Toward a scanning atom probe — computer simulation of electric field -

appled surface science ELSEVIER

Apphed Surface Science 76/77 (1994) 424-430

Toward a scanning atom probe - computer simulation of electric field Osamu N~shlkawa *, Masahlro Klmoto Department of Electromcs, Kanazawa Institute of Technology, 7-10hgtgaoka, Nonotcht, Kanazawa-South, Ishtkawa 921, Japan (Received 2 August 1993 accepted for pubhcatlon 20 September 1993)

Abstract

Development of a new atom probe (AP) named "scanning atom probe (SAP)" is proposed The SAP consists of a funnel-shaped micro-extraction electrode and a thin flat plate grooved or mlcro-photoetched m a checkerboard pattern, shaping up many micro tips The extraction electrode scans over the grooved sample surface and stands still above a particular tip forming a minute field emxsslon or field ion microscope with a high field area well confined ,n a small space between the tip and the electrode The field distribution in the confined space depends on many factors, setting the configuration and arrangement of the tip and electrode Accordingly, the field distribution in the confined space is computed in order to find the optimum tip and electrode configuration, and the relative position of them Variation of the field strength with tip-electrode distances, cone angles of the tip, and the configuration of the electrode, are discussed

1. Introduction In a c o n v e n t i o n a l field 1on m i c r o s c o p e ( F I M ) [1] an a few m m long single s h a r p txp faces a nearby grounded electrode and phosphor screen a b o u t 10 c m away A p p h c a U o n o f a p o s l t w e high voltage, 3 - 3 0 kV, to t h e t~p with t h e tip r a d i u s o f 100 to 1000 A g e n e r a t e s an e x t r e m e l y high field, o 3 - 6 V / A , a b o v e t h e h e m i s p h e r i c a l surface of t h e Up a p e x W h e n t h e field s t r e n g t h is high e n o u g h , " f i e l d - ~ o m z e d " gas a t o m s p r o j e c t an F I M ~mage o f the tip a p e x on t h e s c r e e n with a t o m i c a l l y high r e s o l u t i o n A t h i g h e r fields, surface a t o m s at t h e tip a p e x a r e r e m o v e d in an o r d e r l y f a s h i o n as positively c h a r g e d ions by a p e c u h a r p h e n o m e n o n

* Corresponding author Fax +81 762 946717

c a l l e d " f i e l d e v a p o r a t i o n " [1] This suggests t h a t o n e can directly o b s e r v e the a t o m i c a r r a n g e m e n t on t h e a p e x h e m x s p h e r e with t h e F I M a n d massa n a l y z e l n d w l d u a l e v a p o r a t e d ions with a mass a n a l y z e r a t t a c h e d to the F I M t h e c o m b i n e d Ins t r u m e n t of an F I M a n d a m a s s - a n a l y z e r is c a l l e d t h e a t o m p r o b e ( A P ) [1,2] Since t h e A P is an u l t i m a t e m~cro-massanalyzer, it has b e e n u t l h z e d in the m l c r o a n a l y s l s of v a r i o u s m a t e r m l s such as metals, s e m i c o n d u c tors, a n d c o n d u c t i v e p o l y m e r s a n d c e r a m i c s [2-6] H o w e v e r , It has also b e e n r e a l i z e d t h a t the A P a p p h c a t i o n is intrinsically l i m i t e d b e c a u s e the s a m p l e n e e d s to b e a s h a r p tip a n d t h e a n a l y z e d a r e a is an e x t r e m e l y small h e m i s p h e r i c a l tip a p e x F u r t h e r m o r e , t h e f a b r i c a t i o n o f a s h a r p Up is a n o t h e r p r o b l e m A l t h o u g h the p r e p a r a t i o n p r o cess o f t~ps f r o m a fine m e t a l wire is well devel-

