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EFFECTS OF A GOLD SHANK-OVERLAYER ON THE FIELD ION IMAGING OF SILICON
A. Melmed, W. Schmidt, J. Block, M. Naschitzki, M. Lovisa
To cite this version:
A. Melmed, W. Schmidt, J. Block, M. Naschitzki, M. Lovisa. EFFECTS OF A GOLD SHANK-
OVERLAYER ON THE FIELD ION IMAGING OF SILICON. Journal de Physique Colloques, 1986,
47 (C7), pp.C7-333-C7-336. �10.1051/jphyscol:1986757�. �jpa-00225952�
EFFECTS OF A GOLD SHANK-OVERLAYER ON THE FIELD ION IMAGING OF SILICON
A.J. MELMED, W.A. SCHMIDT*, J.H. BLOCK*, M. NASCHITZKI* and M. LOVISA'
National Bureau of Standards, Gaithersburg, MD 20899, U.S.A.
'~ritz-Ifaber-~nstitut der Max-Planck-Gesellschaft, Faradayweg 4-6, 0-1000 Berlin 33, F.R.G.
Résumé
Un f i l m d ' o r a é t é déposé s u r d e s é c h a n t i l l o n s de s i l i c i u m de t y p e p e t n de r é s i s t i v i t é e n v i r o n 1000 ohm-cm d a n s un microscope i o n i q u e à champ U.H.V. s o u s une t e m p é r a t u r e de 78 K. L ' i n f l u e n c e du champ e t du f l u x lumineux s u r l e s dimensions de l ' i m a g e a é t é é t u d i é e e n f o n c t i o n de l ' é p a i s s e u r du f i l m d ' o r s u r l a s u r f a c e de l ' é c h a n t i l l o n , l a p o i n t e de s i l i c i u m n ' é t a n t p a s r e c o u v e r t e . Pour d e s couches d ' o r d o n t l ' é p a i s s e u r moyenne e s t s u p é y i e u r e à 0.5-1 nm. t o u s l e s e f f e t s
semiconducteurs, dûs aux changements de l ' é c l a i r e m e n t ou du p o t e n t i e l a p p l i q u é au spécimen, s u r l'image de l a p o i n t e de s i l i c i u m e t s u r l e c o u r a n t i o n i q u e , d i s p a r a i s s e n t . L'amplitude du champ n é c e s s a i r e pour former une image d ' u n s i l i c i u m de t y p e n ou p, d a n s l'hydrogène ou d a n s l ' a r g o n , e s t l a même que pour un m é t a l , compte t e n u de l a p r é c i s i o n
e x p é r i m e n t a l e . A b s t r a c t
Gold was d e p o s i t e d o n t 0 p- and n-type s i l i c o n specimens o f
r e s i s t i v i t y a b o u t 1000 ohm-cm i n a UHV f i e l d i o n microscope o p e r a t e d a t 7 8 K . P h o t o - i l l u m i n a t i o n and f i e l d . induced image-size e f f e c t s were observed a s a f u n c t i o n o f g o l d coverage on t h e specimen shank, w i t h t h e s i l i c o n apex uncovered. For g o l d s h a n k - o v e r l a y e r s e s t i m a t e d t o b e i n e x c e s s o f 0.5-1 nm average t h i c k n e s s , a l 1 semiconductor e f f e c t s on t h e f i e l d i o n image o f t h e s i l i c o n apex and t h e i o n c u r r e n t , due t o changes i n i l l u m i n a t i o n o r a p p l i e d v o l t a g e t o t h e specimen, v a n i s h e d . F i e l d s t r e n g t h s f o r imaging p- and n-type s i l i c o n i n hydrogen o r a r g o n were found t o be t h e same a s f o r m e t a l s , w i t h i n o u r e x p e r i m e n t a l accuracy.
