MATERIALS CONSIDERATIONS IN LIQUID METAL ION SOURCE DEVELOPMENT

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MATERIALS CONSIDERATIONS IN LIQUID METAL ION SOURCE DEVELOPMENT

M. Bozack, L. Swanson, A. Bell

To cite this version:

M. Bozack, L. Swanson, A. Bell. MATERIALS CONSIDERATIONS IN LIQUID METAL ION SOURCE DEVELOPMENT. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-95-C2-100.

�10.1051/jphyscol:1986214�. �jpa-00225645�

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JOURNAL D E PHYSIQUE

Colloque C2, suppl6ment au n 0 3 , Tome 47, mars 1986 page c2-95

MATERIALS CONSIDERATIONS IN LIQUID METAL ION SOURCE DEVELOPMENT

M.J. BOZACK*, L.W. SWANSON and A.E. BELL

Oregon Graduate Center, 19600 NW von Neumann Drive, Beaverton, OR 97006, U.S.A.

Abstract

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Successful development of a liquid metal ion source depends upon the satisfactory confluence of several critical design considerations. At least nine desirable properties of the alloy and substrate exist which cannot be fully and simultaneously satisfied in a given contact system. A compromise must usually be made based upon the most important source characteristics. We review these properties, with emphasis on the material and thermodynamic behavior of liquid metal alloy surf aces.

I

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INTRODUCTION

Liquid metal ion sources (MIS) of high melting nonmetals may be achieved by selection of a low melting eutectic alloy and a suitable substrate material. The choice of the alloy and substrate is based on a number of considerations. The alloy must be electrically conductive, have a low vapor pressure at the melting point, and possess low viscosity in order to prevent the formation of jets in the beam. The substrate must allow for good wetting without excessive reaction with the alloy. In this paper we identify several key features of the liquid alloy and substrate which allow for successful development of MIS. Despite the numerous and often conflicting requirements, it is possible to achieve long-lived sources for a variety of technologically important ions.

I1

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REQUIREMENTS FOR THE IDEAL LIQUID METAL ION SOURCE

1. A Low Melting Temperature. The first desirable property of the alloy is that it possess a low melting point. Low melting temperatures are required to minimize reaction between alloy and substrate, and are essential to suppress reaction in many well-wetted contact systems of liquid alloys and refractory metal substrates.

High melting materials may be made compatible by formation of binary and ternary eutectic compositions which lower the melting temperature. The best solution is to choose a shallow eutectic that is low-melting or a low melting compound containing the element of interest. A deep eutectic is less favorable because a slight digression from the eutectic composition (which may occur by interdiffusion between alloy and substrate) begins precipitation of second phase material. A

congruently-melting compound, by contrast, remains liquid during such digressions.

Further, compound formation frequently acts to lower the vapor pressure of highly volatile species through liquid interactions within the compound. On the other hand, deep eutectics usually possess lower melting temperatures than compounds, so often a compromise must be made between low melting points and broad

stoichiometries over which the alloy remains liquid.

'present address : Surface Science Center, University o f Pittsburgh, Pittsburgh, P A 15260, U.S.A.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986214

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JOURNAL DE PHYSIQUE

2. Low V o l a t i l i t y a t t h e Melting Temperature. The second d e s i r a b l e p r o p e r t y of t h e l i q u i d a l l o y i s t h a t i t p o s s e s s low v o l a t i l i t y a t t h e m e l t i n g p o i n t . Low vola- t i l i t y i s n e c e s s a r y t o conserve t h e l i q u i d f i l m supply and promote l o n g l i f e . Elemental boron and a r s e n i c , f o r example, cannot be used f o r IMIS because both of t h e s e e l e m e n t s have h i g h vapor p r e s s u r e s a t t h e i r m e l t i n g p o i n t s and would v o l a t i - l i z e completely i f o p e r a t e d a t t h i s t e m p e r a t u r e f o r any s i g n i f i c a n t p e r i o d of t i m e . A g e n e r a l r u l e of thumb i s t h a t component p a r t i a l p r e s s u r e s g r e a t h e r t h a n t o r r a r e u n a c c e p t a b l e . P r e s e n t e v i d e n c e from s t u d i e s of s e v e r a l l i q u i d a l l o y s s u g g e s t t h a t component vapor p r e s s u r e s may be lowered by choosing combinations which p o s s e s s a low ( i . e . , h i g h l y n e g a t i v e ) Gibbs f r e e energy of formation. Such com- b i n a t i o n s have s u f f i c i e n t s t a b i l i t y t o lower t h e thermodynamic a c t i v i t y of t h e i o n i z i n g component, and hence, t h e component vapor p r e s s u r e above t h e a l l o y . The b e s t s o l u t i o n i s t o s e l e c t a l l o y compositions t h a t a r e c o n g r u e n t l y v a p o r i z i n g a t t h e m e l t i n g p o i n t . T h i s i s because p r e f e r e n t i a l v a p o r i z a t i o n of t h e a l l o y components e v e n t u a l l y a l t e r t h e i n i t i a l s t o i c h i o m e t r y and c a u s e changes i n t h e m e l t i n g temperature.

