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THE COMPUTER-CONTROLLED FIELD ION MICROSCOPE WITH ATOM-PROBE AT THE
HAHN-MEITNER-INSTITUTE
P. Mertens, U. Vidic, Hanna Becker
To cite this version:
P. Mertens, U. Vidic, Hanna Becker. THE COMPUTER-CONTROLLED FIELD ION MICRO-
SCOPE WITH ATOM-PROBE AT THE HAHN-MEITNER-INSTITUTE. Journal de Physique Col-
loques, 1984, 45 (C9), pp.C9-309-C9-313. �10.1051/jphyscol:1984952�. �jpa-00224438�
Colloque C9, supplément au n°12, Tome 45, décembre 198* page C9-309
THE COMPUTER-CONTROLLED FIELD ION MICROSCOPE WITH ATOM-PROBE AT THE HAHN-MEITNER-INSTITUTE
P. Mertens, U. Vidic and H. Becker
Bahn-Meitner-Institut fiir Kemforschung Berlin, Glienieker Str. 100, D-1000 Berlin 39, F.R.G.
Résumé - Un microscope ionique de champ fonctionnant sous un contrô- le informatisé à grande échelle est décrit. Les considérations de base relatives à la conception et à la structure du hardware et aux program- mes de mesure sont soulignées.
A b s t r a c t - A f i e l d ion microscope operated under large scale computer c o n t r o l i s d e s c r i b e d . Basic design c o n s i d e r a t i o n and t h e s t r u c t u r e of e l e c t r o n i c hardware and of measuring programs are o u t l i n e d .
I - INTRODUCTION
The f i e l d ion microscope w i t h atom-probe b r i e f l y sketched here was developed f o r t h e r o u t i n e a n a l y s i s of a l l o y s . In f i e l d ion microscopy an experiment w i t h an a l l o y i s u s u a l l y stopped by the rupture of the t i p specimen at high v o l t a g e s . Therefore a r e l a t i v e l y large number of samples i s involved i n an a n a l y s i s . Thus, an e f f e c t i v e t i p exchange f a c i l i t y i s required which does not cause long-term i n t e r r u p t i o n s . S i m i l a r l y , a high a v a i l a b i l i t y of the instrument has t o be p r o - v i d e d . An a d d i t i o n a l aspect governing t h e design of t h i s instrument was, t h a t i t i s t o be operated by personnel not n e c e s s a r i l y experienced i n f i e l d ion m i c r o - scopy. Right from t h e beginning i t was planned t o be used as a t o o l t o do research w i t h , and not t o do research a t . The basic c o n s i d e r a t i o n s f o r t h i s i n s t r u m e n t ' s design are compiled i n the f o l l o w i n g t a b l e .
Basic Considerations f o r the i n s t r u m e n t ' s design
Requirements P r o v i s i o n s Easy Maintenance
No l i q u i d coolant f o r t i p Temperature c o n t r o l by closed cycle r e f r i g e r a t i o n
Reduction of LN2-use Turbomolecular pumps instead of d i f f u s i o n pumps
High s t a b i l i t y and short Use of commercial components wherever r e p a i r periods p o s s i b l e , instrument and support b u i l t
of two e a s i l y detachable parts
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984952
JOURNAL DE PHYSIQUE
Table cont.
Easy O p e r a t i on
Frequent and r a p i d t i p exchange F i v e - t i p - s t o r a g e a t 10-7 T o r r and f o r s h o r t - 1 i v e d a1 l o y specimens e l e c t r i c a l t r a n s p o r t i n t o t i p h o l d e r FIM u s a b l e by unexperienced
personnel
Computer c o n t r o l f o r c r u c i a l f u n c t i o n s ( v o l t a g e s etc.)
