SPUTTERING OF A FIM SPECIMEN BY IMAGING GASES

HAL Id: jpa-00225700

https://hal.archives-ouvertes.fr/jpa-00225700

Submitted on 1 Jan 1986

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.

SPUTTERING OF A FIM SPECIMEN BY IMAGING GASES

S. Nakamura, T. Hashizume, T. Sakurai

To cite this version:

S. Nakamura, T. Hashizume, T. Sakurai. SPUTTERING OF A FIM SPECIMEN BY IMAGING GASES. Journal de Physique Colloques, 1986, 47 (C2), pp.C2-431-C2-436. �10.1051/jphyscol:1986266�.

�jpa-00225700�

JOURNAL DE PHYSIQUE

Colloque C2, suppl6ment au n03, Tome 47, mars 1986 page c2-431

SPUTTERING OF A FIM SPECIMEN BY IMAGING GASES

S. NAKAMURA', T. HASHIZUME and T. SAKURAI

The Institute for Solid State Physics, The University of Tokyo, Minato-ku, Tokyo, Japan

+The Institute of Scientific and Industrial Research. Osaka University, Suita, Osaka, Japan

Bbstrert - It has been known for some time that irtiaging gases of heavier masses sputter off surface atoms durinq the

imaqing. We have investigated this phenonenon in detail using an atom-probe. We have conclusively shown that the bright spots appeared randomly after exposure to the imaging gas correspond to WHe molecular complex on the surface displaced by the sputtering and that its number density is a unique function of imaging gas pressure, exposure time and d c holding field.

In FIN study of soft metals with low melting-points, neon or argon is used for imaqing because the ionization potential of helium is too high for those metals. In the earlier days of FIN research, people not iced that bright spots appeared random1 y-d ispersed in the course of F1 imaging when Ne or Rr is used. It was speculated that it was caused by either ( 1 ) impurity gases, such as nitrogen and oxygen, are adsorbed on the surface and imaged brightly, or ( 2 ) the surface atoms displaced by Ne or Gr sputtering sit on the surface and are imaged brightly due to the field enhancement. Rdachi and Nakamura/l/

studied this in detail by observing the number density of bright spots as a function of gas pressure, surface field and exposure time and concluded that the latter may be the cause. However, the former explanation could not be ruled out completely because of rather poor vacuum conditions/E/.

In the present study, we attack this problem differently using

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986266

C2-432 JOURNAL DE PHYSIQUE

an atom-probe. We have shown (1) by analyzing individual bright spots by an atom-probe, the bright spots appeared upon exposure to Qr or Ne

represent not impurity gas atoms on the surface but the surface atoms

displaced by sputtering, and ( 2 ) from

layer-by-layer analysis the sputtering yield is a we1 l-def ined funct ion of dc holding field, gas pressure and exposure t irfle.

Rdach i and Nakamura/ l / studied the effects of Rr and Kr on F1 imaging using a simple glass-made FIM.

Their findings are

summarized in Figs. 1 and 2. InFig. 1 thenumber of the bright spots newly appeared upon exposure of the surface to Rr or Kr gas was measured as a function of exposure time.

Rnd in Fig. 2, the number densities of the bright spot6 and dark spots

(vacancies due to the sputtering

were measured as a function of surface electric field for both Rr and Kr. These results suggest that ( 1 ) the number density o f the bright spots is proport ional to the exposure time of Qr or Kr and ( 2 ) the number density of the bright spots is approximate1 y equal to that of the dark spots when both of them can be counted. However, it was known that oxygen or nitrogen impurity gas atoms inside a FIM chamber also cause a similar damage t o the F1 emitter surface/Z/. Because of relatively poor vacuum conditions of the instrument, it was

difficult, then, to draw a clear-cut conclusion. No attempt has been made to

Bornbrding Gas P r n v s

=

=

Tip Rodiv. i 400%

Fidd ; dSOMV cm

I 1

1

200 400 600 800

Time 1 sec. 1

Fig. 1. The number of bright s p o t s created by Ar and Kr g a s e s a r e p l o t t e d a s a function of t h e exposure time a t t h e presence of t h e e l e c t r i c f i e l d of 4 . 5 V/A.

Ne BIF He BIF

I 1 ^{I FEF J}

0300

Field Strength \MV/tm)

F i g . 2 . The numbers o f t h e bright and dark

s p o t s created by t h e sputtering a r e p l o t t e d a s

a function of t h e f i e l d s t r e n g t h . (See ~ e f .

