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MICROANALYSIS TECHNIQUE USING ELECTRON ENERGY LOSS SPECTROSCOPY IN HIGH
VOLTAGE ELECTRON MICROSCOPY
Jean Sévely, Yolande Kihn, G. Zanchi, B. Jouffrey
To cite this version:
Jean Sévely, Yolande Kihn, G. Zanchi, B. Jouffrey. MICROANALYSIS TECHNIQUE USING ELEC-
TRON ENERGY LOSS SPECTROSCOPY IN HIGH VOLTAGE ELECTRON MICROSCOPY. Jour-
nal de Physique Colloques, 1984, 45 (C2), pp.C2-441-C2-444. �10.1051/jphyscol:19842100�. �jpa-
00223767�
JOURNAL
DE PHYSIQUEColloque C2, supplgment au n02, Tome 45, f6vrier 1984 page C2-441
MICROANALYSIS TECHNIQUE USING ELECTRON ENERGY LOSS SPECTROSCOPY I N HIGH VOLTAGE ELECTRON MICROSCOPY
J. Sevely, Y. Kihn, G. Zanchi and B. Jouffrey
Laboratoire d r @ t i q u e EZectronique du
c.u.R.s.*,
29 rue Jeanne Marvig, 31055 Touzouse Cedex, FmnceResume : En vue de l ' u t i l i s a t i o n de l a spectroscopie de p e r t e s d'energie d ' e l e c t r o n s
a
t r @ s haute tension, pour l e developpement de l a microanalyse, on e t u d i e l a v a r i a - t i o n du c o n t r a s t e des d i s t r i b u t i o n s c a r a c t e r i s t i q u e s d ' i o n i s a t i o n du niveau K du car- bone en f o n c t i o n de l ' e n e r g i e des S l e c t r o n s p r i m a i r e s ( e n t r e 0,3 e t 1 MeV). Cette etude permet de conclurea
un avantage des hautes tensions dans ce domaine.A b s t r a c t : With t h e purpose o f using EELS f o r t h e development o f microanalysis,the v a r i a t i o n o f t h e j u m p - r a t i o a t the carbon K-edge i s s t u d i e d as a f u n c t i o n of t h e p r i - mary e l e c t r o n i n c i d e n t energy (0.3
-
1 MeV). This study shows an i n t e r e s t i n using h i g h a c c e l e r a t i n g voltage i n t h a t f i e l d .INTRODUCTION
The a d a p t a t i o n o f e l e c t r o n energy l o s s spectrometers t o h i g h voltage e l e c t r o n micros- copes ( l ) , (2) a l l o w s t o achieve energy l o s s spectra, t h e energy r e s o l u t i o n o f which i s comparable w i t h t h e one obtained on a one hundred k i l o v o l t e l e c t r o n microscope equipped w i t h a thermoionic e l e c t r o n source. These spectra show t h e various kinds o f i n t e r a c t i o n s o f t h e i n c i d e n t e l e c t r o n s w i t h the sample ( 1 ) . The purpose o f t h i s pa- per i s t o study t h e i n f l u e n c e o f t h e a c c e l e r a t i n g v01 tage upon t h e s e n s i b i l i t y o f t h e chemi c a l microanalysi S technique using t h e c h a r a c t e r i s t i c d i s t r i b u t i o n s associated w i t h the i n n e r she1 l e x c i t a t i o n o f t h e specimen atoms. The i n f e r e n c e s we draw from t h i s study a r e i l l u s t r a t e d i n the f i e l d o f b i o l o g y and m e t a l l u r g y , i n some c o n t r i - b u t i o n s o f these proceedings.
THE EXPERIMENTAL DEVICE
The experiments have been performed on a 300
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1000 kV conventional transmission e l e c t r o n microscope equipped w i t h an o m e g a f i l t e r i n g device ( 2 ) . The e l e c t r o n counting system c o n s i s t s o f a charged p a r t i c l e d e t e c t o r connected t o a p u l s e processing sys- tem. An amplitude s e l e c t o r allows t o separate t h e pulses due t o X-rays and t o remove t h e X-ray p a r a s i t e signal from t h e spectra ( 3 ) .CONTRAST OF CHARACTERISTIC DISTRIBUTIONS
The r e l a t i v e importance o f t h e c h a r a c t e r i s t i c s i g n a l can be evaluated by comparing i t t o t h e background i n t e n s i t y . We have chosen t o represent the c h a r a c t e r i s t i c s i g n a l c o n t r a s t C by t h e jump r a t i o (4), C = IM/I
,
where IM i s the maximum i n t e n s i t y o f the c h a r a c t e r i s t i c d i s t r i b u t i o n and 1, i s The background i n t e n s i t y j u s t before the i o n i s a t i o n edge, ( F i g . 1)The experimental r e s u l t s have been compared w i t h t h e t h e o r e t i c a l ones g o t from com- puted spectra. A general procedure has been developed f o r the computation o f the spectra which are described by c o n s i d e r i n g t h r e e d i f f e r e n t kinds o f i n e l a s t i c scat- t e r i n g processes. The Poisson's s t a t i s t i c s a r e a p p l i c a b l e t o t h e d e s c r i p t i o n o f the s c a t t e r i n g i n t h e specimen.
