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CATHODE BOMBARDMENT STIMULATED MICROSTRUCTURE GROWTH - AVERAGE ION
ENERGY
P. Schwoebel, G. Hanson
To cite this version:
P. Schwoebel, G. Hanson. CATHODE BOMBARDMENT STIMULATED MICROSTRUCTURE GROWTH - AVERAGE ION ENERGY. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-59-C2- 66. �10.1051/jphyscol:1986209�. �jpa-00225640�
JOURNAL DE PHYSIQUE
Colloque C2, supplhent au n03, Tome 47, mars 1986 page C Z - 5 9
CATHODE BOMBARDMENT STIMULATED MICROSTRUCTURE GROWTH - AVERAGE ION ENERGY
P.R. SCHWOEBEL and G.R. HANSON
The School of Applied and Engineering Physics and
The National Research and Resource Facility for Submicron Structures, Knight Laboratory, Cornell University, Ithaca, NY 14853, U.S.A.
A b s t r a c t - Surface m i c r o p r ~ t r u s i o n s have been a s s o c i a t e d w i t h cathode i n i t i a - ted vacuum breakdown processes and some i n t e r p r e t a t i o n s of t h e i r f o r m a t i o n i n c l u d e s u r f a c e m i g r a t i o n and f i e l d enhanced growth. M i c r o s t r u c t u r e growth on f i e l d e m i t t e r s has been observed i n a s s o c i a t i o n w i t h low temperature
(5.OK) a d s o r p t i o n of hydrogen o r helium and e l e c t r o n emission c u r r e n t s of 200 t o 1000 nA. The presence of adsorbed f i l m s ( m u l t i l a y e r f i l m s f o r hydrogen) a t 5.OK and t h e g e n e r a l s u r f a c e damage without s t i m u l a t e d growth a t l i q u i d n i t r o g e n temperature s u g g e s t t h a t a high dose of low energy i o n s formed near t h e e m i t t e r s u r f a c e may a c t i v a t e t h e s u r f a c e d i f f u s i o n . The n u c l e a t i o n of t h e growth s i t e may r e s u l t from a number of d i f f e r e n t e v e n t s : i ) Surface d e n s i t y f l u c t u a t i o n s which achieve a c r i t i c a l l o c a l f i e l d enhancement, i i ) Surface damage r e s u l t i n g from t h e impact of a high energy i o n , i i i ) Threshold s u r f a c e e t c h i n g by i o n s p u t t e r i n g . A model c a l c u l a t i o n of t h e i o n energy i n d i c a t e s t h a t a t 5.0K,z86% of t h e hydrogen ions have a k i n e t i c energy l e s s than 100 + ev (0.35 V / A Applied F i e l d , 3300 V). S i m i l a r e f f e c t s a r e demonstrated f o r He cathode bombardment. Low energy helium i o n bombardment a c t i v a t e d s u r f a c e s e l f d i f f u s i o n i n t h e presence of an a p p l i e d f i e l d p r o v i d e s s e v e r a l t e c h n i c a l advantages f o r high b r i g h t n e s s f i e l d i o n source development.
Introduction
The f i e l d ion microscope has been a p p l i e d p r i m a r i l y e x p l o i t i n g its c a p a b i l i t y of imaging s u r f a c e atoms. For p r a c t i c a l a p p l i c a t i o n a s an i o n beam s o u r c e , however, emission c u r r e n t dependencies upon s i n g l e s u r f a c e atoms i s a s e v e r e l i m i t a t i o n . Atomic s u r f a c e rearrangement processes appear a s beam c u r r e n t f l u c t u a t i o n s 1 a s well a s a n g u l a r d i s t r i b u t i o n f l u c t u a t i o n s . The r e l a t i v e f l u c t u a t i o n i s i n v e r s e l y p r o p o r t i o n a l t o t h e s q u a r e r o o t of t h e number of s u r f a c e atoms1 imaged by t h e beam. During l o n g term source o p e r a t i o n , s i n g l e s u r f a c e s i t e e v e n t s such a s n e g a t i v e i o n impact cannot be e l i m i n a t e d . Localized emission from molecular S i z e s u r f a c e f e a t u r e s 2 a r e i n h e r e n t l y very d i f f i c u l t t o c o n t r o l on rounded, well annealed f i e l d e m i t t e r endforms. Such e m i t t e r morphology c o n s t r a i n t s and o t h e r p r a c t i c a l c o n s i d e r a t i o n s of alignment, e m i t t e r p r o c e s s i n g , emission d i s t r i b u t i o n s t a b i l i t y and e m i t t e r l i f e t i m e determine t h e u s e f u l n e s s of gaseous f i e l d emission ion s o u r c e s .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986209
