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Regenerative processes leading to surface flashover in vacuum
Abstracts of 25th International
g r a p h y of the a p p e a r i n g faces, (2) the a d s o r b e d s u b s t a n c e s , (3) the c o v e r a g e degrees, (4) the a d s o r p t i o n structures, and (5) the s u r f a c e e n e r g y changes. We h a v e s t u d i e d this p h e n o m e n o n in p a r t i c u l ar for c a r b o n on tungsten. Equilibrium states w e r e p r o d u c e d on t u n g s t e n crystals d e p e n d i n g on t e m p e r a t u r e (1200 < T < 2600 K) and c o v e r a g e (0 < 8 < 1015 atoms/cm2). Absolute coverages were d e t e r m i n e d by the m e t h o d of P i q u e t et al. I C h a n g e s of the g e o m e t r y of the tip c r y s t a l w e r e m e a s u r e d u s i n g F E M and FIM, to d e t e r m i n e the s u r f a c e - f r e e e n e r g i e s of d l f f e r e n t faces by W u l f f ' s c o n s t r u c tion r e s p e c t i v e l y by an a n a l o g o u s equation. The r e s u l t s a l l o w an e n e r g y class i f i c a t i o n of d i f f e r e n t C on W a d s o r p t i o n sites or states. A first a t t e m p t is d e s c r i b e d for a new m e t h o d to d e t e r m i n e a d s o r p t i o n s t r u c t u r e s by a F E M - F I M technique, i.e., u s i n g data on the c o v e r a g e s and the c r y s t a l l o g r a p h y of the r e c o n s t r u c t e d faces. A few s t r u c ture m o d e l s are p r e s e n t e d . The c a r b o n atoms a d s o r b p r e f e r e n t i a l l y along surface steps r e s p e c t i v e l y on s u r f a c e sites w h e r e the a d a t o m s can e n t e r as deep as p o s s i b l e into the surface. The old idea that c a r b o n forms a t w o - d i m e n s i o n a l g r a p h i t e s t r u c t u r e on {334} has p r o b a b l y to be revised. A c c o r d i n g to a new hypothesis, the {334} faces are f o r m e d by a less dense c a r b o n a d s o r p t i o n structure. Sometimes terrace structures appear whose step h e i g h t e x c e e d s the n o r m a l a t o m i c height. Such s t r u c t u r e s can be e x p l a i n e d as p a r t of s p e c i a l e q u i l i b r i u m shapes of m i n i m u m p o t e n t i a l energy. One of the c o n c l u s i o n s is: The study of c h e m i s o r p tion at t e m p e r a t u r e s w h e r e s u r f a c e selfd i f f u s i o n occurs is i n c o m p l e t e if this does not i n c l u d e i n v e s t i g a t i o n s of the reconstructed adsorption structures with its c h a n g e s of the size and e n e r g y of d i f f e r e n t faces. i.
A.
Piquet,
G.
Praiong,
R. Uzan, M. Drechsler, Suppl. 1977, p. 376.
H.
