A COMBINED FIELD ELECTRON AND FIELD ION MICROSCOPE

HAL Id: jpa-00224430

https://hal.archives-ouvertes.fr/jpa-00224430

Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A COMBINED FIELD ELECTRON AND FIELD ION MICROSCOPE

N. Ernest, G. Ehrlich

To cite this version:

N. Ernest, G. Ehrlich. A COMBINED FIELD ELECTRON AND FIELD ION MICROSCOPE.

Journal de Physique Colloques, 1984, 45 (C9), pp.C9-293-C6-296. �10.1051/jphyscol:1984949�. �jpa-

00224430�

JOURNAL DE PHYSIQUE

Colloque C9, supplément au n°12, Tome 45, décembre 198*

"

A COMBINED FIELD ELECTRON AND FIELD ION MICROSCOPE*

N. Ernst

and G. Ehrlich

Coordinated Science Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A.

RESUME

Une combinaison entre microscope ionique de champ et microscope électronique de champ à trou-sonde équipés avec un iris ajustable et un système informatique d'acquisition de données est décrite. Le diamètre minimum du trou-sonde est compa- rable à la taille de spot de l'image ionique de champ d'un atome de surface et la détection de particules individuelles a été démontrée. Des mesures de travail de sortie sur du tungstène (110) propre ou recouvert de Re sont données.

ABSTRACT

A combined field ion- and field electron probe hole miscroscope equipped with an adjustable iris and computer aided data acquisition is described; the minimum probe hole diameter is comparable to the spot size of the field ion image of a single surface atom, and single particle detection has been demonstrated.

Work function measurements on clean and Re covered W(110) are reported.

Relatively little has been done to examine adsorption on single crystal sur- faces prepared by field evaporation and characterized on the atomic level by field

1-3

ion microscopy. In large measure such studies have been inhibited by the dif- ficulty of carrying out both field ion imaging and work function measurements in one instrument.

We have developed a probe-hole microscope which is designed specifically for field electron emission measurements on single adatoms imaged by field ion micros- copy. This microscope is shown schematically in Figure 1. An externally adjustable

•Supported by the National Science Foundation under grant DMR 82-01884.

+

0 n leave from the Fritz-Haber Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-1000 Berlin 33, F.R.G.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984949

JOURNAL DE PHYSIQUE

u

Video

Camera - _Terminal

Fig. 1 Experimental s e t up.

i r i s 4 d e f i n e s t h e p o r t i o n of t h e e m i t t e r s u r f a c e seen by a Chevron Channel P l a t e (CCP). The CCP s e r v e s a s a d e t e c t o r f o r e l e c t r o n s a s w e l l a s i o n s and t h e ion- o r e l e c t r o n image can b e photographed on a phosphor s c r e e n . I n d i v i d u a l p a r t i c l e s s t r i k i n g the p l a t e a r e r e g i s t e r e d u s i n g f a s t (100MHz) counting e l e c t r o n i c s and computer a i d e d d a t a a c q u i s i t o n .

To avoid contamination of t h e sample, t h e CCP h a s t o be e x t e n s i v e l y outgassed;

t h i s c a u s e s a s i g n i f i c a n t d e c r e a s e of t h e a m p l i f i c a t i o n , a s demonstrated i n Figure 2 . Here, t h e number of p u l s e s r e g i s t e r e d with a f a s t multichannel analyzer

(Le Croy N I M Model 3001) i s p l o t t e d v e r s u s t h e amount of charge generated by t h e

CCP p e r incoming p a r t i c l e . The d i s t r i b u t i o n f o r t h e background n o i s e i s a l s o

shown i n Figure 2. With t h e p r e s e n t d i s c r i m i n a t o r l e v e l a t y p i c a l background

-3O0/0 of pulses above discriminator level

F i g . 2 A m p l i f i c a t i o n of t h e Chevron Channel P l a t e a f t e r e x t e n s i v e e l e c t r o n bomb- bardment. The f u l l l i n e g i v e s t h e background p l u s s i g n a l d i s t r i b u t i o n c u r v e measuring t h e amount of c h a r g e g e n e r a t e d p e r 400 eV f i e l d e l e c t r o n ; t h e dashed c u r v e i s t h e background d i s t r i b u t i o n r e g i s t e r e d w i t h o u t imping- i n g e l e c t r o n s . Curves were measured w i t h 700V a c r o s s t h e f i r s t and l O O O V a c r o s s t h e second c h a n n e l p l a t e .

