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FIELD EMISSION OF ELECTRONS FROM A LIQUID ALLOY SURFACE
K. Hata, S. Nishigaki, M. Watanabe, T. Noda, H. Tamura, H. Watanabe
To cite this version:
K. Hata, S. Nishigaki, M. Watanabe, T. Noda, H. Tamura, et al.. FIELD EMISSION OF ELEC- TRONS FROM A LIQUID ALLOY SURFACE. Journal de Physique Colloques, 1986, 47 (C7), pp.C7- 375-C7-380. �10.1051/jphyscol:1986764�. �jpa-00225959�
FIELD EMISSION OF ELECTRONS FROM A LIQUID ALLOY SURFACE
K. HATA, S. NISHIGAKI, M. WATANABE, T. NODA, H. TAMURA* and H . WATANABE*
Toyohashi University of Technology, Tempaku-cho, ~ o y o h a s h i 440, Japan
'central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan
Electrons were extracted £rom Ga-In-Sn alloy on a liquid metal ion source by reversing polarity of the high electric field. The emission current was kept stable to 2.5 B/hour in a vacuum of 1x10-' Torr by using a high-impedance stabilizer. The 1-V characteristics obey Fowler-Nordheim's law, indicating the field emission mechanism.
The size of effective emission area can be of the order of 10 A I . The angular current density and the brightness amounted to 20 to 40 uA/sr and ca. 1 0 ' O ~ / c m ~ sr, respectively. The energy spread of emitted electrons was comparable to that of solid-phase field emission.
1. Introduction
A liquid metal ion source (LMIS) has attract great attention in the fields of microfabricatlon, surface microanalysis, and so on because of its prominent feature, very high brightness with a small source size. Liquid metal supplied to the apex region of an emitter is known to form a sharp cone (cal1 d the Taylor cone) with the apex radius of the order of 50 to 500 AIT in a high electrostatic field.
Ions are emitted from such a restricted surface area, yielding a very high brightness (ca. 106 A/cmZ-sr). Thus the liquid metal ion source seems to be most suitable for producing submicron ion beamsz).
For electron beams, on the other hand, one usually employs a solid-metal field emitter as a bright electron source. The smallest possible radius of the emitter tip, however, is practically limited by individual tip etching techniques to Ca. 1000 1, resulted in the opti- cal source size of several ten A. Solid emitters sharper than this value are of less practical use because of their shortened lifetime.
Clampitt an6 ~efferies') reported that the electron emission took place very readily from the liquid cesium emitters in 1972. Swanson and schwind4) proposed an idea of extracting electrons from a liquid
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986764
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m e t a l by r e v e r s i n g t h e p o l a r i t y o f a p p l i e d f i e l d . I f t h i s t e c h n i q u e became e s t a b l i s h e d , it would g i v e a n e x c e l l e n t s o u r c e s a t i s f y i n g t h e f o l l o w i n g r e q u i s i t e s :
a ) s m a l l s o u r c e s i z e , b ) h i g h b r i g h t n e s s ,
C ) l o n g l i f e t i m e , a n d d ) wide v a r i e t y of e m i t t e r m a t e r i a l . I n t h i s p a p e r we d e s c r i b e e x p e r i m e n t a l r e s u l t s o f s t a b l e e l e c t r o n e m i s s i o n from a l i q u i d a l l o y s u r f a c e . We o b t a i n e d an e v i d e n c e of t h e f a c t t h a t t h e c u r r e n t c o n s i s t e d r e a l l y o f e l e c t r o n s t u n n e l i n g t h r o u g h t h e s u r f a c e p o t e n t i a l b a r r i e r . The c u r r e n t s t a b i l i t y , t h e e f f e c t i v e s o u r c e s i z e a n d t h e b r i g h t n e s s were deduced.
2 . E x p e r i m e n t a l
F i g . 1 i l l u s t r a t e s c o n s t r u c t i o n o f t h e LMIS we employed5! I t i s e q u i p p e d w i t h an up-and-down mechanism t o move t h e t u n g s t e n e m i t t e r t i p f r o m o u t s i d e o f vacuum, t h r o u g h a s l e e v e c o n t a i n i n g l i q u i d m e t a l , s o a s t o b e e a s i l y w e t t e d w i t h i t . The LMIS h a s a means o f e l e c t r o n bombardment h e a t i n g f o r
t h e t i p and t h e s l e e v e , b u t it was l e f t u n u s e d d u r i n g t h e e x p e r i m e n t o f e l e c t r o n e m i s s i o n . Dia- m e t e r s o f t h e e x t r a c t o r a p e r t u r e , t h e e m i t t e r w i r e a n d t h e s l e e v e h o l e were 1, 0 . 5 a n d 0 . 8 mm, r e s p e c t i v e l y . The ex- t r a c t o r was l o c a t e d 2 mm down £rom t h e e n d o f t h e s l e e v e and was g r o u n d e d a c r o s s a h i g h impedance r e s i s t o r R a s a n emis- s i o n c u r r e n t s t a b i l i z e r , whose r e s i s t a n c e v a l u e was v a r i a b l e £rom O t o 1 0 G R i n 100 M O s t e p s .