0169-4332/94/$07 00 © 1994 Elsevier Science B V All ngbts reserved

SSDI 0 1 6 9 - 4 3 3 2 ( 9 3 ) E 0 2 7 8 - T

0 Ntshtkawa, M Ktmoto/Apphed Surface Sctence 76 / 77 (1994) 424-430

oped and relatwely easy, the tip making from a metal block or a semtconductor wafer lS still fairly comphcated A hopelessly difficult case ts to fabricate a tip wtth a specified spot of a spectmen at the apex or a ttp wtth a multi-layer structure grown on a wafer at the tip end In order to break through these difficulties a new AP n a m e d "scanning atom probe (SAP)" is proposed A specimen of the SAP is a thin plate which is grooved in a checkerboard pattern formlng many tips A funnel-shaped mtcro-extractlon electrode scans over the specimen and forms a m m u t e field emission or field ton microscope when the electrode ts poslttoned above one of the tips Then, the high field generated by a bias voltage apphed to the electrode or the specimen is confined to a small space between the electrode and the specific tip In this study the feaslblhty of developmg the SAP was examined by computing the field distribution between a tip and an extraction electrode for vartous tip lengths, cone angles of the tips, and the positron and configuration of the electrode

AYV'

~

425

>2 lz r n T ~

>1 0 /~m Fig 2 Schematics of the extraction electrode and a t~p formed by grooving

2. Scanning atom probe Untque features of the SAP are a flat specimen and a micro-extraction electrode The spectmen plate ts grooved or mlcro-photoetched in a checkerboard pattern formmg many sharp spikes or tips The assumed depth of the grooves is several to a few tens of microns and the spaces between the ttps are tens to hundreds of microns, Ftg 1 The funnel-shaped extraction electrode scans over the grooved specimen surface and stands sttll above a particular tip, Fig 2 Since the diameter of the open hole at the end of the extraction electrode can be made as small as a few to ten microns, the high field is well confined m a small space between the tip and the electrode when a negattve or positive bias voltage is applied to the sample or the electrode Then, the

'Nf

Fig 1 Schematics of a grooved specimen

Fig 3 Schematics indicating parameters to calculate the field dlstnbuhon m the space between the electrode and the tip

0 Nlshtkawa, M Klmoto /Apphed Surface Sctence 76 / 77 (1994) 424-430

426

NI q~pi-, one

conventtonal field emission microscope ( F E M ) [7,8] and an F I M D e t e c t i o n of mdivldual evaporated ions by a mass-analyzer makes the F I M serve as the SAP [ Sect,

ons 3. Computation of field distribution

TI p / ~ ~ ~ E x t r a c t l 0 n

The confinement of the field d e p e n d s on many factors setting the configuration and a r r a n g e m e n t of the tip and electrode The most effective factor is the t i p - e l e c t r o d e gap, the narrower the gap, the smaller the high field space and the lower the bias voltage However, care must be taken to avoid field emission from the open end of the electrode when the SAP is o p e r a t e d as an F I M because a high-density emission current from the electrode may damage the tip apex Thus, the highest field strength on the electrode surface should be less than one tenth of that of the tip apex Accordingly, the field distribution in the confined space was computed In o r d e r to find the optimum tip and electrode configuration by vary-

Electr0de

F,g 4 Schemat]cs of the dwlded space for computing the field dlstr,buuon by difference equat,on

combination of one of the tips and the electrode enables us to extract field-emitted electrons, field-lontzed gas ions and field-evaporated surface atoms from a tip apex, and to project a field emission or field ion image on a screen like a

Table 1 Variation of field strengths at the tip apex and the electrode end Fxg No

h (pJ

d (/x)

0 (deg)

4) (A)

o/1 (deg)

ola (deg)

R (A)

Et

Ec

Er

22 22 22 22 22 22 32 32 32 32 32 32 32 32 32 64 64

0 1 1 1 1 1 0 0 0 0 1 0 - 0 25 - 05 1 0 0

40 20 20 40 40 60 20 40 60 90 30 30 30 30 30 30 30

24 34 34 24 34 34 24 24 34 34 24 24 24 24 24 24 48

30 30 40 30 30 30 30 30 30 30 30 30 30 30 30 30 30

35 35 45 35 35 35 35 35 35 35 35 35 35 35 35 35 35

6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 6000 9000 6000 6000

73 55 68 53 5 1 61 80 73 61 45 58 75 79 81 59 76 63

0 90 0 68 0 71 0 68 0 66 1 48 0 74 0 78 0 75 1 04 0 60 0 78 0 83 0 88 0 52 0 63 0 51

