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CONICAL EQUIPOTENTIAL SUBSTRATE FOR LIQUID METAL SOURCES
J. Kubby, B. Siegel
To cite this version:
J. Kubby, B. Siegel. CONICAL EQUIPOTENTIAL SUBSTRATE FOR LIQUID METAL SOURCES.
Journal de Physique Colloques, 1986, 47 (C2), pp.C2-107-C2-114. �10.1051/jphyscol:1986216�. �jpa-
00225648�
JOURNAL DE PHYSIQUE
Colloque C2, supplkment au n03, Tome 47, mars 1986 page
c2-107CONICAL EQUIPOTENTIAL SUBSTRATE FOR LIQUID METAL SOURCES
J.A. KUBBY and B.M. SIEGEL
School of Applied and Engineering Physics and
The National Research and Resource Facility for Submicron Structures, Cornell University, Ithaca, NY 14853, U.S.A.
Abstract - Morphological changes that occur at an ion bombarded surface as a result of erosion by sputtering can be utilized for the machining of
cylindrical ly symmetric submicron structures. Such structuring has produced tungsten field emitters of conical configuration with variable cone half angle. A conical equipotential surface with an included half angle of 49.3"
would be a useful emitter substrate for experiments designed to produce an equilibrium conical interface to a conducting fluid in an applied electric field. Taylor used a similar structure to support a Taylor cone so that the electric field near the cone could have the proper distribution when, and if, the fluid surface assumed a conical form.
I - INTRODUCTION
Two different techniques have been used in Liquid Metal Ion Sources (LMIS) for localizing the ion emission using the deformation of a liquid metal interface that occurs when the outward stress due to an electric field,
uE=(1/8n)(E [~/cm1/300)*, on the liquid exceeds the inward stress due to the surface tension, uy=2y/Ro where y is the surface tension and Ro the radius of a hemispherical support. The liquid can be lifted into a conical shape along which these stresses are equal with the formation of a sharp apex from which ion emission occurs. The earliest technique involved anchoring of a cusp like filamentary protrusion /1,2/ onto the end of a needle shaped appropriately to encourage a single cusp only /3,4/. A later approach employed different boundary conditions /5,6/ to encourage the formation and stabilization of a Taylor cone /7/. These two techniques are discussed in detai 1 below.
In a High Voltage Electron Microscope (HVEM) study of the liquid metal emission process, Benassayag and Sudraud /8/ found electrohydrodynamic vibrations of the cusp apex in a LMIS. They concluded that these lateral movements, which are not always associated with current fluctuations, could destroy the brightness
properties of this source. Further, they felt that better control of 1 iquid metal ion source fabrication and hydrodynamical properties would avoid these
instabilities that are incompatible with ion beam technology.
Consideration will be given in this paper as to how the fabrication of the source could be improved. A technique for the generation of conical equipotent ial emitter substrates on a nanometer length scale is described. Such an emitter substrate would allow for a mode of operation in which the boundary conditions to the potential distribution would encourage the formation and stabilization of a Taylor
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986216
JOURNAL DE PHYSIQUE
cone t h a t c o u l d be f e d b y an a d l a y e r o f n e u t r a l m a t e r i a l . O p e r a t i o n w i t h t h i s e m i t t e r would reduce t h e volume o f f l u i d i n t h e apex s t r u c t u r e t o l e s s t h a n 0.001 p e r c e n t o f t h e volume i n a cusp anchored t o t h e end o f a 1 pm r a d i u s o f c u r v a t u r e n e e d l e o p e r a t i n g i n t h e c a p i l l a r y wave mode /1,2,3,4/. O p e r a t i o n around t h e l o c a l e q u i l i b r i u m p o i n t f o u n d by T a y l o r /7/ s h o u l d s t a b i l i z e t h e c o n f i g u r a t i o n which forms a t t h e e m i t t e r apex. As t h i s method i n v o l v e s d i f f e r e n t boundary c o n d i t i o n s r e l a t i v e t o t h e s t a n d a r d LMIS, t h e p o t e n t i a l d i s t r i b u t i o n problem s o l v e d by T a y l o r /7/ w i l l be discussed and c o n t r a s t e d t o t h e c a p i l l a r y wave problem t h a t i s t h e b a s i s o f t h e work b y C l a m p i t t e t a1 /3,4/.
