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MECHANISMS OF ION FORMATION IN LIQUID METAL ION SOURCES
D. Kingham, L. Swanson
To cite this version:
D. Kingham, L. Swanson. MECHANISMS OF ION FORMATION IN LIQUID METAL ION SOURCES. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-133-C9-138.
�10.1051/jphyscol:1984923�. �jpa-00224402�
MECHANISMS OF ION FORMATION IN LIQUID METAL ION SOURCES
+ * D.R. Kmgham and L.W. Swanson
Cavendish Laboratory, Madingley Road, Cambridge CB3 ORE, U.K.
*Oregon Graduate Center, 19600 NW Walker Road, Beaverton, OR 97006, U.S.A.
Résumé - Un modèle théorique sur les sources d'ions à métal liquide a été développé qui permet d'expliquer : 1) la forme et les dimensions de la zone émissive ; 2) les mécanismes de formation des ions ; 3) les propriétés du faisceau ionique. Cet article est principalement consacré au § 2 ) . Nous montrons que l'évaporation de champ est le mécanisme dominant de l'émission ionique et que l'existence d'ions de charge double ou multiple provient d'un phénomène de post-ionisation. Ce modèle est ensuite utilisé pour étudier les propriétés du faisceau émis, telles que : l'intensité angulaire, le déficit et la largeur énergétique, l'abondance relative d'ions de différents états de charge.
Abstract - We have developed a theoretical model of Liquid Metal Ion Source (LMIS) operation to explain consistently; i) the shape and size of the ion emitting region, ii) the mechanisms of ion formation and iii) properties of the ion beam. In this paper we concentrate on ii), we find that field evaporation is the dominant ion formation mechanism and the occurrence of doubly or higher charged ions is attributed to a post-ionization mechanism. We discuss properties of the ion beams from LMIS such as angular intensity, energy deficit, energy spread and the relative abundance of different charge states in terms of our model of LMIS operation.
Liquid Metal Ion Sources (LMIS) are attracting increasing interest at the present time due to the potential applications of high-brightness, quasi-pxiint source ion beams such as microfabrication and microanalysis. Many different liquid metals and alloys have been successfully used in LMIS and a variety of ions can be produced. As yet, however, theoretical understanding of the processes contributing to ion formation is incomplete. Indeed two notable discussions of the mechanism of emission from LMI sources by Gomer /l/ and by Prewett et al. /2/ disagree significantly in their conclusions.
Our objective is to construct a model of LMIS operation which gives a consistent picture of three different, but closely related, aspects of LMI sources:
i. The shape and size of the ion emitting region.
ii. The mechanism of ion formation.
iii. Properties of the ion beam.
In this paper we explain our model of LMIS operation with particular emphasis on the mechanisms of ion formation.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984923
C9-134 JOURNAL DE PHYSIQUE
1. MECHANISMS OF I O N FORMATION
The 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 i s shown s c h e m a t i c a l l y i n f i g . 1. An atom w i t h b i n d i n g e n e r g y H i s f i e l d e v a p o r a t e d o v e r a b a r r i e r of h e i g h t Q(F) r e s u l t i n g i n a s i n g l y charged i o n . Higher c h a r g e s t a t e s may s u b s e q u e n t l y b e formed by p o s t - i o n i z a t i o n a t p o s i t i o n
Xi n f i g . 1. Dixon e t a l . / 3 / have s u g g e s t e d t h a t by
p i
comparing observed r a t i o s of doubly t o s i n g l y charged i o n s i n LMIS t o c a l c u l a t e d p r o b a b i l i t i e s of p o s t - i o n i z a t i o n / 4 / a n e s t i m a t e of apex f i e l d s t r e n g t h may b e made.
