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TRENDS IN EELS WITHIN THE FIELD OF MICROANALYSIS
R. Egerton
To cite this version:
R. Egerton. TRENDS IN EELS WITHIN THE FIELD OF MICROANALYSIS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-423-C2-428. �10.1051/jphyscol:1984296�. �jpa-00224011�
page C2-423
TRENDS IN EELS WITHIN THE FIELD OF MICROANALYSIS
R.F. Egerton
Physics Department, University of Alberta, Edmonton, Alberta, Canada T6G 251
Resume
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On presente l a technique de spectrometrie des pertes d'snergie des electrons en microscopie electronique en transmission. Puis on discute l e s developpements recents de l ' a p p a r e i l l a g e e t on donne plusieurs exemples d'analyse chimique e t s t r u c t u r a l e .Abstract - We begin with a brief introduction t o electron energy-loss spectroscopy carried out using an electron microscope. Recent developments in instrumentation a r e then discussed, followed by examples designed t o i l l u s t r a t e applications of the technique t o elemental analysis and s t r u c t u r e determination.
Electron energy-loss spectroscopy (EELS) i s used t o investigate chemical and structural properties of a specimen, through an analysis of the kinetic- energy d i s t r i b u t i o n within a transmitted or reflected electron beam. Here we discuss only transmission measurements, which a r e conveniently carried out by combining an electron spectrometer with a (conventional or scanning) transmission microscope. The microscope lenses a r e used t o focus incident electrons, typi- c a l l y of lOOkeV energy, onto a small region of a s o l i d specimen ( t y p i c a l l y l0nm-lpm in diameter). After passing through the specimen, the electrons are directed into a high-resolution spectro- meter, often with the help of post- specimen imaging lenses (see Fig. l ) . The spectrometer usually employs a ELECTRON transverse magnetic f i e l d , which
disperses the beam by a few pm per eV of energy loss in the specimen. An elec- tron-detection system converts the resulting spatial d i s t r i b u t i o n i n t o a plot of electron i n t e n s i t y J(E) against energy loss E , which i s the energy-loss spectrum.
ELECTRON SPECTROMETER A simple example of an energy-loss s ~ e c t r u m is shown in Fig. 2. The zero- Fig. 1 - A typical EELS system, based
on a conventional transmission electron microscope (CTEM)
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The CTEM imaging lenses a r e used t o t r a n s f e r transmit- ted electrons into the spectrometer.Z ~ S S peak accounts f o r electrons which have been e l a s t i c a l l y scattered by the specimen, whereas other features r e s u l t from i n e l a s t i c interactions. I n e l a s t i c s c a t t e r i n g by valence or conduction electrons gives r i s e t o one o r more
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984296
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F i g . 2 - E l e c t r o n energy l o s s spectrum f r o m a 30nm f i l m o f amorphous carbon, r e c o r d e d u s i n g 80keV i n c i d e n t e l e c t r o n s and two v a l u e s o f c o l l e c t i o n semi-angle a.
Besides t h e z e r o - l o s s peak, a v a l e n c e - l o s s peak i s v i s i b l e a t E=25eV and a K- i o n i z a t i o n edge a t 285eV. Shown on t h e l e f t a r e t h e ranges o f i n t e n s i t y which can be r e c o r d e d i n 10s b y p a r a l l e l - d e t e c t i o n systems based on a R e t i c o n RL1024S photo- d i o d e a r r a y , o p e r a t e d a t v a r i o u s c l o c k i n g f r e q u e n c i e s and exposed e i t h e r d i r e c t l y t o t h e e l e c t r o n s o r v i a a P4 o r NE102 phosphor.
electron energy loss, EfeV
F i g . 3 - P a r t o f t h e e n e r g y - l o s s spectrum f r o m a Si-AR-0-N-Mg ceramic /28/. The s o l i d l i n e r e p r e s e n t s d a t a r e c o r d e d f r o m t h e i n t e r i o r o f a g r a i n , whereas t h e d o t s r e p r e s e n t measurements f r o m a grain-boundary r e g i o n . Mg, S i and 0 a r e p r e s e n t i n h i g h e r c o n c e n t r a t i o n a t t h e g r a i n boundary, p o s s i b l y as amorphous magnesium s i l i c a t e .
