THE ENERGY SPREAD OF IONS FROM GOLD LIQUID METAL ION SOURCES AS A FUNCTION OF SOURCE PARAMETERS

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THE ENERGY SPREAD OF IONS FROM GOLD LIQUID METAL ION SOURCES AS A FUNCTION

OF SOURCE PARAMETERS

S. Papadopoulos, D. Barr, W. Brown, A. Wagner

To cite this version:

S. Papadopoulos, D. Barr, W. Brown, A. Wagner. THE ENERGY SPREAD OF IONS FROM

GOLD LIQUID METAL ION SOURCES AS A FUNCTION OF SOURCE PARAMETERS. Journal

de Physique Colloques, 1984, 45 (C9), pp.C9-217-C9-222. �10.1051/jphyscol:1984936�. �jpa-00224416�

JOURNAL DE PHYSIQUE

Colloque C9, supplément au n°12, Tome 45, décembre 198* page C9-217

T H E ENERGY S P R E A D OF IONS FROM GOLD LIQUID M E T A L ION S O U R C E S A S A FUNCTION OF SOURCE. P A R A M E T E R S

S. Papadopoulos*, D. Barr, W.L. Brown and A. Wagner

AT&T Bell Laboratories, Murray Hill, New Jersey 07974, U.S.A.

Résumé - Nous avons étudié la dépendance de la distribution énergétique des ions issus d'une (LMIS) d'or en fonction des conditions d'écoulement hydrodynamique imposées par l'état de surface de la pointe. Nous avons étudié également la dé- pendance de la largeur énergétique avec le courant source et l'angle d'émission.

Les changements de largeur en énergie avec la rugosité évalués à l'aide de modè- les et de théories classiques sont faibles. Néanmoins, nos résultats jettent un doute sur les théories existantes fondées sur un champ électrique hémisphérique.

Le A E est bien représenté par une loi puissance ûE^I avec a compris entre 0,32 et 0,39, soit beaucoup moins que le 0,66 mentionné pour les sources Ga et prévu par un effet de charge d'espace longitudinal. La dépendance de AE et celle de la densité de courant en fonction de l'angle d'émission sont remarquablement anti- corrélées. Toutes ces mesures suggèrent que l'image d'une pointe liquide lisse et stable à partir de laquelle 1'évaporation de champ produit un faisceau d'ions atomiques stables est beaucoup trop simple.

Abstract - We have studied the dependence of the energy spread of the ions from Au liquid metal ion sources (LMIS) on the hydrodynamic flow conditions of the liquid imposed by the surface smoothness or roughness of the source tip.

We have also studied the dependence of the energy spread on the source current and angle of emission. The changes in energy spread with source roughness expected on the basis of current models and theory for the range of sources we have examined is relatively small. Nevertheless our results cast doubt on the existing theory for hemispherical electric field geometries. The source

current dependence of energy spread is well represented by a power l a w / ^ ^ I

where " is between 0.32 and 0.39, much less than the anticipated 0.66 which has been reported for Ga sources and expected from the simplest interpretation of longitudinal space-charge effects. The dependence of energy spread on eagle of emission for Au sources shows a remarkable anti-correlation with the angular dependence of current density. All of these measurements suggest that the picture of a smooth and stable liquid tip from which field evaporation produces stable atomic ion beams is much too simple.

f% — 1 —9

Liquid metal ion sources (LMIS) have extremely high brightness -10 Asr cm / I / . Their potential applications are heavily concentrated in finely focused ion beam systems 111 for ion beam lithography / 3 / , micromachining IA1 and direct 'ion implantation in semiconductor microcircuits /5,6,7/. Using electrostatic lenses it is possible to focus ion beams from LMIS's to submicron spots with current densities of the order of 1 Acm 1 2 / . It is in this context that the question of the energy spread from LMIS is of technological as well as scientific interest. The energy

*Permanent Address : Department of Engineering Science, Oxford University, Oxford, United Kingdom

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984936

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spread and t h e c h r o m a t i c a b e r r a t i o n s o f e l e c t r o s t a t i c l e ses i n t h e f o c u s i n g l e n s -9

system l i m i t t h e focused beam c u r r e n t d e n s i t y t o - l A cm . An u n d e r s t a n d i n g o f t h e o r i g i n o f t h e energy spread may l e a d t o i t s c o n t r o l as w e l l as h e l p e x p l a i n t h e processes r e s p o n s i b l e f o r i o n i z a t i o n and mass t r a n s p o r t i n LMIS. I n t h i s paper we examine t h e v a r i a t i o n o f energy spread w i t h t h r e e independent source parameters, source roughness, c u r r e n t and a n g l e of e m i s s i o n and p o i n t o u t m a j o r i n c o n s i s t e n c i e s w i t h e x i s t i n g t h e o r y .

