Mikrochim. Acta 125, 13-19 (1997) Mikrochimica Acta �9 Springer-Verlag 1997 Printed in Austria

Quantitative Chemical Phase Imaging by Means of Energy Filtering Transmission Electron Microscopy*

Werner Grogger, Ferdinand Hofer**, and Gerald KothMtner

Forschungsinstitut ffir Elektronenmikroskopie, Technische Universit~it Graz, Steyrergasse 17, A-8010 Graz, Austria

Abstract. Electron spectroscopic imaging (ESI) in the transmission electron microscope (TEM) is a powerful method to produce 2- dimensional elemental distribution maps. These maps show in a clear way the chemical situation of a small specimen region. In this work we used a Gatan Imaging Filter (GIF) attached to a 200 kV TEM to investigate a Ba-Nd-titanate ceramic. The three phases occuring in this material could be visualized using inner-shell ioniz- ation edges (Ba M45, Nd M45 and Ti L 23). We applied different image correlation techniques to the ESI elemental maps for direct visualiz- ation of the chemical phases. First we simply overlaid the elemental maps assigning each element one colour to form an RGB image. Secondly we used the technique of scatter diagrams to classify the different phases. Finally we quantified the elemental maps by divid- ing them and multiplying them by the appropriate inner-shell ioniz- ation cross-sections which gave atomic ratio images. By using these methods we could clearly identify and quantify the various phases in the Ba-Nd-titanate specimen.

Key words: analytical electron microscopy, electron spectroscopic imaging, atomic ratio images, Ba-Nd-titanate ceramic.

Nowadays electron energy loss spectrometry (EELS) is

a routine method in the transmission electron micro-

cope (TEM) [1]. Because of its spatially-resolved

nature EELS is the basis of energy filtered imaging

(energy filtering transmission electron microscopy =

EFTEM or electron spectroscopic imaging= ESI).

The recent years have brought some developments

which made this technique simpler to use and to be

applied both in biology ([2], [3]) and in materials

science (I-47, [5], [6]). Energy filtering devices for the

TEM have become commercially available: The Zeiss

EM 912 Omega TEM with an operational voltage of

120kV (1,7], 1,-8]) and the Gatan Imaging Filter (GIF) which can be attached to practically any 100-400kV

TEM ([9], [10]).

* Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday

** To whom correspondence should be addressed

With an energy filter some essential advantages for

TEM work become accessible: First the contrast

and resolution of TEM images and electron diffrac-

tion patterns can be improved appreciably by choosing

only elastically scattered electrons with the filter

(zero loss filtering). The inelastically scattered elec-

trons which are very troublesome for conventional

TEM imaging are thus eliminated; chromatic abber-

ations and delocalizations of the inelastic scattering

processes are minimized. Secondly any spectral feature

of the EEL-spectrum can be used for imaging (eg.

plasmons, ionization edges). The inner-shell ionization

edges in the EEL-spectrum are particularly useful be-

cause they contain information about the chemical

composition of the specimen. The ionization edges are

caused by inner-shell excitation of the atoms and the

characteristic onset energies can be used to identify the

chemical elements. Therefore energy filtered images

recorded at the energy of an ionization edge can be

used to derive two dimensional elemental distribution

maps.

In conventional TEM investigations one usually

acquires energy dispersive X-ray (EDX-) or EEL-

spectra from a small selected area (point analysis).

Thus, the chemical composition of a small specimen

region can be obtained quantitatively.

However, the advantage of imaging methods lies in

the simultaneous acquisition of 2-dimensional chemi-

cal composition. With ESI, energy filtered images are acquired which can then be combined to show the

distribution of the chemical elements in the specimen

with nanometre resolution [11]. Similar information

can be obtained using scanning techniques in the TEM

(scanning transmission electron microscope = STEM).

Nowadays field emission guns are widely available for

14 w. Grogger et al.