A s t h e l i q u i d m e t a l m a t e r i a l , we u s e d Ga-In- Sn a l l o y , which i s f l u i d a t t h e room t e m p e r a t u r e . I t f l o w s a l o n g t h e t u n g - s t e n t i p down t o i t s
a p e x . E l e c t r o n e m i s s i o n F i g . 1 C o n s t r u c t i o n of LMIS u s e d i s o b t a i n e d s i m p l y by i n t h i s e x p e r i m e n t .
r e v e r s i n g t h e p o l a r i t y S o u r c e m a t e r i a l :
of a c c e l e r a t i o n v o l t a g e Ga-In-Sn a l l o y w i t h i n c o m p a r i s o n w i t h t h e m e l t i n g p o i n t of 3 O C .
i o n e m i s s i o n mode.
3. R e s u l t s a n d D i s c u s s i o n 3 . 1 C u r r e n t S t a b i l i t y
E l e c t r o n s were e x t r a c t e d o u t when t h e a p p l i e d v o l t a g e t o t h e t i p i n c r e a s e d above a t h r e s h o l d . The c u r r e n t was k e p t s t a b l e w i t h t i m e e v e n i n a medium vacuum c o n d i t i o n of l x l 0 ' 7 ~ o r r , a s shown i n F i g . 2.
The s t a b i l i t y was b e t t e r t h a n 2 . 5 % / h o u r . The r o l e of high-impedance r e s i s t o r i n s t a b i l i z i n g t h e c u r r e n t was u n d e r s t o o d £rom t h e f a c t t h a t t h e c u r r e n t became t o f l u c t u a t e when we removed i t .
D e t a i l e d wave f o r m s of t h e c u r r e n t t r a c e d by an o s c i l l o s c o p e a r e shown i n F i g . 3. The r e p e t i t i o n o f a b r u p t i n c r e a s e and s l o w d e c a y i s o b s e r v e d and i t s f r e g u e n c y l o o k s t o b e d e t e r m i n e d by t h e c o n n e c t e d i m - p e d a n c e a n d t h e s t r a y c a p a c i t a n c e - . A s a c a u s e o f t h e o s c i l l a t i o n , Swanson e t a l p r o p o s e d t h e e x p l o s i o n o f T a y l o r c o n e , t a k i n g t h e r m a l p r o c e s s o f t h e l i q u i d t i p i n t o c o n s i d e r a t i o n . I n c o n t r a s t w i t h t h e i r
c u r r e n t i n
l x l ~ ~ ~ ~ o r r .
e x p e r i m e n t s , however, where upward c o n e formed a t o p a c a p i l l a r y g a v e a c u r r e n t o f c a . 0 . 5 mA above a t h r e s h o l d v o l t a g e a s h i g h a s 8 kV, o u r s d i d a c u r r e n t n o t h i g h e r t h a n s e v e r a l t e n JIA above a t h r e s h o l d of c a . 1 kV. T e m p e r a t u r e c a l c u l a t e d by Swanson e t a l shows no r i s e i n t h i s c u r r e n t r a n q e . Tamura, one o f
t h e c o a u t h o r s , h a s o b t a i n e d a SEM image w i t h a n e m i s s i o n c u r r e n k
5 p A i n a s e p a r a t e e x p e r i m e n t 1.
Through t h e s e f a c t s , DC e m i s s i o n may b e e x p e c t e d i n a low c u r r e n t r e g i o n .
F i g . 3 Measurement o f c u r r e n t wave f o r m . ( a ) : C i r c u i t . (b) t o ( d ) : O s c i l l o g r a m s o b t a i n e d w i t h R = 0 . 4 , 1 . 4 a n d 2.4 Ga, r e s p e c t i v e l y . S c a l e s : v e r t i c a l 2 0 mV/div.
a n d h o r i z o n t a l 1 0 m s / d i v .