A t y p i c a l LMIS c o n s i s t s o f an e l e c t r o c h e m i c a l l y etched w i r e ( u s u a l l y W o r N i ) i n t h e f o r m o f a n e e d l e w i t h an apex r a d i u s o f c u r v a t u r e , R , between 1 and 10gm. The n e e d l e p r o t r u d e s f r o m a r e s e r v o i r of l i q u i d m e t a l an8 i s w e t t e d b y t h e metal i t s e l f , so t h a t a t h i n f i l m o f l i q u i d m e t a l covers i t s s u r f a c e . By a p p l y i n g a s u f f i c i e n t l y p o s i t i v e p o t e n t i a l (2-8 kV) t o t h e n e e d l e w i t h r e s p e c t t o an adJacent e x t r a c t i o n e l e c t r o d e t h e outward s t r e s s due t o t h e e l e c t r i c f i e l d , aE, (=E 1811) on t h e l i q u i d exceeds t h e inward s t r e s s due t o t h e s u r f a c e t e n s i o n ,

(=2)'IR ) and t h e l i q u i d i s suddenly p u l l e d i n t o a c o n i c a l shape. I n t h e i d e a l s t a l ; c s i t u a t i o n t h e mathematical s o l u t i o n o f t h e baAance between t h e s e two s t r e s s e s i s termed a T a y l o r cone whose h a l f - a n g l e i s 49.3 . A t t h e mathematical apex o f t h e cone t h e f i e l d would be i n f i n i t e . I n t h e p h y s i c a l case when t h e cone i s formed i o n s a r e e m i t t e d by a f i e l d e v a p o r a t i o n ( i o n i z a t i o n ) process n o t , as y e t , w e l l understood and t h e shape o f t h e cone i s a l t e r e d . The c u r r e n t a t onset o f i o n e m i s s i o n i s c h a r a c t e r i s t i c o f t h e n e e d l e geometry and s u r f a c e roughness of t h e n e e d l e f o r a g i v e n m e t a l and i t can v a r y between -1 and -20gA.

I n t e r e s t i n t h e v a r i a t i o n o f energy spread w i t h source roughness has been s t i m u l a t e d by Wagner's s u g g e s t i o n 1 8 1 t h a t i t m i g h t be p o s s i b l e t o reduce t h e energy spread o f beams f r o m LMIS a t a g i v e n c u r r e n t by i n c r e a s i n g t h e f l o w impedance o f t h e source.

I n Wagner's model, i o n emission i s c o n t r o l l e d by hydrodynamic l i m i t a t i o n s t o t h e f l o w o f t h e l i q u i d metal towards t h e e m i t t i n g r e g i o n . Major changes i n t h e I - V

c h a r a c t e r i s t i c s o f LMIS by changes i n t h e f l o w impedance s u p p o r t t h i s a s s e r t i o n 181.

Wagner's e x p r e s s i o n f o r t h e e m i t t e d c u r r e n t , I, reads

where T i s t h e f i l m t h i c k n e s s , Ei t h e f i e l d r e q u i r e d f o r f i e l d e v a p o r a t i o n ( i o n i z a t i o n ) and R i s t h e r a d i u s o f t h e assumed h e m i s p h e r i c a l end o f t h e l i q u i d cone. Wagner assuhes t h a t E. i s a p p r o x i m a t e l y c o n s t a n t . A c c o r d i n g t o E q . ( l ) , f o r a g i v e n c u r r e n t a decrease i n f ( e q u i v a l e n t t o an i n c r e a s e i n f l o w impedance, f o r example, by making t h e n e e d l e s u r f a c e smoother) would l e a d t o an i n c r e a s e i n R .

Thus a decrease i n T i m p l i e s t h a t t h e c u r r e n t d e n s i t y a t t h e emission s i t e woufd decrease and, t h e r e f o r e , a decrease i n energy spread s h o u l d occur i f l o n g i t u d i n a l space charge e f f e c t s a r e r e s p o n s i b l e f o r t h e energy spread 191.

I n o r d p r t o examine t h e v a l i d i t y o f t h i s model we have measured t h e energy spread o f t h e Au i o n s e m i t t e d f r o m f i v e Au LMIS as a f u n c t i o n o f c u r r e n t and s o u r c e roughness.

