ENERGY SPREADS IN FIELD EVAPORATION AND LIQUID-METAL ION SOURCES

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ENERGY SPREADS IN FIELD EVAPORATION AND LIQUID-METAL ION SOURCES

G. Mair, T. Mulvey, R. Forbes

To cite this version:

G. Mair, T. Mulvey, R. Forbes. ENERGY SPREADS IN FIELD EVAPORATION AND LIQUID- METAL ION SOURCES. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-179-C9-182.

�10.1051/jphyscol:1984930�. �jpa-00224410�

ENERGY SPREADS IN F I E L D EVAPORATION AND L I Q U I D - M E T A L ION SOURCES

G.L.R. M a i r , T . Mulvey and R.G. Forbes

Department of Mathematics and Physics, The University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET, U.K.

Résumé - Nous présentons des résultats relatifs à l'écart en énergie (largeur à mi-hauteur) des sources d'ions à Gallium liquide. Au dessous de de 2 uA, le graphe log FWHM - log courant a une pente de 0,2 et une valeur extrapolée de 1,2 (î 0,1) eV à 1 nA.

A b s t r a c t - F u r t h e r r e s u l t s on t h e energy spread of l i q u i d g a l l i u m f i e l d i o n sources are p r e s e n t e d . In t h e regime below 2 ^ A , t h e l o g a r i t h m i c (FWHM vs c u r r e n t p l o t ) has a slope of 0 . 2 , and e x t r a p o l a t e s back t o a value of 1.2 ( + 0 . 1 ) eV at 1 nA.

The b r i g h t n e s s and focussed spot s i z e of l i q u i d - m e t a l i o n sources (LMIS) seem, i n p r a c t i c e , t o be l i m i t e d by chromatic a b e r r a t i o n / l / . This i n t u r n i s i n f l u e n c e d by t h e beam energy spread, u s u a l l y assessed by i t s f u l l w i d t h a t half-maximum (FWHM).

In t h e i r o r i g i n a l measurement on g a l l i u m l i q u i d - m e t a l ion sources / 2 , 3 / , Swanson and co-workers found a c u r r e n t dependence of t h e FWHM of Ga+ i o n s , obeying an

approximate 0.7 power l a w . They a t t r i b u t e d t h i s t o coulombic space-charge e f f e c t s i n t h e ion beam, o c c u r r i n g c l o s e t o t h e e m i t t e r . Knauer / 4 / subsequently developed a mathematical model f o r such coulomb i n t e r a c t i o n s i n t h e beam, and p r e d i c t e d a 2/3 power law; t h i s seemed t o c o n f i r m t h e physical o r i g i n of t h e broadening.

Ion energy d e f i c i t s are adequately i n agreement / 3 , 5 / w i t h t h e p r e d i c t i o n s of standard appearance energy theory ( e . g . r e f . 6 ) . This shows t h a t t h e emission process i s some form of s u r f a c e f i e l d - i o n e m i s s i o n , e i t h e r f i e l d e v a p o r a t i o n or t h e s u r f a c e f i e l d i o n z a t i o n of incoming n e u t r a l s produced by some other .means.

The c u r r e n t l y favoured emission mechanism i s some form of f i e l d e v a p o r a t i o n . I n i t i a l l y t h i s was because i t could be s t r o n g l y argued / 7 , 8 / t h a t t h e ' e m i t t e r s u r f a c e t e m p e r a t u r e i s not high enough t o " b o i l o f f " a n e u t r a l f l u x equal t o t h e observed emission c u r r e n t . (Hence f i e l d i o n i z a t i o n of t h e r m a l l y evaporated atoms i s excluded as a s i g n i f i c a n t mechanism.) Atoms c o u l d , of c o u r s e , be produced by o t h e r means, such as t h e break-up of c l u s t e r s , or m i c r o p a r t i c l e s / 9 / . From an a n a l y s i s of the p r o p o r t i o n s of ions i n d i f f e r e n t charge s t a t e s i n a g a l l i u m source c o n t a i n i n g t r a c e s of t i n and copper, Kingham / 1 0 / deduced an e m i t t e r s u r f a c e f i e l d of 19- 20 V/nm. This f i e l d i s more than s u f f i c i e n t t o support f i e l d e v a p o r a t i o n .