D a y l i g h t o p e r a t i o n Video d i s p l a y o f FIM image
Simp1 e a1 ignment and f o c u s s i n g E l e c t r o s t a t i c d e f l e c t i o n p l a t e s , reasona- o f t h e i o n beam b l y we1 1 f o c u s s i n g e i n z e l l e n s , f o c u s
c o n t r o l v i a r a t e m e t e r s and a d d i t i o n a l v i d e o camera
Atom-probe o p e r a t i o n indepen- On-1 i n e p u l s e - h e i g h t a n a l y s i s o f HV-pul- d e n t o f p u l s e r t y p e o r t i m e - ses
dependent performance S t r a i g h t f o r w a r d o p e r a t i o n
Easy d a t a access and e v a l u a t i o n
L a r g e - s c a l e computer c o n t r o l o f t h e expe- r i m e n t , i n d i v i d u a l programs f o r atom- probe, d e s o r p t i o n microscope, and TOF- c a l i b r a t i o n
I n s t r u m e n t coupled t o a POP 11/44 v i a UNIBUS, f i l e s s t o r e d on d i s k s , o n - l i n e and o f f - 1 i n e d i s p l a y o f mass s p e c t r a and d e p t h d i s t r i b u t i o n s
R e l i a b i l i t y and easy maintenance o f t h e i n s t r u m e n t a r e a r e s u l t o f t h e mechanical hardware, w h i l e v e r s a t i l i t y and e f f e c t i v i t y o f t h e o p e r a t i n g a r e most o f a l l due t o t h e e l e c t r o n i c hardware and t h e programs a v a i l a b l e . Here o n l y e l e c t r o n i c hardware and t h e programs w i l l be o u t l i n e d . A comprehensive d e s c r i p t i o n o f t h e i n s t r u m e n t i n c l u d i n g mechanical hardware i s g i v e n i n r e f . 1.
2
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PROCESS INSTRUMENTATION FOR MEASUREMENT AND CONTROLThe f i e l d i o n microscope i s c o n t r o l l e d by a process computer PDP 11/44 as manu- f a c t u r e d by D i g i t a l Equipment C o r p o r a t i o n , Maynard, Mass. The computer i n coup- l e d t o t h e experiment v i s a CA 1 1 - F - i n t e r f a c e , which i s d i r e c t l y a t t a c h e d t o t h e UNIBUS w i t h a l e n g t h o f 75 ft. A t t h e same t i m e , t h i s i n t e r f a c e a c t s as a c r a t e c o n t r o l l e r f o r t h e CAMAC sytem. CAMAC (Computer A p p l i c a t i o n f o r Measurement And C o n t r o l ) i s an i n t e r n a t i o n a l l y standardTzed moduTar i n s t r u m e n t a t T o n system f<r
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e l e c t r o n i c measurements. As a g r e a t v a r i t y o f modules i s a v a i l a b l e , CAMAC i s t h e most v e r s a t i l e system by f a r t o i n t e r f a c e an e x p e r i m e n t ' s e l e c t r o n i c p e r i p h e r a l s t o t h e computer. A b l o c k diagram o f t h e hardware components i s g i v e n i n f i g . 1.A l l v o l t a g e s a p p l i e d t o t h e i n s t r u m e n t can be a d j u s t e d m a n u a l l y as w e l l as under computer c o n t r o l . As t h i s o p t i o n i s s e t by a hardware s w i t c h , an o p e r a t o r can change f r o m manual t o a u t o m a t i c c o n t r o l d u r i n g a measurement, i f i t i s necessary.
B e f o r e a new HV-pulse i s t r i g g e r e d , t h e a c t u a l values f o r t i p v o l t a g e and p u l s e r c h a r g i n g v o l t a g e a r e read a u t o m a t i c a l l y , so t h a t t h i s manual i n t e r v e n t i o n does n o t hamper t h e measurement. Each experiment i s s t a r t e d w i t h t h e manual a d j u s t - ment o f t h e v o l t a g e s f o r t i p , p u l s e r and l e n s . I f a u t o m a t i c c o n t r o l i s s e l e c t e d , t h e s e v a l u e s a r e read by t h e computer, and a p p r o p r i a t e c o n t r o l v o l t a g e s a r e s e t . A f t e r t h e change t o a u t o m a t i c c o n t r o l , t h e computer i n i t i a t e s a l l f u r t h e r adjustments.
