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RECENT PROGRESS IN LOW-VOLTAGE FIELD-EMISSION CATHODE DEVELOPMENT
C. Spindt, C. Holland, R. Stowell
To cite this version:
C. Spindt, C. Holland, R. Stowell. RECENT PROGRESS IN LOW-VOLTAGE FIELD-EMISSION CATHODE DEVELOPMENT. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-269-C6-278.
�10.1051/jphyscol:1984946�. �jpa-00224427�
JOURNAL DE PHYSIQUE
Colloque C9, supplément au n°12, Tome 45, décembre 1984 page C9-269
RECENT PROGRESS IN LOW-VOLTAGE FIELD-EMISSION CATHODE DEVELOPMENT *
C.A. S p i n d t , C.E. H o l l a n d and R.D. S t o w e l l
SRI International, Menlo Park, California 94085
Résumé -Nos récents progrès dans le développement de réseaux de cathodes à émission de champ ont permis d'atteindre des densités de courant supé- rieures à 100 A/cm .
Abstract—Recent progress in field emission cathode array development has yielded current densities greater than 100 A/cm2.
Introduction
This paper describes the further development of a low-voltage, high-current-density field-emission cathode array [1]. We reported earlier that emitter arrays having tip densities of up to 1.2 x 10^ tips/cm2 could be fabricated using microfabrication techniques developed for this purpose. Emission currents averaging 50 /uA/tip
(60 A/cm2) were achieved with small arrays, and large arrays of 10,000 tips have been fabricated.
Efforts are continuing to develop techniques for increasing the tip packing density and, in turn, the current density that can be achieved with the field emitter array.
These efforts have produced emission current densities greater than 100 A/cm2 from small arrays whose tip packing density was 1.2 x 106 tips/cm2. Advanced fabrication technology has shown that packing densities greater than 10? tips/cm2 are achievable, and a corresponding increase in current density may be possible.
The Structure
The cathode structure, shown in Figure 1, consists of a silicon substrate that has been oxidized to a thickness of about 1-1/2 jam of silicon dioxide. .The silicon diox- ide is coated with a 1/2-/urn thick film of molybdenum in which an array of holes is micromachined using electron beam lithography. Holes are then etched through the silicon dioxide to the silicon substrate, using the molybdenum as an etch mask.
Finally, molybdenum cones are formed in the cavities, with their bases on the silicon substrate and their tips in the plane of the molybdenum film, as shown in Figure 1.
Emission Tests
For testing, the cathodes are usually mounted on standard T0-5 headers used in the semiconductor industry. Figure 2 is a cut-away sketch showing the mounting structure and typical electrode geometry.
Tests are usually done in groups of six cathodes in a setup shown in Figure 3. Each cathode has a separate stainless-steel collector for monitoring the emission current.
The cathode is usually driven with a pulsed voltage to keep the average power low, so that heating of the collector is minimized. Each collector is a 4.5-mm-diameter stainless-steel tube, positioned so that the emission current enters one end of the tube. The tubes are bent so that there is no straight line of sigt :hrough the tube, and all the emitted electrons are collected along the inner walls of the tube, thus distributing the impact over a large area.
^Research supported by NASA Lewis Research Center and the Naval Research Laboratories.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984946
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M E T A L GATE
la) SCHEMATIC OF A THIN-FILM FIELD-EMISSION CATHODE ITFFEC) A R R A Y
4"- -
t- 20prn+
(b) SEM MICROGRAPH OF TFFEC A R R A Y
Ic) S E M MICROGRAPH O F TFFEC CATHODE
Figure 1. Thin-film field-emission cathode.
I n t y p i c a l o p e r a t i o n , t h e e x t r a c t o r o r g a t e e l e c t r o d e i s o p e r a t e d a t ground, t h e b a s e o r t i p s a r e d r i v e n n e g a t i v e l y , and t h e c o l l e c t o r t u b e i s b i a s e d t o a b o u t +I200 V t o h e l p i n overcoming s p a c e c h a r g e e f f e c t s n e a r t h e cathode. Emission i s monitored w i t h a d u a l t r a c e o s c i l l o s c o p e . The h o r i z o n t a l sweep o f t h e o s c i l l o s c o p e i s d r i v e n by t h e v o l t a g e a p p l i e d t o t h e e m i t t e r t i p s , and t h e v e r t i c a l d r i v e o f one c h a n n e l i s used t o
/ ARRAY OF TIPS SPOT WELD GATE CONTACT
- GATE F l LM
GLASS INSULATION SILICON CHIP
TO-5 HEADER
u
2.5 mm
Figure 2. Cutaway view of TO-5 cathode mount.