JOURNAL DE PHYSIQUE
For t h e f i e l d ion microscope, t h e a x i a l ion c u r r e n t i s low. A s t h e t o t a l e m i t t e r c u r r e n t i n c r e a s e s , t h e source emittance a l s o i n c r e a s e s , and t h e b r i g h t n e s s remains r e l a t i v e l y low. The d i s t r i b u t i o n of ion emission i n t h e I - V regime above threshold is dominated by s u r f a c e supply m e ~ h a n i s m s 3 , ~ with a l e n g t h s c a l e of an e m i t t e r r a d i u s . Emitter s u r f a c e regions which have reached t h r e s h o l d f o r i o n i z a t i o n due t o higher f i e l d enhancement f a c t o r s a c t a s s i n k s of t h e mobile s u r f a c e supply and sources of f i e l d ions. A s u r f a c e s t r u c t u r e of low f i e l d enhancement f a c t o r , 1.2-1.5, a c t s a s an ion beam source. Angular magnification f a c t o r s 5 show t h a t t h e image of a 0.05 urn diameter s u r f a c e f e a t u r e is compressed by a 0.5 um r a d i u s e m i t t e r i n t o a beam with a half-angle of 0.02 radian. Such a s u r f a c e a r e a c o n s i s t s of =2xlo4 s u r f a c e atoms and produces hydrogen ion c u r r e n t d e n s i t i e s of
= 2 ~ 1 0 3 ~ / c r n ~ . This ion image, compared with the l a r g e a n g l e ion d i s t r i b u t i o n of the FIM, is emittance matched f o r beam forming systems which r e q u i r e l-lOmr acceptance h a l f angles. The s u c c e s s f u l a p p l i c a t i o n of such an emission m i c r o s t r u c t u r e a s an ion source r e l i e s upon techniques t o make and s t a b l i z e t h i s e m i t t e r topography.
C a r a i l l e and ~ r e c h s l e r , 6 and Dranova and ~ i k h a i l o w s k i 7 have demonstrated t h e production of f i e l d s t a b l i z e d growth of p r o t r u s i o n s . The a u t h o r s i n t e r p r e t t h i s phenomenon i n terms of ion bombardment a c t i v a t e d s u r f a c e atom t r a n s p o r t . These experiments were conducted with s u b s t r a t e temperatures 77-100K and apparent d o s e s , ( e l e c t r o n c u r r e n t ) X ( t i m e ) X (gas p r e s s u r e ) , which a r e s i g n i f i c a n t l y l a r g e r than those required i f a base gas and e m i t t e r temperature of 5K i s used.839 This work a t 5K by Schwoebel and ~ a n s o n ~ p 9 p r i m a r i l y with hydrogen cathode bombardment, is l i m i t e d i n i t s a p p l i c a t i o n f o r ion sources because of t h e e m b r i t t l i n g e f f e c t of t h e implanted hydrogen i n t h e s u r f a c e . Further i n v e s t i g a t i o n of the 5K growth is warranted because t h e i n i t i a l hydrogen r e s u l t s d i d not d i s t i n g u i s h c l e a r l y t h e r e l a t i v e importance of ion bombardment, d i e l e c t r i c e f f e c t s or work f unction-electron emission e f f e c t s . I t is suggested t h a t enhanced growth a t 5K is dominated by t h e temperature dependence of t h e ion bombarding energy d i s t r i b u t i o n ; a preponderance of low energy ions (25-500eV) i n c r e a s e s t h e r a t e constant f o r s u r f a c e s e l f d i f f u s i o n r e l a t i v e t o t h a t f o r s p u t t e r i n g and s u r f a c e l a y e r amorphization. Due t o t h e e l e c t r o s t a t i c s t r e s s on t h e e m i t t e r s u r f a c e , t h e l a s t two e f f e c t s a r e t o be minimized f o r a s u c c e s s f u l a p p l i c a t i o n of such m i c r o s t r u c t u r e growth a s a f i e l d ion source f o r beam a p p l i c a t i o n . Observation of microprotrusion growth using helium ions and tungsten or iridium s u b s t r a t e s support the above i n t e r p r e t a t i o n and add d a t a t o t h a t f o r hydrogen ion bombardment a t 5K.
