f <310> Tungsten T i p i n FIM, Jap. J. Appl. Phys. 15 (1976)