SCANNING AUGER MICROANALYSIS AT HIGH ENERGY PROBES

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HAL Id: jpa-00223983

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Submitted on 1 Jan 1984

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SCANNING AUGER MICROANALYSIS AT HIGH ENERGY PROBES

M. Tholomier, P Morin, E. Vicario, N. Mubanga

To cite this version:

M. Tholomier, P Morin, E. Vicario, N. Mubanga. SCANNING AUGER MICROANALYSIS AT HIGH ENERGY PROBES. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-309-C2-312.

�10.1051/jphyscol:1984269�. �jpa-00223983�

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SCANNING AUGER MICROANALYSIS A T HIGH ENERGY P R O B E S

M. Tholomier, P. Morin, E. Vicario and N. Mubanga

Universitg CZaude Bernard, Lyon I , Dgpartement de Physique des Mate'riam,

4 3 BouZevard du 11 Novembre 1918, 6 9 6 2 2 ViZZeurbanne Cedex, France

RQsumk

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Des expkriences de microanalyse Auger sur des ?lots drargent de dimensions de l'ordre de 50 nm

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200 nm sont dgcrites. Le rapport de la hauteur du pic Auger sur le fond continu ainsi que la r6solution spatia- le sont interprgtgs & l'aide des Qlectrons r6trodiffusQs.

Abstract

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MicroAuger analysis on silver islands with 50 nm to 200 nm size, are described. Auger peak height to background ratio and spatial resolution are interpreted by means of backscattered electrons.

I

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Introduction

The last decade has witnessed a phenomenal growth of interest in Auger electron spectroscopy (AES). Today, it is probably the most widely used of all surface analytical techniques. This development has became possible with the advances in vacuum technology which permit to maintain an uncontamined surface and the progress in electronic detection of a spectrum (1). Improvements were achieved by the introduction of high brightness guns based upon LaB6 thermoionic or the W field emission cathodes. Different electron sources have been compared and the limita- tion of spatial resolution by backscattered electrons effects have begun to be investigated (2). The field emission gun (FEG) has emerged as having very conside- rable advantages for all studies requiring a resolution better than 100 nm.

Many surface processes, such as nucleation or crystals growth are extremely inhomo- geneous and complicated. So there is an intense interest, not only to improve the spatial resolution of surfaces techniques, but also to dispose of ultra-high vacuum specimen and/or analysis chambers containing several facilities to characte- rize a sample : possibility of transmission electron microscopy observations, high reflection energy electron diffraction (RHEED), low energy electron diffraction (LEED)

. . . .

In this paper, we present and analyse some microAuger analysis results, in the 50 nm

-

350 nm range, carried out on silver clusters deposited on a silicon subs- trate. The N(E) distributions in the energy range 5

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60 keV around the 356 eV silver M4 VV and 351 eV silver M VV Auger peaks have been recorded and the Auger peak height/background ratio has Fbeen deduced. Change of this ratio as a function of energy has been studied. Interpretation of these variations is made using backscattered electrons distribution. Detection ability of 50 nm size silver clusters at 50 keV primary energy has been shown and this one remains better than 200 nm at 20 keV and lower energies.

I1

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The instrument

The experiments described were performed on HB 50 scanning electron microscope

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(Q), with an ultra high vacuum (UHV) specimen chamber, manufactured by V.G.

Microscope Ltd. In routine conditions, the analysis chamber pressure and the specimen chamber pressure are normally 130 nPa and 650 nPa respectively. After overnight baking at 473 K and outgassing of all filaments and sources, the analysis chamber pressure and the specimen chamber pressure reach 13 nPa and 65 nPa.

These pressures are adequate for many Auger electrons spectroscopy studies without appreciable contamination problems.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984269

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C2-310 JOURNAL

DE

PHYSIQUE

The m i c r o s c o p e u s e s a f i e l d e l e c t r o n e m i t t i n g gun w i t h a L3101 t u n g s t e n t i p . The gun i s a two e l e m e n t e l e c t r o s t a t i c t y p e w i t h c o n t i n u o u s o p e r a t i o n from 2 . 5 up t o 60 keV. A s c i n t i l l a t o r and a p h o t o m u l t i p l i e r assembly a r e u t i l i s e d f o r s t a n d a r d SEM i m a g i n g . For t h i s mode, t h e s p a t i a l r e s o l u t i o n i s b e t t e r t h a n 10 m a t e n e r g i e s h i g h e r t h a n 50 keV. A t 50 keV, t h e sample c u r r e n t d e t e c t e 1s 3 X 10-g A w i t h a 500 p o b j e c t i v e a p e r t u r e f o r a n e m i s s i o n c u r r e n t o f 2 X 10-'

e.