T E M / S T E M s a n d also d e d i c a t e d S T E M s wi th para l l e l

d e t e c t i o n systems. E l e m e n t a l m a p p i n g us ing E D X S in

the S T E M is a w i d e s p r e a d m e t h o d for the i n v e s t i g a t i o n

of e l e m e n t a l d i s t r ibu t ions . I n p rac t i ca l use h o w e v e r

the re are l im i t a t i ons for E D X S such as the very l o n g

acqu i s i t i on t imes (in the r ange of hours ) for o b t a i n i n g

e l e m e n t a l m a p s at h igh r e so lu t ion , and, h e n c e , p r o b -

lems wi th r a d i a t i o n d a m a g e a n d s p e c i m e n dr i f t will

result . Pa ra l l e l E E L S in c o m b i n a t i o n wi th a S T E M is

a n o t h e r poss ibi l i ty , wh ich can be c o n s i d e r e d a c o m p l e -

m e n t a r y m e t h o d to E S I us ing an i m a g i n g fi l ter ( [12] ,

[13], [-14]). T h e i n f o r m a t i o n of the w h o l e E E L - s p e c -

t r u m is r e c o r d e d for each pixel and can the re fo re be

used for ex t r ac t i ng deta i ls (eg. n e a r - e d g e fine s t ructure) .

But aga in one has to m a k e a c o m p r o m i s e b e t w e e n

la te ra l r e s o l u t i o n and a c q u i s i t i o n t ime.

In this p a p e r s o m e of the poss ib i l i t ies of E S I are

e luc ida t ed by the i n v e s t i g a t i o n of a h e t e r o g e n o u s ce-

r a m i c mate r ia l : Th i s m a t e r i a l consis ts of Ba- and N d -

t i t ana t e phases o f v a r y i n g c o m p o s i t i o n , w h i c h c o u l d be

ident i f ied and c h a r a c t e r i z e d by E E L S analys is p rev i -

ous ly [15]. N o w we s h o w h o w to c o m b i n e e l e m e n t a l

d i s t r i b u t i o n images to v isua l ize the c h e m i c a l phases in

the spec imen. W e app l i ed the sca t te r d i a g r a m tech-

n i q u e in c o m b i n a t i o n wi th m a n u a l c lus ter analys is to

the B a - N d - t i t a n a t e c e r amic to s epa ra t e a n d s h o w the

dif ferent phases of the spec imen. A d d i t i o n a l l y we c o m -

b ined the E S I e l e m e n t a l m a p s to give a quan t i f i ed

c o n c e n t r a t i o n i m a g e ( a tomic r a t io image) aga in show-

ing u n e q u i v o c a l l y the c h e m i c a l phases in the spec imen.

Experimental

The specimen was a well known Ba-Nd-titanate ceramic which has been characterized previously [15]. It was prepared using standard TEM preparation techniques with final ion milling. The investiga- tions were performed on a Philips CM20/STEM equipped with a GIF which is mounted beyond the microscope column. The microcope was operated with a LaB 6 cathode at a high voltage of 200 kV.

The images were recorded with a slow-scan CCD camera (1024 x 1024 pixel array) within the GIF. For recording TEM bright field images 1024 x 1024 pixels were used, whereas for reasons of sensitivity energy filtered images were acquired using a binning of 2 x 2 thus resulting in 512 x 512 pixels.

Image processing was performed using Gatan's Digital Micro- graph software running on a Macintosh Quadra 840 AV and the images were subsequently printed on a dye sublimation printer (Tektronix Phaser II SDX).

The calculation and processing of the scatter diagrams (2-dimen- sional histograms) were carried out under Digital Micrograph using its script language. A program was written to calculate scatter diagrams from elemental maps and also to traceback the scatter diagrams to phase images using manually marked areas (clusters).

