INVESTIGATION OF A LIQUID Ga ION SOURCE AT LOW EMISSION CURRENTS

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INVESTIGATION OF A LIQUID Ga ION SOURCE AT LOW EMISSION CURRENTS

H. Mayer, K.-H. Gaukler

To cite this version:

H. Mayer, K.-H. Gaukler. INVESTIGATION OF A LIQUID Ga ION SOURCE AT LOW EMISSION CURRENTS. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-365-C7-370.

�10.1051/jphyscol:1986762�. �jpa-00225957�

INVESTIGATION OF A LIQUID Ga ION SOURCE AT LOW EMISSION CURRENTS

H.P. MAYER and K.-H. GAUKLER

Institut für angewandte Physik der Universitat Tübingen, Auf der Morgenstefle 1 2 , 0-7400 Tübingen, F.R.G.

RESUME

Nous p r é s e n t o n s d e s mesures de l a d i s t r i b u t i o n d ' é n e r g i e e t d e l ' i n t e n s i t é angu- l a i r e d'une s o u r c e d ' i o n s h g a l l i u m l i q u i d e g u i i n d i q u e n t l a p o s s i b i l i t é d e r é d u i r e l a l a r g e u r à mi-hauteur d e l a d i s t r i b u t i o n d ' e n e r g i e e n u t i l i s a n t d e s p o i n t e s f i n e s a v e c un a p p o r t r é d u i t du m e t a l l i q u i d e . En même temps nous t r o u v o n s une p e r t e d ' i n t e n s i t é a n g u l a i r e . E n s u i t e q u a t r e d i f f k r e n t e s t e t r o d e s s o n t comparées.

ABSTRACT

Measurements o f e n e r g y s p r e a d and a n g u l a r i n t e n s i t y o f i o n s e m i t t e d from a l i q u i d g a l l i u m i o n s o u r c e a r e p r e s e n t e d which i n d i c a t e t h a t it is p o s s i b l e t o r e d u c e t h e e n e r g y s p r e a d w i t h o u t l o s i n g much o f t h e a n g u l a r i n t e n s i t y by u s i n g s h a r p e m i t t e r s w i t h a h i g h e r f l o w impedance. F u r t h e r m o r e d i f f e r e n t t e t r o d e l e n s s y s t e m s a r e com- p a r e d f o r t h e i r s u i t a b i l i t y w i t h s u c h s o u r c e s

.

I n t r o d u c t i o n

The performance o f i o n m i c r o p r o b e i n s t r u m e n t s u s i n g l i q u i d m e t a l f i e l d e m i s s i o n s o u r c e s is u s u a l l y l i m i t e d by l e n s a b e r r a t i o n s o f t h e probe-forming o p t i c a l s y s t e m . We s h a l l t r e a t a l i q u i d g a l l i u m i o n s o u r c e c o n s i s t i n g o f t h e f i e l d e m i t t e r and a t e t r o d e s y s t e m which is o f t e n r e s p o n s i b l e f o r most o f t h e l e n s a b e r r a t i o n s .

EMITTER

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F i g 1. S c h e m a t i c drawing o f t h e p r o b e

-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986762

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The c o n t r i b u t i o n o f t h e e x t r a c t i o n o p t i c s t o t h e diameters d, and d , o f t h e spher- i c a l and chromatic d i s c o f c o n f u s i o n a t t h e p o s i t i o n o f t h e o b j e c t i s g i v e n by

where we make use o f t h e r e l a t i o n s

M =

fi * ( 2 ) '

f o

M i s t h e o v e r a l l m a g n i f i c a t i o n o f t h e system. The s p h e r i c a l and chromatic aberra- t i o n c o e f f i c i p n t s C s and C, r e l a t e d t o t h e source depend on t h e source sided f o c a l l e n g t h fo and t h e - r a t i o U , / U , o f e x t r a c t i o n v o l t a g e t o f i n a l beam voltage. The emission h a ï f angle a0 determines t h e probe c u r r e n t Ip

.

There a r e mainly two ways o f o p t i m i z i n g t h e performance o f t h e arrangement :

- t o f i n d optimum values f o r t h e source parameters d I / d Q and

i t s energy spread A E = e * A U by v a r y i n g source geometry, emission c u r r e n t and e m i t t e r temperature.