^{A t}^prima-

r y e n e r g y lower t h a n 7 . 5 keV, t h e s p a t i a l r e s o l u t i o n r e m a i n s b e t t e r t h a n 35 nm.

The r a t i o of t h e sample c u r r e n t w i t h a n 500 pm o b j e c t i v e a p e r t u r e and w i t h o u t a p e r t u r e i s a b o u t 20

-

25 and i s a l m o s t c o n s t a n t on t h e whole e n e r g y r a n g e .

The Auger e l e c t r o n d e t e c t i o n s y s t e m i s made o f c o n c e n t r i c h e m i s p h e r i c a l a n a l y s e r f i t t e d w i t h a r e t a r d i n g g r i d and a n E i n z e l i n p u t l e n s . The a n a l y s e r c a n be o p e r a - t e d i n two modes : t h e c o n s t a n t a n a l y s e r e n e r g y (CAE) and t h e c o n s t a n t r e t a r d i n g r a t i o (CRR) ( 5 ) . I n CAE mode, t h e h e m i s p h e r e s a r e o p e r a t e d a t c o n s t a n t p a s e n e r g y E

.

The r e t a r d i n g f i e l d on t h e i n p u t l e n s i s v a r i e d i n o r d e r t o sweep t h r o u g h a p s p e c t r u m . T h i s mode g i v e s a c o n s t a n t bandwidth AE which i s u s e f u l f o r a f a i t h f u l r e p r o d u c t i o n o f N(E) s p e c t r a and e l i m i n a t e s t h e a r t i f a c t s r e s u l t i n g from peak s h a p e c h a n g e s . I n CRR mode, t h e i n p u t g r i d i s o p e r a t e d a t c o n s t a n t r e t a r d r a t i o and t h e a n a l y s e r p a s s e n e r g y i s swept t o r e c o r d a s p e c t r u m . T h i s mode h a s c o n s t a n t r e s o l v i n g power E /AE and i s u s e f u l t o o b t a i n l a r g e r Auger s i g n a l s a t h i g h e r e n e r g i e s o r t o r g d u c e t h e background i n f l u e n c e a t low e n e r g i e s . The c o n s t a n t r e t a r d r a t i o CRR i s d e f i n e d by CRR ( E /E ) , w i t h E k i n e t i c e n e r g y of sample e m i t t e d Auger e l e c t r o n s . The choosen r a t &

fs

u s u a l l y el o r p o s s i b l y 2. I n normal r u n s , h i g h e r v a l u e s of CRR a r e n o t much used b e c a u s e t h e t o t a l c o l l e c t i o n e f f i - c i e n c y , i n c l u d i n g i n s t r u m e n t a l g e o m e t r i c a l c o l l e c t i o n e f f i c i e n c y , and e n e r g y c o l l e c t i o n e f f i c i e n c y , i s t o o low ( a b o u t I O - ~ ) and d o e s n ' t p e r m i t t o o b t a i n ade- q u a t e Auger p e a k s h e i g h t s . Auger l i n e b r o a d e n i n g i n t r o d u c e d by t h e a n a l y s e r c a n be d e t e r m i n e d u s i n g f o r m u l a d e r i v e d by K u y a t t and Simpson (6). S o f o r t h e mode CRR = 1 , t h e two M VV s i l v e r p e a k s c a n ' t b e c o r r e c t l y s e p a r a t e d (AE

=

7 eV)

.

Auger s p e c t r a were t b l a i n e d i n t h e N(E) mode e i t h e r by a s t a n d a r d l o c k - i n t e c h - n i q u e w i t h beam b l a n k i n g o r by p u l s e c o u n t i n g t e c h n i q u e .

