EDS and EELS using a TEM-FEG microscope

ultramicroscopy ELSEVIER

Ultramicroscopy58 (1995) 35-41

EDS and EELS using a TEM-FEG microscope P. S t a d e l m a n n a, K. Leifer a, C. V e r d o n b a lnstitut Interd~partemental de Microscopie Electronique b Institut de G£nie Atomique, Ecole Polytechnique Fdddrale de Lausanne, CH-1015 Lausanne, Switzerland

Received 10 January 1994; at the Editorial Office 20 September 1994

Abstract

The combination of energy-dispersive X-ray spectroscopy, electron energy-loss spectroscopy and high-resolution electron microscopy provides a very powerful approach to characterizing advanced materials, EDS, EELS and HREM capabilities of an Hitachi HF-2000 FEG fitted with a Noran High-Purity Ge Explorer EDS system, a Gatan Model 666 PEELS and a Gatan Model 694 Retractable Slow-Scan CCD camera have been employed to study the microstructure at the nanometer scale of several materials such as WC-Co thermal-sprayed coatings and Ni/Ti multilayers. HVOF (high-velocity oxy-fuel) thermal-sprayed coatings display a microstructure which is very different from the starting powder due to complex physical and chemical modifications that occur during the spraying process. In particular, the matrix of the coating is no longer only composed of Co, but also of W and C. The simultaneous use of EDS and EELS spectroscopy allowed the quantification of these three elements in the matrix and the characterization of their distribution in the microstructure. The performance of Ni/Ti multilayers for neutron optics can be significantly improved by modifying the layer structure. A systematic EDS, EELS and HREM study of the structure and composition of thin Ni and Ti films with progressive addition of carbon and hydrogen has been made. It is shown that gradual reorientation and smaller grain size in the Ni layers lead to better defined interfaces and that hydrogen incorporation into the Ti layers improves the neutron reflectivity.

I. Introduction

The new FEG-TEMs, that are starting to be used in many Material Science Departments, allow accurate characterization of the nanometerscaled microstructure very often observed in industrial materials. These instruments combine high current density, very small probe size, EDS spectroscopy (140 eV F W H M at 5.6 keV), EELS spectroscopy (0.45 eV F W H M at 200 keV), slowscan CCD camera, . . . , with the ease of use of conventional TEM. All those features added to the excellent high-resolution imaging capability

make the new F E G - T E M ideal for material scientists when the problem of specimen preparation is not a limiting factor. This paper presents two examples of the use of a HF-2000 FEGTEM, where the combination of high-resolution electron microscopy (HREM), energy-dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS) on the same transmission on electron microscope with the ease of use of standard T E M brings new capabilities to investigate nanostructured materials. Since its installation this T E M has been used to characterize several different systems, among them H V O F

0304-3991/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0304-3991(94)00176-6

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P. Stadelmann et al. / Ultramicroscopy 58 (1995) 35-41

(high-velocity oxy-fuel) coatings and N i / T i multilayers.

2. HVOF thermal-sprayed coatings H V O F thermal-sprayed coatings of 88wt% W C - 1 2 w t % C o powder play an important role as a protective thin film that reduces significantly the formation of microcavities in water turbine blades. The initial powder is made of W C grains of diameter in the micrometer range embedded in a Co matrix. After the H V O F thermal-spraying process the microstructure of the coating deposited on the blades is a "pile-up" o; the initial powder grains that have been more or less transformed by the oxygen flame (Fig. 1). F u r t h e r m o r e

the microstructure is heterogeneous at a scale related to that of the initial powder. It has been shown that during the deposition process several percent of the tungsten carbides are dissolved in the initial Co matrix and form a new nanocrystalline phase (Fig. 4b) that constitutes the new binding phase. The new binding phase occupies about 60% volume of the coating and its mechanical properties are of uppermost importance for the impact resistance of the coating [1]. As the new binding matrix is composed of W, Co and C, the use of both X-ray energy-dispersive spectroscopy and electron energy loss spectroscopy allowed us to quantify W versus Co (by EDS) and C versus Co (by EELS). Though our EDS system is able, despite its " N o r v a r " window, to count X-rays emitted by C atoms, the PEELS

Fig. 1. HVOF coating as deposited on water turbine blade (SEM image). Powder grain size is of the order of 100/zm and is made of fine WC grains (about 1/zm) embedded in pure Co matrix. The deposited powder grains are separated by binding phase formed during HVOF deposition that contains W, Co and C.

