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THE EFFECTS OF X-RAY-INDUCED AUGER ELECTRONS IN AUGER MICROANALYSIS
J. Cazaux, D. Gramari, S. Moutou, A. Nassiopoulos
To cite this version:
J. Cazaux, D. Gramari, S. Moutou, A. Nassiopoulos. THE EFFECTS OF X-RAY-INDUCED AUGER
ELECTRONS IN AUGER MICROANALYSIS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-
337-C2-340. �10.1051/jphyscol:1984276�. �jpa-00223990�
JOURNAL DE PHYSIQUE
Colloque C2, supplbment a u n02, Tome 45, fkvrier 1984 page C2-337
THE EFFECTS OF X-RAY-INDUCED AUGER ELECTRONS IN AUGER MICROANALYSIS
J . Cazaux, D. Gramari, S. Moutou and A.G. Nassiopoulos
Laboratoire de Spectroscopie des EZectrons, UER Sciences, 51062 Reirns Cedex, France
RQsumB
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L ' i n t e n s i t B d e s B l e c t r o n s Auger i n d u i t s p a r l e s rayons X c r 6 6 s dans 1 ' 6 c h a n t i l l o n massif p e u t c o n t r i b u e r de fagon non n B g l i g e a b l e P l r i n t e n s i t 6 d e s 6- l e c t r o n s Auger i n d u i t s d i r e c t e m e n t p a r l e s d l e c t r o n s p r i m a i r e s e t l e s B l e c t r o n s r6- t r o d i f f u s d s . N o u s B t a b l i s s o n s pour l a premihre f o i s l e s e x p r e s s i o n s d e s i n t e n s i t e s r e l a t i v e s aux E l e c t r o n s Auger i n d u i t s p a r l e s rayons X c a r a c t d r i s t i q u e s e t p a r l a r a d i a t i o n continue.Nous montrons que c e s c o n t r i b u t i o n s peuvent a t t e i n d r e , d a n s c e r - t a i n s c a s e t Z t e n s i o n p r i m a i r e 6 l e v B e , p l u s i e u r s d i z a i n e s d e % . L t i n f l u e n c e q u ' o n t c e s c o n t r i b u t i o n s s u r l a d B t d r i o r a t i o n de l a r 6 s o l u t i o n s p a t i a l e e n SAM e s t indiquBe.C e r t a i n e s a p p l i c a t i o n s u t i l e s s o n t e n f i n indiquBes.
A b s t r a c t
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The i n t e n s i t y of t h e Auger e l e c t r o n s induced by c h a r a c t e r i s t i c o r c o n t i n u o u s X-rays c r e a t e d i n a b u l k sample by a n e n e r g e t i c i n c i d e n t e l e c t r o n beam c o n t r i b u t e s s i g n i f i c a n t l y t o t h e Auger e l e c t r o n i n t e n s i t y c r e a t e d by primary i n c i - d e n t and b a c k s c a t t e r e d e l e c t r o n s . A n a l y t i c a 1 e x p r e s s i o n s have been e s t a b l i s h e d f o r t h e f i r s t time g i v i n g t h e i n t e n s i t i e s o f c h a r a c t e r i s t i c and c o n t i n u o u s X-ray-indu- ced Auger e l e c t r o n s r e l a t i v e t o e l e c t r o n - i n d u c e d Auger e l e c t r o n s . F o r h i g h e n e r g i e s o f i n c i d e n t e l e c t r o n s t h e s e c o n t r i b u t i o n s may a t t a i n s e v e r a l t e n s o f p e r c e n t . T h e i r i n f l u e n c e o n t h e s p a t i a l r e s o l u t i o n i n SAM (Scanning Auger Microscopy) i s i l l u s t r a - t e d and some u s e f u l a p p l i c a t i o n s a r e i n d i c a t e d .I-INTRODUCTION
When t h e b a c k s c a t t e r i n g f a c t o r R i n Auger q u a n t i t a t i v e a n a l y s i s and t h e e l e c t r o n b a c k s c a t t e r i n g f a c t o r i n Scanning Auger Microscopy (SAM) have been e v a l u a t e d , t h e e f f e c t s of X-ray-induced Auger e l e c t r o n s (XAES e f f e c t s ) have always been n e g l e c t e d , even when t h e methods used a r e s o p h i s t i c a t e d . T h e c o n t r i b u t i o n of c h a r a c t e r i s t i c X-ray-induced Auger e l e c t r o n s (U) and bremsstrahlung-induced Auger e l e c t r o n s (V) t o t h e Auger s i g n a l r e l a t i v e t o t h e e l e c t r o n s d i r e c t l y c r e a t e d by t h e i n c i d e n t beam a l o n e have been e v a l u a t e d by a p p l y i n g t h e c o r r e c t i o n methods c u r r e n t l y used i n Elec- t r o n Probe M i c r o a n a l y s i s ( t a k i n g i n t o a c c o u n t t h e changes imposed by t h e geometry of t h e systern).The U and V v a l u e s used depend s t r o n g l y on b u l k and s u r f a c e compositions ( m a t r i x e f f e c t s ) and i n c r e a s e r a p i d l y w i t h i n c r e a s i n g i n c i d e n t e l e c t r o n e n e r g y E,.