111- R e s u l t s and d i s c u s s i o n

To s t u d y t h e a b i l i t y of SEM t o microAuger a n a l y s i s , we examined a sample comqosed o f s i l v e r c l u s t e r s e v a p o r a t e d w i t h a r a r e g a s atmosphere ( h e l i u m - 1 . 3 X 10 Pa) on a s i l i c o n s u b s t r a t e i n a s e p a r a t e vacuum s y s t e m .

Auger a n a l y s i s were made u s i n g t h e M VV s i l v e r Auger t r a n s i t i o n . A f i r s t s p e c t r a was r e c o r d e d a f t e r a p r e l i m i n a r y e%c%ing ( 2 m i n u t e s ) by a r g o n i o n bombardment [E = 4 keV, I = 20 pA1 t o d e c o n t a m i n a t e t h e s u p e r f i c i a l l a y e r s o f t h e s a m p l e . A f t e r r e p e t i t i v e s c a n s no s i g n i f i c a n t c h a n g e s i n Auger peak h e i g h t and background were s e e n i n d i c a t i n g n e g l i g i b l e e l e c t r o n beam induced e f f e c t s . The r a t i o o f Auger peak h e i g h t t o background h a s been p l o t t e d a s a f u n c t i o n o f e n e r g y i n F i g u r e 1 assuming t h e background p r o p o r t i o n a l t o t h e i n t e n s i t y o f t h e p r i m a r y beam. The o b s e r v a t i o n s were made on a c l u s t e r o f a b o u t 300 nm s i z e i n CRR = 1.

F i g . 1 - The Auger peak h e i g h t t o background r a t i o p l o t t e d a g a i n s t t h e p r i m a r y e n e r g i e s : 351 eV s i l v e r p e a k , mode N ( E ) , CRR = 1 , c u r r e n t : 1 nA, sweep : 0 . 4 V / s , t i m e c o n s t a n t : T = 3 S .

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h i g h e r e n e r g i e s . According t o S h i m i z u ' s c a l c u l a t i o n s ( 7 ) , f o r MVV (- 350 eV) s i l v e r t r a n s i t i o n s , t h e c o n t r i b u t i o n s o f b a c k s c a t t e r e d e l e c t r o n s and o f . p r i m a r y e l e c t r o n s t o t h e i n t e n s i t y o f Auger e l e c t r o n peak a r e a p p r o x i m a t e l y e q u a l f o r a n p r i m a r y e n e r g y o f 15 keV. A t l o w e r e n e r g i e s , t h e c o n t r i b u t i o n o f b a c k s c a t t e r e d e l e c t r o n s i s n e a r l y c o n s t a n t whereas t h e c o n t r i b u t i o n o f p r i m a r y e l e c t r o n s is s t r o n g l y i n c r e a s e d . F o r h i g h e x c i t a t i o n e n e r g i e s , Auger peak i n t e n s i t y d e c r e a s e s owing t o t h e r e d u c t i o n o f t h e c r o s s - s e c t i o n f o r i o n i z a t i o n o f i n n e r s h e l l s . Bethe o r G r y z i n s k i f o r m u l a e ( 8 ) i n d i c a t e t h a t f o r M l e ~ e l _ ~ ~ t h ? i o n i z a t i o n c r o s s - s e c t i o n may y q r y by up t o a f a c t o r o f 3 from

!?:2

^X ¹⁰ m w r t h E = 15 keV t o 0 . 8 X 10- m2 w i t h E = 60 keV. F u r t h e r m o r e , e n e r g e t i c b a c k s c a t t e r e 8 e l e c t r o n s coming from t h e e n t i r e 'range o f t h e p r i m a r y e l e c t r o n beam c o n t r i b u t e f o r t h e most p a r t t o t h e y i e l d o f Auger e l e c t r o n s e m i t t e d by t h e s p e c i m e n . By a n o t h e r way, t h e e x i t a r e a o f b a c k s c a t t e r e d e l e c t r o n s becomes l a r g e r t h a n t h e c l u s t e r s i z e ; s o Auger peak h e i g h t t o background r a t i o i s r e d u c e d .

