X-RAY PHOTOELECTRON MICROPROBE ANALYSIS AND RELATED TECHNIQUES

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X-RAY PHOTOELECTRON MICROPROBE ANALYSIS AND RELATED TECHNIQUES

J. Cazaux, D. Gramari, D. Mouze, A. Nassiopoulos, J. Perrin

To cite this version:

J. Cazaux, D. Gramari, D. Mouze, A. Nassiopoulos, J. Perrin. X-RAY PHOTOELECTRON MICRO- PROBE ANALYSIS AND RELATED TECHNIQUES. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-271-C2-274. �10.1051/jphyscol:1984260�. �jpa-00223974�

JOURNAL DE PHYSIQUE

Colloque C2, supplgment au nb2, Tome 45, fgvrier 1984 page C2-271

X-RAY PHOTOELECTRON MICROPROBE A N A L Y S I S AND R E L A T E D TECHNIQUES

J . Cazaux, D. Gramari, D. Mouze, A.G. Nassiopoulos and J. Perrin Laboratoire de Spectroscopie des EZectrons, U.E.R. Sciences, 51062 Reims Cedex, France

RBsumB - I1 est bien connu que la focalisation et le balayage des faisceaux de photons X sont extrzmement difficiles. On peut surmonter ces difficult&

en focalisant un faisceau d'dlectrons sur une anticathode en forme de film mince et en plasant l'dchantillon (lui-mSme en forme de film mince) au dos de

l'anticathode.

Ce principe a Btd mis en oeuvre dans un spectromstre Auger (16gPrement modi- fih) dans le but de rBaliser les premisres expdriences de :

i) analyse par microsonde photoBlectronique X

ii) analyse par microsonde Auger induiteparles rayons X (radiation caractd- ristique et radiation continue)

iii) microanalyse par microfluorescence X iv) microscopic photo&lectronique X 2 balayage

V) microradiographie X B balayage

vi) spectroscopie Auger induite par des dlectrons sur des films minces.

Abstract - It is a well-known fact that the focusing and scanning X-ray photon beams is an extremely difficult operation. The difficulties which arise can be overcome by focusing and scanning an electron beam on a thin anode and by setting the target (in a thin form) on the back of the anode.

We have done this in a (slightly modified) Auger spectrometer in order to per- form the first experiments in :

i) X-ray photoelectron microprobe analysis (XPMA)

ii)(characteristic and continuous) X-ray-induced Auger microprobe analysis iii) fluorescent X-ray microanalysis (XRF)

iv) scanning X-ray photoelectron microscopy (SXPM) v) scanning X-ray microradiography (SXR)

vi) electron-induced Auger electron spectroscopy in thin films.

1 - INTRODUCTION

The focusing and scanning of an X-ray photon beam is a difficult operation. Thus analytical techniques based on X-ray photons and intended to obtain data concerning the elemental composition of a specimen generally have a poor degree of lateral lo- calization / l / . The aim of the present paper is to show how it is possible with the proposed apparatus to develop microanalytical techniques based on X-ray excitation of inner shell electrons, such as X-ray photoelectron spectroscopy (XPS or ESCA) and X-ray induced Auger electron spectroscopy (XAES) for surface analysis, or X-ray fluo- rescence spectroscopy (XRF) for bulk analysis. The same experimental arrangement is also applied to develop the corresponding scanning microscopies : scanning X-ray photoelectron microscopy (SXPM), scanning X-ray-induced Auger electron microscopy

(SXAM), and scanning X-ray radiography (SXR). The experimental setup and results ob- tained will be presented, including some results obtained by electron-induced Auger spectroscopy (e-AES)

.

2 - EXPERIMENTAL SETUP AND RESULTS OBTAINED

A scannable electron gun is used to provide a focused electron beam (gun 0 ⁱⁿ

figure 1). A cylindrical mirror analyser (CMA) is positioned opposite the focused

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984260

JOURNAL DE PHYSIQUE

Fig. 3 - XAES spectrum of a Fig. 4 - Image of an inhomogeneous deposit silicon deposited on a gold of carbon obtained by Auger microscopy in- anode sputtered on an NaCl duced by continuous X-ray radiation emitted

base. by a silver anode.

electron source. A composite sample

a@

placed between them. In XPS, a thin A1 (or Mg) foil @ is situated normal to the incident electrom beam to create Al(Mg)Ka X-rays, which in turn create photoelectrons from the specimen

a

5 p thick A1 foil with the electron gun volta- ge, beam intensity and spot size equal

ngn, respectively to V = 9 kV, I = 2 uA and

The photoelectron intensity depends on the absorption of the X-rays, which can be used to obtain SXR images. For instance, Fig. 5 shows the radiographic image (obtai- ned in the scanning mode) of a copper grid (200 mesh) interposed between two A1 foils : one of them is the anode, the other is covered by a gold film which acts as a converter.

r \ g h

3m

d = 20 um. The photoelectrons emitted by the photoelectronic process have a kinetic energy EK given by :

2 z

/

(1 ₁

A ~ M S N N

( 1 ) where hV is the photon energy and EB the atomic electron binding energy. So a pho- toelectron spectrum contains photoelectron lines identified using expression (l), but

73

'q

excitation process following the photo-

,."