F i e l d e v a p o r a t i o n and f i e l d i o n i z a t i o n p r o c e s s e s a r e more complicated f o r semiconductors t h a n f o r m e t a l s , a s expected, i n view o f t h e i r more complex e l e c t r o n i c and chemical p r o p e r t i e s . Thus, t h e r e have been s e v e r a l d e s c r i p t i o n s ( 1 ) o f p h o t o - i l l u m i n a t i o n e f f e c t s r e l a t e d t o t h e f i e l d e v a p o r a t i o n and i o n i z a t i o n r a t e s and f i e l d ( o r v o l t a g e ) e f f e c t s r e l a t e d t o image compression o r e x p a n s i o n , and t h e s e e f f e c t s have q u i t e n a t u r a l l y been shown t o depend upon specimen r e s i s t i v i t y , semiconductor t y p e and specimen r e s i s t a n c e . The semiconductor p r o p e r t i e s do n o t c o n s t i t u t e a problem s t r i c t l y f o r imaging p u r p o s e s ; however, c o n v e n t i o n a l atom probe mass
a n a l y s i s o f semiconductors c a n b e made more d i f f i c u l t i f specimen r e s i s t a n c e is v e r y high. Then i t may b e n e c e s s a r y t o u s e l o n g e r - d u r a t f o n p u l s e s ( 2 ) i n o r d e r t o p u l s e f i e l d e v a p o r a t e t h e semiconductor, o r t o u s e i n s t e a d p u l s e d l a s e r f i e l d e v a p o r a t i o n methods ( 3 ) .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986757
JOURNAL DE PHYSIQUE
Conceptually, t h e f i e l d ion microscope (FIM) o r atom probe specimen may be considered a s a n e a r l y hemispherical apex cap, which is t h e p a r t of t h e specimen normally irnaged, a t t a c h e d t o a shank, u s u a l l y 1-10 mm long, which i n t u r n is connected, o r d i n a r i l y by an ohmic c o n t a c t , t o a m e t a l l i c base, a t which t h e specimen imaging voltage is applied. Typically, t h e shank i n c r e a s e s i n diameter from t h e cap value o f 50 nm. f o r example, t o sorne value between and 0.5 mm a t t h e base and t h u s can have a p p r e c i a b l e r e s i s t a n c e and experience a s i g n i f i c a n t f i e l d s t r e n g t h over some of i t s length. This geometric c o n s i d e r a t i o n , along with an e a r l y observation ( 1 ) of photo-illumination enhancement of f i e l d i o n i z a t i o n and evaporation r a t e s which were a f f e c t e d by exposure of a S i specimen t o a i r , motivated u s t o attempt t o cover t h e shank with a m e t a l l i c conducting l a y e r while exposing t h e apex t o t h e usual e l e c t r i c f i e l d f o r imaging; t h i s would e l i m i n a t e shank e f f e c t s . Additionally, t h e procedure was expected t o enable u s t o determine f i e l d s t r e n g t h s f o r i o n i z a t i o n without p o s s i b l e problems due t o t h e semiconducting p r o p e r t i e s of t h e shank. An e a r l i e r experiment ( 4 ) i n which Au was deposited ont0 only one s i d e f o r t h e S i specimen r e s u l t e d i n a s i g n i f i c a n t reduction of t h e voltage drop from base t o apex. However, t h e r e s u l t t h a t when h a l f covered with Au, t h e S i had t h e same p o t e n t i a l drop from base t o apex a s c l e a n S i led t o t h e conclusion t h a t photo-illumination and f i e l d e f f e c t s . when p r e s e n t , a r e due t o oxide on t h e e m i t t e r cap. This conclusion is ques- t i o n a b l e . In t h e p r e s e n t work, Au was chosen f o r t h e metal o v e r l a y e r be- cause it was known t o be e a s i l y evaporated i n UHV and because we wanted t o avoid excessive oxidation of t h e o v e r l a y e r s during specimen t r a n s p o r t i n a i r .
