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HIGH VOLTAGE FIELD ION MICROSCOPE, ITS DESIGN AND FIELD CALCULATION
M. Tagawa, S. Koike, N. Inoue, N. Ohmae, M. Umeno
To cite this version:
M. Tagawa, S. Koike, N. Inoue, N. Ohmae, M. Umeno. HIGH VOLTAGE FIELD ION MICROSCOPE,
ITS DESIGN AND FIELD CALCULATION. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-
583-C6-588. �10.1051/jphyscol:1987695�. �jpa-00226903�
HIGH
VOLTAGEFIELD ION
MICROSCOPE, ITS DESIGNAND FIELD CALCULATION
M.
Tagawa, S. Koike,N.
Inoue,N.
Ohmae andM.
UmenoDepartment of Precision Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan
Abstract
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High voltage field ion microscope with the ultimate applied voltage of 60kV has been developed. One of the primary objectives of this study was to extend the field ion microscopy to bulk materials instead of fine metal wires or whiskers. The atomic configuration of bulk tungsten rod was imaged with He as an image gas. Preliminary study on the atomic structure of carbon fiber reinforced carbon was also carried out. In conjunction with the design and development of the high voltage field ion microscope, the electric field strength and its distribution at the tip surface were computer-simulated with a charge simulation method. It has become clear that the high voltage field ion microscope is able to provide the image from a tip whose radius of curvature is about l p .I
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INTRODUCTIONField ion microscope (FIM) has been potentially used in the research fields of surface science and materials science 11-71. As is well known, FIM resolves individual atoms, and further three dimensional characterization of materials is possible using the field evapolation technique. The sample observable by
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987695
C6-584 JOURNAL DE PHYSIQUE
FIM has been limited to a micro-pointed tip in order to apply a high electric field of 10" V/m to a sample surface. For this reason, the radius of curva- ture of the tip is to be 50 to lOOnm for FIM observations. Therefore, most of the FIM tips have been prepared from fine metal wires using electropolishing.
If it is possible to apply a high electric field, which is enough to cause field ionization of image gases, even to the tip with a larger radius of curva- ture, a further extension of the research fields of FIM will be possible; for example, observation of new materials, bulk materials, interfaces between sub- strate and thin film, and so on. This concept is not new by any means. It was reported that an FIM with the ultimate applied voltage of 50kV was constructed by ~cller /8/ based on the same concept that noted above. However, that FIM was designed using glass components. The glass made FIM has several disadvan- tages compared with the metal made FIM; high transmittance of X-rays, inferior mechanical properties against fracture, difficulties of in-situ experiments inside FIM, and so on. Therefore, from technological points of view, glass made FIM does not appear to satisfy such requirements.
Several image gases have laver ionization fields than He does. So that, FIM observations of the tips, with larger radii of curvature may also be possible by using such image gases as Ne, Ar and Ha. However, the resolution of the FIM image, when using Ne, Ar and Hp, is inferior to that of He. The aim of this study was to develop the high voltage field ion microscopy (W-FIM). Some of the experimental results using the HV-FIM were described. In order to identify the performance of the HV-FIM, a charge simulation method was used to computer- simulate the electric field strength and its distribution on the tip surface.
I1
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APPARATUSBasic requirements for the design of the HV-FIM will be as follows; (1) selection of high voltage vacuum feedthrough, and the design of the cold finger which oper- ates at high voltages, (2) use of bakable stainless steel for structural components except a viewing window, fluorescent screen etc., (3) image gas inlet system which backfills various image gases to the HV-FIM chamber, (4) a convenient exchange of
the tip without leaking the chamber to air, (5) image intensifier in cases where FIM gives a low brightness.
The resolution of'FIM depends greatly upon the tip radius, as indicated in Figure 1. It is obvious that the larger
the tip radius, the lower the tip temper- ature should be, for the purpose of main- s taining a high res.olution of FIM image.
Therefore, the cold finger and high volt- age system were specially designed in 2
order to cool the tip sufficiently.
A schematic diagram of the HV-FIM is -
1
1shown in Figure 2. The HV-FIM chamber oc is approximately 300mm in diameter.