- t o o p t i m i z e t h e o p t i c a l parameters, i.e. C s / fo and C c / fo

O p t i m i z a t i o n o f t h e e m i t t e r

Since chromatic a b e r r a t i o n i s dominant i n t h e range o f acceptance h a l f angles f f o = l t 5 mrad i t would be o f soma i n t e r e s t t o reduce t h e t o t a l energy spread o f the emit- t e d ions. Generally, t h e energy spread decreases w i t h decreasing t o t a l emission c u r r e n t s , and as t h e r e i s a discrepancy between t h e energy spread found f o r emission c u r r e n t s o f about 1pA (1-8) and t h e measurements o f Culbertson e t a l . ( 9 ) f o r which a s a t i s f a c t o r y e x p l a n a t i o n i s s t i l l missing.& have made some measurements a t low emission c u r r e n t s u s i n g r e l a t i v e l y sharp e m i t t e r s

.

Sharp e m i t t e r s have a considerably increased f l o w impedance o f t h e l i q u i d m e t a l due t o t h e s m a l l e r t o t a l surface l e a d i n g t o t h e t i p . For a s u f f i c i e n t g a l l i u m supply a h i g h p o r t i o n o f t h e t o t a l e m i t t e r s u r f a c e must be wetted w i t h t h e l i q u i d metal.

We a r e u s i n g two d i f f e r e n t procedures o f e t c h i n g t h e p o l y c r y s t a l l i n e tungsten e m i t t e r which g i v e t h e same r e s u l t s :

-

-

I n t h e present i n v e s t i g a t i o n we a r e u s i n g t i p r a d i i between 0.2 and 0.5 F m . As wet- t i n g may be impeded by oxide o r hydrocarbon contamination l a y e r s on t h e e m i t t e r (10);

t h e vacuum was k e p t below 5 *IO-' mbar d u r i n g p r e p a r a t i o n and o p e r a t i o n o f t h e t i p . The mass f l o w o f l i q u i d m e t a l can be enhanced by keeping t h e source a t an e l e v a t e d temperature up t o about 400°C d u r i n g o p e r a t i o n . A t t h i s temperature t h e t h e r m a l evaporation r a t e o f g a l l i u m from t h e e m i t t e r i s s t i l l q u i t e low.

Fig.2 shows the c u r r e n t - t o - v o l t a g e c h a r a c t e r i s t i c s o f two d i f f e r e n t t i p s , numbered 12) and 13). The s u f f i x e s a ) and b ) denote two modes o f operation: t h e b)-curves were recorded a f t e r h e a t i n g t h e e m i t t e r up t o 600°C f o r a few seconds w h i l e t h e curves denoted by a) were taken a f t e r about 1 hour o f emission a t low temperatures.

Comparing 12a) and 12b) we n o t e t h a t 12a) i s s h i f t e d towards h i g h e r e x t r a c t i o n v o l - tages which i s probably due t o t h e increased f l o w impedance.

Fig.3 shows t h e angular i n t e n s i t y curves o f t i p 13). The d i s t a n c e between e m i t t e r and e x t r a c t i o n e l e c t r o d e was 0.5 mm f o r 12a), 12b) and 13a) w h i l e t h e t i p was r e -

e m i t t e r had been exposed t o a i r f o r some hours.

I n s u f f i c i e n t supply o f g a l l i u m w i l l l e a d t o an unstable emission a t h i g h e r t i p cur- r e n t s o r even t o sîdeward emission over t h e whole c u r r e n t range. We o b t a i n maximum t i p c u r r e n t s i n t h e order o f 4 p A i n t h e case a ) and about 10 y A f o r the operation mode b) when t h e t i p i s heated.

The c u r r e n t a t t h e onset o f t h e emission i s u s u a l l y i n t h e o r d e r o f .5 PA. Once the emission i s s t a r t e d t h e t i p c u r r e n t i s s t a b l e down t o about .2 PA. Below t h i s value we observe a p u l s a t i n g probe c u r r e n t w i t h v a r y i n g d u t y c y c l e s b u t w i t h n e a r l y con- s t a n t p u l s e h e i g h t and frequency.