The i n f l u e n c e of such i n t e n s i t i e s on t h e Auger p r o f i l e a c r o s s a chemical boundary i s i l l u s t r a t e d and some u s e f u l a p p l i c a t i o n s o f t h e s e e f f e c t s a r e e s t a b l i s h e d .
2-CONTRIBUTION OF AUGER ELECTRON INDUCED BY CHARACTERISTIC AND BREMSSTRAHLUNG X-RAYS TO THE AUGER SIGNAL
F i g u r e 1 i l l u s t r a t e s t h e v a r i o u s c o n t r i b u t i o n s t o t h e Auger s i g n a l when Auger analy- s i s o f a b u l k sample i s performed.Auger e l e c t r o n s a r e c r e a t e d d i r e c t l y by t h e i n c i - d e n t beam b u t a l s o by b a c k s c a t t e r e d e l e c t r o n s and by c h a r a c t e r i s t i c and c o n t i n u o u s X-rays c r e a t e d i n t h e bulk.The c o n t r i b u t i o n s o f t h e c h a r a c t e r i s t i c and c o n t i n u o u s X-ray-induced Auger e l e c t r o n s ( w i t h r e s p e c t t o t h e Auger i n t e n s i t y c r e a t e d d i r e c t l y by t h e i n c i d e n t e l e c t r o n beam a l o n e ) a r e g i v e n r e s p e c t i v e l y by :
$ a (hv) .oX(Ai,hvo)
U - (1)
v
=i$'
g(hv) .aX(A;,hv) .d(hv)I Q . a e ( A ; , E o ) I. .&(Ai ,Eo) ( 2 )
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984276
JOURNAL DE PHYSIQUE
z
u,v,u+v9 9k%! e 3'0 b .lo6 :: " I d e
10'
e-
n
3
3
Fig.1-The causes of Auger lectron production: (1) incident electron beam;(R) back- scattering effect; (U) characteristic X-ray-induced Auger electrons ; (V) bremsstrah-
lung-induced Auger electrons.The paths of photoelectrons,electrons and photons are indicated.
Fig.2-The U and V contributions for the case of a gold substrate.
Fig.3-The variation of U,V and U+V as a function of incident electron energy for a A1 film on a Si substrate (solid lines) and for a Si film on a Au substrate (bro- ken lines).
9,(hv) is the characteristic photon flux striking the surface of the sample,g(hv) is the spectral distribution of the emerging bremsstrahlung,and ox and oe are res- pectively the photoionization and electron ionization cross sections of the atomic level i of a surface atom of the element A. U and V increase when the incident elec- tron beam is increased. This can be illustrated by using the formula first given by Worthington and Tomlin /I/ and reviewed by Green and Cosslett /2/ for the elec- tron ionization cross section oe,the formula given by Green and Cosslett/3/ for the number of characteristic photons created per incident electron and Dyson's formula /4/ for the bremsstrahlung spectral distribution. So one obtains(for 2n steradians):
where Fc and FB are factors which take into account the X-ray absorption effects (absorption of characteristic X-rays (Fc) and absorption of the bremsstrahlung(FB)).
Fc was evaluated by using standard correction methods currently used in EPMA 151 but taking into account the particular geometry of the present problem (the effec- tive X-rays are emitted through a solid angle of 2a steradians) and the expression of the absorption given for a certain angle is integrated over these 2n steradians.
For the bremsstrahlung the absorption correction factor is difficult to evaluate due to the variation of the absorption cross section as a function of energy which is added to the variations of the absorption with the angle of emergence. In this paper we have considered for simplicity that the effective bremsstrahlung radiation is emitted within an angle of n steradians without absorption,so FBtl/2.
Fig.2 illustrates the fact that the most effective X-rays created in the bulk are soft X-rays having a photon energy close to (but slightly greater than) the thres- hold energy of the surface element giving rise to the Auger signal. Fig.3 illustra- tes the results obtained for an A1 film on a Si substrate (solid lines) and for a
Fig.5-Spectrum of a S i sample
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10 nm t h i c k on a gold anode 400 nm t h i c k .F i g 4-Auger e l e c t r o n p r o f i l e c a l c u l a t e d from a Gaussian model
f o r Al/Si and f o r Si/Au. The a-curves correspond t o Auger e l e c t r o n s due t o i n c i d e n t and b a c k s c a t t e r e d e l e c t r o n s ( l + R ) . The b-curves correspond t o Auger e l e c t r o n s due t o i n c i d e n t and b a c k s c a t t e r e d e l e c t r o n s and t o Auger e l e c t r o n s induced by cha- r a c t e r i s t i c and Bremsstrahlung X-rays ( l+R+U+V)
.