F i g . 1 - T a y l o r ' s chamber f o r p r o d u c i n g t h e f i e l d necessary f o r a c o n i c a l i n t e r f a c e t o a c o n d u c t i n g f l u i d .
I 1 - TAYLOR'S BOUNDARY CONDITIONS
I n an a t t e m p t t o f i n d under what c o n d i t i o n s a c o n i c a l l i q u i d p o i n t c o u l d e x i s t i n e q u i 1 ib r i u m w i t h i n an appl i e d f i e l d , T a y l o r /7/ designed t h e e x p e r i m e n t a l dpparatus shown i n f i g u r e 1 t o produce such a p o i n t under c o n t r o l l e d c o n d i t i o n s . To b a l a n c e t h e macroscopic s u r f a c e t e n s i o n which would e x i s t e v e r y h e r e on a c o n i c a l s u r f a c e w i t h t h e s t r e s s on t h a t s u r f a c e produced b y an e l e c t r i c f i e l d , T a y l o r found t h a t t h e p o t e n t i a l would have t o be;
v(r,e ) = v o [ l - ( r / r o ) ' *P~,~(COS(~))I ( 1
where P ~ / ~ ( c o s ( ~ ) ) i s t h e Legendre f u n c t i o n o f o r d e r 1/2, r t h e d i s t a n c e f r o m t h e apex o f t h e c o n i c a l e q u i p o t e n t i a l V ( r , 8 ) = ~ ~ , r o t h e d i s t a n c e f r o m t h e cone apex t o t h e c o u n t e r - e l e c t r o d e d e s c r i b e d below i n e q u a t i o n 2, and e t h e angle measured f r o m t h e a x i s o f t h e c o n i c a l e q u i p o t e n t i a l . The cone a l o n g which t b e p o t e n t i a l i s c o n s t a n t and equal t o Vo has a s e m i - v e r t i c a l a n g l e a=x-Oo=49.3 , where
P112 (cos(eo=130.7))=0. To make it p o s s i b l e t o produce t h e f i e l d r e p r e s e n t e d by t h e p o t e n t i a l i n e q u a t i o n ( 1 ) between two m e t a l l i c s u r f a c e s , T a y l o r /7/ c o n s t r u c t e d t h e chamber shown i n f i g u r e 1, S u r f a c e A i n t h i s f i g u r e was a t r u n c a t e d aluminum cone o f semi-angle 49.3 . S u r f a c e B was an aluminum d i s k h o l l o w e d o u t so t h a t i t s l o w e r s u r f a c e was r e p r e s e n t e d by t h e equation;
r ( 0 )=ro /[P1/2 ( c o s ( e ) )12 ( 2
The o b j e c t o f t h i s d i s k was t o i n s u r e t h a t t h e e l e c t r i c f i e l d near t h e cone c o u l d
have t h e c a l c u l a t e d d i s t r i b u t i o n o f e q u a t i o n ( 1 ) when, and if, t h e f l u i d s u r f a c e
assumed a c o n i c a l form. Cone A was t r u n c a t e d so t h a t i t s upper edge was a
h o r i z o n t a l c i r c l e 1 cm i n d i a m e t e r which c o u l d f o r m t h e lower edge o f a c o n i c a l
l i q u i d surface i f such a s u r f a c e c o u l d i n f a c t be formed. U s i n g t h i s apparatus
T a y l o r found a p o i n t o f l o c a l e q u i l i b r i u m i n t h e s t a t i c l i m i t he was c o n s i d e r i n g
f o r t h e p o t e n t i a l d i s t r i b u t i o n s u r r o u n d i n g an i n f i n i t e cone and c o u n t e r - e l e c t r o d e
of t h e i d e a l i z e d f o r m t h a t he was a b l e t o approximate s u f f i c i e n t l y w i t h h i s
e x p e r i m e n t a l apparatus.
