MICROSCOPIC STUDY OF SURFACES BY AUGER TYPE EMISSION

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MICROSCOPIC STUDY OF SURFACES BY AUGER TYPE EMISSION

C. Le Gressus

To cite this version:

C. Le Gressus. MICROSCOPIC STUDY OF SURFACES BY AUGER TYPE EMISSION. Journal

de Physique Colloques, 1984, 45 (C2), pp.C2-323-C2-328. �10.1051/jphyscol:1984273�. �jpa-00223987�

MICROSCOPIC STUDY OF SURFACES B Y AUGER TYPE E M I S S I O N

C . Le Gressus

DESICP-DPC, CEN SacZay, 911 91 Gif-sup-Yvette, France

Resume L ' e f f e t Auger s e p r S t e l ' e t u d e microscopique d e s s u r f a c e s au t r a v e r s de l a s p e c t r o s c o p i e d e s r a i e s c a r a c t e r i s t i q u e s d e s atomes gmetteurs, d e s s p e c t r o s c o p i e s de s e u i l s ou d e s s p e c t r o s c o p i e s d ' e l e c t r o n s s e c o n d a i r e s accompagnant l a r e l a x a t i o n d ' i o n s e t l a d 6 - e x c i t a t i o n d'atomes m e t a s t a b l e s . Les i n f o r m a t i o n s obtenues s e r a p p o r t e n t B l a composition g l h e n t a i r e , l a l i a i s o n chimique, l a d i s t r i b u t i o n d e s d t a t s B l e c t r o n i q u e s du s o l i d e , e t aux d i s t a n c e s e n t r e proches v o i s i n s . Les retambees d e t e l l e s a n a l y s e s en s c i e n c e s d e s mat6riaux s o n t d 6 j a c o n s i d b r a b l e s .

A b s t r a c t The Auger e f f e c t l e n d s i t s e l f w e l l t o t h e microscopic examination of s u r f a c e s through t h e spectroscopy of t h e c h a r a c t e r i s t i c l i n e s of t h e e m i t t e r atoms, t h r e s h o l d s p e c t r o s c o p i e s , and s p e c t r o s c o p i e s of secondary e l e c t r o n s accompanying t h e r e l a x a t i o n of i o n s and t h e d e - e x c i t a t i o n of m e t a s t a b l e atoms. The d a t a o b t a i n e d p e r t a i n t o t h e elemental composition, t h e chemical bond, t h e d i s t r i b u t i o n of e l e c t r o n s t a t e s of t h e s o l i d , and t h e d i s t a n c e s between c l o s e neighbors. These a n a l y s e s have a l r e a d y had c o n s i d e r a b l e r e p e r c u s s i o n s on t h e m a t e r i a l s s c i e n c e s .

INTRODUCTION

Auger emission was d i s c o v e r e d i n 1923 <1> and i t s a p p l i c a t i o n t o t h e m i c r o a n a l y s i s of s u r f a c e s i n t h e p a s t decade e x p l a i n s i t s p r e s e n t development. Elemental a n a l y s i s , component s u r f a c e d i s t r i b u t i o n , and depth p r o f i l e s o b t a i n e d by i o n

e t c h i n g , a r e t h e d a t a e s s e n t i a l l y o b t a i n e d i n s t a n d a r d i n s t a l l a t i o n s . However, based on t h e form of t h e valence l i n e s and through t h e t h r e s h o l d s p e c t r o s c o p i e s , i t i s a l s o p o s s i b l e t o determine t h e d e n s i t y of e l e c t r o n s t a t e s of t h e s o l i d and t h e d i s t a n c e s between c l o s e neighbors.

The main problems encountered i n Auger microscopy a s s o c i a t e d with scanning microscopy a r e r e l a t e d t o t h e f o l l o w i n g .

.

R e p r o d u c i b i l i t y of r e s u l t s <2> and t h e compilation of r e f e r e n c e d a t a < 3 > . These a s p e c t s r e q u i r e ';he p r e p a r a t i o n of r i g o r o u s experimental methodologies f o r t h e alignment and c a l i b r a t i o n of t h e s p e c t r o m e t e r s and f o r sample p r e p a r a t i o n .

.

Q u a n t i t a t i v e a n a l y s i s : t h e p r e s e n t method s t i l l t a k e s t o o l i t t l e c o n s i d e r a t i o n of e l e c t r o n / s o l i d i n t e r a c t i o n parameters, and t h e absence of s i g n a l n o r m a l i z a t i o n i s a n o t h e r drawback.

.