1 f o r the d e t a i l s . )

s e t t l e t h i s p r o b l e m u n t i l now, p a r t l y b e c a u s e i t is r a t h e r d i f f i c u l t t o p e r f o r m q u a n t i t a t i v e i n v e s t i g a t i o n i n FIM o r a t o m - p r o b e (FIM) /J/.

T h i s k i n d o f d i f f i c u l t y d o e s n o t e x i s t a n y m o r e w i t h o u r a d v a n c e d f o c u s i n g - t y p e ToF a t o m - p r o b e i n w h i c h w e c a n a c h i e v e t h e 100%

d e t e c t i o n e f f i c i e n c y a l m o s t r o u t i n e l y / 3 , 4 , 5 / . Our i r r s t r u r n e n t is c a p a b l e e n o u g h t o c a r r y o u t p r e c i s e one-by-one a t o m i d e n t i f icat i o n e v e n t h o u g h t h e number of a t o m s o f i n t e r e s t is l i m i t e d .

T u n g s t e n s p e c i m e n s w e r e u s e d i n t h i s s t u d y i n o r d e r t o o b t a i n a h i g h q u a l i t y a n d s t a b l e i m a g e f o r p r e c i s e c o u n t i n g o f t h e b r i g h t s p o t s . I m a g i n g w a s c a r r i e d o u t by h e l i u m a t 5 0 K b e f o r e a n d a f t e r t h e s u r f a c e w a s e x p o s e d t o A r . The e x p e r i m e n t s c o n s i s t o f t w o p a r t s : (1) atom-by-atom i d e n t i f icat i o n o f i n d i v i d u a l b r i g h t s p o t s a p p e a r e d o n t h e c e n t r a l area o f t h e ( 1 1 0 ) p l a n e , m a k i n g t h e b e s t u s e o f t h e 1 0 0 % d e t e c t i o n e f f i c i e n c y o f o u r a t o m - p r o b e , a n d ( 2 ) m e a s u r e m e n t o f t h e number o f a t o m s a t t h e t o p m o s t s u r f a c e l a y e r u n d e r v a r i o u s

c o n d i t i arts.

T h e d e t a i l s o f t h e p e r f o r m a n c e o f o u r f o c u s i n g - t y p e ToF a t o m - p r o b e , t h e 100% d e t e c t i o n e f f i c i e n c y i n p a r t i c u l a r , w e r e

d e s c r i b e d e l s e w h e r e 1 3 - 5/. T h e FIM c h a m b e r w h e r e a s p e c i m e n t i p a n d i m a g i n g d e v i c e t o g e t h e r w i t h a p r a b e - h o l e a t t a c h m e n t a r e h o u s e d is e v a c u a t e d f i r s t by a 270 /sec t u r b - m o l e c u l a r pump b a c k e d u p by a s m a l l 1 0 6 /sec t u r b o - m o l e c u l a r pump down t o 1 x 1 0 -l0 Tarr a n d t h e n f i n a l l y by a cryo-pump t o 3

1 0 - l 1 T o r r b e f o r e c u o l i n g down t h e s p e c i m e n t i p t o SQK, t h u s e n s u r i n g t r u l y a c l e a n vacuctrn c u r r d i t i o n a n d m i n i m i z i n g c o n t a m i n a t i o n e f f e c t s a t t h e t i p s u r f a c e / 5 / .

I v - EESubT 1 : I d e n t i f i cat i o r ~ c ~ f irtd i v i d ~ u a l b ~ ~ g ~ & - ~ g g ~ &

I t w a s f o u n d t h a t o n l y a f e w b r i g h t s p o t s show u p i n t h e ( 1 1 0 ) p l a n e u p o n t h e e x p o s u r e o f t h e s u r f a c e t o 1