111 - L I Q U I D METAL SOURCE GEOMETRY; THE CAPILLARY WAVE PROBLEM
C l a m p i t t and J e f f e r i e s / 3 , 4 / , i n developing m i n i a t u r e i o n sources f o r a n a l y t i c a l instruments, saw drawbacks i n t h e mode o f o p e r a t i o n i n v o l v i n g the formation and s t a b i l i z a t i o n o f a T a y l o r cone f o r t h e generation o f ions and i n s t e a d developed a needle geometry w i t h a planar aperture as t h e counter-electrode. The curved surface o f t h e small r a d i u s needle was meant t o reduce t h e v o l t a g e necessary f o r t h e formation o f a cusp, and was shaped a p p r o p r i a t e l y t o encourage a s i n g l e cusp only. An a r r a y o f s i m i l a r cusps form caused by t h e impression o f an e l e c t r o s t a t i c
Fig. 2 - Chamber f o r producing t h e i n i t i a l l y u n i f o r m v e r t i c a l e l e c t r i c f i e l d above a planar conducting surface i n t h e c a p i 11 a r y wave problem.
f i e l d on a plane i n f i n i t e conducting l i q u i d surface /I/. T h i s c a p i l l a r y wave problem was also s t u d i e d by Taylor /2/ w i t h the experimental apparatus shown i n f i g u r e 2. Here a l o c a l v e r t i c a l displacement o f t h e i n t e r f a c e w i l l concentrate t h e 1 ines o f f o r c e and so increase the v e r t i c a l f o r c e on t h i s i n t e r f a c e . I f t h i s increase i s g r e a t enough t o counter-balance t h e pressure drop due t o g r a v i t y and t h e surface tension, t h e i n t e r f a c e becomes unstable and cusps develop. Note t h e s t r i c t e r boundary c o n d i t i o n s imposed a t both t h e anode and counter-electrode o f f i g u r e 1 r e l a t i v e t o t h e anode and counter-electrode system o f f i g u r e 2. The m o t i v e behind t h e C l a m p i t t e t a1 /3,4/ design was a d e s i r e t o reduce t o a minimum t h e s i z e o f the l i q u i d exposed t o t h e h i g h f i e l d /9/, thereby minimizing t h e development o f i n s t a b i l i t i e s on t h e i o n e m i t t i n g s u r f a c e t h a t would l e a d t o t h e emission o f droplets. Presumably they f e l t t h a t the volume o f f l u i d i n a cusp t h a t would form a t t h e apex o f an e l e c t r o c h e m i c a l l y p o l i s h e d needle was l e s s than t h a t which would form w i t h an anode s i m i l a r t o t h a t shown i n f i g u r e 1 and thus avoid d r o p l e t formation. This c o n d i t i o n would c e r t a i n l y be t r u e f o r an anode f a b r i c a t e d w i t h macroscopic dimension as was T a y l o r ' s /7/ o r Kingham and B e l l ' s /5,6/.
However t h i s s i t u a t i o n would n o t o b t a i n f o r a T a y l o r t y p e anode f a b r i c a t e d w i t h dimensions on a l e n g t h scale t h a t i s much smaller than t h e 1 vm r a d i u s of curvature o f t h e needle anode used by C l a m p i t t .
Experimental s t u d i e s /4,5,6,8,10,11,12/ show t h a t c o n i c a l shapes which are s i m i l a r t o those found by Taylor 171 form, i n s i t u a t i o n s t h a t are not t o o f a r removed from t h e s t a t i c l i m i t considered by Taylor, even when t h e boundary c o n d i t i o n s o n l y approximate h i s experimental set up. These r e s u l t s c o n f i r m T a y l o r ' s hypothesis o f a l o c a l e q u i l i b r i u m f o r t h e T a y l o r cone, t h e r e f o r e it would be u s e f u l t o have an e m i t t e r s t r u c t u r e t h a t would support and s t a b i 1 iz e t h i s cone.