I r r a d i a t i o n damage: t h e d o s e s a c c e p t a b l e by a sample a r e i n t h e range of e- Higher doses a l t e r i t s composition, t h e chemical bonds, and t h e e l e c t r i c a l o r o p t i c a l p r o p e r t i e s . These i r r a d i a t i o n damages involve v a r i o u s mechanisms which have g i v e n r i s e t o many i n v e s t i g a t i o n s and a r e beginning t o be b e t t e r understood. They must be checked and t h e i r e f f e c t on t h e s i g n i f i c a n c e of t h e a n a l y t i c a l r e s u l t s cannot be ignored.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984273

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We s h a l l l i m i t o u r s e l v e s in t h i s work t o i n d i c a t i n g a number of p r a c t i c a l d i r e c t i o n s and r e s e a r c h t o p i c s l i k e l y t o h e l p t o s o l v e t h e problems encountered i n Auger spectroscopy.

EXPERIMENTAL METHODS

Auger spectroscopy i s performed i n u l t r a - h i g h vacuum chambers

.

The e l e c t r o n a n a l y z e r may be one of v a r i o u s t y p e s , a c y l i n d r i c a l m i r r o r a n a l y z e r (CMA),

hemispheric a n a l y z e r , o r r e t a r d a t i n g g r i d s . The CMA i s t h e most widely used and can b e coupled t o an e l e c t r o n gun with an o p t i c a l a x i s c o a x i a l with t h e a n a l y z e r . , S p a t i a l r e s o l u t i o n i s a few hundred AngstrGms f o r s t a n d a r d beam c o n d i t i o n s ( l o e 9 A , 10 kV). However, t h i s r e s o l u t i o n i s a c t u a l l y n o t very u s e f u l i n p r a c t i c e , because i r r a d i a t i o n damage i s o f t e n i n s t a n t a n e o u s and e x t e n s i v e , s o t h a t t h e d e t e c t i o n t h r e s h o l d i s reduced t o a few % f o r a n a l y t i c a l times of a few hundred seconds. The Auger image of a s p e c i f i c element r e q u i r e s long exposure times

(500 t o 1000 s ) and only o f f e r s a narrow range of c o n t r a s t s . A s a r u l e , t h e r e f o r e , t h e o p e r a t o r simply works with a defocused probe on t h e zone s e l e c t e d i n t h e u s u a l scanning mode (SEI)

.

^{T h i s i s} why t h e d e s i g n and p r o p e r t i e s ( s e n s i t i v i t y , t r a n s m i t t a n c e ) of t h e secondary e l e c t r o n d e t e c t o r i s of p a r t i c u l a r importance.

The SEI image s e r v e s t o d e t e c t c o n t r a s t v a r i a t i o n s of a b o u t 1% r e f l e c t i n g l o c a l v a r i a t i o n s i n composition o r d e n s i t y of s t a t e s and scanning t h e important zones f o r a n a l y s i s . The response of t h e SEI d e t e c t o r must t h e r e f o r e be p r o p o r t i o n a l t o t h a t of t h e i n t e g r a l of t h e s p e c t r a l d i s t r i b u t i o n n ( E ) ( F i g u r e 1 ) .

I n s t r u m e n t a l problems still n o t thoroughly i n v e s t i g a t e d sometimes make t h e r e s u l t s i n s u f f i c i e n t l y r e p r o d u c i b l e . These problems occur i n alignment, spectrometer c a l i b r a t i o n , and t h e measurement of t h e s y s t e m ' s t r a n s f e r f u n c t i o n . The c o r r e c t i o n of t h e t r a n s f e r f u n c t i o n by deconvolution between t h e experimental spectrum and t h e e l a s t i c peak cannot s u p p l a n t a more i n s t r u m e n t a l approach. On a pure aluminum sample, t h e s o l u t i o n c o u l d c o n s i s t of c o n s t r u c t i n g a r e f e r e n c e

d i s t r i b u t i o n n ( E ) which would s e r v e t o determine t h e secondary emission c o e f f i c i e n t , the b a c k s c a t t e r i n g c o e f f i c i e n t , t h e Auger y i e l d and t h e c h a r a c t e r i s t i c l o s s spectrum, on t h e b a s i s of e x i s t i n g t h e o r e t i c a l d a t a .

* -

8 G R A I N S

I

F i g u r e 1 V a r i a t i o n i n b r i g h t n e s s B of t h e SEI d e t e c t o r a s a f u n c t i o n of primary c u r r e n t I p , measured a t v a r i o u s a c c e l e r a t i o n

v o l t a g e s on d i f f e r e n t g r a i n s of a polycry s t a l l i n e A1 sample

temperature e f f e c t s ( F i g u r e s 2a, 2b and 2 c ) ; secondary e l e c t r o n e m ~ s s i o n c o r r e l a t e d with d e n s i t y of e l e c t r o n s t a t e s : secondary emission, b a c k s c a t t e r i n g y i e l d , and background form r e l a t e d t o t h e atomic and c r y s t a l l o g r a p h i c s t r u c t u r e of the sample

( F i g u r e s 3a and 3b).