The h i g h e r t h e degree o f a u t o m a t i z a t i o n i n a system i s , t h e more i m p o r t a n t f a c i - l i t i e s become by which an o p e r a t o r can i n f l u e n c e t h e c o u r s e o f an experiment.
Here, t h e o p e r a t o r ' s i n t e r v e n t i o n and t h e e x p e r i m e n t ' s s t a t u s c o n t r o l s i g n a l s a r e t r e a t e d i n a common p r i o r i t y i n t e r r u p t r e g i s t e r (Jorway Model 63), which i s
Fig. 1: B l o c k diagram o f t h e microscope w i t h e l e c t r o n i c s u p p l i e s and c o n t r o l s .
r e a d b e f o r e a new HV-pulse i s t r i g g e r e d . T h i s means t h a t t h e s t a t u s c o n t r o l i s accomplished w i t h i n t h e measuring program and n o t by a s e p a r a t e t a s k , which e v e n t u a l l y c o u l d n o t r e a c t t o s t a t u s changes i n s t a n t a n e o u s l y . For t h e FIM expe- r i m e n t , t h e s t a t u s c o n t r o l r e g i s t e r i s merely needed t o t r a n s f e r s c a t u s i n f o r m a - t i o n t o t h e measuring program. D i r e c t hardware p r o t e c t i o n o f , f o r example, t h e channel p l a t e s i n t h e case o f a leakage i n t h e vacuum system i s p r o v i d e d by r e - l a y s , which a r e s e t by t h e p r e s s u r e m o n i t o r i n g system.
computer
The CAMAC modules used f o r t h e t i m e - o f - f l i g h t measurement a r e a Le Croy 2228 A c l o c k w i t h ranges of 2 ps, 5 ~ s and 10 ps, an o u t p u t r e g i s t e r f o r t h e t r i g g e - r i n g o f t h e p u l s e , and a p a r l l e l o u t p u t r e g i s t e r f o r t h e s e t t i n g o f a d i g i t a l delay. A HV-pulse i s t r i g g e r e d , i f a b i t i s s e t i n t h e o u t p u t r e g i s t e r . A synch- ronous p u l s e i s d e r i v e d from t h e HV-pulse, which t h e n r e p r e s e n t s t h e a c t u a l s t a r t s i g n a l f o r t h e t i m i n g c i r c u i t r y . T h i s s i g n a l t r i g g e r s t h e d i g i t a l delay, t h e o u t p u t s i g n a l o f which t h e n i n i t i a t e s t h e t i m i n g c y c l e i n t h e 200 MHz c l o c k and r e s e t s a s h i f t r e g i s t e r . The p u l s e s t o s t o p t h e t i m i n g c y c l e a r e d e r i v e d f r o m t h e d e t e c t o r s i g n a l s o f t h e i m p i n g i n g p a r t i c l e s . I n o r d e r t o measure up t o e i g h t p a r t i c l e s desorbed by one HV-pulse, t h e s h i f t r e g i s t e r i s employed. Thi-s r e g i - s t e r d i r e c t s t h e s i g n a l o f t h e f i r s t p a r t i c l e d e t e c t e d a f t e r t h e r e s e t p u l s e i n t o .the f i r s t channel o f t h e c l o c k , t h e second s i g n a l i n t o t h e second channel and so on. When a f t e r t h e readout t h e i - t h channel o f t h e c l o c k i s found t o have reached i t s maximum c o n t e n t ( 1 1 b i t ) , i t can be concluded t h a t i - 1 p a r t i c l e s have been d e t e c t e d w i t h i n t h e range o f 10 ys. I f even t h e c o n t e n t s o f t h e 8 t h channel i s l e s s t h a n 211
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1, more t h a n e i g h t p a r t i c l e s m i g h t have been desorbed.Such an e v e n t cannot be i n c l u d e d i n t h e c o n s t r u c t i o n o f a mass spectrum. F o r a normal e v e n t w i t h 1 ( i 6 8, t h e masses o f t h e p a r t i c l e s a r e c a l c u l a t e d and accumulated i n t o a mass histogram. Subsequently, a n o t h e r HV-pulse i s t r i g g e r e d .