Images and spectra recorded with the slow-scan CCD camera were corrected for dark current and gain variations. However the images were not corrected for the blurring which is caused by the point-spread function of the scintillator crystal. Since the acquisition times are rather long particularly for higher energy loss images (10-30 s) drift between successive images can occur. This drift was corrected with a cross-correlation algorithm (within Digital Micro- graph).

Since the cross-sections for inner-shell ionizations are very small we had to optimize the experimental conditions to obtain a maxi- mum signal-to-noise ratio (SNR). This was done according to the recommendations of Berger and Kohl [16]. Therefore we chose a large condenser diaphragm (100p.m) and the 40pm objective diaphragm thus giving an acceptance half angle of 7.6mrad. We calculated the window parameters (slit width and position of post- edge window) for each ionization edge in order to get the optimum SNR.

From previous work [16] it is known that the optimum defocus for an energy filtered image differs significantly from that for an elastic image. Accordingly the focus was adjusted in an energy filtered image (at ~ 120 eV energy loss) using the TV-rate camera of the GIF. Immediately after focussing the energy filtered images were acquired with the slow-scan CCD camera.

In order to obtain high intensities in the final images the electron beam (TEM spot size ~ 200 nm) was focused on the specimen region of interest and the emission current was run at a current density of about 6 A/cm 2. This is possible because the investigated material is not radiation sensitive.

To collect chemical information of distinct specimen areas EEL- and EDX-spectra were acquired. For the EDX-spectra we used an HPGe light element X-ray detector with an ultrathin window from Noran. The EEL-spectra were recorded using the GIF operated in spectrum mode (TEM image mode). The EEL-spectra were pro- cessed with Gatan's EL/P program.

Acquisition of Elemental Maps

The inner-shell ionization edges used for elemental mapping sit on top of an exponentially decreasing background (Fig. 1). As a conse- quence it is necessary for elemental mapping to remove the back- ground contribution for each individual pixeL Several methods have been proposed:

Elemental Maps (Three Window Technique)

A true background subtraction below an ionization edge may be achieved with the power-law model I = A.E-r where i is the inten- sity, E is the energy and A and r are two fitting parameters. Elemental maps are obtained by acquiring two energy-filtered background images in front of the edge (pre-edge 1 and pre-edge 2) and one image at the ionization edge (post-edge) of the element of interest. The typical widths of the energy windows lie between 5 and 50eV depending on the edge energy. Afterwards an extrapolated back- ground image is calculated by using the classical A.E -~ fit and subtracted from the ionization edge image thus giving a net image which can be considered as an elemental map (Fig. 1).

Jump Ratio Images (Two Window Technique)

If the element of interest is present only in low concentration the conventional three window method produces noisy images and sometimes unphysical results. An alternative method has therefore been proposed to "enhance" elemental maps by simply dividing the

Quantitative Chemical Phase Imaging 15

post-edge

"0

net intensity .~\~ extrapolated

background

" ~ 1 7 6 1 7 6 . . . . . . . . �9

energy

Fig. 1. EEL-spectrum (schematically) showing the positions of the energy windows needed for the calculation of elemental and jump ratio maps. The windows pre-edge 1 and pre-edge 2 are used for extrapolating the background which is then subtracted from the post-edge window to give an elemental map (net intensity). A jump ratio map is calculated by dividing the post-edge window by pre- edge 2

image at the ionization edge (post-edge) by a pre-edge image (Fig. 1 [17], [18]). This method produces maps with minimum added noise and is not greatly affected by many of the artifacts arising in elemental maps of crystalline materials. Although the contrast of

jump ratio maps is often quite similar to that of elemental maps, it should be considered that jump ratio maps are not always "true" elemental maps and should be used carefully [6].