F i g u r e 2a C r y s t a l l o g r a p h i c e f f e c t s F i g u r e 2b C r y s t a l l o g r a p h i c e f f e c t observed i n S E I a t Ep = 3 kV, and observed a s an image of c h a r a c t e r i s t i c Ep = 230 V, and i n Auger image ( A E I ) on l o s s e s on A l . The r e l a t i v e

t h e A 1 LVV l i n e i n t e n s i t i e s of t h e s u r f a c e and volume plasmons depend on t h e d i f f r a c t i o n

c o

. .

w

F i g u r e 2c Temperature e f f e c t observed between 20 and 400 OC a t Ep = 90 eV on A l . The v a r i a t i o n i n t h e Debye Waller f a c t o r

a f f e c t s b r i g h t n e s s and image c o n t r a s t

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F i g u r e 3b V a r i a t i o n i n s p e c t r a l d i s t r i b u t i o n of A 1 d u r i n g o x i d a t i o n .

I

n-r.1 4.1

' , ' , . , E

L

[q-r*] E . ~ ( E )

- y- h

F i g u r e 3a V a r i a t i o n i n b r i g h t n e s s of SEI L

AUGER EFFECT AND QUANTITATIVE ANALYSIS image d u r i n g s u r f a c e o x i d a t i o n a t Ep = 3 keV.

The i n c o r p o r a t i o n of oxygen i n t h e l a t t i c e i s r e s p o n s i b l e f o r a 20% i n c r e a s e i n b r i g h t n e s s .

The emission of a n Auger e l e c t r o n r e s u l t s from a conversion of p o t e n t i a l energy t o k i n e t i c energy, and t h i s conversion c a u s e s t h e r e l a x a t i o n of an e x c i t e d s t a t e . The e x c i t e d s t a t e may correspond t o an i o n i z e d atomic l e v e l , t o an i o n , t o an atom i n a m e t a s t a b l e ' a t a t e , o r even t o a recombination of an e l e c t r o n / h o l e p a i r .

-

The Auger emission produced by recombination of an e l e c t r o n / h o l e p a i r r e s u l t s in t h e exo-electron emission of i n s u l a t o r s <4>, an emission t h a t o c c u r s a t very low k i n e t i c energy ( 0 . 1 eV)

.

( t h e r m a l l y s t i m u l a t e d )

.

The c r i t i c a l t h i c k n e s s a t which t h i s

-

i n c o r p o r a t i o n i s d e t e c t a b l e v a r i e s w i t h Ep

The Auger emission under i o n o r m e t a s t a b l e atom bombardment <5> i s a secondary e l e c t r o n emission whose energy ( a few eV t o a few dozen eV) depends on t h e p r o c e s s e s involved (Auger r e l a x a t i o n , Auger n e u t r a l i z a t i o n ) , on t h e energy of t h e i n i t i a l e x c i t e d s t a t e s , and on t h e work f u n c t i o n of t h e t a r g e t . The spectroscopy o f t h e s e e l e c t r o n s a l l o w s t h e a n a l y s i s of t h e d e n s i t y of t h e occupied s t a t e s .

Auger emission under e l e c t r o n bombardment <6> c a u s e s t h e n o n - r a d i a t i v e r e l a x a t i o n of e x c i t e d l e v e l s . I n s e v e r a l r e s p e c t s (Auger y i e l d compared with f l u o r e s c e n c e y i e l d , e l e c t r o n escape depth compared with t h a t of X-ray p h o t o n s ) , it i s

complementary t o X-ray a n a l y s i s . I t i s i d e a l f o r t h e q u a l i t a t i v e a n a l y s i s of t h e elemental composition, t h e a n a l y s i s of l i g h t elements (from L i ) , and t h e a n a l y s i s of t h e f i r s t atomic l a y e r s of t h e s u r f a c e s . T h i s emission under e l e c t r o n

bombardment i s c u r r e n t l y t h e most widely used form of t h e Auger effect..