d i s p l a y t h e e m i s s i o n c u r r e n t a s a f u n c t i o n o f d r i v i n g v o l t a g e , w h i l e t h e second chan- n e l i s used t o d i s p l a y any c u r r e n t f l o w t o t h e g a t e s t r u c t u r e . F i g u r e 4 i s a t y p i c a l p r e s e n t a t i o n s e e n on t h e o s c i l l o s c o p e . I n t h i s c a s e , t h e peak e m i s s i o n c u r r e n t i s a b o u t 1 2 mA, and t h e peak g a t e c u r r e n t i s e s s e n t i a l l y z e r o . The d r i v i n g v o l t a g e i s 125 V peak, halfwave 60 Hz. T h i s i s t h e maximum e m i s s i o n l e v e l a c h i e v a b l e w i t h t h e 60-Hz d r i v i n g v o l t a g e w i t h o u t o v e r h e a t i n g t h e s t a i n l e s s - s t e e l c o l l e c t o r t u b e s . Higher l e v e l s of e m i s s i o n a r e e x p l o r e d u s i n g a reduced d u t y c y c l e o r water-cooled c o l l e c t o r system. The maximum c o n t i n u o u s wave (CW) e m i s s i o n l e v e l t h a t i s s a f e w i t h t h i s system i s 2 mA.
F i g u r e 5 i s an o s c i l l o g r a p h o f e m i s s i o n c u r r e n t from a 5000-tip cathode o p e r a t i n g a t 2 mA CW. The e m i s s i o n l e v e l i s d i s p l a y e d a s a f u n c t i o n of t i m e f o r a 5 s/cm sweep and a 5 ms/cm sweep, showing t h e n o i s e l e v e l over two f r e q u e n c y r a n g e s . The s m a l l p e r i o d i c i n c r e a s e s i n c u r r e n t i n t h e 5-ms/cm sweep a r e due t o half-wave 60-Hz i n t e r f e r e n c e from a nearby power s o u r c e . The f l i c k e r n o i s e i n b o t h i s a b o u t 1 p e r c e n t .
Exploring h i g h e r e m i s s i o n l e v e l s would r e q u i r e new c o l l e c t o r systems t o h a n d l e i n c r e a s e d power. A s i m p l e r approach t o i n v e s t i g a t i n g h i g h c a t h o d e l o a d i n g i s t o make s m a l l a r r a y s of cathode t i p s s o t h a t h i g h c u r r e n t d e n s i t i e s can be t e s t e d w i t h o u t having t o d e a l w i t h h i g h t o t a l power.
Small a r r a y s having t e n s of t i p s r a t h e r t h a n thousands were f a b r i c a t e d f o r t h i s pur- pose. F i g u r e 6 i s a n o s c i l l o g r a p h showing t h e a p p l i e d v o l t a g e p u l s e a s a f u n c t i o n of t i m e f o r a small-area c a t h o d e having 1 0 t i p s on 9-pm c e n t e r s (- 1.2 x lo6 t i p s / c m 2 ) and t h e r e s u l t i n g e m i s s i o n c u r r e n t . The peak e m i s s i o n i s a p p r o x i m a t e l y 2.1 mA, o r a b o u t 250 ~ / c m ~ . The same l o a d i n g on a 10,000-tip a r r a y would be 2 A , which i s c l e a r l y w e l l beyond what c o u l d be managed by t h e s t a i n l e s s - s t e e l t u b e c o l l e c t o r system.
C9-272 JOURNAL DE PHYSIQUE
Figure 3 . Apparatus for testing six cathodes using radiation-cooled 304 stainless-steel collectors.
Figure 4. Typical voltage-current characteristic for emission current and gate current for 5000-tip array.
Top: emission current, 2 mAldivision; Bottom: gate current, 0.02 mAldivision; Horizontal: 5 0 Vldivision Cathode: 20C-157-3N.
VERTICAL 0.5 mAIdiv HORIZONTAL 5 msldiv
VERTICAL 0.5 mAldiv HORIZONTAL 5sldiv
Figure 5. CW operation of cathode 20C-170-40 i n test setup with 3116-inch diameter 304 stainless-steel tube collector at 1000 V.