Now consider t h e energy spreads t h e o r e t i c a l l y p r e d i c t e d f o r f i e l d e v a p o r a t i o n on t h e basis of t h e conventional mechanism. For an image-hump mechanism, or t h e c h a r g e - d r a i n i n g mechanism as c u r r e n t l y u n d e r s t o o d , t h e f u l l w i d t h a t half-maximum of t h e ion energy d i s t r i b u t i o n (FWHM) s h o u l d , f o r a given emission s i t e , be of order kT ( i . e . less than 0.1 eV a t room t e m p e r a t u r e ) . In t h i s case t h e observed h a l f - w i d t h might - r e f l e c t v a r i a b i l i t y i n t h e standard energy d e f i c i t as between emission

s i t e s . No i n f o r m a t i o n on t h i s e x i s t s , but i t seems most u n l i k e l y t o exceed one- t h i r d of t h e g a l l i u m s u b l i m a t i o n energy ( 2 . 8 e V ) . For a s i n g l y - c h a r g e d ion formed by t h e charge-hopping mechanism, a crude f i r s t - o r d e r - a p p r o x i m a t i o n t r e a t m e n t gives the formula / l l / :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984930

C9-180 JOURNAL DE PHYSIQUE

where $ i s t h e e m i t t e r work f u n c t i o n and F t h e r e l e v a n t e v a p o r a t i o n f i e l d . F o r g a l l i u m $ i s about 4.5 eV, and F p r o b a b l y i n t h e range 15-20 V/nm. T h i s p r e d i c t s a FWHM o f about 0.6 eV. More d e t a i l e d t r e a t m e n t s m i g h t be expected t o g i v e e s t i m a t e s d i f f e r i n g by a f a c t o r o f around 2.

The p r e c i s e f i e l d - e v a p o r a t i o n mechanism f o r g a l l i u m cannot be p r e d i c t e d 1121. B u t a l l t h e c o n v e n t i o n a l mechanisms would seem t o p r e d i c t a g a l l i u m FWHM o f l e s s t h a n 1 eV.

On t h e e x p e r i m e n t a l s i d e , t h e l o w e s t FWHM measured by Swanson and c o l l e a g u e s /2,3/

f o r a g a l l i u m LMIS was -5 eV a t an observed c u r r e n t o f a b o u t 1 uA. On t h e o t h e r hand C u l b e r t s o n and co-workers 113,141 r e p o r t an FWHM o f a b o u t 1.5 eV a t c u r r e n t s o f 1-10 nA, f o r f i e l d e v a p o r a t i o n f r o m a l i q u i d g a l l i u m f i l m on a t u n g s t e n e m i t t e r .

I n t h e s e circumstances i t seemed u s e f u l t o l o o k a g a i n a t t h e e x p e r i m e n t a l energy spreads. G a l l i u m i s a convenient m a t e r i a l because i t i s w e l l - behaved. The main f e a t u r e s o f o u r r e s u l t s have a l r e a d y been r e p o r t e d 15,111, and we b r i e f l y

r e c a p i t u l a t e them here.

The a n a l y s e r was o f a r e t a r d i n g - f i e l d t y p e , s i m i l a r t o t h a t employed by Van Oostrom 15,151. The i o n s o u r c e was o f t h e Culham "needle type", w i t h an e x t r a c t o r e l e c t r o d e c l o s e t o t h e e m i t t e r . Energy d i s t r i b u t i o n s o f t h e ~ a + i o n beam were o b t a i n e d by e l e c t r o n i c a l l y d i f f e r e n t i a t i n g t h e r e t a r d a t i o n curves.

A l o g a r i t h m i c p l o t of FWHM vs c u r r e n t i s shown i n Fig. 1. A break i n s l o p e a t about 2 IJA i s c l e a r l y v i s i b l e . Above 2 uA and up t o around 25 IJA t h e s l o p e i s -0.72.

This i s somewhat h i g h e r t h a n t h e v a l u e o f 213 p r e d i c t e d by Knauer's t h e o r y , b u t v e r y n e a r t o t h a t r e p o r t e d by Swanson e t a1 131; t h e d i f f e r e n c e may l i e i n t h e changing

Fig. 1

-

^FWHM ( A E ~ ) vs i o n c u r r e n t ( i ) o f t h e Gat i o n beam produced by a " n e e d l e t y p e " Ga LMIS; i n g e t : energy d i s t r i b u t i o n o f t h e Gaf i o n beam f o r i = 0.66 uA.

Arrow i n d i c a t e s t h e v a l u e measured by C u l b e r t o n e t a1 113,141 f o r Ga on W, f o r i

-

^{1-10 nA.} V e r t i c a l l i n e i n i n s e t i n d i c a t e s z e r o energy d e f i c i t .

2 MA t h e s l o p e i s 0.2, and t h e c u r v e e x t r a p o l a t e s back t o 1.2 (20.1) eV a t a c u r r e n t of l nA. T h i s v a l u e s h o u l d be compared w i t h t h e r e s u l t s o f C u l b e r t s o n e t a1 f o r a G a l l i u m f i l m on a sharp t u n g s t e n needle, namely an FWHM o f about 1.5 eV a t c u r r e n t s of 1 t o 10 nA.

The i n s e t i n F i g . 1 shows an energy d e f i c i t (peak p o s i t i o n ) o f about 5 eV a t a near e x t i n c t i o n c u r r e n t o f -66 $4. T h i s v a l u e was v a r i a b l e , sometimes b e i n g n e a r e r t o 4 eV, and as a l r e a d y mentioned, i s i n s a t i s f a c t o r y agreement w i t h t h e o r y .