from i n n e r - s h e l l e l e c t r o n s causes s t e p - l i k e f e a t u r e s ( i o n i z a t i o n edges), s i m i l a r i n appearance t o X-ray a b s o r p t i o n edges. They occur a t energy losses approximately equal t o t h e b i n d i n g energies o f t h e i n n e r s h e l l s , which vary from 50eV up t o many thousands o f eV, and have been t a b u l a t e d f o r a l l elements /l/. Therefore, EELS can be used t o r e v e a l t h e presence o f i n d i v i d u a l elements i n a specimen (see Fig. 3 ) . Further, by measuring a s u i t a b l e area beneath each i o n i z a t i o n edge (making allowance f o r the background i n t e n s i t y ) and c a l c u l a t i n g t h e a p p r o p r i a t e i o n i z a t i o n cross section, t h e r e l a t i v e o r absolute c o n c e n t r a t i o n o f each element can be determined /2,3/. Consequently, EELS provides an a l t e r n a t i v e t o X-ray emission spectroscopy
(XEDS) f o r elemental microanalysis. I n t h i s r o l e , EELS has the advantage o f higher s i g n a l - c o l l e c t i o n e f f i c i e n c y (- 50% compared t o
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1% f o r X-ray d e t e c t o r s ) and i n t r i n s i c a l l y h i g h e r s i g n a l ( s i n c e the number o f energy-loss e l e c t r o n s i s equal t o the swn o f the X-ray photons and t h e Auger e l e c t r o n s c r e a t e d ) . I t s main disadvan- tages are t h a t the c h a r a c t e r i s t i c i o n i z a t i o n edges a r e accompanied by a r e l a t i v e l y h i g h background, and t h a t ( f o r minimum background) very t h i n specimen areas (< 50nm) a r e required, unless h i g h e r i n c i d e n t energies (e.g. 1MeV) a r e employed /4/.During t h e l a s t few years, several advances i n i n s t r u m e n t a t i o n have enabled t h e p o t e n t i a l advantages o f EELS t o be more f u l l y r e a l i z e d .
(1) E l e c t r o n spectrometers have been designed w i t h a low a b e r r a t i o n c o e f f i c i e n t 15-91. This i s u s u a l l y achieved by c u r v i n g t h e entrance and e x i t boundaries o f t h e magnetic f i e l d , a l l o w i n g an entrance angle y - 1 0 mrad t o be used w i t h o u t appreciable l o s s o f energy r e s o l u t i o n .
(2) Post-specimen lenses have been used t o increase t h e c o l l e c t i o n angle a measured a t t h e specimen (see Fig. 1 ) . For example, i f t h e specimen i s imaged onto t h e spectrometer o b j e c t plane w i t h a m a g n i f i c a t i o n M, one has a = yM /10,11/. I n t h i s way, values o f a i n excess o f 100 mrad a r e possible, a l l o w i n g n e a r l y a l l o f t h e s c a t t e r e d e l e c t r o n s t o be c o l l ected by t h e spectrometer.
( 3 ) P a r a l l e l - d e t e c t i o n systems have been developed which r e c o r d p r a c t i c a l l y a1 l o f t h e e l e c t r o n s which have passed through t h e spectrometer, r a t h e r than t h e 1% o r l e s s which i s t y p i c a l o f s e r i a l (spectrum-scanni ng) techniques. These new d e t e c t o r s are based on a s i l i c o n - d i o d e array, which responds t o t h e e l e c t r o n s e i t h e r d i r e c t l y o r f o l l o w i n g conversion t o photons by a h i g h - r e s o l u t i o n phosphor screen. They are s t i l l i n a p r o t o t y p e stage 112-151, one o f t h e remaining problems being t h a t o f r e c o r d i n g t h e very l a r g e range o f e l e c t r o n i n t e n s i t i e s present i n an energy-loss spectrum (see F i g . 2 ) .
A r e l a t e d technique, which has r e c e n t l y received renewed a t t e n t i o n , uses t h e e l e c t r o n spectrometer as a f i l t e r , i n order t o o b t a i n electron-microscope images from e l e c t r o n s which have i n c u r r e d a chosen range o f energy l o s s . By s e l e c t i n g a range corresponding t o t h e i o n i z a t i o n edge o f a p a r t i c u l a r element and making proper allowance f o r t h e background i n t e n s i t y , an elemental map can be produced showing t h e s p a t i a l d i s t r i b u t i o n o f t h a t element. The technique i s r e l a t i v e l y easy t o implement i n the case o f a scanning-transmission microscope .(STEM). Using a computer t o c o n t r o l b o t h t h e i n c i d e n t beam and t h e spectrometer, background s u b t r a c t i o n can be performed a t each p i x e l o f a 64x64-pixel image and t h e s p a t i a l d i s t r i b u t i o n o f several elements mapped simultaneously /l 61 (see Fig. 4 ) .
Energy f i l t e r i n g can a l s o be achieved i n fixed-beam (CTEM) mode, by u t i l i z i n g the image-forming p r o p e r t i e s o f a magnetic spectrometer. Even the simple magnetic prism can be used i n t h i s way /17/. However, improved performance i s obtained w i t h a p r i s m / m i r r o r arrangement 1181 where t h e e l e c t r o n s pass t w i c e through t h e magnetic f i e l d , o r w i t h t h e s o - c a l l e d omega f i l t e r /19,20/ where f o u r magnetic d e f l e c t i o n s a r e involved. I n b o t h cases, t h e beam emerges p a r a l l e l t o i t s o r i g i n a l d i r e c t i o n o f t r a v e l , a1 lowing the devices t o be i n s e r t e d i n t o t h e middle o f a microscope column.