S i f i l m on a Au s u b s t r a t e (broken l i n e s ) . I t can be shown t h a t U and V can reach v a l u e s of s e v e r a l t e n s of p e r c e n t when Eo reaches 60 keV. These e f f e c t s obviously have t o b e taken i n t o account when q u a n t i t a t i v e s u r f a c e a n a l y s i s i s performed.
3-INFLUENCE ON THE SPATIAL RESOLUTION
I n o r d e r t o show how t h e s e e f f e c t s can a f f e c t t h e s p a t i a l r e s o l u t i o n , w e have used t h e a n a l y t i c a l model suggested by one of us 161.111 t h i s model t h e r a d i a l d i s t r i b u - t i o n s of a l l t h e c o n t r i b u t i o n s c o n s i d e r e d above t a k e Gaussian forms. By c o n s i d e r i n g normal i n c i d e n c e and t h e U and V v a l u e s given above f o r an i n c i d e n t e l e c t r o n ener- gy E,=60 keV,and by weighting t h e above Gaussian expressions,we have c a l c u l a t e d t h e p r o f i l e s f o r an A 1 sample on a S i s u b s t r a t e and a S i sample on a Au s u b s t r a t e when t h e i n c i d e n t e l e c t r o n beam parameter i s 0,=20 nm.For t h e f i r s t c a s e , t h e back- s c a t t e r i n g c o e f f i c i e n t l+R i s taken e q u a l t o 1.6, U=0.56 ,V=0.02. The parameter or a s s o c i a t e d with b a c k s c a t t e r e d e l e c t r o n s i s deduced from Archard's model / 7 /
(ur=4.5 pm). For t h e second case 1+R=1.6 ,U=0.07 ,V=0.07 and ur=1.91 um. I n each c a s e , t h e parameters a s s o c i a t e d with X-rays induced Auger e l e c t r o n s a r e taken equal t o 2 . 0 ~ . The corresponding ~ r o f i l e s o b t a i n e d a r e shown i n f i g u r e s 4. A s expected t h e broad U and V c o n t r i b u t i o n s i n t r o d u c e a broadening of t h e Auger emisslve a r e a which s l i g h t l y a f f e c t s t h e s p a t i a l r e s o l u t i o n when t h e c r i t e r i o n A50 i s used,but s t r o n g l y a f f e c t s i t i f t h e c r i t e r i o n 6(10%-90%) i s used. For q u a n t i t a t i v e a n a l y s i s t h e above r e s u l t s show t h a t i t i s necessary t o t a k e l+R+U+V i n s t e a d of 1+R ( = r ) when s t a n d a r d methods a r e used. This means t h a t t h e X-rays-induced Auger e l e c t r o n e f f e c t s have t o be taken i n t o account when s o p h i s t i c a t e d methods such a s Monte Car-
C2-340 JOURNAL DE PHYSIQUE
lo simulation are used. The additional contributions are calculated in this paper and the only difficulty that remains for the quantitative analysis of a small preci- pitate in SAM is the largespreading of the X-ray-induced and backscattered contribu- tions.
4-OTHER EFFECTS
Though X-ray-induced Auger electrons are in general a parasitic phenomenon which de- grade the spatial resolution in e-AES, the$- may be beneficial for other microanaly- tical methods: -(i)-in SAM at high operating voltages,the surface composition may be deduced from the kinetic energy of the Auger 1ines.The energetic position EK of the associated true photoelectron lines (excited by the characteristic X-ray radiation hv) makes it possible to deduce the energy of the soft X-ray photons created in the bulk (hv=Eg+EK). So it is possible to perform simultaneous bulk and surface analysis of such a samp1e;this has been done for a thin foil by using a partEcukar experimen- tal arrangement /8/,where X-ray analysis was performed by using X-ray photoelectron spectroscopy.-(ii)-in conventional X-ray photoelectron spectroscopy when an unfilte- red X-ray radiation is used,the characteristic X-ray induced Auger electron intensi- ty is reinforced by the Bremsstrahlung effect 191. -(iii)-in microanalytical X-ray- induced Auger electron spectroscopy(XAES) developed in our laboratory /lo/ the Brems- strahlung contribution is also large for a heavy-material anode : this technique consists of attaching the sample on the back of the anode in order to use the emit- ted characteristic and continuous X-rays to excite Auger electrons on the surface of a thin sample. We have pointed out /11/ that the ratio of the contributions of con- tinuous to characteristic X-ray-induced Auger electron,equivalent to the U/V ratio, in certain cases can become several times unity. The spatial resolution is also im- proved by the use of the thin heavy-material anode due to autoabsorption of the emit- ted radiation and we have succeeded in obtaining the first image by scanning X-ray- induced microscopy
1111.
An XAES spectrum of a Si sample obtained with a gold anode and a 1 1 k V incident elec- tron beam is shown in Fig.5. In this case a theoretical and experimental evaluation gives equivalent characteristic and continuous X-ray contributions to the Auger si- gnal.
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