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Most components of the HV-FIM are made
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f 1.5
of 304 stainless steel. The HV-FIM is L evacuated by a turbo molecular pump #.a
(ULVAC, UT-36) and a Ti-getter pump
(ULVAC, PGT-3F), and an oil-free vacuum 0.2 was obtained with the ultimate pressure of lo-'Pa. The cold finger contains 0.1
about 2 liter liquid-nitrogen. Not only 10 10' I o'
T l p radlur lmnl
the cold stage, but also the high volt- age leads are cooled by the cold finger
in order to avoid thermal loss transmit- Fig. 1
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Relation between tip radius and resolution of FIM.ting through the high voltage leads (TORISHA, COLD FINGER ji203). The high voltage vacuum feedthrough which has a
&@
\nrFig. 3
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Tip h o l d e r and cold s t a g e used i n t h e HV-FIM.maximum v o l t a g e of 60kV (Ceramaseal, 808c 6525-1) i s mounted on t h e top of t h e apparatus. A m e t a l cone was n o t f i t t e d t o t h e c o l d f i n g e r f o r a v o i d i n g a b r u p t e l e c t r i c discharge. The image i n t e n s i f i e r (ITT F4727) photographed weak images.
Figure 3 i n d i c a t e s t h e schematic d i a - gram of a c o l d s t a g e and a t i p h o l d e r
~ 22 ~
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.Schematic diagram of t h e of t h e HV-FIM. The micro-pointed t i pHV-FIM. was mounted on t h e t i p holder. The t i p
holder and a p a r t of c o l d s t a g e a r e made of s a p p h i r e , because of its h i g h thermal c o n d u c t i v i t y and e l e c t r i c i n s u l a t i o n . The t i p h o l d e r assembly is attached t o t h e cold stag6'which l o c a t e s a t t h e bottom of t h e c o l d f i n g e r . The t i p temper- a t u r e was measured by a thermocouple and i t was found t h a t t h e temperature a t t h e t i p reached 78K a f t e r one o r two hours a f t e r pouring l i q u i d - n i t r o g e n i n t o t h e c o l d f i n g e r .
I11
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SAMPLES FOR HV-FIM OBSERVATIONTwo m a t e r i a l s were used a s t h e samples f o r t h e HV-FIM o b s e r v a t i o n s . One i s t h e tungsten rod with a p u r i t y of 99.95% whose diameter was 2mm, and t h e o t h e r was t h e carbon f i b e r r e i n f o r c e d carbon (CFRC) block. I n t h e l a t t e r c a s e , a n approxi- mate diameter of t h e specimen t i p was 0.3mm w i t h a l e n g t h of 10mm. One end of t h e tungsten rod was roughly ground i n t o a shape of n e e d l e , w h i l e CFRC t i p was c u t from t h e bulk. Both of them w e r e attached t o t h e Mo-loop, and were e l e c t r o - polished by a s o l u t i o n c o n t a i n i n g 3 % KOH. The FIM t i p s t h u s prepared were r i n s e d i n CzH50H and d i s t i l l e d w a t e r , and d r i e d i n a i r .
I V
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RESULTSFigure 4 shows a HV-FIM image of t h e t u n g s t e n rod. The b e s t image v o l t a g e of t h i s f i g u r e was 27kV w i t h 0.1PaHe. Misalignments of atoms may i n d i c a t e an incorpo- r a t i o n of l a t t i c e d e f e c t s . The s t r e a k must be caused d u r i n g g r i n d i n g procedures.
The n e t p l a n e r i n g a t , t h e c e n t e r was i d e n t i f i e d a s t h e [Oll] plane, because of t h e e x i s t e n c e o f t h e zone l i n e d e c o r a t i o n . The e s t i m a t i o n of t h e t i p r a d i u s was n o t p o s s i b l e from counting t h e number of n e t p l a n e r i n g s between t h e [Oil] and t h e [I121 planes due t o an i n c o r p o r a t i o n of s o many l a t t i c e d e f e c t s . However, From
C6-586 JOURNAL DE PHYSIQUE
scanning e l e c t r o n microscopical observa- t i o n of t h i s t i p a s w e l l a s from t h e b e s t image f i e l d of He, i t was found t h a t t h e approximate r a d i u s of c u r v a t u r e of t h i s t i p was 0.1 p.
Figure 5 shows a HV-FIM image and a scanning e l e c t r o n micrograph of t h e CFRC.