T h i s p o s s i b i l i t y h a s been f u r t h e r c o n s i d e r e d by Kingham / 5 / f o r v a r i o u s d i f f e r e n t LMI s o u r c e s . T a b l e 1 shows a comparison of t h e p o s t - i o n i z a t i o n model e s t i m a t e of t h e apex f i e l d s t r e n g t h , Fpi, w i t h a n e s t i m a t e , Fih, f o r f i e l d e v a p o r a t i o n of a s i n g l y charged i o n based o n t h e image-hump model. For most of t h e e l e m e n t a l i o n s o u r c e s i n t a b l e 1 (Al, Ga, I n and B i ) t h e v a l u e s of F and Fih a r e i n r e a s o n a b l e agreement and a r e a l l
p i
c l o s e t o 2 V I A . E l e m e n t a l Au i s a n e x c e p t i o n t o t h i s w i t h F
=3.5 V I A which i s i n p i
agreement w i t h t h e observed low t e m p e r a t u r e e v a p o r a t i o n f i e l d of Au, whereas t h e v a l u e of F i s known t o be t o o h i g h . I n t h e c a s e of t h e Au
i h gosilo a l l o y t h e F
p i v a l u e s d i s a g r e e , a p p a r e n t l y i n c o n t r a d i c t i o n t o t h e p o s t - i o n i z a t i o n model, b u t t h i s i s t h o u g h t t o b e due t o t h e f o r m a t i o n of some S i by f i e l d i o n i z a t i o n . F i e l d +
e v a p o r a t i o n and s u b s e q u e n t p o s t - i o n i z a t i o n can produce c h a r g e s t a t e s up t o a t l e a s t M* i n some c a s e s , whereas f i e l d i o n i z a t i o n c a n o n l y produce s i n g l y charged i o n s
( w i t h a n e g l i g i b l e p r o b a b i l i t y of s u b s e q u e n t p o s t - i o n i z a t i o n ) .
F i g . 1 - P o t e n t i a l e n e r g y diagram f o r f i e l d e v a p o r a t i o n of a s i n g l y charged i o n and p o s s i b l e p o s t - i o n i z a t i o n a t
X.. AE1 and bE2 a r e t h e i o n e n e r g y d e f i c i t s .
P=
observed by Swanson and co-workers. The c a l c u l a t e d v a l u e s of apex f i e l d s t r e n g t h based on t h e p o s t - i o n i z a t i o n model, Fpi, and t h e image-hump model, Fih, a r e shown.
Elements R e l a t i v e Abundance F (VIA) Fih (V/A)
p i i n LMIS M ~ + / M + a t 10 UA
( a ) e s t i m a t e of F t a k e n from r e f . 5 p i
( b ) r e s u l t s f o r a n Au gosilo a l l o y w i t h Fih based on p u r e m e t a l v a l u e s
-
( c ) r e s u l t s f o r a Y 62 N i 23B15 a l l o y w i t h Fih b a s e d on p u r e m e t a l v a l u e s 2+ +
( d ) r e l a t i v e abundance f o r Y i s y3+/y2+ n o t Y /Y
F i e l d e v a p o r a t e d i o n s h a v e a n energy d e f i c i t , AE, g i v e n by ( s e e f i g . 1 )
AEn
=H. + Cn Ii - n O c - Q(F) (1)
where O i s t h e work f u n c t i o n of t h e r e t a r d i n g e l e c r o d e and Ii i s t h e i t h i o n i z a t i o n p o t e n t i a l . T h i s f o r m u l a i s a p p r o p r i a t e t o t h e p o s t - i o n i z a t i o n model of f i e l d e v a p o r a t i o n a s w e l l a s t o t h e c o n v e n t i o n a l image-hump and charge-exchange models.