3. High R e l a t i v e Bulk C o n c e n t r a t i o n of Ion S p e c i e s of I n t e r e s t . The t h i r d d e s i r a b l e p r o p e r t y of t h e a l l o y i s t h a t i t p o s s e s s a high r e l a t i v e bulk c o n c e n t r a - t i o n of t h e i n t e n d e d i o n i z i n g element. It is t h e bulk, and n o t t h e s u r f a c e , con- c e n t r a t i o n of t h e i n t e n d e d element t h a t i s i m p o r t a n t because t h e s t e a d y s t a t e e m i s s i o n c h a r a c t e r i s t i c s of M I S r e f l e c t t h e bulk s t o i c h i o m e t r y of t h e m a t e r i a l ( s e e Table I ) f o r IMIS where l i t t l e a l l o y / s u b s t r a t e i n t e r a c t i o n o c c u r s and which o p e r a t e under c o n d i t i o n s of congruent f i e l d e v a p o r a t i o n . F u r t h e r , assuming t h e f i e l d e v a p o r a t i o n mechanism of i o n f o r m a t i o n t o be v a l i d , t h e c u r r e n t of i o n spe- c i e s k i s l i n e a r l y dependent upon t h e f r a c t i o n a l bulk c o n c e n t r a t i o n of atom k /1/.

Best y i e l d s o f , s a y , B+ o r AS+ i n a LMIS of B and As a r e t h e r e f o r e o b t a i n e d by choosing a n a l l o y combination having t h e h i g h e s t mole f r a c t i o n of B o r A s com- p a t i b l e w i t h low m e l t i n g . S u r f a c e s e g r e g a t i o n of low s u r f a c e t e n s i o n a l l o y components i n t h e l i q u i d / 2 / does n o t appear t o c o n t r i b u t e t o t h e beam s t o i c h i o m e t r y . 4. Low S u r f a c e F r e e Energy and Good Wetting. Fourth, i t i s d e s i r a b l e f o r t h e a l l o y t o p o s s e s s a low s u r f a c e f r e e energy. Low s u r f a c e t e n s i o n m a t e r i a l s promote w e t t i n g of t h e a l l o y t o t h e s u b s t r a t e . Good w e t t i n g , d e f i n e d a s a c o n t a c t a n g l e c o n s i d e r a b l y l e s s t h a n 90 d e g r e e s , i s n e c e s s a r y t o f a c i l i t a t e l i q u i d flow from t h e r e s e r v o i r t o t h e apex t i p where e m i s s i o n occurs. The governing r e l a t i o n i s Young's e q u a t i o n :