Results and Discussion

Figure 2a shows a T E M br ight field image of the

Ba -Nd- t i t ana t e ceramic. By s imply look ing at the mor -

pho logy of the specimen one can dis t inguish between

three different gra in types: F i r s t crystals of r o u n d shape

can be found (e.g. 1) and some which are more ob long

in shape (e.g. 2). One can also see a gra in b o u n d a r y

phase between the crystals (e.g. 3). As k n o w n from the

previous invest igat ion, region 1 co r responds to Ba -Nd-

t i tanate , and region 2 to a Nd- t i t ana te . Region 3 turns

out to be an amorphous , Ba-r ich phase of sl ightly

vary ing compos i t ion . This can also be deduced look ing

at the EDX- and EEL-spec t r a show in Fig. 3.

The i m p o r t a n t e lements in this specimen are Nd, Ti

and Ba. Consequent ly , we acqui red e lemental maps of

these elements using the three window technique (Fig.

2b, c and d). F igure 2b shows the d i s t r ibu t ion of Nd:

The Ba -Nd- t i t ana t e and the N d - t i t a n a t e have a lmos t

the same mean greylevel (1000 and 1300 respectively),

Fig. 2. TEM bright field image and elemental maps (Nd, Ti and Ba) of a Ba-Nd-titanate ceramic: a TEM bright field image, zero loss filtered (slit width: 20eV, exposure time: 1 s), b elemental map of Nd using the Nd-M4s edge (prel: 865eV, pre2: 925eV, post: 999eV, slit width: 40eV, exposure time: 20s), c elemental map of Ti using the Ti-L23 edge (prel: 393eV, pre2:437 eV, post: 466 eV, slit width: 16 eV, expo- sure time: 20 s), d elemental map of Ba using the Ba-M45 edge (prel: 675 eV, pre2: 745eV, post: 799 eV, slit width: 30 eV, exposure time: 20 s)

16 W. Orogger et al.

3.0

, ~ 2,5

2,0 %

. ~ 1.5

0.5

0,0

Ba-k= + Ti-K=

O-K

J A Nd~

j !i~'%' ~-~-, . . . . . . . ,~.~ . . . . . .,,.:~

il

i'L;?

., : :: . . . . , . . . . . . . .:'".. i ": ,i'.. " ' " 1 ' " " ' " " " ' 1 ' " ' , , " ' " " t " " . . . . . . . . . . . . r-- ' " "" i" - , " % r ' " l , r . . . . . . . .

1 2 3 4 5 6

energy [ke V]

9O

50 '

,.~ X_

" ~ 20

0 400

TL" L~3 Ba -Nd - t i t ana te (1) Nd-titanate (2)

. . . . . . Ba-rich phase (3)

O-K

�9 -, t l . j -_ , . . .- :;

.......... ~';: .....

L I I I 500 600 700 800 90(3 1000 1100

energy [e V]

Fig. 3. EDX- (top) and EEL-spectra (bottom) of the Ba-Nd-titanate ceramic. 1, 2 and 3 correspond to the regions marked in Fig. 2a. For comparison the spectra were shifted along the intensity axis

but the Ba-rich phase appears dark (560)�9 A similar statement can be deduced from the Ti elemental map (Fig. 2c): The Ba-Nd-titanate and the Nd-titanate have nearly the same intensity (mean greylevels: 3000 and 2400) and the Ba-rich phase again appears dark (1300). However all three phases can be clearly separated by looking at the Ba elemental map (Fig. 2d): the bright areas (800) correspond to the Ba-rich phase, the Ba- Nd-titanate appears grey (450) and the Nd-titanate remains dark (mean greylevel: 40).

Consequently these maps show the distribution of the three phases: The Ba-Nd-titanate appears bright in the Nd and Ti elemental maps and grey in the Ba map. Brightness in the Nd and Ti elemental maps and darkness in the Ba elemental map characterize the Nd-titanate. The amorphous phase with a big amount of Ba appears bright in the Ba elemental map and dark in the two other maps. Using and combining the information in these maps the three phases in the

specimen can unambiguously be identified and their distribution shown�9

However the greylevels of the various crystals be- longing to one particular phase differ quite a lot (Fig. 2b, c and d). This artifact is caused by the different orientation of thie crystals: this diffraction contrast to a certain extent remains in the elemental maps. There- fore we suggested to use jump ratio maps where the diffraction effects almost completely cancel out. How- ever artifacts may appear in jump ratio maps as well: e.g. thickness effects may cause unwanted contrast in the images [6]. As we wanted to quantify the maps - this is only possible using elemental maps -we did not use any jump ratio maps in this study.