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JOURNAL DE PHYSIQUE
3
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THE SOFTWARE SYSTEMThe program system comprises t h r e e t a s k s , r u n n i n g under t h e c o n t r o l o f t h e o p e r a t i n g system RSX-11M v e r s i o n 4.0:
'-
MUMFIM = t a s k f o r c o n t r o l and measuring o p e r a t i o n s-
FIMSTO = t a s k f o r d a t a s t o r a g e-
FIMGRA = t a s k f o r t h e g r a p h i c i n p u t and o u t p u t on t h e d i s p l a y .A c t u a l experiment c o n t r o l i s performed by MUMFIM i n a d i a l o g u e w i t h t h e o p e r a t o r who can choose between t w o d i f f e r e n t modes:
-
atom probe mode-
d e s o r p t i o n mode ( w i t h i n t e g r a t e d atom probe mode).I n a d d i t i o n t o t h e s e t w o modes, t h e r e i s a t h i r d program
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c a l i b r a t i o nw h i c h can b e used t o a d j u s t t h e parameters f o r atom probe mode.
FIMSTO and FIMGRA a r e l o a d e d o n l y i f r e q u i r e d by t h e measuring programs.
The f u n c t i o n o f FIMSTO i s t o s t o r e mass s p e c t r a and d e p t h p r o f i l e s on t h e d i s k i n t h e f o r m a t which i s g e n e r a l l y used f o r s p e c t r a i n t h e HMI (CESSNA f o r m a t as d e f i n e d i n t h e i n t e r n a l r e p o r t by W.L. B a u e r f e l d , DM 52
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J u l y 1977). FIMSTO i s a l s o employed t o r e a d mass s p e c t r a f r o m t h e d i s k and t o make them a v a i l a b l e f o r c a l i b r a t i o n purposes.The f u n c t i o n o f FIMGRA i s t o p l o t mass s p e c t r a and c o n c e n t r a t i o n d e p t h p r o f i l e s on t h e g r a p h i c d i s p l a y .
MUMFIM i s programmed i n MUMTI, which i s a H M I - w r i t t e n i n t e r p r e t a t i v e language, whereas FIMSTO and FIMGRA a r e FORTRAN programs.
The program f o r atom probe mode i s designed t o a n a l y z e t h e elemental c o m p o s i t i o n o f t h e t i p under i n v e s t i g a t i o n . Fig. 2 p r e s e n t s a rough diagram o f t h i s mode.
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Fig. 2
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S o f t w a r e components f o r atom probe modeparameters and c o n t r o l s t h e hardware components a p p l i e d . Then t h e program t r i g g e r s t h e t i p p u l s e a t a p r e - s e l e c t e d frequency. B e f o r e a new p u l s e i s t r i g g e r e d , t h e s t a t u s o f th'e experiment i s checked and t h e a c t u a l h i g h v o l t a g e s f o r t h e t i p and f o r t h e p u l s e r a r e measured. From t h e t i m e - o f - f l i g h t d a t a o f t h e i o n s r e g i s t e r e d a t t h e i o n d e t e c t o r , t h e r e s p e c t i v e mass-to-voltage r a t i o s m/n a r e c a l c u l a t e d , accumulated t o a spectrum and l i s t e d on t h e g r a p h i c d i s p l a y (atom probe l i s t i n g ) . By means o f manual i n t e r v e n t i o n by t h e o p e r a t o r ('show spectrum'), t h e spectrum i s p l o t t e d o n t o t h e g r a p h i c d i s p l a y d u r i n g t h e measurement. The m/n l i m i t s f o r t h e g r a p h i c d i s p l a y a r e n o r m a l l y chosen by t h e program i n a way t h a t c u t s o f f t h e p a r t s o f t h e spectrum b e f o r e t h e f i r s t and a f t e r t h e l a s t peak. But o t h e r s e c t i o n s can be s e l e c t e d a l t e r n a t i v e l y by t h e o p e r a t o r ( ' e n t e r mass d i s p l a y l i m i t s ' ) . A d e p t h p r o f i l e o f t h e t i p can be c o m p i l e d by r e g i s t e r i n g t h e number o f i o n s p e r atom l a y e r f o r . s p e c i f i c elements. Up t o 8 elements w i t h up t o 3 charge s t a t e s each can be included. I n t h e presence o f imaging gas, t h e s i g n a l
' l a y e r desorbed' can be g i v e n manually o r a u t o m a t i c a l l y (see 3.3). For o p e r a t i o n s w i t h o u t imaging gas, an atom l a y e r i s a r b i t r a r i l y d e f i n e d by a f i x e d number o f atoms p e r l a y e r . Every atom l a y e r i s l i s t e d on t h e t e r m i n a l . A g r a p h i c d i s p l a y i n t h e f o r m o f c o n c e n t r a t i o n p r o f i l e s can be r e q u e s t e d m a n u a l l y ('show d e p t h p r o - f i l e ' ) . S p e c t r a and d e p t h p r o f i l e s ( i n t h e form o f i n d i v i d u a l s p e c t r a f o r s e l e c - t e d specimens) can be s t o r e d on t h e d i s k .