R. A. A n d e r s o n ,
Albuquerque,
C h a r g i n g of the s u r f a c e of an elect r i c a l l y s t r e s s e d i n s u l a t o r in v a c u u m by an a v a l a n c h e of s e c o n d a r y e m i s s i o n e l e c t r o n s is w e l l e s t a b l i s h e d , I and there is e x p e r i m e n t a l e v i d e n c e that the a v a l a n c h e is i n v o l v e d in the f o r m a t i o n of the f l a s h o v e r path. 2,3 However, the t r a n s i t i o n from the l o w - c u r r e n t p r e b r e a k down s e c o n d a r y e m i s s i o n a v a l a n c h e to a h i g h l y c o n d u c t i v e p l a s m a c h a n n e l has not been satisfactorily explained. A model b a s e d on e l e c t r o n - s t i m u l a t e d d e s o r p t i o n of gas f r o m the i n s u l a t o r s u r f a c e is p r e s e n t e d w h i c h a c c o u n t s for the transition to b r e a k d o w n in f a s t - p u l s e d s u r f a c e flashover. The time delay p r e c e d i n g b r e a k d o w n in this m o d e l is l a r g e l y det e r m i n e d by the rate of a c c u m u l a t i o n of a layer of p o s i t i v e charge, c o m p o s e d of d e s o r b e d ions and d e s o r b e d n e u t r a l s i o n i z e d w h i l e d r i f t i n g t h r o u g h the layer of e l e c t r o n s in the a v a l a n c h e . Event u a l l y the p o s i t i v e c h a r g e e n h a n c e s the e l e c t r i c field n e a r the c a t h o d e end of the i n s u l a t o r and a r e g e n e r a t i v e g r o w t h of d e s o r p t i o n and i o n i z a t i o n occurs. As the e l e c t r i c field rises other r e g e n e r ative p r o c e s s e s b e c o m e p o s s i b l e , w h i c h r a p i d l y lead to b r e a k d o w n . Time delays a c c o r d i n g to this m o d e l are in good a g r e e m e n t w i t h our e x p e r i m e n t a l r e s u l t s for p o l y m e t h y l m e t h a c r y l a t e and a l u m i n a c e r a m i c i n s u l a t o r s w h i c h w e r e of the o r d e r of i0 mm long and b r o k e down in 4 ns at 107 V/m. In addition, this m o d e l a c c o u n t s for the a p p r o x i m a t e prop o r t i o n a l i t y we o b s e r v e b e t w e e n the time d e l a y and the i n v e r s e s q u a r e of a p p l i e d v o l t a g e for time d e l a y s b e t w e e n 1 and 20 ns, as w e l l as the more r a p i d i n c r e a s e in time delay w i t h d e c r e a s i n g v o l t a g e b e y o n d 20 ns.
Roux,
Le Vide
185
i.
J.
P.
Phys.
Karl G. H e r n q v i s t Princeton,
J. P. B r a i n a r d
S a n d ia L a b o r a t o r i e s , New Mexico 87185
New
3260
D.
Jensen,
Appl.
3.
R. A. Anderson, 4210 (1977).
J. Appl.
Work
supported
J.
by u.s.
Phys.
Dept.
48,
of Energy.
ION M I C R O S C O P Y OF S I L I C O N e
~. S a k u r a i %, A. J. M e l m e d %t
Lett.
Phys.
METALLURGY
FIELD
Appl.
(1974).
R. A. Anderson, 24, 54 (1974).
* A b s t r a c t not a v a i l a b l e at time of publlcation.
R E G E N E R A T I V E P R O C E S S E S L E A D I N G TO S U R F A C E F L A S H O V E R IN V A C U U M ~
Brainard,
45,
2. E F F E C T S OF G L A S S E L E C T R O L Y S I S ON E L E C T R I C A L B R E A K D O W N IN HIGH V A C U U M *
RCA Laboratories, Jersey 08540
379
Field Emission Symposium
380
Abstracts of 25th International Field Emission Symposium
"[Department of P h y s i c s , P e n n s y l v a n i a S t a t e U n i v e r s i t y , U n i v e r s i t y Park, P e n n s y l v a n i a 16802; % % S u r f a c e S c i e n c e D i v i s i o n , N a t i o n a l B u r e a u of S t a n d ards, W a s h i n g t o n , D.C. 2 0 2 3 4
We have been i n v e s t i g a t i n g o r i e n t e d silicon w h i s k e r s using field ion m i c r o s c o p y (FIM), a t o m - p r o b e FIM, and field d e s o r p t i o n m i c r o s c o p y (FDM), and have found that s i l i c o n behaves quite d i f f e r e n t l y from a m e t a l in the p r e s e n c e of a high field. For instance, Si field e v a p o r a t e s as r a n d o m clusters of Si ions, w h e n e v a p o r a t e d at or b e l o w room t e m p e r a ture in vacuum, instead of e v a p o r a t i n g in an orderly m a n n e r from the surface kink sites. This anomaly in field evapo r a t i o n is b e l