T h e r e i s a n i n t r i n s i c e n e r g y s p r e a d of about 1 eV due t o v a r i a t i o n i n H between
d i f f e r e n t e v a p o r a t i o n s i t e s and t o t h e r m a l energy. The p o s t - i o n i z a t i o n p r o c e s s may
i n t r o d u c e a n e x t r a b r o a d e n i n g which s h o u l d be l e s s t h a n 0.5 eV due t o t h e f i n i t e
w i d t h of t h e p o s t - i o n i z a t i o n zone / 5 / . The major c a u s e o f t h e observed e n e r g y
broadening i n a n LMIS i o n beam i s a s p a c e c h a r g e e f f e c t of coulomb i n t e r a c t i o n s
between i o n s i n t h e beam which r e s u l t s i n a t o t a l e n e r g y s p r e a d which i s u s u a l l y
observed t o be a t l e a s t 5 eV 161. It h a s r e c e n t l y been p o i n t e d o u t by Gesley and
Swanson / 7 / t h a t t h i s e f f e c t a l s o r e s u l t s i n a mass and c u r r e n t dependent s h i f t i n
t h e mean i o n energy. Doubly charged i o n s a r e e x p e c t e d t o have a reduced energy s p r e a d
b e c a u s e t h e y t r a v e l f a s t e r t h r o u g h t h e r e g i o n of h i g h s p a c e c h a r g e and t h e
p o s t - i o n i z a t i o n mechanism h a s o n l y a s m a l l e x t r a b r o a d e n i n g e f f e c t . T h i s i s i n
C9-136 JOURNAL DE PHYSIQUE
agreement w i t h o b s e r v a t i o n and i t h a s i m p o r t a n t i m p l i c a t i o n s f o r f i n e f o c u s s e d beam a p p l i c a t i o n s where t h e i o n beam p r o b e s i z e i s l i m i t e d by c h r o m a t i c a b b e r a t i o n / 8 / . Doubly c h a r g e d i o n s a l s o g i v e t h e a d v a n t a g e of d o u b l i n g t h e a v a i l a b l e i o n e n e r g y f o r a g i v e n v o l t a g e .
The p o s s i b i l i t y o f d e v e l o p i n g i o n s o u r c e s s p e c i f i c a l l y t o g i v e u s e f u l p r o p o r t i o n s o f doubly and h i g h e r c h a r g e d i o n s h a s been c o n s i d e r e d by Kingham / 5 / f o l l o w i n g Van d e Walle and Sudraud's o b s e r v a t i o n of mainly U'+ and u3+ i o n s from a uranium LMIS / g / .
The j e t - l i k e p r o t r u s i o n model of LMIS s h a p e ( s e e s e c t i o n 2 ) , which s u c c e s s f u l l y overcomes Gomer's / l / o b j e c t i o n t o a 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 , does n o t e l i m i n a t e t h e p o s s i b i l i t y of some c o n t r i b u t i o n of f i e l d i o n i z a t i o n t o t h e t o t a l i o n c u r r e n t . The t e m p e r a t u r e r i s e a t t h e apex o f s u c h a s h a p e i s much g r e a t e r t h a n t h a t f o r a t r u n c a t e d T a y l o r cone s h a p e f o r a g i v e n h e a t i n p u t t o t h e apex. Mair and A i t k e n /10/ have c o n s i d e r e d t h i s f o r a Ga s o u r c e and t h e y s u g g e s t t h a t t h e r m a l e v a p o r a t i o n would be n e g l i g i b l e . The p r e s e n t a u t h o r s a g r e e w i t h t h i s , b u t have found t h a t f o r o t h e r e l e m e n t s t h e r m a l e v a p o r a t i o n f o l l o w e d by f i e l d i o n i z a t i o n may make a s i g n i f i c a n t , b u t n o t dominant c o n t r i b u t i o n t o t h e t o t a l i o n c u r r e n t . F i e l d i o n i z a t i o n , u n l i k e f i e l d e v a p o r a t i o n , c a n o c c u r i n a r e g i o n many A wide above t h e apex of t h e LMIS s o t h a t t h e s e i o n s have a broad energy d i s t r i b u t i o n even b e f o r e t h e e f f e c t s of s p a c e c h a r g e . T h i s may i n c r e a s e t h e o v e r a l l e n e r g y s p r e a d of s i n g l y charged i o n s and c a n g i v e r i s e t o a two-peak o r peak and s h o u l d e r s t r u c t u r e i n t h e
+ +
e n e r g y d i s t r i b u t i o n a s h a s been o b s e r v e d f o r Ga /11/ and f o r S i / 6 / . F i e l d i o n i z a t i o n does n o t c o n t r i b u t e t o t h e c u r r e n t of doubly charged i o n s .