To visualize the distribution of the phases even more clearly the elemental maps can be combined in an RGB image�9 There each elemental map (Ba, Ti, Nd) is assig- ned to a colour: red, green and blue. The result of superimposing the images is shown in Fig. 4a. The information is compressed into one image without any loss and the relevant phases can be seen more easily: the Ba-Nd-titanate appears greenish, the Nd-titanate blue and the amorphous phase is red.

Overlaying images to form an RGB image is a rather simple method of combining elemental maps. However in some cases the RGB images might not be convenient eg. when more than three elements have to be consider- ed. Sometimes the resulting mixed colours may also lead to misinterpretation. And again diffraction effects may confuse the viewer: As can be seen in Fig. 4a, not all the Nd-titanate crystals show the same shade of blue although they belong to the same chemical phase. Therefore other disciplines (e.g. scanning Auger spec- troscopy and imaging secondary ion mass spec- trometry) use more sophisticated methods such as scatter diagrams with subsequent cluster analysis sometimes in combination with principal component analysis (e.g. [19-22]). Since these techniques are well known we applied them to elemental maps obtained by ESI.

For this method scatter diagrams have to be cal- culated which can be considered as two-dimensional or bivariate histograms ([23, 24]). Each pixel (x, y) in image A is associated with the corresponding pixel in image B. The greylevels in both images a (x, y) and b (x, y) are accumulated in the scatter diagram at (a, b). The scatter diagram usually consists of a 256 x 256 pixel array (image), where the abscissa corresponds to the greylevels in image A and the ordinate corresponds to the greylevels of image B. The greylevels on their part

Quantitative Chemical Phase Imaging 17

Fig. 4. a RGB image created by superimposing the elemental maps of Ba (red), Ti (green) and Nd (blue), b quantified composi- tional map (atomic ratio map) showing the Ba/Nd concentration ratio, e scatter diagram calculated using the elemental maps of Ba and Nd, d phase image recal- culated from e using the marked areas: bright green (cluster 1): Ba- Nd-titanate, blue (cluster 2): Nd- titanate, red (cluster 3): Ba-rich phase

correspond to concentrations of the elements A and B respectively. Clusters appearing in the scatter dia- grams may correspond to particular phases. After cal- culation of a scatter diagram the clusters can be traced back and coloured in the original images.

Using the elemental maps in Fig. 2b, c and d we calculated scatter diagrams. As an example the scatter diagram of Ba and Nd is shown in Fig. 4c. The cluster in the centre of the scatter diagram (l) corresponds to the Ba-Nd-titanate. There we find the highest intensity in the scatter diagram because this phase covers most of the investigated specimen region. The points in this cluster lie along a straight line with the slope of this line being directly proportional to the concentration ratio of Ba and Nd within the Ba-Nd-titanate phase. A sec- ond cluster (2) can be found at lower Ba greylevels (left side of scatter diagram); this cluster can be ascribed to the Nd-titanate crystals. Again the points lie along a line at intensity (Ba) ~ 0 indicating a Ba/Nd concen- tration ratio near 0. The Ba-rich phase can be found in

the top, right corner of the scatter diagram (high Ba intensity, low Nd intensity). Because this phase covers only about 3% of the whole investigated region the intensity in the scatter diagram is only weak. As the concentration ratio Ba/Nd varies within the amor- phous phase the corresponding points in the scatter diagram are spread towards cluster (1). The bright spot in the top, left corner corresponds to a hole in the specimen and gives approximately the origin of the scatter diagram. If the three clusters are traced back into the original images the three phases can be shown: Cluster 1 corresponding to Ba-Nd-titanate was col- oured bright green, cluster 2 (Nd-titanate) blue and cluster 3 (Ba-rich phase) red. Superimposing the phase images the three phases can be visualized (Fig. 4d). This figure shows the distribution of the different chemical phases in the specimen. However a problem may occur there as well: The Ba-rich phase (red) in Fig. 4d is always surrounded by a green zone (Ba-Nd-titanate) although this phase mostly touches directly the Nd-