i e v e d to be due to a comb i n a t i o n of field p e n e t r a t i o n in the n e a r - s u r f a c e bulk and the unique b o n d i n g geome t r y of the t e t r a h e d r a l d i a m o n d structure. We also have o b s e r v e d a szrong effect of light i l l u m i n a t i o n on the field ion image i n t e n s i t y and quality, unique to s e m i c o n d u c t o r emitters. This m y s t e r i o u s effect, k n o w n for some years, can be s u m m a r i z e d as follows: (i) The i n t e n s i t y of a FI image u s i ng H2, He, Ar and Kr imaging gases increases m a r k e d l y (a factor of up to i0) by infrared i l l u m i n a t i o n w h e n a Si e m i t t e r surface is not fully developed. (2) As field e v a p o r a t i o n continues, the i l l u m i n a t i o n effect decays out gradually. (3) When the Si surface is c o m p l e t e l y d e v e l o p e d to show a fully o r d e r e d p a t t e r n there is p r a c t i c a l l y no i l l u m i n a t i o n effect. We show that this effect results from an increase in surface potential, w h i c h causes an i n c r e a s e d surface field due to the sharp r e s i s t a n c e drop across the surface oxide (Si02) layer, all due to the well known p h o t o c o n d u c t i v i t y effect. *Work p a r t i a l l y
supported
by NSF.
H A N D L I N G D A M A G E OF FIM S P E C I M E N S CAUSED BY F I E L D C O R R O S I O N H.-O.
Andr@n,
A.
Henjered,
D e p a r t m e n t of P h y s i c s , v e r s i t y of T e c h n o l o g y , Goteborg, Sweden
H. N o r d ~ n C h a l m e r s UniS - 4 0 2 20
Many p o l y m e r s acquire s u b s t a n t i a l surface charges w h e n making sliding contact w i t h metals. 1 The time for the subsequent d i s c h a r g e may be very long, but if the m a t e r i a l is not kept u n d e r v a c u u m the surface charge gets n e u t r a l i z e d after some time by a layer of ions from the s u r r o u n d i n g atmosphere. This layer is
loosely b o u n d and it can e a s i l y be rem o v e d by friction. 2 C o n s e q u e n t l y , if an u n s h i e l d e d fieldion s p e c i m e n is p a s s e d through a polymeric gasket (e.g. a V i t o n O - r i n g or a PTFE sliding seal), the tip may be exposed to a c o n s i d e r a b l e field strength, and specimen damage from field c o r r o s i o n is very likely to occur. The shape of a damaged s p e c i m e n is a p p r o x i m a t e l y that of a truncated cone. The edge of the cone is u s u a l l y s u f f i c i e n t l y sharp to give a r i n g - s h a p e d image in the f i e l d - i o n microscope, and if the damage is severe, it is not p o s s i b l e to obtain a fully developed image. These findings have i m p o r t a n t consequences for the design of s p e c i m e n changers for f i e l d - i o n and a t o m - p r o b e instruments. E x a m p l e s of dangerous and safe designs will be shown. i. A. W~hlin, G. B~ckstr6m, J. Appl. Phys. 45, 2058 (1974). 2. M. I. Kornfeld, Sov. Phys. Solid State 17 (3), 596 (1975).
A STUDY OF S P I N O D A L D E C O M P O S I T I O N Cu-Ti A L L O Y S BY MEANS OF THE FIMA T O M - P R O B E AND FDM
IN
R. W a g n e r I n s t i t u t e f o r M e t a l P h y s i c s and S F B 126, U n i v e r s i t y o f G ~ t t i n g e n , G~ttingen, West Germany
In l i t e r a t u r e there exists a controversy about the p r e c i p i t a t i o n m e c h a n i s m in Cu-Ti above 330°C. Some authors propose a c l a s s i c a l n u c l e a t i o n and selective g r o w t h mechanism, I others p r o p o s e a spinodal m e c h a n i s m . 2 F r o m a p r e l i m i n a r y study of v a r i o u s l y h e a t - t r e a t e d C_~u-Ti alloys with the F I M - a t o m p r o b e , we concluded that the d e c o m p o s i t i o n r e a c t i o n is the spinodal one. The m e c h a n i c a l h a r d e n i n g of these alloys supports this conclusion, and one should be able to q u a n t i f y the h a r d e n i n g m e c h a n i s m in m o d u l a t e d s t r u c t u r e s by m e a s u r i n g the m o d u l a t i o n a m p l i t u d e and w a v e l e n g t h with the F I M atom-probe. i. Tsujimoto, Trans. JIM i, 445 (1975) . 2. D. Laughlin, Acta Met., 23, 329 (1975).