F o r t h i s purpose we have made use o f a 60' magnetic s e c t o r mass spectrometer. A l l sources b u t one were o f n e a r l y i d e n t i c a l geometry ( w i t h r e s p e c t t o n e e d l e r a d i u s o f c u r v a t u r e ) . The magnitude o f t h e s l o p e 6116V o f t h e sources i s l a r g e r f o r r o u g h e r needles. R V - t h e onset v o l t a g e , V(10) - t h e v o l t a g e f o r 1OgA c u r r e n t and 6116V o f each souPteOare l i s t e d i n T a b l e I below.

I n t h e experiments c a r e was t a k e n t o m i n i m i z e v a r i a t i o n s i n t e m p e r a t u r e s i n c e

t e m p e r a t u r e a l s o i n f l u e n c e s f l o w impedance t h r o u g h v i s c o s i t y and s u r f a c e t e n s i o n .

The r e s u l t s o f t h e energy spread measurements w i t h t h e s e f i v e sources a r e p l o t t e d i n

F i g . 1. I t can be seen t h a t t h e r e i s no c l e a r r e l a t i o n s h i p between [E and 61/6V. I n

f a c t , i t appears t h a t a t almost a l l c u r r e n t s t h e l a r g e s t v a l u e s o f were found f o r

Au12 which i s t h e n e x t t o t h e smoothest o f t h e f i v e . The l a c k o f dependence o f f E on

6 1 1 6 ~ i s s i m i l a r t o t h a t found by M a i r e t a1 I 1 0 1 f o r Ga LMIS.

TABLE I

Source

Au18 Au12 Au7 Au8 A u l l

F i g . 1 - Energy spread (FWHM) o f

beams from f i v e g o l d LMIS's ( T a b l e I ) as a f u n c t i o n o f t o t a l source c u r r e n t .

To e s t i m a t e t h e magnitude o f t h e change i n L E which m i g h t b e expected from t h e s e f i v e sources, we u t i l i z e K n a u e r ' s e x p r e s s i o n I 9 1

2/3R -1/3

A E % I

( 2 a )

By assuming t h a t Ei i s c o n s t a n t , we e s t i m a t e Rt f r o m Ei%V/Rt. Thus:

From Table I we see t h a t f o r 10pA o f s o u r c e c u r r e n t t h e s o u r c e v o l t a g e f o r t h e f i v e sources v a r i e s by a p p r o x i m a t e l y a f a c t o r o f 2. T h e r e f o r e , maximum changes o f 25%

a r e expected. T h i s corresponds t o o n l y - 5 eV which i s o f t h e same o r d e r as t h e v a r i a t i o n s among t h e f i v e sources. Other u n c o n t r o l l e d v a r i a b l e s i n t h e s e experiments may dominate t h e measured LE'S (e.g., dependence o f L!£ on a n g l e o f emission, as w i l l b e d i s c u s s e d below). N e v e r t h e l e s s , t h e l a c k o f a s y s t e m a t i c dependence of !E on f l o w

impedance c a l l s i n t o q u e s t i o n t h e p r e d i c t i o n s o f Wagner's model.

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F i g . 2 - L o g - l o g p l o t o f t h e d a t a o f F i g u r e 1 f o r t h e source Au 18.

F i g . 2 shows t h e d a t a f o r s o u r c e Au18 p l o t t e d on a l o g - l o ~ ale t o r e v e a l a p o s s i b l e power dependence. &E i s found t o v a r y a p p r o x i m a t e l y as I

. The o t h e r f o u r sources habe6zlopes v a r y i n g between 0.32 and 0.39. T h i s i s a much weaker dependence t h a n t h e I -dependence commonly expected f r o m Knauer's space charge t h e o r y I 9 1 and s h o ~ n ~ q n F i g . 2. R e s u l t s o b t a i n e d w i t h t h e same apparatus f r o m Ga LMIS have y i e l d e d a I -dependence which r o u g h l y agrees w i t h e x p e r i m e n t a l o b s e r v a t i o n s r e p o r t e d f o r Ga /10,11,12/, a l t h o u g h two d i f f e r e n t dependences f o r t h e same Ga LMIS h a v i n g powers o f 0.4 and 0.66 have a l s o been r e p o r t e d 1131.

K n a u e r ' s f i n a l e x p r e s s i o n reads.

where i s t h e a n g l e o f emission w i t h i n which t h e c u r r e n t d e n s i t y i s assumed bo6be c o n s t a 3 t . The expected energy dependence has been w i d e l y i n t e r p r e t e d as an I -

dependence. However, b e p j s e o f t h e presence o f a s i t more a p p r o p r i a t e l y should be i n t e r p r e t e d as a ( d I I d 0 ) dependence where i s t h e s o l i d a n g l e o f emission.