S e v e r a l t e s t s have been made a t h i g h t i p l o a d i n g s . F i u r e 7 shows pul'se and CW o p e r a t i o n o f two c a t h o d e s , w i t h one p r o d u c i n g 320 A/cm 4 peak e m i s s i o n and t h e o t h e r 5 0 ~ / c m ~ peak p u l s e e m i s s i o n i n b o t h p u l s e and CW modes. F i g u r e 8 i s a s c a n n i n g e l e c t r o n m i c r o g r a p h o f t h e c a t h o d e t h a t produced t h e 320 ~ / c m ~ p u l s e d e m i s s i o n , t a k e n a f t e r t h e test. The peak e m i s s i o n l e v e l was 6 mA, o r a n a v e r a g e o f 0.5 mA/tip. T h i s i s t h e h i g h e s t a v e r a g e e m i s s i o n l e v e l we h a v e measured t o d a t e , a n d , a s c a n be s e e n , t h e r e i s n o damage t o a n y o f t h e t i p s .
Advanced F a b r i c a t i o n P r o c e s s e s
The a r r a y shown i n F i g u r e 8 h a s 12.7 p m betweeri t i p c e n t e r s . C l e a r l y t h e r e i s room f o r many more t i p s i n t h e same s p a c e i f t h e e t c h u n d e r c u t c a n be minimized and l i t h o g r a p h y t e c h n i q u e s c a n be improved t o produce a h i g h e r p a c k i n g d e n s i t y of h o l e p a t t e r n s . Both a r e p o s s i b l e by means o f s t e p and r e p e a t e x p o s u r e s w i t h o u r s c r e e n
JOURNAL DE PHYSIQUE
Figure 6. High peak current density in the pulsed emission mode. Peak current density is 250 AIcm2 (cathode 29A-128-41-1. 10 tips on 9-/m centers).
l e n s e l e c t r o n beam e x p o s u r e s y s t e m , and r e a c t i v e i o n e t c h i n g (RIE) t o machine t h e h o l e s i n t o t h e molybdenum and s i l i c o n . RIE c a n be made t o e t c h a n i s o t r o p i c a l l y , t h u s e l i m i n a t i n g u n d e r c u t t i n g o f t h e g a t e s t r u c t u r e d u r i n g e t c h i n g o f t h e s i l i c o n d i o x i d e l a y e r .
F i g u r e 9 shows a n improved m u l t i p l e e l e c t r o n beam e x p o s u r e s y s t e m t h a t c a n produce p a t t e r n s o f submicron h o l e s w i t h g r e a t l y i n c r e a s e d p a c k i n g d e n s i t i e s . T h i s system c o n s i s t s o f a t u n g s t e n h a i r p i n f i l a m e n t c a t h o d e , E i n z e l l e n s , beam b l a n k e r , o b j e c t a p e r t u r e , d u a l d e f l e c t o r s , t h e s c r e e n l e n s , and t a r g e t o r c a t h o d e wafer. The o p t i c s o f t h e s y s t e m a r e s u c h t h a t t h e o b j e c t a p e r t u r e i s d e m a g n i f i e d i n t h e image by a f a c t o r o f 2(Zo/Z1), o r 1 0 0 t i m e s 121. Thus a 50-pm-diameter o b j e c t a p e r t u r e
w i l l produce a 0.5-pm d i a m e t e r image on t h e t a r g e t . F u r t h e r m o r e , i f t h e s c r e e n l e n s h a s a s c r e e n opening o r l e n s l e t s p a c i n g o f 12.7 p m between o p e n i n g s , t h e p a t t e r n exposed on a t a r g e t c o a t e d w i t h a n e l e c t r o n - s e n s i t i v e polymer w i l l c o n s i s t o f a n a r r a y of 0.5-pm d i a m e t e r s p o t s on 12.7-pm c e n t e r s (6.4 x 1 0 ~ / c m ~ ) , and a l l w i l l b e p r i n t e d s i m u l t a n e o u s l y .
P a t t e r n s h a v i n g h i g h e r p a c k i n g d e n s i t i e s c a n be formed i n t h e polymer by a s t e p and r e p e a t p r o c e s s , making a n e x p o s u r e and t h e n d e f l e c t i n g t h e beam w i t h t h e d u a l d e f l e c - t o r system and r e p e a t i n g t h e e x p o s u r e . The r e q u i r e d e x p o s u r e d o s e i s a b o u t
coul/cm2, which c a n b e accomplished i n a b o u t 1 second--so s e v e r a l s e q u e n t i a l expo- s u r e s c a n b e done v e r y c o n v e n i e n t l y .