81 80 95 78 8 1 4 1 10 8 94 8 1 43 97 97 95 93 11 5 12 2 12 3

Fig No indicates the number of the figure shown m this report Tip radius r = 1000 .~, h hp length, d height difference between the hp apex and the electrode end, 0 cone angle of the tip, 05 diameter of the open hole at the electrode end, cone angles and end curvatures of the electrode are al, a 2 and R, respectwely E t and E e are the field strengths ( V / , ~ ) at the tip apex and the electrode end for V = 10 kV, respectwely, and E r = Et//Ee

0 Ntshtkawa, M Ktmoto/Applted Surface Scwnce 76/77 (1994) 424-430 Ee[I//A] Et 10-

10

09

9

08-

8

07-

7

06-

6

05-

5

04

-05

viA]

E Er

10 9

0=30 ° h=3 ?[um]

8

7

Et

6 5

0

05

4

d ~um]

427

a n g l e s a n d e n d c u r v a t u r e s o f t h e e l e c t r o d e a i, a 2 and R, respectively, Fig 3 Since the tip-elect r o d e a s s e m b l y is s y m m e t r i c a r o u n d t h e tip axas, c y l i n d r i c a l c o o r d i n a t e s w e r e e m p l o y e d to c a l c u l a t e t h e e l e c t r i c p o t e n t i a l V T h e n , L a p l a c e ' s diff e r e n t i a l e q u a t i o n w a s t r a n s f e r r e d to d i f f e r e n c e equations and the r - z space was divided into M × N s e c t i o n s to c o m p u t e t h e p o t e n t i a l s at e a c h 1 - j t h c r o s s i n g p o i n t V,,j, F i g 4 I n t h e p r e s e n t c a l c u l a t i o n M a n d N w e r e 800 a n d 1600, r e s p e c tively T h e c a l c u l a t i o n o f a f i e l d d i s t r i b u t i o n w i t h a M a c i n t o s h Q u a d r a 950 t o o k 10 to 40 h, d e p e n d ing o n initial s e t t i n g

Fig 5 Variation of Et, E r and E~ with d, 0 = 30 °

4. Field strength and field distribution lng the and the h, c o n e h o l e at

h e i g h t d i f f e r e n c e b e t w e e n t h e tip a p e x e l e c t r o d e e n d d, tip r a d i u s r, tip l e n g t h a n g l e o f t h e tip 0, d i a m e t e r o f t h e o p e n t h e e n d o f t h e e l e c t r o d e ~b, a n d c o n e

A n i n t u i t i v e e s t i m a t e o f t h e f i e l d s t r e n g t h sugg e s t s t h e f i e l d s t r e n g t h at t h e tip a p e x , E t, inc r e a s e s w i t h d e c r e a s i n g r a n d 0, a n d i n c r e a s i n g

Fig 6 Equlpotentlal and field hnes around the hp and the electrode r = 1000 A, h = 3 2 /~, 0 = 20°, d = 0, ~b = 2 4 t~, a 1 = 30 °, a 2 = 35° and R = 6000 .~ E t and E e for V = 10 kV are 8 02 and 0 74 V/.~, respectively, and E, = 10 8 The potential difference between two eqmpotentlal hnes AV is 200 V