FIM STUDY OF M o - R e
ALLOYS
K. Teramoto, H. Morikawa, N. igata~, A. K o h y a m a %
Y. Hashiro,
Department of Coordinated Science, N a g o y a I n s t i t u t e of T e c h n o l o g y , G o k i s o - c h o , S h o w a - k u , N a g o y a 466
g r a p h y of the a p p e a r i n g faces, (2) the a d s o r b e d s u b s t a n c e s , (3) the c o v e r a g e degrees, (4) the a d s o r p t i o n structures, and (5) the s u r f a c e e n e r g y changes. We h a v e s t u d i e d this p h e n o m e n o n in p a r t i c u l ar for c a r b o n on tungsten. Equilibrium states w e r e p r o d u c e d on t u n g s t e n crystals d e p e n d i n g on t e m p e r a t u r e (1200 < T < 2600 K) and c o v e r a g e (0 < 8 < 1015 atoms/cm2). Absolute coverages were d e t e r m i n e d by the m e t h o d of P i q u e t et al. I C h a n g e s of the g e o m e t r y of the tip c r y s t a l w e r e m e a s u r e d u s i n g F E M and FIM, to d e t e r m i n e the s u r f a c e - f r e e e n e r g i e s of d l f f e r e n t faces by W u l f f ' s c o n s t r u c tion r e s p e c t i v e l y by an a n a l o g o u s equation. The r e s u l t s a l l o w an e n e r g y class i f i c a t i o n of d i f f e r e n t C on W a d s o r p t i o n sites or states. A first a t t e m p t is d e s c r i b e d for a new m e t h o d to d e t e r m i n e a d s o r p t i o n s t r u c t u r e s by a F E M - F I M technique, i.e., u s i n g data on the c o v e r a g e s and the c r y s t a l l o g r a p h y of the r e c o n s t r u c t e d faces. A few s t r u c ture m o d e l s are p r e s e n t e d . The c a r b o n atoms a d s o r b p r e f e r e n t i a l l y along surface steps r e s p e c t i v e l y on s u r f a c e sites w h e r e the a d a t o m s can e n t e r as deep as p o s s i b l e into the surface. The old idea that c a r b o n forms a t w o - d i m e n s i o n a l g r a p h i t e s t r u c t u r e on {334} has p r o b a b l y to be revised. A c c o r d i n g to a new hypothesis, the {334} faces are f o r m e d by a less dense c a r b o n a d s o r p t i o n structure. Sometimes terrace structures appear whose step h e i g h t e x c e e d s the n o r m a l a t o m i c height. Such s t r u c t u r e s can be e x p l a i n e d as p a r t of s p e c i a l e q u i l i b r i u m shapes of m i n i m u m p o t e n t i a l energy. One of the c o n c l u s i o n s is: The study of c h e m i s o r p tion at t e m p e r a t u r e s w h e r e s u r f a c e selfd i f f u s i o n occurs is i n c o m p l e t e if this does not i n c l u d e i n v e s t i g a t i o n s of the reconstructed adsorption structures with its c h a n g e s of the size and e n e r g y of d i f f e r e n t faces. i.
A.
Piquet,
G.
Praiong,
R. Uzan, M. Drechsler, Suppl. 1977, p. 376.
H.