The s p a t i a l r e s o l u t i o n i n Auger s p e c t r o s c o p y must be i n v e s t i g a t e d i n t e r m s o f l a t e r a l d i s t r i b g t i o n o f t h e Auger e l e c t r o n s which a r e e m i t t e d from t h e t h i n s u r - f a c e l a y e r ( 8 A e s c a p e d e p t h f o r 350 eV s i l v e r Auger e l e c t r o n s ) by b o t h prima- r y e l e c t r o n s and b a c k s c a t t e r e d e l e c t r o n s . B e f o r e d e t e c t i o n by t h e a n a l y s e r , t h e t r u e Auger p r o f i l e r e s u l t s from t h e c o n v o l u t i o n o f t h e i n c i d e n t beam w i t h t h e a n g u l a r b a c k s c a t t e r e d e l e c t r o n s d i s t r i b u t i o n . According t o Gomati and P r u t t o n ( g ) , a t low e n e r g i e s s p a t i a l r e s o l u t i o n s h o u l d s c a l e t o be a b o u t t w i c e t h e beam s i z e p r o v i d e d t h a t t h e beam s i z e is s m a l l compared w i t h t h e b a c k s c a t t e r i n g a r e a . A t h i g h e n e r g i e s , l a t e r a l d i s t r i b u t i o n of b a c k s c a t t e r e d e l e c t r o n s becomes impor-

t a n t . The d e t e c t i o n a b i l i t y on s m a l l a r e a s i s i l l u s t r a t e d i n F i g u r e 2 and 3.

F i g . 2

-

S i l v e r p a r t i c l e on a s i l i c o n s u b s t r a t e : ( a ) m i c r o g r a p h a t h i g h m a g n i f i c a - t i o n ; ( b ) Auger s p e c t r a o b t a i n e d on t h i s p a r t i c l e . [ P r i m a r y e n e r g y : 20 keV ; CRR = 1 ; c u r r e n t :

-

0 . 6 nA ; sweep : 0 . 4 V / s e c . ; T = 3 s ; d e t e c t i o n s e n s i b i - l i t y : 3 mV].

t

Fig. 3

-

^{( a )} and ( b ) : S i l v e r p a r t i c l e on a s i l i c o n s u b s t r a t e . Auger a n a l y s i s on a 75 nm s i z e and s p e c t r a o b t a i n e d f o r a 50 keV p r i m a r y e n e r g y i n t h e same d e t e c t i o n c o n d i t i o n s ( c u r r e n t

-

3 . 2 5 nA ; d e t e c t i o n s e n s i b i l i t y : 10 m V ) .

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C2-312 JOURNAL

DE

PHYSIQUE

I n t h i s c a s e , b a c k s c a t t e r e d e l e c t r o n e f f e c t s r e p r e s e n t t h e f u n d a m e n t a l l i m i t a t i o n of Auger a n a l y s i s r e s o l u t i o n .

F i g u r e 2 shows t y p i c a l s p e c t r a o b t a i n e d w i t h a 200 nm s i l v e r c l u s t e r a t 20 keV p r i m a r y e n e r g y . F i g u r e 3 shows t h e s p e c t r a , o b t a i n e d i n t h e same d e t e c t i o n c o n d i - t i o n s , w i t h 50 keV p r i m a r y e n e r g y on a a n a l y s i s s i z e o f 75 nm. The s m a l l e s t p a r t i - c l e s which have been d e t e c t e d a t 50 keV a r e 50 nm s i z e .

B a c k s c a t t e r e d e l e c t r o n s i n f l u e n c e i s p o i n t e d o u t on t h e s p e c t r a o f f i g u - r e 4 : i n s p o t p o s i t i o n on t h e

I V

-

C o n c l u s i o n o

MicroAuger a n a l y s i s o f 50 nm s i l v e r i s l a n d d e p o s i t e d on a s i l i c o n s u b s t r a t e h a s been d e m o n s t r a t e d . Auger peak h e i g h t t o background r a t i o h a s been d e t e r m i n e d e x p e r i m e n t a l l y i n t h e 5

-

60 keV r a n g e . O p t i m i z i n g t h i s r a t i o n e e d s e x c i t a t i o n c o n d i t i o n s u s i n g p r i m a r y e n e r g i e s lower t h a n 20 keV. For i n c r e a s e t h e beam c u r r e n t i n t h i s e n e r g y r a n g e and t o a v o i d e m i s s i o n c u r r e n t d r i f t , t h e r m a l - f i e l d b u i l t up t i p i s n e c e s s a r y . Tip h e a t e r c i r c u i t h a s been e l a b o r a t e d and checked i n o u r m i c r o s c o p e : beam c u r r e n t d r i f t i s e l i m i n a t e d and t h e r e s i d u a l f l u c t u a t i o n s o f t h e c u r r e n t a r e a b o u t 5 $.