428

0 Ntvhlkawa. M Klmoto /Apphed Surface Sctence 76/77 (1994) 424-430

h Similarly, t h e field s t r e n g t h at the e n d of the e l e c t r o d e E e d e c r e a s e s with i n c r e a s i n g 4', d a n d R A c c o r d i n g l y , t h e r a t i o o f t h e field strengths, E r = E t / E e , s h o u l d i n c r e a s e with d e c r e a s i n g r a n d 0, a n d with i n c r e a s i n g h, 4', d a n d R H o w ever, t h e c o n f i n e m e n t of t h e h~gh field in a small s p a c e b e t w e e n t h e t~p a p e x a n d the e l e c t r o d e r e q u i r e s small 4', d a n d R, a n d large 0, which is f a v o r a b l e for f a b r i c a t i n g t h e txps by grooving T h e c a l c u l a t e d values o f E t , E e a n d E r for the tip bias v o l t a g e Vt = 10 k V a n d r = 1000 .& a r e h s t e d m T a b l e 1 for v a r i o u s h, 0, d, 4' a n d R T h e varmtxon r a n g e s of E t , E~ a n d E r a t oVt = 10 k V a r e r a t h e r narrow, from 8 1 to 4 5 V / A , f r o m 1 48 to 0 51 V / , ~ a n d f r o m 12 3 to 4 1, r e s p e c twely, for t h e v a r m t l o n of h from 2 2 to 6 4 / x , 0 f r o m 20 ° to 90 °, d f r o m - 0 5 to 1 0 / x , ~ from 2 4 to 4 8 /x a n d R f r o m 6000 to 9000 A A s pres u m e d , E t i n c r e a s e s as t h e e l e c t r o d e a p p r o a c h e s the tip, b u t E r r a t h e r stays c o n s t a n t b e c a u s e E c also i n c r e a s e s with E t, F i g 5 A s 0 increases, E t

d r o p s s h a r p l y a n d E e increases, lowering E r O n the o t h e r h a n d E r i n c r e a s e s from 9 7 to 11 5 by the i n c r e a s e o f R from 6000 to 9000 A T h e largest E~, 12 3, was o b t a i n e d for h = 6 4 / x F i e l d a n d e q u l p o t e n t l a l hnes a n d t r a j e c t o r i e s of H e ions f i e l d - i o n i z e d above t h e tip surface at Vt = 10 kV a r e shown in Figs 6 - 9 A l t h o u g h t h e hlgh field regions a r e well c o n f i n e d in a n a r r o w s p a c e a r o u n d t h e tip a p e x a n d t h e field hnes are s m o o t h l y curved, the 1on t r a j e c t o r i e s s p r e a d fairly straightly into the r a d i a l d i r e c t i o n s of the tip apex, i n d i c a t i n g the p r o j e c t i o n of an e n l a r g e d i m a g e of the tip a p e x on a s c r e e n hke an o r d i n a r y FIM

5. Discussion and conclusion T h e tip voltage, Vt, r e q u i r e d to g e n e r a t e E t = o 4 5 V / A , the l o m z a t l o n field of He, is above 20 k V for a c o n v e n t i o n a l F I M with a tip of r = 1000

Fig 7 Equlpotentlal hnes of Fig 6 and trajectories of He ions field-ionized above the tip surface covering the area 45° from the tip apex, V= 10 kV

0 Ntshtkawa, M Ktmoto/Apphed Surface Sctence 76 / 77 (1994) 424-430 .~ However, the p r e s e n t t i p - e l e c t r o d e configuration yields E t > 4 5 V / A at Vt = 10 kV b e c a u s e the high field is c o n f i n e d m a small space This l m p h e s that even the apex of a n obtuse tip with 0 = 90 ° can be observed a n d m a s s - a n a l y z e d by the S A P T a b l e 1 also indicates that E e is high a n d few t i p - e l e c t r o d e c o n f i g u r a t i o n s gwe E r > 10 If we VlSuahze the tip as a core cable a n d the electrode as a n o u t e r cyhnder, the field distribution b e t w e e n the tip a n d the electrode is somewhat s~mllar to the d t s t r l b u t l o n expected in a cyhndrlcally symmetric space T h e analogy becomes obvious as d r e d u c e d to 0 a n d b e c o m e s negative w h e n the t~p is i n s e r t e d into the o p e n hole of the electrode T h e n , E r should be p r o p o r tional to the ratio c b / 2 r which is 2 0 for ~b = 2 4 tx a n d r = 6000 A, a n d is a b o u t 1 / 5 of E r T h e p r o p o r U o n a l c o n s t a n t varies gently with h, 0, d, & a n d R, as shown in T a b l e 1 o