R. A. A n d e r s o n ,
Albuquerque,
C h a r g i n g of the s u r f a c e of an elect r i c a l l y s t r e s s e d i n s u l a t o r in v a c u u m by an a v a l a n c h e of s e c o n d a r y e m i s s i o n e l e c t r o n s is w e l l e s t a b l i s h e d , I and there is e x p e r i m e n t a l e v i d e n c e that the a v a l a n c h e is i n v o l v e d in the f o r m a t i o n of the f l a s h o v e r path. 2,3 However, the t r a n s i t i o n from the l o w - c u r r e n t p r e b r e a k down s e c o n d a r y e m i s s i o n a v a l a n c h e to a h i g h l y c o n d u c t i v e p l a s m a c h a n n e l has not been satisfactorily explained. A model b a s e d on e l e c t r o n - s t i m u l a t e d d e s o r p t i o n of gas f r o m the i n s u l a t o r s u r f a c e is p r e s e n t e d w h i c h a c c o u n t s for the transition to b r e a k d o w n in f a s t - p u l s e d s u r f a c e flashover. The time delay p r e c e d i n g b r e a k d o w n in this m o d e l is l a r g e l y det e r m i n e d by the rate of a c c u m u l a t i o n of a layer of p o s i t i v e charge, c o m p o s e d of d e s o r b e d ions and d e s o r b e d n e u t r a l s i o n i z e d w h i l e d r i f t i n g t h r o u g h the layer of e l e c t r o n s in the a v a l a n c h e . Event u a l l y the p o s i t i v e c h a r g e e n h a n c e s the e l e c t r i c field n e a r the c a t h o d e end of the i n s u l a t o r and a r e g e n e r a t i v e g r o w t h of d e s o r p t i o n and i o n i z a t i o n occurs. As the e l e c t r i c field rises other r e g e n e r ative p r o c e s s e s b e c o m e p o s s i b l e , w h i c h r a p i d l y lead to b r e a k d o w n . Time delays a c c o r d i n g to this m o d e l are in good a g r e e m e n t w i t h our e x p e r i m e n t a l r e s u l t s for p o l y m e t h y l m e t h a c r y l a t e and a l u m i n a c e r a m i c i n s u l a t o r s w h i c h w e r e of the o r d e r of i0 mm long and b r o k e down in 4 ns at 107 V/m. In addition, this m o d e l a c c o u n t s for the a p p r o x i m a t e prop o r t i o n a l i t y we o b s e r v e b e t w e e n the time d e l a y and the i n v e r s e s q u a r e of a p p l i e d v o l t a g e for time d e l a y s b e t w e e n 1 and 20 ns, as w e l l as the more r a p i d i n c r e a s e in time delay w i t h d e c r e a s i n g v o l t a g e b e y o n d 20 ns.
Roux,
Le Vide
185
i.
J.
P.
Phys.
Karl G. H e r n q v i s t Princeton,
J. P. B r a i n a r d
S a n d ia L a b o r a t o r i e s , New Mexico 87185
New
3260
D.
Jensen,
Appl.
3.
R. A. Anderson, 4210 (1977).
J. Appl.
Work
supported
J.
by u.s.
Phys.
Dept.
48,
of Energy.
ION M I C R O S C O P Y OF S I L I C O N e
~. S a k u r a i %, A. J. M e l m e d %t
Lett.
Phys.
METALLURGY
FIELD
Appl.
(1974).
R. A. Anderson, 24, 54 (1974).
* A b s t r a c t not a v a i l a b l e at time of publlcation.
R E G E N E R A T I V E P R O C E S S E S L E A D I N G TO S U R F A C E F L A S H O V E R IN V A C U U M ~
Brainard,
45,
2. E F F E C T S OF G L A S S E L E C T R O L Y S I S ON E L E C T R I C A L B R E A K D O W N IN HIGH V A C U U M *
RCA Laboratories, Jersey 08540
379
Field Emission Symposium
380
Abstracts of 25th International Field Emission Symposium
"[Department of P h y s i c s , P e n n s y l v a n i a S t a t e U n i v e r s i t y , U n i v e r s i t y Park, P e n n s y l v a n i a 16802; % % S u r f a c e S c i e n c e D i v i s i o n , N a t i o n a l B u r e a u of S t a n d ards, W a s h i n g t o n , D.C. 2 0 2 3 4
We have been i n v e s t i g a t i n g
supported
by NSF.