10 -5 T o r r f o r R r f o r 3 0 sec ( F i g . 3). One of t h e m w a s s e l e c t e d a n d p l a c e d o n t h e

p r o b e - h o l e p r e c i s e l y u s i n g t h e s e c o n d i r n a g i n g d e v i c e a t t h e a u x i l i a r l y c h a m b e r w h i c h is l o c a t e d b e h i n d t h e p r o b e - h o l e . T h e n

h i g h - v o l t a g e t r i g g e r p u l s e s w e r e a d m i n i s t e r e d k e e p i n g t h e p u l s e rat i o c o n s t a n t a t 0 . 2 3 i n t h i s e x p e r i m e n t u n t i l t h e i m a g e o f t h e b r i g h t s p o t d i s a p p e a r e d . T h i s p r o c e d u r e is v e r y s i l i m a r t o t h e o n e w e u n d e r t o o k i n t h e s t u d y o f b r i g h t s p o t s i n a MO-0.3atX R e a l l o y s p e c i m e n / 4 , 6 / . I n a l l 25 c a s e s w e h a v e t r i e d t o i d e n t i f i e d , t h e b r i g h t s p o t s b e i n g a i m e d w e r e i d e n t i f i e d a s ~ ~ e + , 3 i n s t e a d o f W i o n s . The r e a s o n t h e t u n g s t e n a t o m s w e r e d e t e c t e d a s t u n g s t e n - h e 1 ium i n t h i s case may b e t h a t t h o s e

b r i g h t l y i m a g e d ~ W a t o m s w e r e s i t t i n g o n t h e s u r f a c e a n d e x p e r i e n c e d s l i g h t l y a h i g h e r f i e l d t h a n t h e rest t o a t t r a c t s u f f i c i e n t a m o u n t s o f h e l i u m g a s a t o m s a n d t o a b s o r b . I n a n y case, w e h a v e d o c u m e n t e d , f o r t h e first

t i m e , w i t h o u t a n y d o u b t t h a t F i g . 3. F1 micrographs of t h e (110) a r e a those b r i g h t l y imaged s p o t s upon i t s exposure t o A r ( 1 x 1 0 - ~ T o r r ) f o r u p o n its e x p o s u r e t o R r g a s 30 s e c , showing a b r i g h t s p o t i n each case.

r e p r e s e n t t h e W a t o m s These i n d i v i d u a l s p o t s were aimed by a d i s p l a c e d by t h e s p u t t e r i n g probe-hole and analyzed.

a n d s i t t i n g o n t h e t o p s u r f a c e

JOURNAL DE PHYSIQUE

Now t h a t w e know t h a t t h e r a n d o m l y d i s p e r s e d b r i g h t s p o t s i n FIM i m a g e s upcan s p u t t e r i n g c o r r e s p o n d t o t h e d i s p l a c e d s u r f a c e a t o m s a n d n o t d u e t o t h e g a s a d s o r b a t e s , t h e d a t a p u b l i s h e d by R d a c h i f

Nakamura ( F i g s . l a n d 2 ) c a n b e i n t e r p r e t e d s t r a i g h t f o r w a r d a l o n g t h i s u n d e r s t a n d i n g . T h e s p u t t e r i n g r a t e

> number- of b r i g h t s p o t s )

is p r u p c w t i o n a l t o t h e Flr g a s p r e s s u r e a n d i t s e x p o s u r e time a s is s e e n i n t h e f i g u r e s . However, u n d o u b t e d l y some o f t h e a t o m s s p u t t e r e d f r o m t h e o r i g i n a l s i t e s d e s o r b i m m e d i a t e l y w i t h o u t r e m a i n i n g o n t h e s u r f a c e . T h e d e s u r b i n g p r o b a b i l i t y o f t h e a t o m s , s p u t t e r e d a n d s u b s e q u e n t l y s i t t i n g o n t h e t a p s u r f a c e l a y e r * , is a s t r o n g f u n c t i o n o f t h e l o c a l s u r f a c e f i e l d a n d , t h e r e f o r e , is d i f f i c u l t t o b e e v a l u a t e d p r e c i s e l y .

Fl b e t t e r m e t h o d t o m e a s u r e t h e s p u t t e r i n g r a t e is t o c o u n t t h e nurnber o f t h e t o t a l r e m a i n i n g a t o m s a t t h e s u r f a c e i n s t e a d o f

c o u n t i n g t h e b r i g h t d i s p l a c e d a t o m s s i n c e s o m e o f t h e m easi l y d e s o r b frarfl t h e s u r f a c e . T h i s c a n b e p e r f o r m e d r e a d i l y i f o n e h a s a n a t o m - p r o b e w i t h t h e 100 % d e t e c t i o n e f f i c i e n c y / 4 / . By s e t t i n g t h e s y s t e m i n a l a y e r - b y - l a y e r f i e l d e v a p o r a t i o n mode, t h e n u m b e r (No 1 o f t h e t o t a l a t o m s a t e a c h s u r f a c e l a y e r c a n b e o b t a i n e d . T h e nurnbet- o f t h e s p l r t t e r e a t o m s N is, t h e n , t h e d i f f e r e n c e b e t w e e n N a n d t h e nurnber o f t h e remaining s t o r n s (N