As a d e m o n s t r a t i o n o f t h e s t e p and r e p e a t p r o c e s s , a p a t t e r n o f 16 e x p o s u r e s i n a 4 x 4 s q u a r e a r r a y was made t h r o u g h e a c h l e n s l e t w i t h 2.5-pm d e f l e c t i o n s between e x p o s u r e s . The e l e c t r o n - s e n s i t i v e polymer was t h e n d e v e l o p e d t o form h o l e s wherever t h e polymer had been exposed t o t h e e l e c t r o n beam, and a molybdenum f i l m u n d e r t h e polymer was e t c h e d where i t was n o t p r o t e c t e d by t h e polymer. The r e s u l t i n g a r r a y o f h o l e s i s shown i n F i g u r e 10. The h o l e s i n e a c h 4 x 4 s q u a r e a r e s p a c e d 2.5-pm a p a r t , and t h e 16-hole s q u a r e a r r a y s a r e s p a c e d on 12.7-/*m c e n t e r s . T h i s i s 16 t i m e s t h e p a c k i n g d e n s i t y shown i n F i g u r e 8 ( o r 10'/cm2), and i n p r i n c i p l e c o u l d produce 1 6 t i m e s t h e c u r r e n t d e n s i t y t h a t was a c h i e v e d w i t h t h e a r r a y shown i n F i g u r e 8. I n o r d e r t o r e a l i z e t h i s p o t e n t i a l improvement, we must e t c h t h e s i l i c o n d i o x i d e l a y e r u n d e r t h e molybdenum a n i s o t r o p i c a l l y , s o t h a t t h e o x i d e s u p p o r t u n d e r t h e molybdenum between t h e h o l e s w i l l n o t be removed. T h i s r e q u i r e s t h e a p p l i c a t i o n o f a n a p p r o p r i - a t e d r y e t c h i n g p r o c e s s , s u c h a s t h e r e a c t i v e i o n e t c h mentioned e a r l i e r [ 3 , 4 ] .
25A-121-5-C
25A-121-5-0 (PULSE [60 Hz1 )
250 200 150 140 130 120 110 100
APPLIED VOLTAGE - l / V scale
Figure 7. 1 as a function of v - l f o r two cathodes operated at high current density in the pulse mode (25A-121-1501 and the pulse and CW mode (25A-121-5C).
RIE p r o c e s s e s have been h i g h l y developed by t h e semiconductor i n d u s t r y f o r a p p l i c a - t i o n t o i n t e g r a t e d c i r c u i t f a b r i c a t i o n . To t e s t t h e a p p l i c a t i o n o f t h i s t e c h n o l o g y t o o u r p r o c e s s e s , samples were p r e p a r e d h a v i n g 1 t o 1 / 2 p m o f s i l i c o n d i o x i d e i n a s i l i c o n s u b s t r a t e and a b o u t 1 / 2 p m o f molybdenum o v e r t h e o x i d e . Hole p a t t e r n s were formed i n t h e molybdenum by t h e e l e c t r o n l i t h o g r a p h y p r o c e s s d e s c r i b e d above. The o x i d e was t h e n e t c h e d i n a n RIE system u s e d f o r i n t e g r a t e d c i r c u i t p r o c e s s i n g t o machine v e r t i c a l w a l l s i n t h e s i l i c o n d i o x i d e , w i t h t h e molybdenum g a t e f i l m s e r v i n g a s t h e e t c h mask.
Cones were t h e n formed i n t h e h o l e s by t h e u s u a l p r o c e s s . A c a t h o d e f a b r i c a t e d i n t h i s way was broken and examined i n a s c a n n i n g e l e c t r o n microscope t o s e e i f t h e r e s u l t conformed t o e x p e c t a t i o n s . F i g u r e 11 shows t h e RIE-etched s t r u c t u r e , a l o n g w i t h a f r a c t u r e d wet-etched sample f o r comparison. Note t h a t t h e molybdenum g a t e f i l m on t h e RIE-etched sample, h a s v i r t u a l l y no u n d e r c u t a s compared t o a n u n d e r c u t o f a b o u t two h o l e - d i a m e t e r s w i t h t h e wet-etched p r o c e s s .