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3.2 1 - V C h a r a c t e r i s t i c s
The mean e m i s s i o n c u r r e n t 1, d e f i n e d a s t h e a r i t h m e t i c mean of p e a k - t o - p e a k a m p l i t u d e above z e r o i n t h e wave f o r m , i n c r e a s e d w i t h t h e a c c e l e r a t i o n v o l t a g e Vacc a p p l i e d t o k h e l i q u i d t i p above i t s t h r e s - h o l d Vth of Ca. 1 kV, a s shown i n F i g . 4 . M a j o r i t y o f e l e c t r o n s f l o w i n t o t h e e x t r a c t o r e l e c t r o d e a n d h e n c e r a i s e i t s p o t e n t i a l which i s t h e p r o d u c t o f t h e c u r r e n t 1 a n d t h e s t a b i l i z e r impedance R . The e f f e c t i v e e x t r a c t i o n v o l t a g e Veff i s e x p r e s s e d a s
Veff = Vacc - I . R . ( 1 )
The c u r r e n t i s r e p l o t t e d a g a i n s t t h e v o l t a g e Veff i n F i g . 5 a c c o r d i n g t o Fowler a n d Nordheim. I n t h e r e g i o n A where t h e c u r r e n t i s r e l a - t i v e l y low, m e a s u r e d v a l u e s w e l l f i t t h e s t r a i g h t l i n e w i t h t h e s l o p e m = - 0 . 2 3 a n d t h e i n t e r s e c t i o n w i t h t h e v e r t i c a l a x i s , q = - 1 0 . 6 5 . The l i n e a r r e l a t i o n s h x p between l o g ( 1 / v 2 ) and 1/V means t h a t e l e c t r o n s a r e e m i t t e d by t u n n e l i n g t h r o u g h t h e s u r f a c e p o t e n t i a l , t h a t i s , t h e normal f i e l d e m i s s i o n mechanism. From t h e s l o p e m w i t h t h e i n t e r - s e c t i o n q , we e s t i m a t e d t h e e f f e c t i v e s u r f a c e a r e a S o f t h e e m i s s i o n t o b e of t h e o r d e r of 1 0 A2 . The f i e l d s t r e n g t h a t t h e c o n e apex o f t h e l i q u i d m e t a l i s a p p r o x i m a t e d t o b e a b o u t 1.1 v/% a t t h e t h r e s h o l d v o l t a g e . T h i s v a l u e seems r a t h e r h i g h C O p a r e d t o t h a t i n t h e c a s e o f c o n v e n t i o n a l s o l i d f i e l d e m i t t e r s ( 4 1 V/!), f o r which f i e l d s t r e n g t h i n t h e v i c i n i t y of t h e c o n e a p e x i s a p p r o x i m a t e d by
F = V / 5 r , ( 2 )
where V a n d r a r e a p p l i e d v o l t a g e a n d r a d i u s o f c u r v a t u r e a t t h e a p e x , r e s p e c t i v e l y . Using F 4 x 10' V/cm t y p i c a l l y and V = 1 kV, Our d e d u c t i o n g i v e s r = 500 A. S i n c e t h e d i s c r e p a n c y c a n h a r d l y b e e x - p l a i n e d a t p r e s e n t , a u t h o r s i n t e n d t o o b s e r v e t h e a c t u a l s h a p e o f t h e c o n e a p e x i n o p e r a t i o n .
0 i i i j i s 6 i 8 ! VJe
Applied voltage (kV)
F i g . 4 T o t a l e m i s s i o n c u r r e n t v s . a p p l i e d v o l t a g e f o r v a r i o u s s t a b i l i z e r R ' s .
F i g . 5 Fowler-Nordheim p l o t a t R = 200 Ma.
3.4 B r i g h t n e s s
I n o r d e r t o g e t t h e ' a n g u l a r c u r r e n t d e n s i t y dI/dCL, we p r o b e d a c u r r e n t t o w a r d a r e s t r i c t e d s o l i d a n g l e b y u s i n g a n a p e r t u r e w i t h t h e h a l f a n g l e of 2 . 3 mrad. The r a t i o of t h e p r o b e c u r r e n t t o t h e t o t a l o n e was n e a r l y c o n s t a n t , a s shown i n F i g . 7. T h i s means t h a t t h e m i c r o g e o m e t r y i n t h e v i c i n i t y of t h e c o n e a p e x i n c l u d i n g t h e e m i s s i o n a r e a d o e s n o t c h a n g e e s s e n c i a l l y a t t h e c u r r e n t up t o s e v e r a l t e n PA.