Because d l l d n near t h e s o u r c e a x i s i n g e n e r a l i n c r e a s e s l e s s r a p i d l y t h a n t h e c u r r e n t , a much s t r o n g e r &-dependence on I t h a n a 0.66 power s h o u l d i n g e n e r a l b e expected. T h i s p o i n t s up t h e importance o f examining as a f u n c t i o n o f d I I d n and o f c o n s i d e r i n g t h e space charge consequences o f a beam w i t h n o n - u n i f o r m a n g u l a r c u r r e n t d e n s i t y .

We have measured t h e energy spread as a f u n c t i o n o f a n g l e o f emission,

a,

a t f o u r c u r r e n t l e v e l s , namely 2, 4, 10 and 20uA (see F i g . 3a). For purposes of comparison we have a l s o c a l c u l a t e d t h e t o t a l c u r r e n t , IT, under t h e energy

d i s t r i b u t i o n curves f r o m which t h e energy w i d t h s o f F i g .

were determined. These r e s u l t s as a f u n c t i o n o f

a r e p l o t t e d i n F i g . 3b. I t can be c l e a r l y seen f r o m Figs.

3a and 3b t h a t t h e r e i s an a n t i - c o r r e l a t i o n o f &E@) and I p). The magimum o f I E b ) corresponds t o a l o c a l minimum i n t h e I T b ) c u r v e i n t h e v l c i n i t y o f 0 and /!E reaches a minimum a t angles which correspond t o a maximum i n I . T h i s e f f e c t becomes more pronounced f o r l a r g e r s o u r c e c u r r e n t s . The o f f s e t o f The a n g u l a r c u r r e n t and [!E d i s t r i b u t i o n s f r o m 0' i n d i c a t e s e m i s s i o n from a cone n o t p e r f e c t l y a l i g n e d w i t h t h e a x i s o f t h e source needle.

The shape o f t h e energy s p e c t r a a l s o changes w i t h a n g l e o f e m i s s i o n and t h e s e changes

become more apparent as t h e e m i s s i o n c u r r e n t i s increased: a t angles where

F i g . 3 - ( a ) Energy spread as a f u n c t i o n o f a n g l e o f e m i s s i o n o f

beams f o r source Au 12 a t f o u r d i f f e r e n t t o t a l source c u r r e n t s . ( b ) D i f f e r e n t i a l c u r r e n t p e r u n i t s o l i d a n g l e f o r Au+ beams as a f u n c t i o n o f a n g l e o f e m i s s i o n f o r source Au 12.

becomes a minimum t h e c o r r e s p o n d i n g energy d i s t r i b u t i o n d i s p l a y s a much s h o r t e r l o w energy t a i l t h a n t h a t o f a d i s t r i b u t i o n a t maximumLE. F i g . 4 i l l u s t r a t e s t h i s e f f e c t a t a s o u r c e c u r r e n t o f 10gA.

F i g . 4 - Energy s p e c t r a f o r Au from source Au 12 a t two a n g l e s o f e m i s s i o n +

a t 10pA t o t a l source c u r r e n t .

To a t t e m p t t o understand b o t h t h e v a r i a t i o n o f w i t h

and t h e change i n t h e shape o f t h e energy d i s t r i b u t i o n s we n o t e t h e f o l l o w i n g : I t i s a w e l l - e s t a b l i s h e d

e x p e r i m e n t a l f a c t t h a t t h e e m i s s i o n o f i o n s f r o m LMIS's i s accompanied by a mass l o s s which i s more t h a n can be accounted f o r by atomic i o n s /14,15/. I t i s a l s o known /15,16/ t h a t t h e a n g u l a r d i s t r i b u t i o n b f t h i s mass i s narrower t h a n t h a t o f t h e i o n s . The a n i s o t r o p i c s p a t i a l d i s t r i b u t i o n o f t h i s mass suggests t h a t i t cannot c o n s i s t o f n e u t r a l atoms because i f i t d i d , i t would be d i s t r i b u t e d much more i s o t r o p i c a l l y . I t seems more l i k e l y , i n s t e a d , t h a t t h i s mass i s e m i t t e d i n t h e f o r m o f charged d r o p l e t s whose s i z e and s p a t i a l d i s t r i b u t i o n changes w i t h t h e emission c u r r e n t and whose maximum charge i s determined by t h e R a y l e i g h c r i t e r i o n /17/.