c o s C = (Tsv

-

T s ~ ) Tlv

where Tsv, Tsl, and Tlv a r e t h e s o l i d - v a p o r , s o l i d - l i q u i d , and l i q u i d - v a p o r s u r f a c e t e n s i o n s r e s p e c t i v e l y and C i s t h e c o n t a c t a n g l e . Young's e q u a t i o n shows t h a t t h e b e s t w e t t i n g is r e a l i z e d when t h e l i q u i d a l l o y has t h e l o w e s t p o s s i b l e s u r f a c e f r e e energy. Otherwise s t a t e d , w e t t i n g i s a consequence of t h e r e l a t i v e magnitudes of cohesion and a d h e s i o n i n a c o n t a c t system. I f t h e a d h e s i v e f o r c e s between a l l o y and s u b s t r a t e a r e g r e a t e r t h a n t h e c o h e s i v e f o r c e s of t h e a l l o y and s u b s t r a t e s e p a r a t e l y , then w e t t i n g i s l i k e l y t o occur. Our s t u d i e s i n d i c a t e t h a t t h i s s i m p l e model of w e t t i n g i s complicated by s u r f a c e s e g r e g a t i o n of low-level i m p u r i t i e s i n t h e l i q u i d a l l o y s which i n h i b i t r e a c t i o n between a l l o y and s u b s t r a t e / 3 / .

U t i l i z i n g s u r f a c e c o a t i n g s composed of m a t e r i a l s r e a c t i v e t o t h e i m p u r i t i e s a l l o w s w e t t i n g t o occur by t y i n g up t h e i m p u r i t i e s a t t h e i n t e r f a c e .

5. Low S o l u b i l i t y of Alloy i n S u b s t r a t e . F i f t h , i t i s i m p o r t a n t t h a t w e t t i n g o c c u r w i t h o u t e x c e s s i v e a t t a c k of t h e s u b s t r a t e . The r a t e and e x t e n t of d i f f u s i o n and r e a c t i o n of a l l o y components i n t o t h e s u b s t r a t e m a t e r i a l must be minimal. Our s t u d i e s have confirmed t h a t t h i s mechanism i s t h e major l i f e t i m e - l i m i t i n g mechanism f o r c o n t a c t systems composed of l i q u i d m e t a l a l l o y s of B w e t t e d t o r e f r a c t o r y m e t a l s u b s t r a t e s . A r e p r e s e n t a t i v e example is shown i n F i g u r e 1. I n t h i s s t u d y , t h e molten s u r f a c e of a PtB e u t e c t i c w e t t e d t o an Re s u b s t r a t e was monitored o v e r time by Auger s p e c t r o s c o p y . Excessive r e a c t i o n between a l l o y and s u b s t r a t e was observed t o o c c u r w i t h i n m i n u t e s a f t e r c o n t a c t , and t h e a l l o y r e q u i r e d s u c c e s s i v e l y h i g h e r t e m p e r a t u r e s t o m a i n t a i n i t i n t h e molten s t a t e . Auger s p e c t r a d u r i n g t h i s p e r i o d

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showed monotonically d e c r e a s i n g s u r f a c e c o n c e n t r a t i o n s of B w i t h c o r r e s p o n d i n g i n c r e a s e s i n P t . The system e v e n t u a l l y s u f f e r e d c a t a s t r o p h i c breakdown a f t e r a few hours. Auger s p e c t r a of t h e corroded s u r f a c e a f t e r breakdown a r e shown i n F i g u r e 1. I n a d d i t i o n t o d e p l e t e d s u r f a c e c o n c e n t r a t i o n s of B, t h e s u r f a c e shows r e l a t i - v e l y small amounts of Re. A h i g h r e s o l u t i o n Auger a n a l y s i s of a f r a c t u r e d c r o s s - s e c t i o n of t h e f a i l e d c o n t a c t system i s shown i n F i g u r e 2. A c e n t r a l r e g i o n t h a t is p r i m a r i l y Re and P t i s surrounded by a second r e g i o n which has l a r g e amounts of B ( t h e a l l o y had wet b o t h s i d e s of t h e R e ) . It is c l e a r t h a t both B and P t have d i f f u s e a i n t o t h e s u b s t r a t e , and t h a t P t a p p e a r s t o be p r e f e r e n t i a l l y a t t a c k i n g t h e Re. We conclude t h a t i t is d i f f i c u l t t o i n h i b i t r e a c t i o n a t t h e m e l t i n g tem- p e r a t u r e s of most c o n t a c t systems composed of b o r i d e m e t a l a l l o y s and r e f r a c t o r y m e t a l s . D i f f u s i o n of a l l o y components i n t o t h e s u b s t r a t e a l t e r s i t s e l e c t r i c a l r e s i s t i v i t y , l e a d i n g t o a s i t u a t i o n where t h e c i r c u i t power must be c o n s t a n t l y monitored t o p r e v e n t r a d i c a l changes i n t e m p e r a t u r e f o r r e s i s t i v e l y heated s o u r c e c o n f i g u r a t i o n s .