*associ6 Z l'Universit6 Paul Sabatier
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842100
C2-442 JOURNAL DE PHYSIQUE
electron intensity
.----. . . . . .
. .
F i g . 1 : D e f i n i t i o n o f t h e con- t r a s t o f t h e c h a r a c t e - r i s t i c d i s t r i b u t i o n c o r r e s p o n d i n g t o a K- i o n i s a t i o n edge.
The F o u r i e r transfor&w,ed ) o f t h e i n t e n s i t f l ~ ,
ed
) t r a n s m i t t e d b y t h e sample t h r o u g h t h e o b j e c t i v e l e n s a p e r t u r e (Qd) can be w r i t t e n :where, X i s t h e mean f r e e p a t h f o r t h e main i n e l a s t i c s c a t t e r i n g process ( b u l k p l a s - mon e x c l h a t i o n ) , h2 t h e mean f r e e p a t h f o r a secondary i n e l a s t i c process o f m i n o r importance, which a l s o c o n t r i b u t e s t o t h e background f o r m a t i o n and XK t h e mean f r e e p a t h f o r t h e i o n i s a t i o n p r o c e s s . tjl(w), q2(w) and QK(w) a r e t h e F o u r i e r t r a n s f o r m s o f t h e energy l o s s f u n c t i o n s gl(E), g (E), g (E) a s s o c i a t e d r e s p e c t i v e l y w i t h t h e
i n e l a s t i c processes d e f i n e d prevjous?y ( S ) . K ~ h e s e n o r m a l i z e d f u n c t i o n s r e p r e s e n t t h e p r o b a b i l i t y f o r an e l e c t r o n t o undergo a g i v e n energy l o s s E a f t e r a n i n t e r a c t i o n o f t h e c o r r e s p o n d i n g k i n d . F(O ) i s a f u n c t i o n o f t h e a n g l e Od d e f i n e d - b y t h e o b j e c t i v e l e n s diaphragm. I t r e p o r t s $he a n g u l a r s c a t t e r i n g i n t h e specimen. Gs(w) i s t h e F o u r i e r t r a n s f o r m o f t h e apparatus t r a n s f e r f u n c t i o n G ( E ) ; th e l a t e r t a k e s i n t o account t h e energy f l u c t u a t i o n s o f t h e i n c i d e n t beam ahd t h e s e l e c t i n g s l i t e f f e c t s o f t h e d i s p e r s i v e system.
The f i r s t two i n e l a s t i c processes o r i g i n a t e t h e background i n t e n s i t y which i s i n t e r p r e t e d i n t e r m o f p l u r a l e l e c t r o n s c a t t e r i n g . The t h i r d i n e l a s t i c e v e n t i s l i a - b l e f o r t h e c h a r a c t e r i s t i c d i s t r i b u t i o n o f t h e s p e c t r a .
~ f f l ~ , Od) i s t h e background i n t e n s i t y computed f r o m t h e r e v e r s e F o u r i e r t r a n s f o r m o f t h e f u n c t i o n
t h e f l ~ , Od) can be w r i t t e n :
where
*
r e p r e s e n t s t h e c o n v o l u t i o n p r o d u c t o f two f u n c t i o n s .These r e l a t i o n s a l l o w t o compute t h e energy l o s s s p e c t r a and to-compare them t o t h e e x p e r i m e n t a l ones. The d o t t e d c u r v e ( F i g . 2) corresponds t o a computed spectrum fit- t e d w i t h an e x p e r i m e n t a l spectrum ( f u l l c u r v e ) g o t w i t h a 410 nm t h i c k carbon f o i l a t 1000 kV. The a v a i l a b i l i t y o f t h e p r e v i o u s r e l a t i o n s and o f t h e models chosen t o r e p r e s e n t t h e energy l o s s f u n c t i o n s , has been checked w i t h v a r i o u s k i n d s o f samples, t h i c k n e s s e s and e x p e r i m e n t a l c o n d i t i o n s : a c c e l e r a t i n g v o l t a g e and s e l e c t i n g s l i t w i d t h . I n any case we reached a good f i t t i n g between e x p e r i m e n t a l and computed spec- t r a ( t o be p u b l i s h e d ) .
F i g . 2 : Comparison o f an expe- r i m e n t a l ( f u l l c u r v e ) and c a l c u l a t e d ( d o t t e d c u r v e ) energy l o s s spec- t r u m f o r a 410 nm t h i c k evaporated carbon f i l m a t 1000 kV.
F i g . 3 : V a r i a t i o n o f t h e K - i o n i - s a t i o n edge c o n t r a s t as a f u n c t i o n o f t / X 1 f o r carbon e v a p o r a t e d f o i 7 s . F u l l c u r v e : c a l c u l a t e d r e s u l t s . P o i n t s : expe- mental r e s u l t s g o t a t l 0 0 0 kV (.), 500 kV ( A ) ,
300 kV (a).