A number o f consequences o f i n t e r a c t i o n s o f d r o p l e t s w i t h a t o m i c i o n s have been considered. The atomic i o n s o v e r t a k e d r o p l e t s ( t h o u g h w i t P / l i m i n i s h e d r e l a t i v e l y v e l o c i t y , e s p e c i a l l y f o r s m a l l d r o p l e t s , because o f t h e N dependence o f v e l o c i t y , where N i s t h e number o f atoms i n a d r o p l e t ) . They can cause s p u t t e r i n g which has been i d e n t i f i e d as t h e source o f e x c i t e d n e u t r a l atoms r e s p o n s i b l e f o r o p t i c a l emission f r o m LMIS's /18/. Atoms e j e c t e d by such s p u t t e r i n g may be i o n i z e d b y f i e l d i o n i z a t i o n where t h e y a r e produced o r i f t h e f i e l d i s t o o weak a t t h e p o i n t of

s p u t t e r i n g , may be a t t r a c t e d by p o l a r i z a t i o n f o r c e s t o t h e h i g h e r f i e l d r e g i o n c l o s e r

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t o t h e l i q u i d m e t a l t i p and i o n i z e d as t h e y approach i t . Such f i e l d i o n i z a t i o n processes m i g h t c o n t r i b u t e t o t h e l o w energy t a i l o f t h e energy d i s t r i b u t i o n o f atomic i o n s . Small d r o p l e t s may be evaporated by t h e h e a t i n t r o d u c e d b y s i n g l e o r m u l t i p l e atomic i o n impacts and c o n t r i b u t e t o an e n e r g e t i c n e u t r a l atomic component o f t h e beam because o f t h e d r o p l e t v e l o c i t y b e f o r e e v a p o r a t i o n . A x i a l l y e m i t t e d d r o p l e t s can s e r v e t o b l o c k a t o m i c i o n s and reduce t h e i r c u r r e n t a t s m a l l emission angles. T h i s may c o n t r i b u t e t o t h e c e n t r a l minima i n t h e a n g u l a r c u r r e n t

d i s t r i b u t i o n s a t h i g h e r c u r r e n t s . D r o p l e t s may a l s o s e r v e t o d e f l e c t atomic i o n s by Coulomb r e p u l s i o n and by d i r e c t s c a t t e r i n g ( w i t h Thomas Fermi c r o s s s e c t i o n s t h a t may be q u i t e l a r g e , e s p e c i a l l y f o r keV Au on Au c o l l i s i o n s ) . N e u t r a l atoms s p u t t e r e d o r evaporated f r o m d r o p l e t s may undergo charge exchange w i t h atomic i o n s , a g a i n adding energy spread t o t h e a t o m i c i o n d i s t r i b u t i o n . A l l o f t h e s e phenomena depend c r i t i c a l l y on t h e s i z e and charge d i s t r i b u t i o n o f d r o p l e t s .

We suggest t h a t even more i m p o r t a n t than any o f t h e above processes a r e t h e enormous t r a n s i e n t p e r t u r b a t i o n s o f t h e s u r f a c e geometry and e l e c t r i c f i e l d a t t h e l i q u i d t i p as a d r o p l e t i s e m i t t e d and i n t h e dynamic r e l a x a t i o n a f t e r a d r o p l e t i s p u l l e d away.

The t r a n s i e n t e l e c t r i c f i e l d a t t 2 e l J q u i d t i p due t o t h e m o t i o n o f a charged d r o p l e t p e r s i s t s f o r 2-100 psec as N = 10 -10 atom d r o p l e t s a c c e l e r a t e o v e r d i s t a n c e s o f a few hundred angstroms. The t r a n s i e n t f i e l d s due t o t h e hydrodynamic shape changes o f t h e t i p w i l l p e r s i s t f o r much l o n g e r . The p r o b a b i l i t y o f f i e l d e v a p o r a t i o n i s l i k e l y t o be d r a m a t i c a l l y m o d i f i e d d u r i n g t h e s e times. Furthermore, a t o m i c i o n s t h a t a r e e m i t t e d a r e a c c e l e r a t e d i n a time-dependent e l e c t r i c f i e l d ( n o t a c o n s e r v a t i v e , s t a t i c f i e l d as i s u s u a l l y assumed) and t h e i r e n e r g i e s f r o m t h i s e f f e c t a l o n e w i l l be m o d i f i e d .

Our r e s u l t s emphasize t h e importance o f examining n o n - s t a t i o n a r y e f f e c t s i n LMIS's f r o m b o t h t h e o r e t i c a l and e x p e r i m e n t a l p o i n t s o f view.

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