Experimental
Using a helium cooled ion source assembly,8 helium ion imaging of helium cathode bombardment induced m i c r o s t r u c t u r e growth has been done on tungsten and iridium by standard f i e l d emission techniques. Figure 1 is a sequence of helium i o n , threshold emission p a t t e r n s from a tungsten m i c r o s t r u c t u r e , a p p l i e d average f i e l d - F=2.6V/A. The diameter of t h e s t r u c t u r e is =200A. Figure 2 shows a f i e l d e l e c t r o n p a t t e r n of an iridium m i c r o s t r u c t u r e grown by 5K helium bombardment. In t h i s c a s e , growth occured a t very low t o t a l apparent dose (=100nA f o r 1 minute a t 5x10-5 t o r r helium). This occurred a t one of t h e f o u r f i e l d enhanced s u r f a c e p o i n t s which a r e i n proximity t o both the (100) and (211) f a c e t edges. Figure 3 shows a sequence of helium ion p a t t e r n s before and a f t e r growth on the (100) r e g i o n s of a f i e l d evaporated tungsten e m i t t e r by helium cathode bombardment a t 5K. The e m i t t e r was f i e l d evaporated (7021V) before s u r f a c e s e l f - d i f f u s i o n was a c t i v a t e d by helium bombardment a t 5K. Figure 3a shows t h e s u r f a c e before bombardment. Figure 3b shows helium ion emission concentrated i n t h e r e g i o n s of enhanced f i e l d a f t e r ion bombardment. Figures 3c and 3d show t h e ion image a f t e r p a r t i a l desorption of t h e m i c r o s t r u c t u r e with s u r f a c e af;oms i n high f r e e energy s i t e s a c r o s s the e m i t t e r s u r f a c e . Figure 3e is an image a f t e r f i e l d evaporation a t 6978V. There is no evidence of l a r g e s c a l e s u r f a c e damage or b l i s t e r i n g . The experiment showed a s u r f a c e l a y e r which is scrambled by t h e ion bombardment. This i s made apparent by a lowering of t h e evaporation f i e l d (4.5-5V/A) of r e d i s t r i b u t e d s u r f a c e atoms.
Helium must a l s o be present i n t h e s u b s t r a t e s u r f a c e l a y e r s a s a r e s u l t of t h e bombardment.
The region which is field-enhanced by t h e ion bombardment shows a high degree of c r y s t a l l i n i t y under t h e s u r f a c e selvedge r e f e r r e d t o above. For t h e magnification a v a i l a b l e , no change of e m i t t e r r a d i u s is d e t e c t a b l e a f t e r growth and removal of t h e f i e l d enhanced region.
Dicussion and Theory
Work f u n c t i o n and d i e l e c t r i c e f f e c t s of helium on iridium a r e small compared t o those of hydrogen on tungsten (5K). Therefore, c u r r e n t changes during cathode bombardment experiments using helium-iridium a t 5K a r e more e a s i l y i n t e r p r e t e d i n terms of ion-solid i n t e r a c t i o n s . Adlayers have t h e e f f e c t of decreasing ion-substrate c o l l i s i o n e n e r g i e s and s c a t t e r i n g i n c i d e n t ion-momentum i n t o d i r e c t i o n s t r a n s v e r s e t o t h e s u b s t r a t e s u r f a c e . Heavy helium a d l a y e r s cannot e x i s t a t 5 ~ ~ p a r t i c u l a r 1 y under cathode bombardment and the n e u t r a l gas d i s t r i b u t i o n must be considered a s t h e only dependent v a r i a b l e of temperature. Although t h e s e r i e s i n Figure 3 was observed using a highly curved e m i t t e r , and t h e r e f o r e low voltage, t h e energy d i s t r i b u t i o n of the bombarding ions ranges from 25-790eV. For helium i o n s t o c r e a t e a bulk i n t e r s t i a l , 480eV ions a r e required. Surface d i f f u s i o n a c t i v a t i o n e n e r g i e s span values from -.l t o 3 . l e v l l and t h e r e f o r e r e q u i r e much lower t h r e s h o l d ion e n e r g i e s , -37eV He+ ion. Figure 4 shows t h a t t h e s p u t t e r i n g threshold is -110eV f o r a s p u t t e r a c t i v a t i o n energy of 8.8eV. The damage mechanisms o p e r a t i v e f o r l a r g e r a d i u s e m i t t e r s t o be used i n source r e s e a r c h a r e o p e r a t i v e i n t h i s case.