O f course, t h i s makes t h e i r i n s t a l l a t i o n more complicated; however, as a r e s u l t o f t h e symmetry o f the e l e c t r o n t r a j e c t o r i e s , c e r t a i n a b e r r a t i o n s vanish. Using a p r i s m / m i r r o r system, t h e d i s t r i b u t i o n o f calcium and phosphorous has been mapped i n various b i o l o g i c a l samples /21,22/ w i t h a s p a t i a l r e s o l u t i o n estimated t o be
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0.4nm,which corresponds t o an elemental s e n s i t i v i t y below 10-'Og o r l e s s than 100 atoms /23/
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Besides elemental a n a l y s i s , EELS can provide o t h e r kinds of information, via an a n a l y s i s of fine structure. For small a , t h e energy-loss i n t e n s i t y J ( E ) can be shown t o be proportional t o t h e imaginary p a r t of t h e reciprocal of t h e complex p e r m i t t i v i t y E . By Kramers-Kroning a n a l y s i s , both t h e r e a l and imaginary p a r t s of
E can be r e t r i e v e d , together with r e l a t e d q u a n t i t i e s ( r e f r a c t i v e index, o p t i c a l r e f l e c t a n c e and o p t i c a l absorption c o e f f i c i e n t ) . The low-loss region o f t h e spectrum t h e r e f o r e contains t h e same kind of information a s would be obtained from o p t i c a l measurements extending between t h e v i s i b l e and t h e f a r u l t r a v i o l e t region of t h e electromagnetic spectrum.
Inner-shell i o n i z a t i o n edges a l s o contain f i n e s t r u c t u r e , which can be divided i n t o near-edge s t r u c t u r e (ELNES) and extended f i n e s t r u c t u r e (EXELFS). EXELFS i s t h e electron-beam equivalent of EXAFS modulations found i n X-ray absorption s p e c t r a and can y i e l d s i m i l a r information, namely t h e r a d i a l d i s t r i b u t i o n function of a p a r t i c u - l a r element and i t s s e p a r a t i o n from nearest-neighbour atoms. 111 p r i n c i p l e , t h e EXELFS technique i s capable of analysing smaller volumes of material than EXAFS /24,25/ p a r t i c u l a r l y i f a p a r a l l e l - d e t e c t i o n system i s used. However, e x i s t i n g measurements have used s e r i a l d e t e c t i o n and have been r e s t r i c t e d t o energy l o s s e s
UNFI LTERED IMAGE
SILICON-K (1840-188U~V)
Fig. 4 - Energy-filtered images of a phase-separated Na-B-Si-0 ? l a s s , forned from K- s h e l l l o s s e s of boron, s i l i c o n and oxygen. Whereas t h e oxygen appears uniformly d i s t r i b u t e d , t h e boron concentration i s lower (by a f a c t o r
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4 . 5 ) and t h e s i l i c o n concentration higher (by a f a c t o r-
2.2) within t h e c i r c u l a r regions, which appear dark i n t h e b r i g h t - f i e l d u n f i l t e r e d micrograph. The i n c i d e n t energy was 200keV, t h eincident-beam c u r r e n t 3nA and t h e t o t a l recording time 10 mins. (sample courtesy of D. Cann and P. Taylor; f i l t e r e d images by C . F i o r i , K . Gorlen and R. Leapman).
systems.
Near-edge s t r u c t u r e o c c u r s w i t h i n 50eV o f an edge and c o n s i s t s o f t y p i c a l l y two o r t h r e e w e l l - d e f i n e d peaks. These were o r i g i n a l l y t h o u g h t t o a r i s e from t h e energy dependence o f t h e d e n s i t y o f e l e c t r o n s t a t e s above t h e Fermi l e v e l ; however, t h i s would i m p l y t h a t t h e peak s t r u c t u r e s h o u l d be s i m i l a r i n t h e d i f f e r e n t edges measured f r o m a compound, which i s g e n e r a l l y n o t observed (see F i g . 5). A more g e n e r a l t h e o r y o f ELNES (and o f t h e r e l a t e d XANES s t r u c t u r e p r e s e n t a t an X-ray a b s o r p t i o n edge) i s s t i l l under d i s c u s s i o n , b u t t h e r e i s e x p e r i m e n t a l evidence t h a t (as w i t h EXELFS) nearest-neighbour atoms e x e r t t h e m o s t i n f l u e n c e on t h e s t r u c t u r e . I n some cases, t h e r e seems t o be a c o r r e l a t i o n between t h e t y p e o f ELNES s t r u c t u r e observed and t h e coordination o f t h e atoms g i v i n g r i s e t o t h e edge ( s e e F i g . 6 ) .
F i g . 5 - Energy-loss near-edge s t r u c - F i g . 6 - K-edge ELNES from o c t a h e d r a l Mg t u r e observed a t t h e K-edge o f each ( i n 01 i v i n e ) , o c t a h e d r a l AR ( i n s p i n e l ) , element i n c r y s t a l 1 in e BN and g l a s s y t e t r a h e d r a l #g ( i n s p i n e l ) , t e t r a h e d r a l AQ 8203. Note t h a t t h e peak s t r u c t u r e i s i n ( o r t h o c l a s e ) and t e t r a h e d r a l S i ( i n s u b s t a n t i a l l y d i f f e r e n t f o r t h e two 01 i v i n e ) . Note t h e apparent c o r r e l a t i o n elements i n t h e same compound /26/. o f edge shape w i t h c o o r d i n a t i o n /27/.
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