The c h e m i s t r y o f t h e RIE e t c h p r o c e s s i s complex, and h a s a t e n d e n c y t o produce poly- mers on t h e s u r f a c e s o f t h e r e a c t i o n chamber, i n c l u d i n g t h e s u r f a c e s of t h e sample.
These polymers t e n d t o b e v e r y r o b u s t and a r e d i f f i c u l t t o remove w i t h o u t damaging t h e s u b s t r a t e s u r f a c e . A s a r e s u l t , o n l y one o f s e v e r a l c a t h o d e g r o u p s t h a t h a s been
C9-276 J O U R N A L DE PHYSIQUE
Figure 8. Scanning electron micrograph of a 12-tip cathode after operating at 6 m A peak with a 60-Hz driving voltage (320 Alcmz peak).
CATHODE
ANODE
FIRST DEFLECTOR
V 2 Vl Z O z l m
SECOND DEFLECTOR
BEAM-LIMITING APERTURE, RADIUS r,
SCREEN LENS
LENS APERTURE, RADIUS R IMAGE PLANE
WAFER
LASER-CONTROLLED STAGE MOTIONS ( X . Y, 0)
Figure 9. Diagram of the multiple-electron-beam exposure system.
Figure 10. Array of holes etched in a molybdanum film with a packing density of 107 holesfcm2; 2.5 p m center-to-center spacing between holes and 12.7 p m center-to-center spacing between each square array of 16 holes.
fabricated using RIE has produced satisfactory results in emission tests. This group was very encouraging, however, because of 15 cathodes tested, all produced 20 mA or more peak pulse emission--and one produced 110 mA peak emission. These cathodes were our standard array patterns consisting of 5000 tips on 12.7-pm centers. This result showed only that it is possible to use RIE etching without causing excessive residual contamination on the sample surface; more work is needed to use RIE routinely. Addi- tional testing will be also needed to demonstrate performance at high packing
densities.
Summary
Current densities greater than 100 A/cm using a 60-Hz driving voltage have been pro- duced many times with small arrays of emitter tips (approximately 30 tips) spaced on 9 - p centers. These large current densities have not yet been achieved with large- area arrays (of the order of 10,000 tips on 9 - p centers), because of power-handling problems with collector systems and the structure's intolerance to electrical break- down in the test chamber when excessive heating and outgassing occurs.
Improved fabrication technology has shown that it is possible to fabricate arrays with packing densities about 10 times higher than have been achieved with the origi- nal technology, and it is possible that this may eventually produce cathode arrays with a tenfold increase in current density over the reported values.
JOURNAL DE PHYSIQUE
(al CATHODE FABRICATED USING WET CHEMISTRY TO ETCH THE Su02 LAYER
(bl CATHODE FABRICATED USING REACTIVE 10n BEAM ETCHING O N THE Si02 LAYER.
Figure 11. Comparison of wet and dry etct.,ng of the SiOz layer in field emission cathode arrays.
R e f e r e n c e s
1 C.A. S p i n d t , I. B r o d i e , L. Humphrey, a n d E.R. W e s t e r b e r g , 1976: " P h y s i c a l Prop- e r t i e s o f T h i n F i l m F i e l d E m i s s i o n C a t h o d e s , " J. Appl. Phys., Vol. 47, No. 12, p. 5248 (December).
2 I. B r o d i e , E.R. W e s t e r b e r g , D.R. Cone, J.J. Muray, N. W i l l i a m s , a n d L. G a s i o r e k , 1981: " A M u l t i p l e - E l e c t r o n - B e a m E x p o s u r e System f o r High-Throughput, D i r e c t - W r i t i n g Submicrometer L i t h o g r a p h y , " IEEE T r a n s . E l e c t r o n D e v i c e s , Vol. ED-28, No. 11, p. 1422 (November).
3 J.W. Coburn, 1982: P l a s m a E t c h i n g a n d R e a c t i v e I o n E t c h i n g , AVS Monograph S e r i e s , N. Rey W h e t t e n , ed. (Amer. I n s t . o f P h y s i c s , I n c . , New York).
4 D.L. Flamm a n d V.M. D o n n e l l y , 1981: " D e s i g n o f Plasma E t c h a n t s , " P l a s m a Chemis- t r y a n d P l a s m a P r o c e s s i n g , Vol. 1, No. 4 (December).
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