The c h a r a c t e r i s t i c s i g n a l c o n t r a s t has been measured f r o m t h e computed s p e c t r a and compared w i t h t h e e x p e r i m e n t a l r e s u l t s . The f u l l c u r v e o f F i g . 3 r e p r e s e n t s t h e va- r i a t i o n o f t h e computed c o n t r a s t values as a f u n c t i o n o f t/X1.
The p o i n t s correspond t o e x p e r i m e n t a l v a l u e s g o t a t v a r i o u s a c c e l e r a t i n g v o l t a g e s w i t h carbon f o i l s o f d i f f e r e n t t h i c k n e s s e s . The e x p e r i m e n t a l r e s u l t s a r e i n good agreement w i t h t h e c a l c u l a t e d values.
The c o n t r a s t of t h e c h a r a c t e r i s t i c d i s t r i b u t i o n depends on t h e a c c e l e r a t i n g v o l t a g e t h r o u g h t h e mean f r e e p a t h s a s s o c i a t e d w i t h t h e v a r i o u s i n e l a s t i c s c a t t e r i n g proces- ses. 11, X2 and hK a r e i n c r e a s i n g f u n c t i o n s o f t h e a c c e l e r a t i n g v o l t a g e . The curves which d e s c r i b e t h i s e v o l u t i o n have n e a r l y t h e same shape, so t h a t t h e r a t i o s A1/X2 and X1/XK remain s c a r c e l y c o n s t a n t and t h e c o n t r a s t depends on t h e a c c e l e r a t i n g v o l - t a g e o n l y t h r o u g h t h e parameter t/X1.
JOURNAL
DE
PHYSIQUEF i g . 4 : V a r i a t i o n o f t h e K - i o n i - s a t i o n c o n t r a s t as a f u n c t i o n o f the accelara- t i n g v o l t a g e f o r various thicknesses o f carbon evaporated f i l m s . (a) :
a 16 nm, (b) : 32 nm, ( c ) :
6 100 nm, ( d ) : 410 nm.
4
0 .l .2 .3 .4 .5 .6 .7 .8 .Q 1
The conclusion may be drawn t h a t i t i s b e t t e r t o work w i t h h i g h energy e l e c t r o n s , mainly when t h e specimen cannot be made t h i n n e r enough. These inferences are i n good agreement w i t h t h e curves o f F i g . 4 which show t h e experimental v a r i a t i o n o f t h e con- t r a s t as a f u n c t i o n o f t h e a c c e l e r a t i n g v o l t a g e f o r v a r i o u s thickness values o f t h e sample.
CONCLUSION
The technique o f chemical microanalysis using e l e c t r o n energy losses i s a l l t h e more s e n s i t i v e as t h e t / X 1 value i s small. Consequently, t h e use o f h i g h a c c e l e r a t i n g v o l - tages i n t h i s f i e l d presents two p o i n t s o f i n t e r e s t :
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For a given thickness o f t h e specimen, t h e s e n s i t i v i t y o f t h e method increases.That i n v o l v e s an extension o f t h e range of energy losses a v a i l a b l e t o i d e n t i f y the elements o f the specimen, ( u s u a l l y up t o 3000 eV which i s s u f f i c i e n t t o i d e n t i f y any element).
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When t h e r e are d i f f i c u l t i e s i n making t h i n n e r enough t h e specimen, f o r instance i n t h e f i e l d o f ceramics o r geology, i t may then be necessary t o use h i g h a c c e l e r a t i n g voltages. Nevertheless i t does n o t seem t o be o f a g r e a t i n t e r e s t t o go beyond 500 kV, as beyond t h i s value, t h e mean f r e e path o f t h e main i n e l a s t i c s c a t t e r i n g process l i a b l e f o r t h e background remains n e a r l y constant ( 6 ) . The use o f h i g h a c c e l e r a t i n g v o l t a g e (up t o 500 kV) a l l o w s chemical m i c r o a n a l y s i s o f 200 o r 300 nanometer t h i c k specimens.REFERENCES
(1) SEVELY J., PEREZ J.Ph., JOUFFREY B., C.R. Acad. Sc. P a r i s , (1973), 276, 515.
(2) ZANCHI G., SEVELY J., JOUFFREY B., J. Microsc. Spectrosc. Electronique, (1976), 2, 95.
(3) KIHN Y., PEREZ J.Ph., SEVELY J., ZANCHI G., JOUFFREY B., Proc. 6 t h I n t e r . Conf.
H.V.E.M., Antwerp, (1980), 42.
(4) MAHER D.M., JOY D.C., EGERTON R.F., MOCHEL P., J. Appl. Phys., (1979), 50, 5105.
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EGERTON R.F., SEVELY J., J. o f Microsc., (1983), 129, RP 1.(5) MISEL D.L., Advances i n e l e c t r o n i c s and e l e c t r o n physics, (Academic Press, New- York
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London), (1973), 32.( 6 ) SEVELY J., PEREZ J.Ph., JOUFFREY B., Proc. 3rd I n t e r n . Conf. on H.V.E.M.. Oxford, (1974), (Academic Press, London and New-York)