A t l i q u i d n i t r o g e n temperature (77K), t h e r e q u i r e d apparent dose (e- c u r r e n t X time f o r growth X chamber p r e s s u r e ) f o r growth of s i n g l e m i c r o s t r u c t u r e s using hydrogen or helium is s u f f i c i e n t l y high t h a t within t h e normal o p e r a t i n g c o n s t r a i n s , 10-6-10-~ t o r r chamber pressure and emission c u r r e n t s of 100-1000nA, only generalized s u r f a c e damage i s found. Previous workers r e p o r t r e s u l t s with higher t o t a l apparent dose a t pressures where ion-neutral c o l l i s i o n s may a f f e c t t h e incident ion energy d i s t r i b u t i o n .
Ions with e n e r g i e s below t h e s p u t t e r threshold w i l l a c t i v a t e s u r f a c e d i f f u s i o n without a s p u t t e r l o s e f r a c t i o n . The d e n s i t y of t h e s e i o n s is s t r o n g l y temperature dependent through a boltzmann f a c t o r of the p o l a r i z a t i o n . I t is suggested t h a t s i n g l e m i c r o s t r u c t u r e growth occurs when t h e r e l a t i v e dose of low energy t r a n s f e r s which a c t i v a t e s u r f a c e s e l f d i f f u s i o n without s p u t t e r i n g is g r e a t e r than t h e r a t e of l o s s of s u r f a c e m a t e r i a l f o r a l l impinging (ion energy range, 25V t o a p p l i e d e m i t t e r v o l t a g e ) i o n s . This r e l a t i v e dose is g r e a t e s t i f the t o t a l energy d i s t r i b u t i o n is peaked a t low values. For continued growth, t h e time r e q u i r e d t o grow a m i c r o s t r u c t u r e must be l e s s than t h e time f o r d i r e c t impact a t t h e m i c r o s t r u c t u r e by a high energy event which o b l i t e r a t e s t h e c l u s t e r nucleus or changes its shape. With i n c r e a s i n g chamber p r e s s u r e s , t h e dose of lncoming i o n s having a k i n e t i c energy equal t o t h e ion p o t e n t i a l energy a t t h e i n s t a n t of e l e c t r o n impact would become l e s s frequent because of i n c r e a s i n g m u l t i p l e ion-neutral c o l l i s i o n s which d i s t r i b u t e t h e f i e l d energy of t h e ion.
A t low p r e s s u r e and low temperature (5K), t h e d i s t r i b u t i o n of the n e u t r a l gas p a r t i c l e concentration i s s t r o n g l y weighted t o t h e high p o t e n t i a l energy region of t h e e m i t t e r p o l a r i z a t i o n f i e l d . The e l e c t r o n impact c r o s s s e c t i o n is peaked a t e l e c t r o n e n e r g i e s of -1 00eV. These two f a c t o r s determine t h e ion e n e r g y d i s t r i b u t i o n f u n c t i o n . I n v e s t i g a t i o n s of t h e ion backbombardment dose have been given by s e v e r a l a u t h 0 r s , ~ * 7 * 2 * l 3 but without c o n s i d e r a t i o n of the n e u t r a l concentration e f f e c t s induced by the e m i t t e r . An a x i a l e l e c t r o n c u r r e n t , I , expands a s depicted i n Figure 5 where Z is t h e a x i a l d i s t a n c e from t h e e m i t t e r and V is t h e p o t e n t i a l . For t h i s model c a l c u l a t i o n , t h e d e v i a t i o n of t h e bombarding ion t r a j e c t o r y 1 3 from t h e f i e l d l i n e s w i l l be neglected; an approximation a p p r o p r i a t e only f o r t h e low energy ions. Ions formed f a r from the s u r f a c e do not follow t h e e l e c t r o n t r a j e c t o r y back t o t h e emission point. For such an approximation t o the r e a l c a s e , t h e s u r f a c e ion bombardment r a t e produced by ions o r i g i n a t i n g a t Z is:
JOURNAL DE PHY SlQUE
where C ( Z ) is t h e concentration of n e u t r a l s and o is t h e e l e c t r o n impact i o n i z a t i o n c r o s s s e c t i o n . This equation does not consider mu1 t i p l e i o n i z a t i o n or i o n - n e u t r a l c o l l i s i o n s . The k i n e t i c energy of the bombarding ion i s determined by i t s i n i t i a l d i s t a n c e from t h e s u r f a c e when t h e e l e c t r o n impact occurs.