2. SHAPE AND SIZE OF THE I O N EMITTING REGION
I t was shown by Gomer / l / t h a t t h e u s u a l assumption of a rounded o f f T a y l o r cone s h a p e i s i n c o n s i s t e n t w i t h a 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 . Gomer i n t e r p r e t e d h i s r e s u l t s a s meaning t h a t f i e l d e v a p o r a t i o n c o u l d n o t b e o c c u r i n g a t c u r r e n t s above 10 pA. An a l t e r n a t i v e view i s t h a t f i e l d e v a p o r a t i o n i s o c c u r r i n g , b u t t h a t t h e s h a p e i s t h a t o f some j e t - l i k e p r o t r u s i o n on t h e end of a T a y l o r cone. We adopt t h i s a l t e r n a t i v e view h e r e , i n agreement w i t h Kang and Swanson's 1121 c a l c u l a t i o n s and w i t h t h e d i r e c t TEM o b s e r v a t i o n s of Gaubi e t a l . / 1 3 / .
3 . _CALCULATION OF PROPERTIES OF THE I O N BEAM
We have made a s e l f c o n s i s t e n t c a l c u l a t i o n o f LMIS s h a p e and t h e i o n t r a j e c t o r i e s
i n c l u d i n g e f f e c t s of s p a c e c h a r g e , u s i n g a m o d i f i e d v e r s i o n o f t h e SCWIM program used
by Kang and Swanson / 1 2 j . Examples o f r e s u l t s o f t h e s e c a l c u l a t i o n s a r e g i v e n i n f i g s
2 and 3. F i g . 2 shows t h a t t h e c a l c u l a t e d a n g u l a r i n t e n s i t y a s a f u n c t i o n of beam
Atomic Mass B e o m Angle ( D e g r e e s )
t'
0.
m
-
DSO-
Fig. 2 - Comparison of calculated Fig. 3 - Comparison of calculated and and measured angular intensity experimental beam divergence at 10pA for for a Ga source. different elements.
A
=C o l c u l o t e d v a l u e s
I o p AO = E x p e r r m e n l o l p o i n t s
0 =t o l c u l o t e d v o l u e s with
f i x e d l e n p t h protrusion
angle for a Ga LMIS is in resonable agreement with both the width and shape of the measured angular intensity. The FWHM of the angular intensity for different elements is shown in fig. 3. The results of the present calculation, which include a variable length protrusion on the end of a Taylor cone shape that adjusts to offset the effects of space charge, are in much better agreement with the experimental results than the fixed length protrusion model adopted by Kang and Swanson / 1 2 / .
W
l K o n g ond S w o n s o n )
0
m
P 60-
>
.- n
& 4 0 -
-
3*, --- a--
CI, c
a K o n g and S w o n s o n
4. SUMMARY AND CONCLUSIONS
A schematic view of Liquid Metal Ion Source operation is shown in fig. 4. The shape is that of a jet-like protrusion which assymptotically approaches a Taylor cone shape. The dominant mechanism of ion formation is field evaporation from the apex producing mainly singly charged ions, M , ' and perhaps some singly charged small clusters. A few A from the tip the M+ ions may, in some cases, be post-ionized to M 2+
and subsequently to M3+ or even higher charge states. Small field evaporated clusters may also be post-ionized to higher charge states. A subsidiary mechanism of ion formation is field ionization of thermally evaporated neutral atoms and clusters.
Polarization forces tend to attract these neutrals into the high field region above the apex where they may be field ionized at a distance of order 10 A above the surface. This field ionization mechanism only gives rise to singly-charged ions.
Our conclusion is that the mechanism of ion formation in LMIS is predominantly field
evaporation and that the source shape consists of a jet-like protrusion on the end of
a Taylor cone shape, which grows steadily as the ion current increases.
JOURNAL DE PHYSIQUE
field ionization of neutrafs produces M+ and singly
charged clusters
some neutrals charged clusters
escape
field evaporation mainly M+
some clusters M ~ + , M3+ etc.
thermal evaporation of neutral atoms
and clusters
Fig. 4 - A schematic view of the present model of Liquid Metal Ion Source operation.
Acknowledgments - This work was partly supported by NSF grant no: ELS-8303095. DRK is grateful for financial support from a Royal Society University Research Fellowship REFERENCES
+