The s p e c t r a l d i s t r i b u t i o n n ( E ) of any t a r g e t i s very complex because it i n c l u d e s :

.

e l a s t i c a l l y r e f l e c t e d e l e c t r o n s ,

.

i n e l a s t i c a l l y b a c k s c a t t e r e d e l e c t r o n s ,

The r u l e s governing t h e p e n e t r a t i o n of t h e primary beam and t h e e l e c t r o n emission depend a l s o on t h e c r y s t a l l o g r a p h i c p r o p e r t i e s of t h e s o l i d , and on p o s s i b l e emission a n i s o t r o p i e s . The number and complexity of t h e mechanisms encountered e x p l a i n why none of t h e problems of s i g n a l n o r m a l i z a t i o n , e x t r a c t i o n of t h e l i n e from i t s background, and of q u a n t i t a t i v e a n a l y s i s , h a s y e t been r e a l l y r e s o l v e d . The o l d e s t s i m p l i f i e d method f o r q u a n t i t a t i v e a n a l y s i s employs ' s e n s i t i v i t y c o e f f i c i e n t s ' measured on d e r i v a t e d peaks ( t o e l i m i n a t e background). Peak/

background n o r m a l i z a t i o n was r e c e n t l y proposed. C o n s i d e r a t i o n was given s u c c e s s i v e l y t o normalizing t h e l i n e w i t h t h e emission i n t e g r a l between t h e high energy background and t h e peak b a s e background. T h i s type of n o r m a l i z a t i o n , t o w i t h i n p r a c t i c a l l i m i t s , i s probably s a t i s f a c t o r y f o r a homogeneous sample. I n e f f e c t , experience h a s shown t h a t , even i n t h e p r e s e n c e of a d i f f r a c t i o n e f f e c t , a l l t h e s i g n a l s (Auger peak, background, energy l o s s e s e x c e p t f o r s u r f a c e plasmon) vary s i m i l a r l y i n t h e f i r s t o r d e r a s a f u n c t i o n of a c c e l e r a t i o n v o l t a g e and beam/sample a n g l e .

On t h e o t h e r hand, f o r a heterogeneous sample ( s e g r e g a t i o n l a y e r , a d s o r p t i o n l a y e r e t c ) t h e form of t h e s p e c t r a l d i s t r i b u t i o n depends on t h e atomic numbers and t h i c k n e s s e s of t h e l a y e r s p e n e t r a t e d by t h e primary beam and by t h e e l e c t r o n s emitted. T h i s i s observed, f o r example, i n t h e a l t e r a t i o n of emission y i e l d s

(secondary, Auger, b a c k s c a t t e r i n g ) of aluminum p r o g r e s s i v e l y exposed t o oxygen.

T h i s m o d i f i c a t i o n a p p e a r s a t a c r i t i c a l oxide t h i c k n e s s which depends on t h e a n a l y t i c a l v o l t a g e , a s implied by t h e g e n e r a l form, and t h e v a r i a t i o n s i n e f f e c t i v e c r o s s - s e c t i o n s a s a f u n c t i o n of energy. One s o l u t i o n t o t h i s problem would be t o c a l c u l a t e t h e envelope of n ( E ) by t h e i t e r a t i v e method, a s a l r e a d y r e p o r t e d f o r a p u r e element<7>.

The p r e s e n t q u a n t i t a t i v e a n a l y s i s method n e v e r t h e l e s s a p p l i e s s a t i s f a c t o r i l y t o t h e simple c a s e of o r d e r l y l a y e r s and s o l i d s o l u t i o n s . Comparisons of compositions measured by t h e Auger method w i t h t h o s e measured by X-ray a n a l y s i s have d i s p l a y e d s y s t e m a t i c d i f f e r e n c e s o f a b o u t 10%. The s e n s i t i v i t y c o e f f i c i e n t s necessary f o r measurement were remeasured i n situ.

CONCLUSIONS

Auger microscopy and t h e s u r f a c e s c i e n c e s in g e n e r a l c o n t r i b u t e t o t h e development of t e c h n o l o g i e s r e q u i r i n g c l e a n s u r f a c e s . The d i s c o v e r y of contamination, e s p e c i a l l y i n t h e semiconductor i n d u s t r y , s u f f i c e s t o improve t h e p r o c e s s e s and t o enhance t h e r e l i a b i l i t y of t h e d e v i c e s . C a t a l y s t s , u l t r a - h i g h vacuum m a t e r i a l s of p a r t i c l e a c c e l e r a t o r s , m a t e r i a l s of c o n t r o l l e d f u s i o n , and m a t e r i a l s s u b j e c t e d t o i n t e n s e f i e l d s , a r e a l l advancing t h a n k s t o a b e t t e r understanding of s u r f a c e r e a c t i o n s . T h i s knowledge n e v e r t h e l e s s r e q u i r e s i n v e s t i g a t i o n s t o reach beyond t h e s t a g e of elemental a n a l y s i s and t o implement supplementary i n v e s t i g a t i v e methods.

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