The Au evaporator was constructed using W wires which could be r e s i s t i v e l y heated. Two s e p a r a t e l y degassable loops supported t h e main, approximately c i r o u l a r , loop which was about 2 cm i n diameter and had s e v e r a l s m a l l v-shaped bends around which were wrapped s h o r t l e n g t h s of Au wire. After degassing procedures, t h e s e were melted t o f o r a small Au beads. The assembly included a s t a i n l e s s s t e e l s h i e l d geometrically shaped t o s h i e l d t h e FIM micro-channel p l a t e image d e t e c t o r and having an 11 mm diameter c e n t r a l a p e r t u r e , so a s not t o i n t e r f e r e with t h e t r a j e c t o r i e s of ions c o n t r i b u t i n g t o t h e image. Extensive degassing of t h e Au evaporator a t temperatures and f o r times i n excess of those a n t i c i p a t e d i n a c t u a l experiments was done i n a s e p a r a t e vacuum chamber. I n t h e FIM, t h e evaporator assembly was p o s i t i o n e d using e x t e r n a l screw a d j u s t e r s so t h a t t h e specimen apex was approximately i n t h e c e n t e r of t h e c i r c u l a r plane defined by t h e Au beads, a s s u r i n g t h a t t h e e n t i r e specimen and i t s base c o n t a c t d i r e c t l y received Au from t h e
evaporator. The s h i e l d a p e r t u r e was then l o c a t e d 5.5 mm from t h e specimen.
This p o s i t i o n r e l a t i v e t o t h e specimen was maintained during t h e subsequent course o f t h e experiments. P a r t i c u l a r a t t e n t i o n was given t o a s s u r i n g t h a t t h e r e was a good e l e c t r i c a l l y conducting c o n t a c t a t t h e S i t o base
connection.
The FIM used f o r t h i s work was a bakeable UHV mostly metal microscope using l i q u i d n i t r o g e n cooling and micro-channel p l a t e image d e t e c t i o n . The procedure f o r e x p l o r i n g t h e e f f e c t s of shank-overlayers c o n s i s t e d of f i r s t c l e a n i n g and smoothing t h e S i FIM specimen by slow f i e l d evaporation i n hydrogen, and t e s t i n g f o r t h e occurrence of photo-illumination and f i e l d e f f e c t s by varying i l l u m i n a t i o n ( u s i n g red o r white l i g h t ) and by r a p i d l y i n c r e a s i n g or decreasing t h e a p p l i e d v o l t a g e , i n t h e dark. Eye o b s e r v a t i o n s of t h e FIM image and measurement of t h e image d e t e c t o r output c u r r e n t were used f o r t h i s purpose. Si specimens were prepared from 1000 ohm-cm p-type bulk m a t e r i a l and a 960 ohm-cm n-type S i whisker; both c l e a r l y e x h i b i t e d photo-illumination and f i e l d e f f e c t s during imaging with hydrogen and a l s o d u r i n g imaging with argon. After f u l l FIM c h a r a c t e r i z a t i o n , t h e imaging gas was removed with t h e specimen imaging voltage l e f t on t o rninimize contamina- t i o n ; t h e n t h e voltage was reduced t o z e r o and Au was evaporated ont0 t h e specimen. The Au covering o f t h e apex was completely removed by f i e l d evapo- r a t i o n i n hydrogen, while observing t h e FIM image. Once again, FIM charac- t e r i z a t i o n was done i n hydrogen and i n argon, This c y c l e was r e p e a t e d u n t i l
l a t e r by transmission e l e c t r o n microscopy (TEM). The f i e l d compression f a c t o r f o r t h e FIM equipped with t h e Au evaporator was determined by t h e ring-counting method ( 5 ) using a W e m i t t e r with a geometry s i m i l a r t o t h a t of t h e S i specimens.
For both S i specimens, a f t e r t h e f i r s t dose of Au was deposited and t h e Au was removed from t h e apex, photo-illumination and f i e l d e f f e c t s were s t i l l c l e a r l y evident during hydrogen and argon imaging. A r a p i d i n c r e a s e i n ion c u r r e n t accompanied i l l u m i n a t i o n of t h e specimen and a slow (seconds) decrease t o t h e o r i g i n a l ion c u r r e n t occurred when t h e l i g h t was turned o f f . Also, a r e v e r s i b l e expansion and c o n t r a c t i o n of t h e image, accompanied by i n c r e a s e s and decreases i n t h e number o f image s p o t s , occurred when t h e applied voltage was r a p i d l y (manually) increased o r decreased. A l 1 of t h e s e e f f e c t s completely vanished a f t e r about 4 or 5 doses of Au. We estimated t h a t t h e average t h i c k n e s s of t h e Au l a y e r was then about 0.5-1 nm.