I V - I O N BEAM PROCESSING OF LMIS EMITTERS
By bombarding an e l e c t r o c h e m i c a l l y p o l i s h e d e m i t t e r t i p along i t s axis, t h e
v a r i a t i o n o f t h e s p u t t e r y i e l d e f f i c i e n c y w i t h the angle o f incidence between a
c o l l i m a t e d i o n beam and t h e normal t o t h e l o c a l macroscopic surface o f t h e t i p can
3 0 U R N A L DE PHYSIQUE
be used t o contour t h e apex o f t h e e m i t t e r i n a p r e d i c t a b l e manner. V a r i a t i o n i n s p u t t e r y i e l d w i t h angle o f incidence has been described p r e v i o u s l y /13/ so it w i l l o n l y be discussed b r i e f l y here. E s s e n t i a l l y , f o r angles o f incidence which are n o t t o o f a r removed from t h e normal, t h e s p u t t e r e f f i c i e n c y i n i t i a l l y increases w i t h i n c r e a s i n g angle o f incidence as t h e mean depth of energy d e p o s i t i o n nears t h e t a r g e t surface where s p u t t e r i n g occurs; a near l/cos(O) e f f e c t o b t a i n s as shown i n f i g u r e 3(a). However as a c h a r a c t e r i s t i c g l a n c i n g angle i s approached, determined by t h e p r o j e c t i l e / t a r g e t combination as w e l l as the i o n ' s i n c i d e n t energy /14/, t h e beam i s r e f l e c t e d o f f the t a r g e t surface decreasing t h e energy d e p o s i t i o n w i t h i n t h e t a r g e t and thus reducing t h e s p u t t e r e f f i c i e n c y t o zero a t t o t a l r e f l e c t i o n . T h i s combination o f e f f e c t s leads t o the c h a r a c t e r i s t i c form f o r t h e s p u t t e r y i e l d shown i n f i g u r e 3 ( b ) . The o r i e n t a t i o n between t h e beam and t a r g e t surface t h a t i s
F i g u r e 3 F i g u r e 4
C O S ( B )
P -
0 3 0 6 0
ap
908 deg
Fig. 3 - For a given p r o j e c t e d range Rp, as t h e angle o f incidence increases away from t h e normal t h e energy d e p o s i t i o n occurs nearer the surface as shown i n (a).
I n (b) i s shown t h e i n i t i a l r i s e o f s p u t t e r e f f i c i e n c y w i t h increased energy d e p o s i t i o n near t h e surface, and t h e f i n a l r e d u c t i o n beyond Bp which i s t o be associated w i t h i o n r e f l e c t i o n o f f the t a r g e t .
Fig. 4 - The hemisphere i n (a) i s approximated by t h e s t r a i g h t l i n e segments a t t h e o r i e n t a t i o n s marked on t h e s p u t t e r y i e l d e f f i c i e n c y curve i n ( b ) . As e r o s i o n proceeds t h e l i n e segment a t Bp, which s p u t t e r s most e f f i c i e n t l y , overtakes a l l o t h e r o r i e n t a t i o n s a t time
t 3 .marked
O pi n t h i s f i g u r e i s t h e o r i e n t a t i o n which s p u t t e r s most e f f i c i e n t l y ; i t i s steep enough t o b r i n g the energy d e p o s i t i o n close t o t h e surface but not so steep as t o r e f l e c t o f f a l l o f t h e beam. A segment o f t h e surface t h a t i s a t t h i s o r i e n t a t i o n w i l l recede r a p i d l y along t h i s d i r e c t i o n under i o n beam erosion.
F i g u r e 4 ( a ) shows t h e morphological development t o be expected f o r an i n i t i a l l y
hemispherical t a r g e t bombarded from above. The hemisphere i s approximated by t h e
f o u r o r i e n t a t i o n s t h a t are marked on t h e s p u t t e r y i e l d curve i n f i g u r e 4 ( b ) . As e r o s i o n proceeds, these surface elements move p a r a l l e l t o themselves a t t h e r e l a t i v e v e l o c i t i e s determined by t h e i r s p u t t e r e f f i c i e n c i e s . By t h e t i m e marked t2, th e o r i e n t a t i o n a t 8, has overtaken t h e slower (lower e f f i c i e n c y f o r
s p u t t e r i n g ) o r i e n t a t i o n a t 81 forming a c o n i c a l cap o f semi-angle 82. As t h e e r o s i o n proceeds, the element a t o r i e n t a t i o n
Op,which s p u t t e r s most e f f i c i e n t l y , overtakes a l l other elements t o form the e q u i l ~ b r i u m endform o f a cone w i t h h a l f angle e p a t time t3. A s i m i l a r morphological development i s expected f o r an i n i t i a l p r o f i l e t h a t i s p a r a b o l i c f o r it also includes a l l surface o r i e n t a t i o n s i n c l u d i n g t h a t which s p u t t e r s most e f f i c i e n t l y . A f t e r e l e c t r o p o l i s h i n g , a t i p i s approximately p a r a b o l o i d a l . The c o n i c a l endform which r e s u l t s a f t e r erosion, w i t h cone h a l f angle equal t o 49.3 degrees, provides t h e proper geometry a t t h e anode t o s a t i s f y t h e p o t e n t i a l d i s t r i b u t i o n boundary c o n d i t i o n s o f t h e T a y l o r cone problem.