D X+ W5 1m trum is given in Fig. 2. Contrary to pho- Fig. 2 - XPS spectrum of silver toelectron lines, the energetic position

(A1 anode : 5pm thick). of Auger lines is fixed and independent of the photon energy. Thus XAES can also be performed using the continuous radiation created in a heavy material anode. Figure 3 shows the XAES spectrum of a silicon film deposited on a gold ano- de, 0.4 p thick, sputtered on an NaCl base. Na and C1 contaminants are present in the spectrum and al- so 0 and C lines are visible. Figure 4 shows an image of an inhomogeneous deposit of carbon on a silver anode 0.5 u m thick. It was obtained by the use of the Auger line of carbon excited by a con- tinuous radiation issued from the anode.

eV 50 200 400 600

Relation (1) can also be used to deduce the energy of X-ray photons when the atomic composition of the surfa- ce is known. In this case, X-rays are analysed by using photoelectron spectroscopy. This has been done to obtain the bulk composition of a Mg/Al alloy anode by electron probe microanalysis (EPMA) 141. X-ray fluorescence ana- lysis (XRF) of a Mg foils, 6 pm thick, excited by the X-rays issued from an A1 anode has been performed inthe same way, the identification of the fluorescent MgKa photons being deduced from the photoelectrons they exci- te on the surface (oxygen atoms in figure 6).

radiographic image of a copper grid sandwiched between two A1 foils.

-

- h ^U)(X] rgk*

CMA e-

-

Rm

Q) mm

rs.ro6

Fig. 6 - X-ray fluorescence analysis of thin films m Jbo a) experimental arrangement

b) corresponding photoelectron spectrum.

Fig. 7 - Auger spectrum of a thin Electron-induced Auger spectroscopy by reflec- silver film ; V = 10 kV ;

tion on and transmission through a thin film I = 3.4 pA ; obtained by trans- makes it possible to decrease the background mission.

and consequently to increase the signal/noise ratio compared with the results obtained on a

bulk sample. So in the case of the thin silver film (figure 7), the signal/noise ra- tio relative to the Auger line is ten times better than in the case of the XPS spec- trum (figure 2) of sivler on the aluminium anode.

3 - DISCUSSION

In the reported experiments, the spatial resolution is determined by the X-ray spot size which irradiates the sample. The divergence of the X-ray beam is limited by self-absorption and the cone of the efficient X-rays has a semi-apex angle of about 30' - 45'. This resolution depends on the distance between the X-ray source and the sample but also on the size of the source itself. Due to the diameter of the incident electron beam we used (20 pm), the spatial resolution = 30 pm ; this can be improved by using a finer probe (a probe with I. = 1 pA, do = 1 pm, V, = 30 kV is achievable and would give the same X-ray intensity). The ultimate limit is in the micron range in XAES with a thin anode of a heavy metal and a thin sample 151.

Though for convenience we have used electron spectroscopy for X-ray analysis, we have planned to use a conventional EDX system for XRF. To detect X-rays, the value of electron spectroscopy lies in its intrinsic energy resolution (which is almost that of the CMA), but it has a poor quantum-efficiency : in figure 6, the 0 (Is) photoelectron signal corresponds to 106 - 107 emerging (MgKcl) photons/sec. in XRF, despite the fact that Mg has a poor fluorescence yield (3.10-2) and the analysed volume is less than 10-5 mm3.

All the reported techniques are characterised by the fact that the sample (being as near as possible to the X-ray source) is bombarded by an X-ray photon density of about 106-107 photons/sec./pm2 ; a density which can be improved by a factor of

100 with the use of a finer probe. This is far greater than is given by synchrotron radiation (in the present state of the art) even if the brightness (photons/mm2/ste- rad.) of synchrotron radiation is better, due to the small divergence of the X-rays

C2-274 JOURNAL DE _PHYSIQUE

beams produced.

The t e c h n i q u e s d e s c r i b e d r e q u i r e t h e u s e o f a sample i n a more o r l e s s t h i n f i l m form. I n XPMA, t h i s t h i c k n e s s can r e a c h 15 pm and a homogeneous anode/sample com- b i n a t i o n can be used f o r t h e s u p e r f i c i a l a n a l y s i s ( n i t r i d a t i o n , o x i d a t i o n , e t c ...)

of e l e m e n t s s u c h a s A l , Mg, S i , t h a t c u r r e n t l y s e r v e a s anodes.

F o r t h e XAES microprobe, where c h a r a c t e r i s t i c r a d i a t i o n i s n o t u s e d , t h i s t y p e of a n a l y s i s may b e e x t e n d e d t o e l e m e n t s of more v a r i e d c o m p o s i t i o n s (heavy m e t a l s , a l l o y s , e t c . . . 1 5 1 ) .

The i d e a s developed h e r e f o r SXR 1 6 1 , f l u o r e s c e n t X-ray m i c r o a n a l y s i s / 7 / a r e n o t r e a l l y new, h a v i n g been s t u d i e d i n t h e 1950's and modernized by t h e u s e o f t h e s y n c h r o t r o n r a d i a t i o n 181. N e v e r t h e l e s s , t h e i r a p p l i c a t i o n s h e r e do n o t r e q u i r e t h e mechanical s c a n n i n g of t h e sample o r t h e u s e o f a s m a l l a p e r t u r e . The most i n t e r e s - t i n g f e a t u r e of t h e s e t e c h n i q u e s i s t h e i r p o k e n t i a l a b i l i t y t o l o c a t e and i d e n t i f y s m a l l n o d u l e s o f e l e m e n t s b u r i e d d e e p l y ( s e v e r a l microns) i n a m a t r i x : t h i s may be v e r y u s e f u l f o r a n a l y s i s and q u a l i t y c o n t r o l o f semi-conductor d e v i c e s / g / . We a r e now c o n t i n u i n g i n t h i s d i r e c t i o n .

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