Fig. 1

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Auger s u r f a c e e l e m e n t a l composition f o r t h e Pt72B28 on Re a l l o y a t 300 K f o l l o w i n g c a t a s t r o p h i c f a i l u r e .

Fig. 2

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Auger s u r f a c e e l e m e n t a l composition of t h r e e p o s i t i o n s w i t h i n a f r a c t u r e d c r o s s - s e c t i o n of t h e f a i l e d Pt72B28 on Re system above a t 300 K.

-

Z eo-

-

72.5

70

-

z Position @ Position @

2 6 0 - ^56.0

43.9

6. Low S o l u b i l i t y of S u b s t r a t e i n Alloy. The s i x t h d e s i r a b l e p r o p e r t y of t h e con- t a c t system i s t h a t t h e e x t e n t of d i s s o l u t i o n of t h e s u b s t r a t e i n t o t h e a l l o y be minimal. I n t h e c a s e of a e u t e c t i c m i x t u r e on a r e f r a c t o r y m e t a l s u b s t r a t e , d i s s o - l u t i o n of t h e s u b s t r a t e i n t o t h e a l l o y changes t h e composition of t h e e u t e c t i c and a c t s t o i n c r e a s e t h e m e l t i n g p o i n t of t h e l i q u i d a l l o y . The e m i t t e r t h e n s h u t s o f f . To r e m e l t t h e a l l o y , an i n c r e a s e i n c i r c u i t power i s n e c e s s a r y , b u t t h i s i n c r e a s e s t h e r a t e of d i s s o l u t i o n and r e s u l t s i n a runaway c o n d i t i o n which ter- m i n a t e s w i t h c a t a s t r o p h i c breakdown of t h e e m i t t e r . F u r t h e r , we have observed a

z 4 0 - 390

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14 7 10.5

7 2

1.6 3.7

a ^or,

13 8 3.4

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~ 2 - 9 8 JOURNAL DE PHYSIQUE

number of c a s e s where t h e d i s s o l v i n g m e t a l forms m e t a l b o r i d e s i n t h e l i q u i d a l l o y and s u p p r e s s e s t h e r a t i o of B i n t h e beam. To c o r r e c t t h i s problem, s e l e c t i o n of an i n e r t s u b s t r a t e i s n e c e s s a r y , b u t t h i s i s u s u a l l y i n c o m p a t i b l e w i t h good w e t t i n g . Wetting o c c u r s by l o w e r i n g t h e t o t a l energy of t h e c o n t a c t system d u r i n g chemical bond f o r m a t i o n , s o t h e l i q u i d a l l o y must i n t e r a c t w i t h t h e s u b s t r a t e t o some e x t e n t . The d u a l r e q u i r e m e n t s of good w e t t i n g and low r a t e s of t h e chemical r e a c t i o n a r e p a r t i c u l a r l y c h a l l e n g i n g i n l i g h t of t h e mutual e x c l u s i v i t y of t h e s e p r o p e r t i e s . It may be t h a t t h e b e s t s o l u t i o n i s t o have no s u b s t r a t e a t a l l by development of a consumable e l e c t r o d e w i t h l o c a l i z e d h e a t i n g of t h e apex r e g i o n . I t i s q u e s t i o n a b l e , however, whether such a c o n f i g u r a t i o n would be a d e q u a t e f o r focused beam a p p l i c a t i o n s .