c o u n t i n g r a t e of f i v e e v e n t s p e r second i s r e g i s t e r e d . The o v e r a l l d e t e c t i o n e f f i c i e n c y of t h e CCP i s a b o u t 1 / 1 0 and t h e u l t i m a t e s e n s i t i v i t y f o r probe h o l e c u r r e n t measurements i s 10 -18 A . C l e a n l i n e s s of t h e CCP and of o t h e r p a r t s mounted i n t h e microscope h a s been t e s t e d by measurements of e l e c t r o n 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 of t i m e . These t e s t s do n o t show any c o n t a m i n a t i o n of t h e e m i t t e r s u r - f a c e , a t l e a s t d u r i n g a t i m e p e r i o d of f i f t e e n m i n u t e s . T y p i c a l l y , Fowler-Nordheim- r u n s o v e r s e v e r a l d e c a d e s of e m i s s i o n c u r r e n t c a n b e completed i n l e s s t h a n t e n m i n u t e s .

The i n s t r u m e n t i s c u r r e n t l y b e i n g u s e d f o r a d s o r p t i o n s t u d i e s of m e t a l o v e r -

l a y e r s on W f l l O ) . D i r e c t f i e l d i o n m i c r o s c o p i c o b s e r v a t i o n s have e s t a b l i s h e d t h a t

t h e minimum d i a m e t e r of t h e i r i s p r o b e h o l e is a s s m a l l a s t h e a p p a r e n t s i z e of a

s i n g l e s u r f a c e atom. T h i s i s i l l u s t r a t e d i n F i g u r e 3 ( a ) f o r t h e p a r t i c u l a r example

C9-296 JOURNAL DE PHYSIQUE

of a s i n g l e Re-atom i n a n o f f - c e n t e r p o s i t i o n on W(110). There two Fowler-Nordheim p l o t s a r e shown, o b t a i n e d w i t h and w i t h o u t Re p r e s e n t on t h e s u r f a c e . Though t h e

F i g . 3 . Fowler-Nordheim p l o t s ( i

probe h o l e c u r r e n t , V

t i p v o l t a g e ) of e x p e r i - P r o

m e n t a l d a t a t a k e n w i t h a p r o b e h o l e d i a m e t e r a s s m a l l a s t h e f i e l d i o n image s p o t of a s i n g l e Re-adatom: ( a ) For t h e c l e a n s u r f a c e and f o r t h e e m i s s i o n o r g i n a t i n g from t h e r e g i o n of t h e Re-adatom and (b) f o r c l e a n W(110) w i t h d i f f e r e n t t i p p o s i t i o n s .

Re-adatom c a u s e s a s l i g h t i n c r e a s e of t h e l o c a l work f u n c t i o n , a s i g n i f i c a n t i n c r e a s e i n probe h o l e c u r r e n t i s d e t e c t e d , i n agreement w i t h e a r l i e r o b s e r v a t i o n s . 3

A f t e r removing t h e Re-adatom by f i e l d e v a p o r a t i o n t h e (110) c e n t e r was a l i g n e d w i t h t h e probe h o l e . A s shown i n F i g u r e 3 ( b ) t h e l o c a l work f u n c t i o n i n c r e a s e s when e m i s s i o n from t h e c e n t e r i s probed. T h i s c a n b e e x p l a i n e d by t h e d e c r e a s i n g c o n t r i b u t i o n of t h e s u r r o u n d i n g low work f u n c t i o n a r e a s t o t h e probe h o l e c u r r e n t .

REFERENCES

1. x. W. Plummer and T. N. Rhodin, Appl. Phys. L e t t e r s 11, 194 (1967) 2. C. J. Todd a n d T. N . Rhodin, S u r f a c e S c i . 36, 353 (1973)

3. G. L. Kellogg and T. T. Tsong, S u r f a c e S c i . 63, 343 (1977)

4. A. J. Melmed, M. Martinka and R . K l e i n , P r o c e e d i n g s of t h e 2 9 t h IFES e d i t e d

by H.O. Andrgn and H. Norden (Almqvist

W i k s e l l , I n t e r n a t i o n a l , Stockholm,

1 9 8 2 ) , p . 243