The b e s t image v o l t a g e was 32.5kV with 0.12PaHe. The a r r a y of b r i g h t l i n e s , which i s c h a r a c t e r i s t i c of g r a p h i t e , a r e observed. CFRC c o n s i s t s of carbon f i b e r and p i t c h matrix. It was found by t h e scanning e l e c t r o n microscopical observa- t i o n t h a t t h e apex of t h e t i p l o c a t e d a t t h e carbon f i b e r . I n o u r p r e v i o u s s t u d i e s u s i n g a conventional FIM, Ne-ion image of p o l y a c r y l o n i t r i l e carbon f i b e r d i d n o t show t h e a r r a y of b r i g h t l i n e s 191.
Therefore, t h e carbon f i b e r i n CFRC may have high g r a p h i t i z a t i o n . However, i t i s
u n c l e a r whether t h i s h i g h g r a p h i t i z a t i o n Fig. 4
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HV-FIM image of a tungstenoccurred a t t h e manufacturing process of the best image 27 kV
carbon f i b e r s themselves o r a t t h e pyro- with O s l PaHe. evaporation l i z a t i o n process of CFRC. Although pre- carried Out at 29 kV-
l i m i n a r y o b s e r v a t i o n o f CFRC by HV-FIM was s u c c e s s f u l l y made, much work i s
needed f o r c l a r i f y i n g t h e s t r u c t u r e s of carbon f i b e r and m a t r i x . Such a n a t t e m p t i s b e i n g c a r r i e d o u t i n our l a b o r a t o r y u s i n g high v o l t a g e t r a n s m i s s i o n e l e c t r o n microscopy a s w e l l a s HV-FIM.
V
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COMPUTER SIMULATIONE l e c t r i c f i e l d s t r e n g t h and i t s d i s t r i b u t i o n on t h e t i p s u r f a c e determine f i e l d i o n i z a t i o n of image gases, and t h i s process i s of primary importance t o t h e HV-FIM.
A number o f s t u d i e s have been r e p o r t e d w i t h r e s p e c t t o t h e e l e c t r i c f i e l d s t r e n g t h and e l e c t r i c f i e l d d i s t r i b u t i o n on t h e t u n g s t e n f i e l d e m i t t e r o r a FIM t i p 110-121.
Fig. 5
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HV-FIM image of a CFRC ( a ) , and scanning e l e c t r o n micrograph of t h e t i p (b); t h e b e s t image v o l t a g e 32.5 kV with 0.12 PaHe.models a r e 3'. 30' and 60'.
Most of t h e s e s t u d i e s d e a l t with t h e t i p a s a hyperboloid o r a paraboloid of revo- l u t i o n in o r d e r t o c a l c u l a t e t h e e l e c t r i c f i e l d s t r e n g t h and i t s d i s t r i b u t i o n on t h e t i p s u r f a c e a n a l y t i c a l l y . However, t h i s method i s not applicable t o t h e HV- FIM t i p . I n t h e HV-FIM, t h e t i p s a r e f r e q u e n t l y prepared from bulk m a t e r i a l s , s o t h a t t h e shape of t h e t i p has a wide v a r i e t y .
The computer simulation of e l e c t r i c f i e l d s t r e n g t h and e l e c t r i c f i e l d d i s t r i - bution was c a r r i e d o u t by using a charge simulation method. This simulation has been known a s an a c c u r a t e c a l c u l a t i n g method, when compared w i t h o t h e r numeri- c a l s o l u t i o n s r e l a t e d t o t h e system con- s i s t i n g of a t i p and a f l a t screen.
Charges on t h e s u r f a c e of t h e t i p were replaced by charges arranged i n s i d e t h e t i p . The method of arranging charges were i n d i c a t e d elsewhere 1131. Three models of t h e t i p were considered. The shank a n g l e of each model was 3', 30' and 60°, respectiveljr, a s shown i n Figure 6.
E l e c t r i c f i e l d s t r e n g t h and e l e c t r i c f i e l d d i s t r i b u t i o n f o r t h r e e models were c a l c u l a t e d . E r r o r s i n t h e e l e c t r i c f i e l d could be estimated by those of p o t e n t i a l s 1141. It i s noted t h a t t h e r e l a t i v e e r r o r of t h e e l e c t r i c f i e l d s t r e n g t h was l e s s than t h e o r d e r of a t t h e apex of t h e t i p .