W i t h i n t h e s t a b l e r e g i o n o f o p e r a t i o n t h e l o n g term d r i f t o f t h e angular i n t e n s i t y i s about 1 5 % per hour (Fig.4) and t h e n o i s e i n t h e probe c u r r e n t i s below l0Io i n t h e frequency range between 50 Hz and 5 kHz. We have t h e r e f o r e operated t h e source w i t h o u t any f u r t h e r s t a b i l i s a t i o n o f t h e emission c u r r e n t . L i k e o t h e r authors (7) we have found t h a t t h e s t a b i l i t y i s a f f e c t e d by poorer vacuum c o n d i t i o n s .

Fig. 2 and 3: Emission c u r r e n t I , vs. e x t r a c t i o n v o l t a g e and angular i n t e n s i t y curves f o r two d i f f e r e n t e m i t t e r s 12) and 13). 12a) and 13a) show measurements w i t h reduced supply o f l i q u i d metal. The d i s t a n c e between e m i t t e r and e x t r a c t i o n elec- t r o d e was 0.5mm f o r H a ) , 12b) and 13a) and 4.5mm f o r 13a) and l 3 c ) .

F i g . 4 : S t a b i l i t y o f t h e angular i n t e n s i t y i n o p e r a t i o n mode a)

We have measured t h e energy spread o f t h e e m i t t e d i o n s u s i n g a M o l l e n s t e d t analyzer which p r o v i d e s an energy r e s o l u t i o n b e t t e r t h a n .1 eV (11). I n order t o keep t h e source a t good vacuum i t was placed i n t o an u l t r a h i g h vacuum chamber mounted on top o f the h i g h vacuum column c o n t a i n i n g t h e analyzer. A 200 ~m diaphragm between HV and UHV served as a d i f f e r e n t i a l pumping a p e r t u r e and a t t h e same t i m e as a l i m i t i n g a p e r t u r e f o r t h e measurement o f t h e angular i n t e n s i t y .

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The i n t e n s i t y d i s t r i b u t i o n below t h e analyzer i s swept over t h e r e c o r d i n g s l i t by means o f e l e c t r o s t a t i c d e f l e c t i n g p l a t e s . The c u r r e n t i s c o l l e c t e d i n a Faraday cage below t h e r e c o r d i n g s l i t .

The energy d i s t r i b u t i o n curves a r e n e a r l y gaussian-shaped. Fig.5 shows t h e FWHM o f the energy d i s t r i b u t i o n f o r two d i f f e r e n t e m i t t e r s vs. t h e t o t a l emission c u r r e n t . We n o t e t h a t i n case a) t h e increase o f t h e energy spread o f t h e i o n s w i t h t h e t o t a l emission c u r r e n t i s approximately l i n e a r w h i l e i n case b ) where t h e l i q u i d m e t a l supply i s more abundant t h e r e i s a decrease i n t h e slope o f t h e energy spread curves a t low emission c u r r e n t s known from e a r l i e r measurements (2,121. Kuby and S i e g e l (13) have r e c e n t l y t r i e d t o a t t r i b u t e t h e energy spread t o t h e volume o f f l u i d ex- posed t o t h e h i g h f i e l d .

Fig.5 and 6: Energy spread A E F W H M and chromatic angular i n t e n s i t y

The r e p r o d u c i b i l i t y o f measurements w i t h one t i p l i e s w i t h i n 10-15%. We a s c r i b e t h i s v a r i a t i o n t o f l u c t u a t i o n s o f t h e mass flow.

As t h e emission c u r r e n t i n a) s t i l l shows a sharp onset a t a g i v e n v o l t a g e and as t h e angular i n t e n s i t y curves a r e q u i t e s i m i l a r t o t h e ones i n b ) we assume t h a t t h e sources s t i l l operate i n an electrohydrodynamic mode although t h e e m i t t i n g f e a t u r e may be a l t e r e d by an increased f l o w impedance compared w i t h b).