7. F a v o r a b l e Mechanical, E l e c t r i c a l , and Vacuum P r o p e r t i e s . Seventh, i t i s d e s i r a b l e f o r t h e c o n t a c t system t o p o s s e s s f a v o r a b l e mechanical, e l e c t r i c a l , and vacuum p r o p e r t i e s . The s u b s t r a t e m a t e r i a l must be e l e c t r i c a l l y c o n d u c t i v e f o r r e s i s t . i v e l y heated n e e d l e s o u r c e s , and be r e a d i l y machined o r e t c h e d i n t o s h a r p t i p s . The s t r u c t u r e must have s u f f i c i e n t mechanical s t r e n g t h t o t o l e r a t e e l e c t r i c f i e l d s of t h e o r d e r of t e n t h s of v o l t s p e r angstrom. I f t h e s u b s t r a t e i s too b r i t t l e , a s i s t h e c a s e f o r many f i n e g l a s s y n e e d l e s , i t may n o t s u r v i v e w e t t i n g and assembly o p e r a t i o n s . The c o e f f i c i e n t of thermal expansion of t h e s u b s t r a t e and i t s a b i l i t y t o w i t h s t a n d t e m p e r a t u r e s n e a r 1000 K must be a d e q u a t e t o p r e v e n t t h e r - mal d r i f t i n t h e f o c u s i n g column. The s u b s t r a t e must be e a s y t o c l e a n i n o r d e r t o e l i c i t c o n s i s t e n t w e t t i n g by t h e a l l o y . For example, i t i s well-known t h a t o x i d e f o r m a t i o n which forms on exposure of m e t a l l i c s u b s t r a t e s t o atmosphere can dramati- c a l l y a f f e c t t h e c o n t a c t a n g l e / 4 / . Auger s t u d i e s we have performed on many com- m e r c i a l l y produced r e f r a c t o r i e s s u c h a s W and Re showed s i g n i f i c a n t c o n c e n t r a t i o n s of s u r f a c e i m p u r i t i e s even a f t e r h i g h t e m p e r a t u r e thermal a n n e a l i n g . F i n a l l y , t h e LMIS should be a b l e t o f u n c t i o n e f f e c t i v e l y i n l e s s t h a n u l t r a - h i g h vacuum con- d i t i o n s .

8. Congruent Ion Formation. E i g h t h , t h e e x i s t e n c e of congruent i o n f o r m a t i o n i s an i m p o r t a n t requirement f o r a M I S . Congruent i o n f o r m a t i o n means t h a t t h e s t o i c h i o m e t r y of t h e beam r e f l e c t s t h e s t o i c h i o m e t r y of t h e l i q u i d a l l o y producing t h e i o n s . A noncongruent i o n forming LMIS w i l l n o t o p e r a t e f o r t h e f o l l o w i n g r e a s o n s : (1) a h i g h l y noncongruent i o n beam w i l l l e a d t o l o c a l c o n c e n t r a t i o n gra- d i e n t s n e a r t h e e m i t t e r apex c a u s i n g l o c a l s o l i d i f i c a t i o n of t h e IMIS; ( 2 ) a moderately noncongruent i o n beam w i l l g r a d u a l l y a l t e r t h e s o u r c e r e s e r v o i r com- p o s i t i o n and u l t i m a t e l y r e s u l t i n (1); (3) c o n c u r r e n t w i t h (1) and (2), a noncongruent i o n forming M I S s o u r c e may e x h i b i t a beam s t o i c h i o m e t r y u n a c c e p t a b l y low i n t h e i o n s p e c i e s of i n t e r e s t .

It i s c l e a r t h a t a s t a b l e , l o n g l i v e d LMIS must e x h i b i t congruent o r n e a r congruent i o n formation. Assuming a f i e l d e v a p o r a t i o n mechanism f o r i o n f o r m a t i o n , t h e a c t i - v a t i o n b a r r i e r f o r f i e l d e v a p o r a t i o n Q(F) may be w r i t t e n a s :

where Ii i s t h e f i r s t i o n i z a t i o n p o t e n t i a l of t h e i t h e v a p o r a t i n g s p e c i e s , W i s t h e s u r f a c e work f u n c t i o n , Hi i s t h e v a p o r i z a t i o n energy of t h e i t h e v a p o r a t i n g spe- c i e s , and f ( F ) depends on t h e d e t a i l s of t h e e v a p o r a t i o n model /5,6/. For t h e c a s e of a b i n a r y a l l o y of components A and B o p e r a t i n g a s a LMIS, i t may be shown /7/

t h a t t h e e x p r e s s i o n QA

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^QB ⁼⁰p r o v i d e s a measure of t h e congruency of i o n f o r - mation. I f one assumes t h a t b o t h a l l o y components e x p e r i e n c e t h e same work func- t i o n and t h a t f A ( F )