Figure 7 shows t h e r e s u l t s of t h e com- p u t e r simulation of t h e e l e c t r i c f i e l d s t r e n g t h on t h e apex of t h e t h r e e types of t h e t i p . The l a t e r a l t h r e e l i n e s cor- responed t o t h e e l e c t r i c f i e l d s t r e n g t h which is needed f o r t h e f i e l d i o n i z a t i o n of image gases; H e , Ne and Hz. From t h i s f i g u r e , He, f o r example, i s a b l e t o image t h e t i p with a r a d i u s of curvature of 600nm, while Ne images t h e t i p with l p r a d i u s of curvature.
The r e s u l t s of t h e e l e c t r i c f i e l d d i s - t r i b u t i o n on t h e t i p s u r f a c e a r e i n d i c a t e d
Fig. 7
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Calculated e l e c t r i c f i e l d s t r e n g t h a t t h e apex of t h e t i p . Oblique and l a t e r a l t h r e e l i n e s i n d i - c a t e t h e e l e c t r i c f i e l d s t r e n g t h a t t h e apex of t h e t i p and i o n i z a t i o n f i e l d s of image gases, r e s p e c t i v e l y .Fig. 8
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Calculated e l e c t r i c f i e l d d i s t r i b u t i o n on t h e t i p surface.@=O corresponds t o t h e apex of t h e t i p . The e l e c t r i c f i e l d s t r e n g t h a t t h e apex of t h e t i p h a s been normal- ized t o t h e i o n i z a t i o n f i e l d of He, i . e . , 44 V/nm.
C6-588 JOURNAL
DE
PHYSIQUEin Figure 8. The electric field strength at the apex of the tip is normalized to the ionization field of He. The electric field strength decreases Gjith increasing polar angle, especially for the tip having a larger shank angle. It is therefore important to make a shank angle small in order to obtain a wide view of the HV-FIM images on a fluorescent screen.
VI
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SUMMARYThe high voltage field ion microscope has been developed, and preliminary studies using the HV-FIM were carried out for a tungsten rod and a CFRC.
Results of the computer simulation of electric field strength and its distri- bution indicated that a tip with a radius of curvature of l p could be ob- served using the high voltage field ion microscope.
ACKNOWLEDGEMENTS
The authors thank ULVAC CO.,LTD. and Torisha CO.,LTD. for their contributions to the development of the HV-FIM. Sincere thanks are due to C.Mings for her help during the preperation of this manuscript. This work was supported by the Grant-in-Aid for Developmental Scientific Research I, from the Ministry of Education, Science and Culture, Japan, (60850031).
REFERENCES
/I/ Ayralt,G. and Ehrlich,G., J.Chem.Phys.
60
(1974) 281./2/ Nishikawa,O. and Utsumi, T., J.Appl.Phys.
46
(1973) 945./3/ Adachi,T. and Nakamura,S., Surf.Sci.
59
(1976) 61./4/ Sakurai,T., &ller,E.W. and Colbertson,R.J., Phys.Rev.Letters
29
(1977) 578./5/ Tsong,T.T. and Kellog,G.L., Phys.Rev. (1975) 1343.
161 Walko,R. J., Surf-Sci.
70
(1978) 302./7/ Ohmae,N., Nakamura,A., Koike, S. and Umeno,M., J.Vac .Sci. Technol.
A5
(1987) 1367.181 Gller ,E.w. and T,song,T.T., Field Ion Microscopy: Principles and Appli- cations, Elsevier, New York, 1969, 106.
/9/ Ohmae,N., Tagawa,M., Umeno,M. and Gumi,K., paper presented at the ASLE 1987 Annual Meeting, Anaheim, CA, May 1987, will appear in the ASLE Trans.
/lo/ Birdseye,P.J. and Smith,D.A., Surf.Sci.
2
(1970) 198.1111 Gipson,G.S., Yannitel1,D.W. and Eaton,H.C., J-Phys. (1979) 987.
/12/ Dyke,W.P., Trolan,J.K., Dolan,W.W. and Barnes,G., J.Appl.Phys. (1953) 570.
1131 Tagawa,M., Takenobu,S., Ohmae,N. and Umeno,M., Appl.Phys.Letter
50
(1987) 545./14/ Kato,S., Int. High Voltage Symp. Milan, No. 11.09 1987-8.