Changes o f t h e Auger peak h e i g h t t o background r a t i o and s p a t i a l r e s o l u t i o n h a v e been e x p l a i n e d by means o f b a c k s c a t t e r e d e l e c t r o n s i n f l u e n c e . To l i m i t t h i s e f f e c t , t h i n f i l m Auger a n a l y s i s s t u d i e s w i t h s i m u l t a n e o u s STEM o b s e r v a t i o n s have been s t a r t e d .

t

-

s i l v e r c l u s t e r , A u g e r a n a l y s i s

!

r e v e a l s a l a r g e s i l i c o n peak ( 9 2 eV)

i n i t i a t e d by t h e b a c k s c a t t e r e d e l e c t r o n s i n t h e s u b s t r a t e . L i k e w i s e , i f a n Auger a n a l y s i s r e a l i z e d on S i s u b s t r a t e i s made s u f f i c i e n t l y n e a r a c l u s t e r , a s m a l l s i l v e r peak c a n be o b s e r v e d . L a t e r a l d i s t r i - b u t i o n of b a c k s c a t t e r e d e l e c t r o n s o n a s p e c i m e n s u r f a c e h a s been o b t a i n e d u s i n g Monte-Carlo c a l c u l a - t i o n s ( 1 0 ) ( 1 1 ) ( 1 2 ) . A s a n example,

5

w i t h a 30 keV i n c i d e n t e l e c t r o n

R e f e r e n c e s

-

560 50 eV

beam h i t t i n g t h e specimen a t a n i n c i d e n c e o f 45O, t h e d i a m e t e r a t h a l f maximum o f t h e l a t e r a l F i g . 4

-

Primary e n e r g y 40 keV. d i s t r i b u t i o n i s a p p r o x i m a t e l y 1 , 4 pm

Same d e t e c t i o n c o n d i t i o n s . f o r s i l v e r and 4 , 6 pm f o r s i l i c o n .

l . Harris L . A . , J . Appl. Phys.

2

(1968) 1419.

2. C h r i s t o u A . , J . Appl. Phys.

9

(1976) 5464.

3. Venables J . A . , J a n s e n A.P., Akhter P . , D e r r i e n J . and H a r l a n d C . J . , J. Micros- copy

118

( 1979) 351.

4. Venables J . A . , J a n s s e n A.P., U l t r a m i c r o s c o p y . 5 ( 1 9 8 0 ) 297.

5. Browning R . , B a s s e t t P . J . , E l Gomati M . M . , and P r u t t o n M . , P r o c . R . Soc. Lond.

A. 357 ( 1 9 7 7 ) 213.

6. K u y a t t C.E. and Simpson J . A . , Rev. S c i . I n s t r .

3

(1967) 103.

7. Shimizu R . , Q u a n t i t a t i v e a n a l y s i s by Auger e l e c t r o n s p e c t r o s c o p y , Toyota Founda- t i o n R e s e a r c h R e p o r t . Rep. N O 1

-

006

-

^{N o}⁷⁶

-

0175.

8 . P o w e l l C . J . , Rev. o f Modern P h y s i c s .

3

( 1 ) (1976) 34.

9 . E l Gomati M . M . and P r u t t o n M . , S u r f a c e S c i e n c e

72

( 1 9 7 8 ) 485.

10. Murata K . , J. Appl. Phys. ( 4 ) ( 1 9 7 4 ) 110.

11. V i c a r i o E. and E s c u d i e B . , J o u r n a l d e m i c r o s c o p i e e t s p e c t r o s c o p i e Q l e c t r o - n i q u e s ,

5

⁽1980) 357.

12. V i c a r i o E . , P r i v a t e communication.