Subsequent TEM imaging enabled u s t o measure t h e f i n a l average r a d i i of curvature f o r t h e S i specimens.
Figure 1 shows one view o f each specimen.
The c l u s t e r i n g o f gold, evident i n t h e s e micrographs, almost c e r t a i n l y occurred a f t e r t h e specimens were removed from t h e UHV cold c o n d i t i o n s i n t h e FIM.
For each S i specimen, two views about 40 degrees r o t a t e d from each o t h e r were obtained t o guard a g a i n s t p o s s i b l e asymmetry of t h e specimen cross-sections.
TEM magnif i c a t i o n s of 220,000 were used f o r the r a d i u s measurements. The p-type S i apex r a d i u s was 50.0 nm and t h e n-type S i r a d i u s was 45.5 nm. Ring counting between (01 1 ) and (1 1 2 ) poles of t h e W specimen, imaged i n helium r e s u l t e d i n t h e value of 4.8 f o r t h e f i e l d compression f a c t o r . We t h e n c a l c u l a t e d t h e f i e l d s t r e n g t h s f o r imaging p-type and n-type
S i i n hydrogen t o be 2.1 and 2.2 VIA, micrograph of t h e p-type s i l i c o n r e s p e c t i v e l y . The same values were specimen with gold on t h e shank.
found f o r argon ion imaging. Overall, Magnif i c a t i o n marker r e p r e s e n t s we e s t i m a t e about 10-15% u n c e r t a i n t y i n 100 nm.
t h e s e values.
The r e s u l t s o f t h i s work c l e a r l y demonstrate t h e l a r g e i n f l u e n c e o f t h e shank o f a semiconductor specimen i n determining t h e magnitude of
photoillumination and f i e l d e f f e c t s observed by means of t h e FIM image of t h e apex and t h e measurement of ion c u r r e n t . These e f f e c t s can be eliminated by t h e i n s i t u d e p o s i t i o n of a m e t a l l i c conducting l a y e r on t h e shank. I t would be i n t e r e s t i n g t o t r y ex s i t u depositon a s well. The r e s u l t t h a t t h e f i e l d s t r e n g t h s f o r hydrogen and argon ion imaging of S i a r e i n d i s t i n g u i s h a b l e from t h e r e l e v a n t f i e l d s t r e n g t h s f o r imaging metals is i n t e r e s t i n g because it d i f f e r s considerably from t h e r e s u l t s o f e a r l i e r measurements f o r Ge ( 6 ) and f o r & A s ( 7 ) where t h e f i e l d s t r e n g t h s were found t o be much lower, approximately one-half of t h e metal value. A t t h i s time, t h e reason f o r t h e d i f f e r e n c e between Our r e s u l t s and t h e e a r l i e r r e s u l t s is not c l e a r .
Acknowledgement
The a u t h o r s extend t h e i r thanks t o Dr. M.J. Kaufman f o r doing t h e transmission e l e c t r o n microscopy a s s o c i a t e d with t h i s work.
JOURNAL DE PHYSIQUE
References
1. For example, A.J. Melmed and R.J. S t e i n , S u r f . S c i .
3
(1975) 645-648;A.J. Melmed, J.J. C a r r o l l and E . I . Givargizov, 2 6 t h I n t e r n a t i o n a l F i e l d Emission Symposium, W. B e r l i n , S e p t . 1979, unpublished.
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( 5 ) (1981) 416-417.3. W. Drachsel, S. Nishigaki and J.H. Block, I n t . J , Mass Spectrom. Ion Phys. (1980) 333-343.
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5 . E.W. Müller and T.T. Tsong, " F i e l d Ion Microscopy P r i n c i p l e s and Applica- t i o n s , I 1 American E l s e v i e r P u b l i s h i n g Co., I n c . New York (1969).
6. L . E r n s t and J.H. Block, S u r f . S c i .
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(1975) 293-309.7. Y. Ohno, S. Nakamura, T . Adachi and T. Kuroda, S u r f . S c i .
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(1977)521 -532.