H A N D L I N G D A M A G E OF FIM S P E C I M E N S CAUSED BY F I E L D C O R R O S I O N H.-O.
Andr@n,
A.
Henjered,
D e p a r t m e n t of P h y s i c s , v e r s i t y of T e c h n o l o g y , Goteborg, Sweden
H. N o r d ~ n C h a l m e r s UniS - 4 0 2 20
Many p o l y m e r s acquire s u b s t a n t i a l surface charges w h e n making sliding contact w i t h metals. 1 The time for the subsequent d i s c h a r g e may be very long, but if the m a t e r i a l is not kept u n d e r v a c u u m the surface charge gets n e u t r a l i z e d after some time by a layer of ions from the s u r r o u n d i n g atmosphere. This layer is
loosely b o u n d and it can e a s i l y be rem o v e d by friction. 2 C o n s e q u e n t l y , if an u n s h i e l d e d fieldion s p e c i m e n is p a s s e d through a polymeric gasket (e.g. a V i t o n O - r i n g or a PTFE sliding seal), the tip may be exposed to a c o n s i d e r a b l e field strength, and specimen damage from field c o r r o s i o n is very likely to occur. The shape of a damaged s p e c i m e n is a p p r o x i m a t e l y that of a truncated cone. The edge of the cone is u s u a l l y s u f f i c i e n t l y sharp to give a r i n g - s h a p e d image in the f i e l d - i o n microscope, and if the damage is severe, it is not p o s s i b l e to obtain a fully developed image. These findings have i m p o r t a n t consequences for the design of s p e c i m e n changers for f i e l d - i o n and a t o m - p r o b e instruments. E x a m p l e s of dangerous and safe designs will be shown. i. A. W~hlin, G. B~ckstr6m, J. Appl. Phys. 45, 2058 (1974). 2. M. I. Kornfeld, Sov. Phys. Solid State 17 (3), 596 (1975).
A STUDY OF S P I N O D A L D E C O M P O S I T I O N Cu-Ti A L L O Y S BY MEANS OF THE FIMA T O M - P R O B E AND FDM
IN
R. W a g n e r I n s t i t u t e f o r M e t a l P h y s i c s and S F B 126, U n i v e r s i t y o f G ~ t t i n g e n , G~ttingen, West Germany
In l i t e r a t u r e there exists a controversy about the p r e c i p i t a t i o n m e c h a n i s m in Cu-Ti above 330°C. Some authors propose a c l a s s i c a l n u c l e a t i o n and selective g r o w t h mechanism, I others p r o p o s e a spinodal m e c h a n i s m . 2 F r o m a p r e l i m i n a r y study of v a r i o u s l y h e a t - t r e a t e d C_~u-Ti alloys with the F I M - a t o m p r o b e , we concluded that the d e c o m p o s i t i o n r e a c t i o n is the spinodal one. The m e c h a n i c a l h a r d e n i n g of these alloys supports this conclusion, and one should be able to q u a n t i f y the h a r d e n i n g m e c h a n i s m in m o d u l a t e d s t r u c t u r e s by m e a s u r i n g the m o d u l a t i o n a m p l i t u d e and w a v e l e n g t h with the F I M atom-probe. i. Tsujimoto, Trans. JIM i, 445 (1975) . 2. D. Laughlin, Acta Met., 23, 329 (1975).
FIM STUDY OF M o - R e
ALLOYS
K. Teramoto, H. Morikawa, N. igata~, A. K o h y a m a %
Y. Hashiro,
Department of Coordinated Science, N a g o y a I n s t i t u t e of T e c h n o l o g y , G o k i s o - c h o , S h o w a - k u , N a g o y a 466