The c l o s e c o n n e c t i o n o f t h e f i e l d i o n microscope w i t h a computer m i g h t be r e g a r - ded as a r e s t r i c t i o n on p e r f o r m i n g t e s t s and e l e c t r o n i c a d j u s t m e n t s a t t h e i n s t r u m e n t . But a m u l t i t u d e o f f u n c t i o n s can, i n f a c t , be o p e r a t e d manually, if d e s i r e d . A l l v o l t a g e s can be a p p l i e d i n t h i s way, and a l l p u l s e s can be t r i g g e - r e d by an o f f - 1 i n e p u l s e r w i t h p r e s e l e c t a b l e p u l s e numbers and r e p e t i t i o n r a t e s . Only t h e TOF spectrum cannot be o b t a i n e d l i k e t h a t . But t h e number o f p a r t i c l e s a r r i v i n g a t t h e d e t e c t o r w i t h i n t h e TOF range o f 10 p s can be r e g i s t e r e d i n s t e a d . F o r t h i s purpose, t h e g a t e o f t h e NIM c o u n t e r f o r t h e d e t e c t o r p u l s e s i s opened f o r 10 ps, s i m u l t a n e o u s l y w i t h t h e p u l s e s t a r t i n g t h e c l o c k . Thus, v i r t u a l l y a l l f u n c t i o n s o f t h e i n s t r u m e n t can be o p e r a t e d o f f - l i n e . Computer programs, on t h e o t h e r hand, can be debugged w i t h o u t t h e experiment attached. For t h e v o l t a g e s t o be read, d e f a u l t v a l u e s a r e programmed. The TOF events can be s i m u l a t e d by a p u l s e generator, which i s t r i g g e r e d by t h e CAMAC o u t p u t r e g i s t e r l i k e t h e HV- p u l s e r . Employed i n b u r s t mode, t h i s g e n e r a t o r (HP 8015 A) produces a p r e s e l e c t - a b l e number o f o u t p u t p u l s e s s i m u l a t i n g t h e desorbed p a r t i c l e s w i t h a d j u s t a b l e d e l a y t i m e s between them. By t h i s o f f - 1 i n e o p e r a t i o n and TOF s i m u l a t i o n , i n s t r u - ment and program can be t e s t e d i n d e p e n d e n t l y i n an e f f e c t i v e way.
Having o p e r a t e d t h i s f i e l d i o n microscope w i t h atom-probe f o r t w o y e a r s , i t can be s t a t e d now t h a t t h e i n i t i a l goal o f b u i l d i n g a r e l i a b l e i n s t r u m e n t w h i c h i s easy t o s e r v i c e and t o o p e r a t e has been f u l l y reached. Examples f o r t h e p e r f o r - mance i n imaging and atom p r o b i n g a r e g i v e n i n r e f . / I / .
REFERENCES
/I/ P. Mertens, U. V i d i c and H. Becker, A Computer-Controlled F i e l d I o n M i c r o - scope w i t h Atom-Probe, Report of t h e H a h n - M e i t n e r - I n s t i t u t Fur Kernforschung B e r l i n CmbH