Fig. 5 - SEM
micrograph o f t h e c o n i c a l e q u i l i b r i u m c o n f i g u r a t i o n a f t e r a long erosion time.
The marker shows a 100 nm length.
V - EXPERIMENTAL
A d e t a i l e d d e s c r i p t i o n of t h e i n i t i a l e m i t t e r p r e p a r a t i o n as w e l l as t h e
c h a r a c t e r i s t i c s o f t h e m i l l i n g apparatus was given elsewhere /13/. Only an o u t l i n e w i l l be given here.
The e m i t t e r t i p was i n some cases s i n g l e c r y s t a l <100> o r i e n t e d tungsten 5 m i l f o u r PZR wire, and i n o t h e r cases drawn p o l y c r y s t a l l i n e tungsten o r i r i d i u m .
E l e c t r o p o l i s h i n g sharpened t h e t i p t o an approximate 1 pm r a d i u s o f curvature, s i m i l a r t o the sharpness o f a sewing needle.
The ion m i l l t h a t was used f o r the erosion experiments, w i t h a hollow anode source and r o t a t i n g specimen, i s s i m i l a r t o t h e c o n f i g u r a t i o n used i n i o n t h i n n i n g o f TEM specimens. As much as 500 P A o f Ar+ as w e l l as o t h e r species o f i n e r t and r e a c t i v e gases i n t h e energy range o f 3 t o 15 keV were provided by t h i s source. A hollow anode c o n f i g u r a t i o n provides a w e l l c o l l i m a t e d beam o f approximately 1 mn
diameter. F u r t h e r beam d e f i n i t i o n i s given by an aperture a t t h e f r o n t o f a t a r g e t
chamber t h a t contains the e m i t t e r t i p as w e l l as an a p e r t u r e a t t h e e x i t o f the
source. Mechanical r o t a t i o n o f t h e e n t i r e t a r g e t assembly a t an angular v e l o c i t y w
o f 1 radian/sec averages the ion f l u x t o give beam heterogeneity and provides a
reference a x i s f o r morphoTogica1 development.
JOURNAL DE PHYSIQUE
V I - RESULTS
An SEM micrograph, f i g u r e 5, shows the e q u i l i b r i u m c o n i c a l endform obtained f o r an Ar+/W p r o j e c t i l e / e m i t t e r combination. The included h a l f angle i s a f u n c t i o n o f t h e
i n c i d e n t i o n ' s energy as w e l l as t h e mass r a t i o M1 /M2 where M1 i s the mass o f the i n c i d e n t i o n and M;! i s t h a t o f t h e t a r g e t atoms /14/. The t r a n s i e n t cone h a l f angle i s a f u n c t i o n o f dose /14/. A bar i n t h e lower corner d e l i n e a t e s a length o f 100 nm. The c o n i c a l s e c t i o n extends t o a base g r e a t e r than 1 pm. A s e r i e s o f SEM micrographs i n /13/ show t h e dynamical development o f the e m i t t e r contour as t h i s e q u i l i b r i u m endform i s approached. The computer generated p l o t s i n /13/ show t h e morphological development expected i n accordance t o t h e f i r s t order erosion theory /15/. D e t a i l s o f t h e cone apex are a t t h e r e s o l u t i o n l i m i t o f t h e SEM and can be masked by the u b i q u i t o u s carbon contamination. I n t h e TEM micrograph included i n f i g u r e 6 these d e t a i l s can be seen. Here t h e cone apex i s seen t o be rounded w i t h a 20 nm r a d i u s o f curvature t h a t i s comparable t o t h e ion p e n e t r a t i o n depth o f t h e Ar+ i o n w i t h i n t h e W t a r g e t m a t r i x /16,17/.