I n t h e case where chromatic a b e r r a t i o n i s dominant t h e chromatic angular i n t e n s i t y d I / ^{( d o} * A E ~ ) (Fig.6) i s t h e source parameter t h a t determines t h e c u r r e n t a v a i l a b l e i n a probe o f a g i v e n diameter (2,8,12). The chromatic angular i n t e n s i t y curves show a sharp maximum a t t i p c u r r e n t s o f about . 8 p A i n case a) w h i l e i n case b) t h i s maximum i s l e s s pronounced and s h i f t e d t o t i p c u r r e n t s o f about 2.3 PA.

I o n o p t i c a l c o n s i d e r a t i o n s

The used e x t r a c t i o n o p t i c s was s i m i l a r ta t h e asymetrlc l e n s (14) which has been i n - v e s t i g a t e d f o r f i e l d emission a p p l i c a t i o n s f o r s e v e r a l t i m e s (15,161. We have c a l - c u l a t e d t h r e e o t h e r e l e c t r o d e arrangements and compared them f o r t h e i r s u i t a b i l i t y f o r i o n microbeam a p p l i c a t i o n s such as primary i o n sources f o r SIMS a p p l i c a t i o n s . Lens no.2 i s i d e n t i c a l w i t h no.1 except t h e c e n t e r e l e c t r o d e which has been t u r n e d around. No.3 i s t h e l e n s w i t h an asymetry parameter n=5 described by Shimizu (17) and no. 4 i s s i m i l a r t o Riddle's l e n s no. 16 (18).

We have c a l c u l a t e d t h e source sided f o c a l i e n g t h f o , t h e source p o s i t i o n 20, t h e s p h e r i c a l a b e r r a t i o n c o e f f i c i e n t C, and t h e chromatic a b e r r a t i o n c o e f f i c i e n t Cc r e f e r r e d t o the source s i d e f o r t h e case o f h i g h m a g n i f i c a t i o n . The r e s u l t o f t h e c a l c u l a t i o n o f t h e a b e r r a t i o n s we have made u s i n g t h e LINPROG program w i l l be pub- l i s h e d elsewhere i n more d e t a i l .

Fig. 8 r e p r e s e n t s t h e r e l a t i v e a b e r r a t i o n c o e f f i c i e n t s Ca/ fo and C c / fo o f t h e four lenses f o r d i f f e r e n t v i r t u e l source p o s i t i o n s a.,.

an o b j e c t i v e l e n s i d e n t i c a l t o no.2 operated a s an e i n z e l lens.

Fig.7 Four d i f f e r e n t lenses Fig.8 : R e l a t i v e a b e r r a t i o n c o e f f i c i e n t s i n v e s t i g a t e d . o f t e t r o d e systems 1+4 vs. source p o s i t i o n .

F i g . 9: Schematic drawing o f t h e Fig.10 : Spot s i z e and b r i g h t n e s s o f

l e n s system used. complete system vs. probe c u r r e n t and

acceptance h a l f angle.

On the h o r i z o n t a l a x i s we have p l o t t e d t h e acceptance h a l f angle a0 and t h e beam c u r r e n t I, on t h e b a s i s o f an angular i n t e n s i t y o f 10 wA/sr. For t h e c a l c u l a t i o n o f d and R we used an energy spread o f 3 eV, an e x t r a c t i o n v o l t a g e o f U,=3kV, a f i n a l beam v o l t a g e o f U, =10 kV and a v i r t u a l source diameter o f 30nm. The b r i g h t - ness on t h e s i d e o f t h e o b j e c t i s s t r o n g l y i n f l u e n c e d by t h e l e n s a b e r r a t i o n s . For s m a l l values o f a0 i t converges towards t h e source b r i g h t n e s s which depends on t h e estimated diameter o f 30 nm f o r t h e v i r t u a l source.

ACKNOWLEDGEMENTS

The authors a r e very indebted t o t h e members o f t h e L e h r s t u h l f u r Theoretische Elek- t r o n e n o p t i k i n our i n s t i t u t e f o r s u p p l y i n g t h e LINPROG program f o r t h e c a l c u l a t i o n o f l e n s a b e r r a t i o n s and e s p e c i a l l y t o Dr.F.Lenz f o r very h e l p f u l discussions.

JOURNAL DE PHYSIQUE

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