18 w. Grogger et al.

titanate (blue) as can be seen in Fig. 4a. This artifact arises because the interface between the Barich phase

and the Nd-ti tanate does not lie perpendicular to the

specimen surface. Consequently the resulting signals are a mixture between these two phases giving a con-

centration ratio of Ba/Nd that goes continuously from

a high value within the Ba-rich phase to zero in the Nd-titanate.

To gain a better understanding of the concentration of the elements we also use an alternative approach

which is primarily based on the EELS k-factor method

for the quantification of EEL-spectra ([25], [26]).

Usually quantification of an EEL-spectrum is per- formed due to the following relationship

CA O'B I A = k "IA

where c: concentration (at%), a: ionization cross-sec-

tion, I: net intensity under ionization edge and kAB:

k-factor of the elements A and B respectively. The

k-factor is the cross-section ratio which can be cal- culated but also determined experimentally using stan-

dards. The quantification formula shows the direct relationship between intensity ratio and concentration

ratio. For quantification of elemental maps we divided

two images and multiplied the result by a suitable k-factor. As k-factors are usually measured for energy

windows of 50 or 100eV [-27] we had to recalculate

them using the correct energy windows (16, 30 and 40eV for Ti-L23, Ba-M45 and Nd-M4s, respectively) and window positions. This was performed using EEL

reference spectra. The resulting atomic ratio image of Ba and Nd is shown in Fig. 4b. This image has grey-

levels of 0.03 • 0.04 (dark areas), 0.35 • 0.07 (grey areas) and 1.2 • 0.4 (bright areas) for the Nd-titanate,

Ba-Nd-titanate and the Ba-rich phase respectively

(mean _+ standard deviation). These concentration ra- tios agree very well with that previously determined using point analysis 1-15]: 0, 0.355 and 1.08. Although

this method has not been used very frequently in the past (only Bonnet et al. [28]) the division of elemental maps used for quantification offers the additional ad- vantage that diffraction artifacts cancel out to a certain extent (compare Fig. 4b to Fig. 4a with a view to the variation in greylevel/colour within the three phases).

Conclusion

We investigated a Ba-Nd-titanate ceramic using en- ergy filtering TEM. The specimen consists of three

different phases (Nd-titanate, Ba-Nd-titanate and an

amorphous Ba-rich phase) which were found by previ- ous EELS point analysis. However it would have been

very time consuming to get the phase distribution by

using just point analysis. By acquiring elemental maps of Nd, Ti and Ba we could confirm the findings of the

point analysis for a 2.8 x 2.8 gm 2 specimen area within some minutes. For the visualization of the three phases

we used different methods: First we superimposed the

elemental maps to form an RGB image which is a rather simple process and in many cases gives good

results. Secondly we applied the technique of scatter

diagrams to elemental maps, marked clusters in the scatter diagrams manually and could show the differ-

ent phases by tracing back the clusters, Finally we quantified the elemental maps using the EELS k-factor

method. The results of the quantification agree very

well with previous EELS point analysis.

Acknowledgements: We would like to thank Peter Warbichler for the preparation of the Ba-Nd-titanate ceramic specimen and we grate- fully acknowledge financial support by the ForschungsfSrderun- gsfonds ffir die gewerbliche Wirtschaft, Vienna, Austria and by the Steierm~irkische Landesregierung, Graz, Austria.

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Quantitative Chemical Phase Imaging 19

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