The e l e c t r o n impact i o n i z a t i o n c r o s s s e c t i o n i s t h e r e f o r e e x p r e s s i b l e i n terms of t h e ion energy using Eqn. 2 ; o=o(K). The f a c t o r C(Z), assuming e q u i l i b r i u m is given by t h e boltzmann f a c t o r :
The a x i a l d i s t a n c e dependence of t h e e l e c t r o s t a t i c f i e l d is approximated by:
where D i s t h e emitter-cathode d i s t a n c e and R is an e m i t t e r c u r v a t u r e parameter ( t h e focus of t h e hyperboloid r e p r e s e n t i n g t h e e m i t t e r ) .
Figure 6 shows t h e r e l a t i v e ion dose r a t e a s a f u n c t i o n of ion k i n e t i c energy,
The d i s t r i b u t i o n f o r a r e l a t i v e l y l a r g e e m i t t e r r a d i u s (Figure 6 ) d i v i d e s i n t o two d i s t i n c t regimes: t h e low energy (25-500eV) regime dominated by t h e boltzmannfactor and t h e high energy regime which i n t h e one-dimensional approximation is dominated by t h e i n v e r s e f i e l d dependence. I t is apparent from Figure 6 t h a t ion bombardment a t very low temperatures s i g n i f i c a n t l y i n c r e a s e s t h e low energy ion dose r a t e which a c t i v a t e s s u r f a c e migration without s p u t t e r i n g . The 77K d i s t r i b u t i o n i n Figure 6 i s not s i g n i f i c a n t l y d i f f e r e n t from the 294K d i s t r i b u t i o n f u n c t i o n . Its shape is p r i m a r i l y determined by t h e energy dependence of t h e e l e c t r o n impact i o n i z a t i o n c r o s s s e c t i o n . f 4 For small e m i t t e r r a d i i , t h e r a p i d f i e l d decrease with a x i a l d i s t a n c e decreases the e f f e c t of t h e c o n c e n t r a t i o n f a c t o r ; however, f o r small e m i t t e r r a d i i , t h e low e l e c t r o n emission voltages i n h e r e n t l y make a low energy ion d i s t r i b u t i o n .
Conclusion
Activation of s u r f a c e s e l f d i f f u s i o n by ~ e + impact with r e s u l t i n g f i e l d s t a b l i z e d growth can occur a t 5K without e x t e n s i v e r a d i a t i o n damage. Gas b l i s t e r s a r e not observed and bulk damage such a s microcrystal formation can be avoided.
P o s s i b i l i t i e s f o r ion beam source a p p l i c a t i o n of d i f f u s i o n l i m i t e d e m i t t e r morphological changes is g r e a t l y enhanced by helium ion a c t i v a t i o n (5K He gas) which minimizes chemical e f f e c t s of t h e implanted gas s p e c i e s ; 5K hydrogen ion a c t i v a t e d s t r u c t u r e s a r e s u b j e c t t o implanted hydrogen embrittlement. Surface selvedge o r s u r f a c e damage is observed and must be minimized. R e c r y s t a l l i z a t i o n of the s u r f a c e l a y e r and protuberance by t h e r m a l - f i e l d techniques w i l l f u r t h e r i n c r e a s e t h e s t r u c t u r a l i n t e g r i t y of t h e processed endform. Successful
p r o t u b e r a n c e growth on i r i d i u m h a s t h e a d v a n t a g e o f i n c r e a s e d r e s i s t a n c e t o chemical e f f e c t s which d e g r a d e l o n g term s o u r c e performance and c o m p l i c a t e e m i t t e r o p e r a t i o n .
I n v e s t i g a t i o n 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 a c t i v a t i o n by v e r y low energy i o n s , from 10-100eV, i s w a r r a n t e d because s u c h p r o c e s s i n g would e l i m i n a t e s p u t t e r i n g and b e n e f i t t h e growth p r o c e s s i n s e v e r a l t e c h n i c a l ways: i ) Decrease l a r g e e n e r g y t r a n s f e r s which l i m i t t h e t o t a l growth time f o r a s i n g l e m i c r o s t r u c t u r e , i i ) Decrease t h e implanted f r a c t i o n , i i i ) D e c r e a s e t h e d e p t h and e n e r g y of t h e s u b s t r a t e damage d i s t r i b u t i o n , and i v ) Decrease t h e a c t i v a t i o n energy n e c e s s a r y f o r t h e r m a l d e s o r p t i o n l 5 of t h e implanted helium.