Our o b s e r v a t i o n o f a break i n s l o p e a t about 2 $4 r a i s e s two q u e s t i o n s . F i r s t , i s i t r e a l , o r i n s t r u m e n t a l i n o r i g i n ? I n t h i s c o n t e x t , we n o t e t h a t Fox and c o - workers 1 1 6 1 found a " l e v e l l i n g - o u t " a t l o w c u r r e n t s , b u t towards a FWHM o f 6 eV, o r somewhat h i g h e r . T h i s r e s u l t i s almost c e r t a i n l y a consequence o f l i m i t e d a n a l y z e r r e s o l u t i o n . Our a n a l y z e r has a demonstrated a b i l i t y t o r e s o l v e t h e r m i o n i c and f i e l d - e l e c t r o n energy d i s t r i b u t i o n s , which imp1 i e s good ( C 0.1 eV) a n a l y z e r

r e s o l u t i o n f o r e l e c t r o n s . W i t h t h e i o n source i n p l a c e , t h e r e i s an e x t r a e l e c t r o d e near t h e e m i t t e r , b u t we do n o t t h i n k t h i s c o u l d d r a m a t i c a l l y a f f e c t t h e a n a l y z e r r e s o l u t i o n . The o n l y p o s s i b i l i t y o f a n a l y z e r d e g r a d i n g we can t h i n k o f i s space- charge broadening a t a cross-over w i t h i n t h e r e t a r d i n g r e g i o n , a r i s i n g f r o m t h e d i f f e r e n c e i n mass between i o n s and e l e c t r o n s . Moreoever, t h e C u l b e r t s o n e t a1 r e s u l t s were a l s o t a k e n w i t h a r e t a r d i n g p o t e n t i a l a n a l y z e r , and g i v e

FWHM = -1.5 eV; s i m i l a r h a l f - w i d t h s ( o f l e s s t h a n 2 eV) can be deduced f r o m t h e work o f E r n s t and co-workers /17/ on Ni, Cu and Fe.

We a r e t h u s i n c l i n e d t o b e l i e v e t h a t o u r o b s e r v a t i o n s a r e r e a l , and t h e f a c t t h a t t h e y e x t r a p o l a t e towards t h e Cul b e r t s o n e t a1 r e s u l t a t l o w c u r r e n t s encourages us i n t h i s b e l i e f . The break i n s l o p e suggests t h a t t h e regime below 2 $4 i s n o t a s s o c i a t e d w i t h Knauer-type broadening. Various s p e c u l a t i o n s a r e p o s s i b l e ; f o r example: ( i ) t h e f i e l d e v a p o r a t i o n mechanism i s d i f f e r e n t i n t h e LMIS case 1181, o r

( i i ) some f o r m o f coulomb broadening mechanism o t h e r t h a n t h e Knauer t y p e i s a t work, p o s s i b l y o c c u r r i n g v e r y c l o s e t o t h e e m i t t e r s u r f a c e , and p o s s i b l y i n v o l v i n g v e r y r a p i d a t o m i c - s c a l e changes i n s u r f a c e p o s i t i o n .

One s h o u l d a1 so n o t e t h a t m i n o r d i s c r e p a n c i e s e x i s t c o n c e r n i n g " o r d i n a r y " f i e l d e v a p o r a t i o n . Thus E r n s t and co-workers have measured an energy spread o f l e s s t h a n 2 eV f o r N i c k e l . However, Tsong and K i n k u s 1191, u s i n g a p u l s e d - l a s e r t i m e - o f - f l i g h t t e c h n i q u e , w i t h a r e s o l u t i o n c l a i m e d t o be a b o u t 0.5 eV, show a ~ i + d i s t r i b u t i o n w i t h a h a l f - w i d t h of around 4.5 eV. It i s n o t c l e a r whether t h e b a s i c r e s o l u t i o n o f t h e Tsong and Kinkus i n s t r u m e n t i s worse t h a n t h e c l a i m e d 0.5 ev, O r

whether t h e t i p h e a t i n g by t h e l a s e r p u l s e causes an e m i s s i o n process t h a t i s n o t

"normal

"

f i e 1 d-evaporation.

We t h u s b e l i e v e t h a t a d d i t i o n a l energy-spread measurements would be d e s i r a b l e , b o t h on l i q u i d - m e t a l i o n sources and on o r d i n a r y f i e l d e v a p o r a t i o n . I d e a l l y , t h e s e s h o u l d i n v o l v e b o t h a range o f m a t e r i a l s and a range o f t e c h n i q u e s .

Acknowledgements

We thank Dr P D P r e w e t t , f o r m e r l y o f UKAEA (Culham L a b o r a t o r y ) and now o f D u b i l i e r S c i e n t i f i c (UK), f o r p r o v i d i n g t h e Ga LMIS. We a l s o thank Mr A E M a r r i o t t - R e y n o l d s f o r drawing t h e diagram i n F i g . 1. T h i s work was f i n a n c e d by t h e SERC (UK).

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