The a n g u l a r c u r r e n t d e n s i t y was d e d u c e d £rom F i g . 7 t o b e Ca. 1 ) i A / s r a t t h e t o t a l c u r r e n t o f 1 8 PA, b u t it f a i r l y v a r i e d by t i l t i n g t h e t i p a s s e m b l y a g a i n s t t h e a p e r t u r e . T h i s i m p l i e s inhomogeneous a n g u l a r d i s t r i b u t i o n o f t h e d e n s i t y . The maximum d ~ / d A v a l u e s o f a r o b t a i n e d i s 20 t o 40 p A / s r . B r i g h t n e s s B i s d e f i n e d a s t h e r a t i o ( d I / d A ! / S , where S i s t h e e m i s s i o n a r e a , b e c a u s e it i s t o o s m a l l t o i m a g i n e t h e o p t i c a l s o u r c e s i z e s e p a r a t e l y . We t h e n o b t a i n B o f Ca. 10" ~/cm'.sr, e n e r g y a n a l y z e r ' ) was s e t
up on t h e beam a x i s . - m
D i f f e r e n t i a t i n g c u r r e n t * - c
t h r o u g h t h e f i l t e r w i t h 3
r e t a r d i n g f i e l d , e n e r g y JZJ L
d i s t r i b u t i o n o f e m i t t e d - m e l e c t r o n s was o b t a i n e d
a s shown i n F i g . 6. The ??.
f u l l w i d t h a t h a l f maxi- Ü mumwas 0 . 4 6 e V a t I = 1 7 PA. As t h e a n a l y z e r
-
'
- - - -
F i g . 7 P r o b e c u r r e n t v s . t o t a l c u r r e n t f o r a p e r t u r e w i t h 2.3 mrad h a l f a n g l e . r e s o l u t i o n i s e s t i m a t e d -1.0 -0.8 -0.6 -0.4 -02 00 0.2 0.4 0.6 0.8 1.0
t o b e c a . 0 . 4 e V , t h e
e n e r g y s p r e a d of e l e c - RETARDING VOLTAGE ( V 1
t r o n s i s A E A 0 . 3 eV.
T h i s v a l u e i s c o m p a r a b l e F i g . 6 Energy s p e c t r u m of e m i t t e d t o o r s l i g h t l y l a r g e r e l e c t r o n s . H o r i z o n t a l s c a l e t h a n t h a t f o r t h e s o l i d - i s r e l a t i v e t o i n c i d e n c e p h a s e f i e l d e m i s s i o n . e n e r g y of 1.26 keV.
Very h i g h i n t e n s i t y o f e l e c t r o n s i n t h e p r e s e n t
f i e l d e m i s s i o n may c a u s e a d d i t i o n a l e n e r g y s p r e a d e i t h e r b y Coulomb r e p u l s i o n i n t h e beam o r by a t e m p e r a t u r e r i s e n e a r t h e c o n e a p e x .
30
- a : -
C 20
C .
2 .
L .
3
U .
w 'O-
n .
a O
- *
-
: .
O ~ . " " " " ' " 10 X) 30 40 50
Total current (gA)
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much higher than that of an ordinary solid field emitter. With r obtained from eq.(2), on the other hand, B is defined as (dI/dQ)/So with So = 7t.r; and ro = d m , where Ea, A E and ro are acceleration voltage, energy spread and optical source radius, respectively. Even with Ea of 10 kV typically used in conventional SEM, B gives the same value as above.
4. Conclusion
Under development is a liquid metal electron source with very small emission area and very high brightness. It endures a long term operation even in a conventional vacuum. This is fairly important in view of practical application. Some problems such as oscillation in the current wave form still remain unsettled. These may also depend on properties of source material employed as the liquid-phase emitter.
Acknowledgements
The authors are grateful to the late Dr. T. Okutani for his invaluable guidance. They also wish to thank Prof. N. Yukawa for supplying the liquid alloy material. This work was supported by the joint research project of Toyohashi University of Technology and Hitachi, Ltd.
Reference
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J. Vac. Sci. Technol., B4(1), 143 (1986).
3) R. Clampitt and D. K. Jefferies:
Journees d'etudes (Toulouse, juin 1972) p.145.
4) L. W. Swanson and G. A. Schwind: J. Appl. Phys.,e, 5655 (1978).
5) T. Noda, Y. Sugiura, T. Okutani, H. Tamura and H. Watanabe:
Int. J. Mass Spectrometry and Ion Processes (1986) in press.
6) Private communication.
7) J. A. Simpson and L. Marton: Rev. Sci. Instrum., Z r 802 (1961).