429

P r e s e n t discussions i n d i c a t e that the SAP allows the o b s e r v a t i o n of atomic a r r a n g e m e n t s a n d the mass-analysis of individual atoms of gentle p e a k apexes wlth 0 > 90 ° o n a grooved s p e c t m e n surface P r e s e n t results also suggest that it is desirable to set & > 5r, h > 2& a n d R > 5r T h e n the S A P can respect each p e a k apex a b o u t 10 /x apart o n a grooved surface R e a h z a t l o n of the p r o p o s e d S A P requtres the fabrication of the grooved s p e c i m e n surface a n d the micro-extraction electrode T r m l fabrication was c o n d u c t e d by drflhng t h r o u g h a gold-plated S~ t~p of an atomic force microscope by lrradmtlng a 30 keV G a 1on b e a m for a b o u t 1 m l n T h e b e a m d i a m e t e r was 0 2 /zm a n d the ton c u r r e n t was ~ 30 p A A f u n n e l - s h a p e d electrode with an o p e n hole of less t h a n 2 /zm m d t a m e t e r was f a b r i c a t e d T h e success in the trial fabrication ~s a p r o m i s i n g step for the d e v e l o p m e n t of a SAP

F i g 8 Equlpotentml and field hnes around the t~p w,th a large cone angle and the electrode r = 1000 ,~, h = 3 2 ~, 0 = 90°, d = 0, 0 5 = 3 4 / z , a 1=30 °, ot2=35 ° and R=6000,~ E t and E e for V=10kV are 45 and 104V/A, respectwely, and E r = 4 3 , AV is 200 V

430

0 Nzshlkawa, M Ktmoto/Apphed Surface Sctence 76 / 77 (1994) 424-430

F~g 8 Equlpotentlal and field hnes around the tip with a large cone angle and the electrode r = 1000 A, h = 3 2/z, 0 = 90°, d = 0, & = 3 4 ~, t~I = 30°, a2 = 35° and R = 6000 A E t and E e for V = 10 kV are 4 5 and 1 04 V/,~, respectively, and E r = 4 3, AV is 200 V

with unique features in the mlcro-analysls of various flat specimens which cannot be investigated b y t h e eyastlng A P

6. Acknowledgements The authors are grateful to Professor T KanajI of Kobe University for his helpful advice The authors also wish to express their deep gratitude to Dr S Hosaka of the Central Research Laboratory of Hitachi, Ltd for his efforts at trial fabrication of the micro-extraction electrodes T h i s w o r k is s u p p o r t e d b y a G r a n t - i n - A i d f o r Scientific Research on Priority Areas "Tunnehng Characteristics of Individual Atoms" from the Ministry of Education, Science and Culture

7. References [1] E W Muller and T T Tsong, Fxeld Ion Microscopy, Principles and Apphcatlons (Elsevier, New York, 1969) [2] M K Miller and G D W Smith, Atom Probe Mlcroanalysis Prlnoples and Applications to Materials Problems (Materials Research Society, Pittsburgh, PA, 1989) [3] O Nlshtkawa, Y Tsunashlma, E Nomura, S Hone, M Wada, M Shlbata, T Yoshlmura and R Uemon, J Vac So Technol B 1 (1983) 6, O Nlshtkawa, E Nomura, M Wada, Y Tsunashlma, S Horle, M Shlbata, T Yoshlmon and R Uemon, J Vac Scl Technol B 1 (1983) 10 [4] O Nlshlkawa, O Kaneda, M Shlbata and E Nomura, Phys Rev Lett 53 (1984)1252 [5] O Nlshlkawa and H Kato, J Chem Phys 85 (1986) 6758 [6] O Nlshlkawa and M Nagal, Phys Rev B 37 (1988) 3685 [7] R H Good and E W Muller, Handb Phys 21 (1956) 176 [8] R Gomer, Field Emission and Field Iomzatlon (Harvard University Press, Cambridge, MA, 1961)