F i g . 4 s h o w s Nr a s a f a - t n c t i o n o f 8 , t h e f 11.tence o f i r ~ l p i n g i n g

n r a t o m s . The s t r a i g h t l i n e is c l e a r e v i d e n c e t h a t t h e s p u t t e r i n g r a t e

" N is p r o p o r t i c n a l t o t h e impinging r a t e c ' f t h e

i n d i v i d u a l s u r f a c e t l - t n g s t e n atarn

0

W e e s t i n l a t e d t h e i m p i n g i n g r a t e t o b e 0 " " P ( F ) u s i n g a c u n v e n t i o n a l g a s s u p p l y

f u n c t i o n / 7 / , w h e r e 0 is t h e o r d i n a r y g a s 51-tpply f c t n c t i c ~ n i n t h e p r e s e n c e c ~ f f i e l d a n d PCF) is t h e s u r v i v i n g p r n b a b i l i t y o f n e u t r a l Flt- g a s a s n e u t r - a l . L e t t; be t h e s p u t t e r i n g y i e l d t h e s u r f a c e W atarn by Flr i m p i n g i m e n t . T h e n N is g i v e n b y

whet-e p, , T, a n d m a r e g a s p r e s s u r e , p o l a r i z a b i l i t y , temper-atitt-e a n d m a s s o f R r g a s , r e s p e c t i v e l y .

a,

r;'

W ( 1 10) ~n A r gass

2 30 ^IOOK, I5.Bpulses/sec a

\ ( VE = 8.23KV

2 25 ^{pulse factor = 0.23}

0 .d

W

20

3

; I5

*

loo 100 200 300

( Ar atoms / W atom

F i g . 4. Number of W atoms

d e t e c t e d from i n d i v i d u a l

l a y e r s a r e p l o t t e d a s a

f u n c t i o n of Ar impingiment

r a t e , which c a n b e c o n t r o l -

l e d by changing A r g a s

p r e s s u r e d u r i n g t h e atom-

probe a n a l y s i s .

S u b s t i t u t i n g t h e n u m e r i c a l v a l u e s f o r o u r e x p e r i m e n t , s u c h a s

P = l x we

-4

F = 4 . 3 V/R, T = 50 K,

10 Tot-r a n d 5 = 1. 0,

o b t a i n P ( F = 4 . 3 V / R ) = 1.3

.

T h i s v a l u e is smaller t h a t h e

c a l c u l a t e d v a l u e ( 2

10-Y

a s m

1 -6

s h o w n i n F i g . 5 . T h i s d i s c r e p a n c y -

is p r o b a b l y d u e t o s e v e r a l f a c t o r s

n o t c o n s i d e r e d i n t h e p r e s e n t 0-l s i m p l e c a l c u l a t ictn. T h e m o s t 0 i m p o r t a n t factor may b e t h e

e n h a n c e m e n t f a c t o r a f g a s s u p p l y

f u r r c t i o n d u e t o t h e m i g r a t , i o r r of -8

U r

t h e s h a n k , w h i c h ct:.uld b e l a r g e e n o u g h t o a c c o u n t f o r t h i s d i s c r e p a n c y .

U s t o t h e c o n t r i b u t i o n mf t h e

r e s i d u a l i m p u r i t y g a s s p e c i e s , w e -10

c a n r u l e o u t c o m p l e t e l y i t s e f f e c t o n t h e c o r r o s i v e react i o n a t t h e

400 420 440

s u r f a c e . T h e b a c k g r o u n d p a r t i a l

g a s p r e s s u r e s a r e i n t h e r a n g e of F. (MV/cm)

1 0 'l0 T o r r t h r o u g h o u t t h e

e x p e r i m e n t w i t h t h e h e l p of t h e Fig. 5. Surviving p r o b a b i l i t y of A r a u x i l i a r y c r y u - p u m p a t t a c h e d n e a r atoms as n e u t r a l is p l o t t e d as a t h e F 1 s p e c i m e n a n d t h e p r u b a b i f t y f u n c t i o n of t h e e l e c t r i c f i e l d of t h e a d s o r p t i o n o f c u r r o s i v e s t r e n g t h .

i m p u r i t y g a s e s s u c h a s o x y g e n o r n i t r c t g e n d u r i n g t h e e x p e r i m e n t w h i c h t a k e s o n l y l e s s t h a n s e v e r a l m i n u t e s is n i l l .