-

fB(A), which i s approximately t r u e f o r t h e image hump model, t h e n t h e requirement t h a t

must be s a t i s f i e d f o r congruent i o n formation. T h i s e x p r e s s i o n p l a c e s a fundamen- t a l requirement on t h e i o n i z a t i o n and v a p o r i z a t i o n e n e r g i e s of t h e a l l o y components

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TABLE I TABLE 11

A l l o y and Beam Compositions f o r V a r i o u s Summary o f H e a t s o f V a p o r i z a t i o n A l l o y s a t 20 FA T o t a l C u r r e n t ( a t 1300 K) H and I o n i z a t i o n

P o t e n t i a l s I f o r V a r i o u s Elements

A l l o y Composition/ Beam

S u b s t r a t e Composition Element H (eV) I (eV) H

+

I (eV)

* I n d i c a t e s a l l o y w i t h minimal s u b s t r a t e l a l l o y i n t e r d i f f u s i o n and n e a r c o n g r u e n t i o n f o r m a t i o n .

I n T a b l e I1 a r e l i s t e d t h e v a l u e s o f I

+

^H f o r v a r i o u s e l e m e n t s where H is t h e h e a t of v a p o r i z a t i o n a t 1300 K of t h e p u r e e l e m e n t s . The e n e r g e t i c s of t h e f i e l d e v a p o r a t i o n mechanism r e q u i r e t h a t H s h o u l d be t a k e n from t h e a l l o y and n o t from t h e p u r e e l e m e n t s . However, a s a f i r s t a p p r o x i m a t i o n , we c a n e s t i m a t e t h e com- p a t i b i l i t y of v a r i o u s a l l o y c o m b i n a t i o n s w i t h r e s p e c t t o c o n g r u e n t i o n f o r m a t i o n based on t h e T a b l e I1 v a l u e s . On t h i s b a s i s , t h e model p r e d i c t i o n s f o r v a r i o u s c o m b i n a t i o n s of t h e d i f f e r e n c e i n t h e Q-values must be s a t i s f i e d . Those a l l o y com- b i n a t i o n s i n d i c a t e d by

*

i n T a b l e I11 g i v e v a l u e s of QA

-

QB n e a r e s t t o z e r o and t h e r e f o r e might b e e x p e c t e d t o show n e a r c o n g r u e n t i o n f o r m a t i o n . On t h i s b a s i s , t h e o n l y p r o m i s i n g a l l o y c o m b i n a t i o n f o r a s o u r c e of B i s t h e PtB a l l o y system. As a s o u r c e of Be and S i , t h e AuSiBe i s p r e d i c t e d t o be a p r o m i s i n g a l l o y

combination.

S e v e r a l o f t h e T a b l e I11 a l l o y c o m b i n a t i o n s have been i n v e s t i g a t e d f o r t h e i r poten- t i a l a s LMIS. The r e s u l t s of t h e s e i n v e s t i g a t i o n s a r e g i v e n i n T a b l e s I and I V . T a b l e I compares t h e s t o i c h i o m e t r i e s of t h e beam w i t h t h a t of t h e a l l o y

.

^{The pre-}

d i c t i o n s of t h e f i e l d e v a p o r a t i o n model a r e g e n e r a l l y f o l l o w e d . F o r example, t h e YNiB a l l o y d o e s n o t p r o v i d e any B i n t h e beam a s e x p e c t e d . On t h e o t h e r hand, t h e PtB a l l o y i s b o t h s t a b l e and n e a r s t o i c h i o m e t r y . The somewhat lower r a t i o of B/Pt i n t h e beam compared t o t h e o r i g i n a l a l l o y i s due t o l o s s o f B by d i f f u s i o n i n t o t h e Re s u b s t r a t e .

The PdB and NIB a l l o y a r e n o t e x p e c t e d t o be good s o u r c e s o f B. Although t h e B/Pd r a t i o i n t h e beam d o e s n o t d i f f e r from t h a t of t h e a l l o y as much as t h e YNiB a l l o y , i t has v e r y l i t t l e B+ i n t h e beam a s shown i n T a b l e I V . Most of t h e B i n t h e beam i s t i e d up a s P ~ B + . S i n c e we have no v a l u e s f o r H o r I f o r PdB, t h e model c a n n o t e used t o e v a l u a t e t h i s p o s s i b i l i t y . N e v e r t h e l e s s , t h e f a v o r a b l e f o r m a t i o n of P ~ B + , w h i l e i t c a n h e l p t o m a i n t a i n c o n g r u e n t f i e l d e v a p o r a t i o n and a smooth r u n n i n g s o u r c e , i s of l i t t l e u s e i n p r o v i d i n g a l a r g e f r a c t i o n of B+ i n t h e beam.