Acknouledgements
T h i s r e s e a r c h i s d i r e c t e d t o t h e development o f g a s e o u s f i e l d e m i s s i o n under a program o f t h e N a t i o n a l Research and Resource F a c i l i t y f o r Submicron S t r u c t u r e s , NSF Grant ECS-8200312.
The c o n t r i b u t i o n s of i n d i v i d u a l s i n MSC, NRRFSS, and LASSP on t h e t e c h n i c a l s u p p o r t s t a f f a r e g r a t e f u l l y acknowledged.
R e f e r e n c e s
1 H.D. Beckey, F i e l d I o n i z a t i o n Mass S p e c t r o m e t r y (Pergamon, Oxford, 1971 p. 26.
2 G.R. Hanson and B.M. S i e g e l , J. Vac. S c i . Technol. 16, 1875 ( 1 9 7 9 ) .
3 A. J a s o n , B. H a l p e r n , M.G. Inghram, and R. Gomer, J . Chem. Phys. 52, 2227 (1 970).
4 Y .C. Chen and D . N . Seidman, S u r f . S c i . 7, 231 (1 971 ) .
5 J.C. Wiesner and T.E. E v e r h a r t , J . Appl. Phys. 411, 2142 ( 1 9 7 3 ) . 6 J . Y . C a v a i l l e and M . D r e c h s l e r , S u r f . S c i . 15, 342 (1978).
7 Zh.1. Dranova and I . M . M i k h a i l o v s k i i , S o v i e t Phys.-Solid S t a t e 12, 104 ( 1 9 7 2 ) . 8 P. Schwoebel and G . Hanson, J . Appl. Phys. 56, 2101 ( 1 9 8 4 ) .
9 P.R. Schwoebel and G.R. Hanson, J . Vac. S c i . Technol. B 3 ( 1 ) , 214 ( 1 9 8 5 ) . 10 B. Halpern and R . Gomer, J. Chem. Phys. 51, 3043 (1969).
11 P.C. B e t t l e r and F.M. C h a r b o n n i e r , Phys. Rev. 2, 8 5 (1960).
12 J.M. W a l l s , R.M. Boothby and H.N. Southworth, S u r f . S c i . 61, 419 (1976).
1 3 R. S m i t h , J . Phys. D.: Appl. Phys. 2, 1045 (1984).
14 A. von E n g e l , I o n i z e d Gases (Oxford, Clarendon P r e s s , 1955) p. 52.
1 5 E.V. Kornelsen, Can. J. Phys. 48, 2812 (1970).
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Figures
Figure 1. Hei f i e l d i o n image - l o c a l i z e d e m i s s i o n from a s u r f a c e m i c r o s t r u c t u r e grown on an a n n e a l e d t u n g s t e n e m i t t e r by 5 K helium c a t h o d e bombardment. The p a t t e r n was photographed e v e r y 15 m i n u t e s .
Figure 2. F i e l d e l e c t r o n micrograph o f <TOO> I r i d i u m a f t e r t h e growth of a m i c r o s t r u c t u r e by 5 K helium c a t h o d e bombardment.
Figure 3. He' f i e l d ion image of s u b s t r a t e e f f e c t s due t o 5K helium cathode bombardment. a ) before growth: evap. voltage - 7021V, image voltage - 56738.
b ) a f t e r cathode bombardment induced growth on the (100) f a c e s : image voltage -
2676V. c ) a f t e r some evaporation: evap. voltage - 5766V, image voltage - 5766V.
d ) a f t e r f u r t h e r evaporation: evap. voltage - 6700V, image v o l t a g e - 5812V.
e ) a f t e r r e t u r n i n g evaporation voltage t o t h e i n i t i a l value: evap. voltage -
6978V, image v o l t a g e - 5927V..
Figure 4. S p u t t e r y i e l d measurements Figure 5. Schematic r e p r e s e n t i n g one- summary, H.H. Anderson and H.L. Bay, dimensional ion dose r a t e c a l c u l a t i o n . S p u t t e r i n g by P a r t i c l e Bombardment I
(Springer-Verlag, B e r l i n , 1981 p. 145.
JOURNAL DE PHYSIQUE
1
0.01 0,l 1 10
ELECTRON KINETIC ENERGY / ION &YBAlUXNT ENERGY (KEV)
Figure 6. Ion dose rate as a function of ion energy for a one dimensional calculation using a hyperbololdal endform.