I t w a s a l s o s u g g e s t e d t h a t t h e c u l l i s i s n a l c h a r g e e x c h a n g ~ d u r i n g t h e f l i g h t p a t h may r e s u l t i n t h e r e d u c t i o n af t h e s i g n a l l e v e l s i n c e t h e r e s u l t i n g n e u t r a l a t o m s w i l l n o t b e d e t e c t e d b y t h e f u c u s i n g - t y p e ToF a t o m - p r o b e . T h e a t o m - p r o b e a n a l y s i s w a s , h o w e v e r , c a r r i e d o u t a t

1 x 1 0

T o t - r i n t h e m a i n c h a m b e r a n d less t h a n 1

1 0 -7 T o r r i n t h e l e n s s e c t i o n . T h e mean free p a t h o f W i o n s is l a r g e e n o u g h u n d e r t h i s e x p e r i m e n t a l c o n d i t i o n a n d t h e p r o b a b i l i t y of t h e c o l l i s i o n w i t h i m p u r i t y g a s a t o m s is e x t r e m e l y r e m o t e . F u r t h e r m o r e e v e n i f w e a s s u r f l e t h a t t h i s may t a k . e p l a c e , t h i s e f f e c t a c c a u n t f u r t h e p r e s s u r e

d e p e n d e n c e a l o n e a n d c a n n o t e x p l a i n t h e d e p e n d e n c e o f N arr t h e e x p o s i l t - e t i m e . T h e r e f o r e , t h e c h a r g e e x c h a n g e d u e t o t h e col l i s i o n w i t h i r n p u t X i t y g a s e s c a n b e n e g l e c t e d .

U s i n g t h e f o c u s i n g - t y p e T o F a t o m - p r o b e w h i c h a t t a i n s t h e l O O X d e t e c t i o n e f f i c i e n c y , w e h a v e s t u d i e d t h e s p u t t e r i n g e f f e c t s of Or g a s i n d e t a i l . W e h a v e f o u n d t h a t

( 1 ) T h e b r i g h t s p g ~ t s a p p e a r e d r a n d o m l y u p o n t h e e x p o s u r e t a R r g a s i n t h e p r e s e n c e o f d c f i e l d c o r r e s p o n d t o WHe m o l e c u l a r c o m p l e x s i t t i n g iln t h e s u r f a c e d u e t o t h e s p u t t e r i n g b y R r a t o r n s .

( 2 ) T h o s e b r i g h t a t o m s f i e l d e v a p o r a t e m o r e e a s i l y t h a n t h e rest of t h e s c a r f a c e atonts a n d a l w a y s e v a p o r a t e a s WHe 4-3 i o n s . ( 3 ) T h e s p u t t e r i n g r a t e w a s e v a l u a t e d t o b e close t o t h e u n i t y

a n d t h e s u r v i v a l r a t e of Flr g a s a t o n l s is 1.3

10 -5 i n out-

e x p e r i m e n t a l c o n d i t i o n (F = 4 . 3 V / R a n d T = 50 K > .

JOURNAL DE PHYSIQUE

/ l / T. R d a c h i a n d S. Nakaniura, R p p l . Phys. 22, 474 ( 1 9 7 3 ) .

/ 2 / S. Nakarnura, 7. B d a c h i , t T. Kctroda, B p p l . P h y s . 22, 5 2 6 ( 1 9 6 8 ) . /3/ T. S a k u r a i a n d T. Hashizurne, Rev. Sci. I n s t r u r n .

/ 4 / T. S a k u r a i , T. Hashirclme & R. Jirnbo, B p p l . Phys. L e t t . 54, 38 (1984)

/ 5 / T. S a k u r a i et a l . , Rev. Sci. I n s t r u m . , t o b e s u b m i t t e d . /6/ T. S a k u r a i , T. H a s h i z u n i e & H. Mcrrikawa, S u r f . S c i . to b e

subrni t t e d .

/7/ E. W. M u e l l e r a n d T. T. T s o n g , F i e l d I c r n M i c r o s c o ~ ~ . &&nche&

&n&-gtn_8p~l i c a t i o n s . ( E l s e v i e r ) 1969.