The AuSi a l l o y forms Au and S i c o n g r u e n t l y i n t h e beam a s p r e d i c t e d by t h e model.

Thus, w i t h t h e l i m i t e d r e s u l t s a v a i l a b l e a t t h i s t i m e , i t a p p e a r s t h a t t h e f i e l d e v a p o r a t i o n model u s i n g t h e v a l u e s of H of t h e p u r e e l e m e n t s , r a t h e r t h a n t h e more a p p r o p r i a t e v a l u e s of H from t h e r e s p e c t i v e a l l o y s , g i v e s a f i r s t o r d e r b a s i s f o r s e l e c t i o n of c o m p a t i b l e a l l o y c o m b i n a t i o n s f o r M I S .

9. F a v o r a b l e E l e c t r o n O p t i c a l C h a r a c t e r i s t i c s

Assuming i t h a s been p o s s i b l e t o s a t i s f y t h e m a t e r i a l and congruency r e q u i r e m e n t s l i s t e d above, i t is s t i l l p o s s i b l e f o r t h e ZMIS t o f a i l by way o f i n a d e q u a t e e l e c t r o n o p t i c a l p r o p e r t i e s . For f o c u s e d beam and i m p l a n t a t i o n a p p l i c a t i o n s , i t i s

(7)

JOURNAL DE PHYSIQUE

TABLE 111

E4odel P r e d i c t i o n s f o r V a r i o u s A l l o y Combinations

A l o y (AB) QB

-

^QA ( e v )

BNi BPd BP t BeAu*

SiAu*

AuSiBe*

BeAu BeSi SiAu YNiB

BNi BY N i Y

*Near c o n g r u e n t i o n f o r m a t i o n e x p e c t e d

TABLE I V

% Abundance o f V a r i o u s Elements i n t h e Beam a t 20 microamps f o r a Pd72B28 LMIS on Si/Carbon (T = 1165 K)

% 66.9 16.2 6.4

3.7 T o t a l Pd 77.4

3.7

1.7 T o t a l B 16.5

1.2

. 3 T o t a l S i 6.0 .2

.2 .17 .01

n e c e s s a r y f o r t h e s o u r c e t o have a s m a l l v i r t u a l s o u r c e s i z e and h i g h a n g u l a r i n t e n s i t y . F u r t h e r , t h e r e l a t i v e i n t e n s i t i e s of t h e v a r i o u s i o n s p e c i e s must be t a v o r a b l e and e x h i b i t low e n e r g y s p r e a d s .

111

-

C o n c l u s i o n s

I n view o f t h e numerous and o f t e n c o n f l i c t i n g r e q u i r e m e n t s f o r l i q u i d a l l o y i o n s o u r c e s , i t is r e m a r k a b l e t h a t s o u r c e s may be developed which s a t i s f y many o f t h e s e r e q u i r e m e n t s t o a h i g h d e g r e e .

REFEKENCES

/1/ M u l l e r E.W. and Tsong T.T., F i e l d I o n M i c r o s c o p y - P r i n c i p l e s and A p p l i c a t i o n c ; (Amsterdam: E l s e v i e r , 1969).

/ 2 / Miedema B.R., 2. M e t a l l .

9

(1978) 455.

1 3 1 Bozack M . J . , Swanson L.W., and B e l l A.E., e l s e w h e r e i n t h i s volume.

/ 4 / Addisson C.C., and I b e r s o n E., J. Chem. Soc.

116

(1965) 1437.

/ 5 / Gomer R. and Swanson L.W., J. Chem. Phys.

2

(1963) 1613.

1 6 1 McKinstry D., S u r f . S c i .

2

(1972) 37.

1 7 1 Bozack M . J . , Swanson L.W., and O r l o f f J., SEM, t o be p u b l i s h e d .