Fig. 6 - Anode and
counter-electrode system f o r producing t h e f i e l d necessary f o r a c o n i c a l i n t e r f a c e t o a conducting f l u i d i n t h e L i q u i d Metal I o n Source. The anode i s a TEM micrograph o f the e m i t t e r apex t h a t has a 20 nm r a d i u s o f curvature.
I
E Q U I P O T E N T I A L SCALE: rim'sV I I - DISCUSSION
T a y l o r had found t h a t a conducting f l u i d c o u l d e x i s t i n s t a t i c e q u i l i b r i u m i n t h e form o f a cone under t h e a c t j o n o f an e l e c t r o s t a t i c f i e l d , but o n l y when the s e m i - v e r t i c a l angle was 49.3 . H i s apparatus was c o n s t r u c t e d i n such a way t h a t t h e necessary f i e l d could be s e t up and f l u i d surfaces t h a t were very n e a r l y c o n i c a l w i t h t h e hypothesized h a l f angle of 49.3' were observed. Other
experimental observations o f the LMIS e m i t t i n g s t r u c t u r e /4,5,6,8,10,11,12/ also show t h i s s t r u c t u r e t o be a shape o f l o c a l e q u i l i b r i u m . Therefore it would be u s e f u l t o have an e m i t t e r s t r u c t u r e t h a t would support and s t a b i l i z e t h i s l i q u i d metal cone s i m i l a r t o t h e apparatus o f T a y l o r /7/, b u t reduced i n size. The i o n e r o s i o n f a b r i c a t i o n technique discussed has produced c o n i c a l e m i t t e r e q u i p o t e n t i a l substrates as shown i n f i g u r e s 5 and 6. C o n t r o l o f t h e e m i t t e r apex i n t h e LMIS has been suggested p r e v i o u s l y by Yamaguchi e t a1 /18/ where the t o p curvature o f t h e t i p i n a needle t y p e G a l l i u m i o n source was re-etched t o improve c u r r e n t s t a b i l i t y , although no d e t a i l s of t h e process were given.
The combination o f anode and counter-electrode i s shown i n f i g u r e 6. The boundary
c o n d i t i o n s t h a t are imposed by t h e electrodes i n t h i s f i g u r e should be compared t o
t h e boundary c o n d i t i o n s imposed by the system o f e l e c t r o d e s shown i n f i g u r e 1. The
e l e c t r o d e s i n f i g u r e 6 are meant t o r e p l i c a t e , a l b e i t on a reduced l e n g t h scale,
those i n f i g u r e 1. The boundary c o n d i t i o n s imposed by t h e system o f e l e c t r o d e s
shown i n f i g u r e 6 should a l s o be c o n t r a s t e d w i t h t h e boundary c o n d i t i o n s imposed by
t h e electrodes o f f i g u r e 2. The s t r i c t e r boundary c o n d i t i o n s imposed by the set o f
e l e c t r o d e s i n f i g u r e 6 are meant t o be an improvement on t h e boundary c o n d i t i o n s
imposed by the electrodes o f f i g u r e 2 t h a t are s i m i l a r t o t h e boundary c o n d i t i o n s
imposed by t h e needle geometry of t h e conventional LMIS /3,4/.
A t h i n a d l a y e r has been drawn on t h e e m i t t e r i n f i g u r e 6 which s u p p l i e s t h e e m i s s i o n f r o m t h e T a y l o r cone formed a t t h e e m i t t e r apex. An o p t i m a l o p e r a t i n g t e m p e r a t u r e /19/ would a l l o w t h e l e v e l o f s u p p l y t o equal t h e r a t e o f e m i s s i o n and
i n t h i s way a v o i d t h e i n s t a b i l i t i e s a s s o c i a t e d w i t h l a r g e volumes o f f l u i d b e i n g exposed t o h i g h f i e l d /9/. R e d u c t i o n o f t h e base dimension o f t h e e m i t t e r t h a t s u p p o r t s t h e T a y l o r cone f r o m t h e 1 pm o r l a r g e r r a d i u s o f c u r v a t u r e n e e d l e i n t h e C l a m p i t t e t a1 /3,4/ c o n f i g u r a t i o n t o t h e 8 0 nm r g d j u s o f c y r v a t u r e e m i t t e r shown i n f i g u r e 6 would g i v e a f a c t o r o f (20*10- /1*10- ) o r 10- r e d u c t i o n . i n t h e volume o f f l u i d exposed t o t h e h i g h f i e l d w i t h i n t h e T a y l o r cone.
The volume o f f l u i d exposed t o t h e h i g h f i e l d can a l s o a f f e c t t h e energy spread o f t h e LMIS source. With t h e r e d u c t i o n o f f l u i d exposed t o t h e h i g h f i e l d i n b o t h t h e s u p p l y l a y e r and t h e cone s t r u c t u r e i t i s expected t h a t t h e e m i s s i o n o f d r o p l e t s s h o u l d be reduced /4,9/. These d r o p l e t s and t h e n o n - s t a t i o n a r y e f f e c t s a s s o c i a t e d w i t h t h e d r o p l e t emission process; i.e. t r a n s i e n t p e r t u r b a t i o n s o f 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 e m i t t e r t i p and t h e a c c e l e r a t i o n o f atomic i o n s i n t h i s t i m e dependent ( n o n - c o n s e r v a t i o n ) e l e c t r i c f i e l d can a f f e c t t h e energy spread i n t h e LMIS as d i s c u s s e d b y Papadopoulos e t a1 i n /20/.
O p e r a t i o n about a p o i n t o f l o c a l e q u i l i b r i u m s h o u l d a l s o h e l p suppress grqss movements o f t h e e n t i r e 1 iq u i d p r o t r u s i o n /21/ t h a t s h o u l d be avoided f o r i o n o p t i c a l a p p l i c a t i o n s . As t h e cone, i n t h e f a b r i c a t i o n t e c h n i q u e t h a t has been described, i s a l i g n e d t o t h e a x i s o f t h e needle i t s h o u l d reduce t h e o f f s e t o f 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 f r o m 0 degrees found by Papadopoulos e t a1 /19/.
L o c a l i z a t i o n o f t h e h i g h f i e l d r e g i o n by t h e e m i t t e r s u b s t r a t e s h o u l d decrease s e n s i t i v i t y t o c o n t a m i n a t i o n away f r o m t h e cone apex. C o n t a m i n a t i o n can cause t h e p o s i t i o n o f t h e cusp t o be u n s t a b l e on t h e t i p c o n f i g u r a t i o n o f t h e c o n v e n t i o n a l LMIS as w e l l as p r o v i d e growth s i t e s f o r p a r a s i t i c cusps which can occur even a t low v o l t a g e s /8/.
V I I - ACKNOWLEDGMENTS
The a u t h o r s wish t o express t h e i r g r a t i t u d e t o t h e s t a f f o f t h e M a t e r i a l s Science Center; John Hunt, Margaret C r a f t , and Ray Coles, n o t o n l y f o r t h e i r s e r v i c e and p a t i e n t t r a i n i n g on t h e SEM and TEM b u t a l s o f o r p r o v i d i n g t h e i o n m i l l , Jane Jorgensen f o r t h e drawings, and M a r c i a K e l l e y f o r t h e photography. We would a l s o l i k e t o thank E l i z a b e t h A. Roach and L i n d a H a b e t l e r o f t h e C l a r k H a l l P h y s i c a l Sciences L i b r a r y f o r a s s i s t a n c e i n l i t e r a t u r e searches and d i v e r s i o n . We
a p p r e c i a t e d t h e h e l p f r o m members o f t h e Department o f A p p l i e d Physics; t h e t y p i n g done by Bonni Jo Davis, t h e guidance o f Dr. Gary R. Hanson, and Paul R. Schwoebel f o r i n t e r e s t i n g and i n f o r m a t i v e d i s c u s s i o n s .
T h i s work was s u p p o r t e d b y DARPA under ONR c o n t r a c t N00014-80-C-0587.
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