P. Stadelmann et al. / Ultramicroscopy 58 (1995) 35-41

system has a much higher carbon detection efficiency and hence has been preferred to quantify C versus Co. The very weak W N45 and W N23 edges (Fig. 2) that appear as shoulders at 245 and 425 eV, respectively, could not be used to perform quantitative chemical analysis. The quantification of the EDS spectra was performed using the standard Cliff-Lorimer scheme while the quantification of the EELS spectra, done using the E L / P program by Gatan, was complicated by the following facts: (i) the specimens were rather thick even very close to the edge (as a matter of fact the thinning rate is much lower than that of the Co matrix); (ii) a strong contamination was initially observed despite that ion milling was conducted at liquid-nitrogen temperature; (iii) the carbon quantification is complicated by the overlap of the C K edge and the W N45 shoulder. These difficulties were worked around in the following manner: (i) the zero-loss peak was always recorded and the experimental spectra showing a t/A ratio (the ratio of the thickness and the mean free path for inelastic events) larger than 0.9 rejected;

• 0

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EELS spectrum

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lo'oo '~2bo

Energy Loss {eV) Fig. 2. Typical EELS spectrum acquired using a Gatan PEELS spectrometer and showing the WN45 shoulder situated just below the C K edge. The EELS analysis allows the quantification of the C / C o ratio of the binding phase.

37

3.s 3

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w ...............

i i i

I

l .......................... ~ ~i

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1.5

2 ratio

2.5

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Fig. 3. C / C o ratio versus W / C o ratio (measured by EDS). The upper line marks the limit of the WC phase.

(ii) the specimen was systematically mounted on a Gatan double-tilt LN2-cooled specimen holder and cooled to liquid-nitrogen temperature (reading of the thermometer was 100

K); (iii) the W N45 edge appears as a slope change (at 245 eV) of the background in front of the C K edge and makes the removal of the background under the C K edge difficult. In order to overcome this difficulty the background was fitted before the C K edge in a window of 20 eV placed just in front of it (energy 260-280 eV). This procedure introduces a carbon quantification error of 20 at%. Fig. 3 shows the correlation between the C / C o and W / C o atomic ratios in the nanocrystalline matrix of the deposited coating obtained using more than 50 EDS and EELS combined spectra. It clearly demonstrates that the HVOF process is responsible for a partial fusion of WC grains and the formation of a binding matrix that contains a significant concentration of W and C together with a few at% of oxygen (Fig. 5). The study of the interfaces between parts of the coating originating from different initial powder grains shows the existence of an amorphous layer of thickness in the range 5-50 nm (Figs. 4a and 4b). This layer which is formed during the projection of the transformed powder grains was proved to be the mechanically weakest part of the coating and does very often contain microcracks

38

P. Stadelmann et al. / Ultramicroscopy 58 (1995) 35-41

Fig. 4. (a) Binding phase (TEM bright-field image): an amorphous layer (2) rich in oxygen separates the two powder grains (1 and 3). It has a thickness between 5 and 50 nm and could be responsible for the fragility of the HVOF coating. (b) Binding phase (TEM dark-field image) is made of very fine crystalline domains of size of the order of 1 nm.

P. Stadelmann et al. / Ultramicroscopy 58 (1995) 35-41

Co_L3 "

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Energy Loss (eV)

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400

660

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Fig. 5. EELS spectrum of the amorphous (point 2 of Fig. 4a) layer of figure 4, showing an important OK edge.

Fig. 6. EELS spectrum of the binding phase (point 1 of Fig. 4b) showing the typical O K edge observed in this phase.

[1]. EDS and EELS chemical analysis has also shown that this amorphous layer does not only contain W / C o and C but also a large percentage

of oxygen (Fig. 6). Table 1 shows the result of the analysis at points 1, 2 and 3 of Fig. 4. Analysis point 2 shows C / C o and O / C o ratios 10 times larger than in the Co-based binding matrix.

Fig. 7. Ni/Ti multilayers deposited on (111) Si wafer, single layer thickness 20 nm.

40

P. Stadelmann et aL / Ultramicroscopy 58 (1995) 35-41

Table 1 Relative compositional analysis from regions indicated on Fig. 4 Point number

W/Co

C/Co

O/Co

1 2 3

2.4 3.3 2.6

0.93 10.5 1.1

0.29 5.1 0.45

3. Ni / Ti multUayers Ni/Ti multilayers used as optical components for neutron beams (monochromators, neutron guides, filters and polarizers [2,3]), were studied by HRTEM, EDS and EELS using the HF-2000 FEG-TEM. The layers, single layer thickness 20 nm, were deposited by DC magnetron sputtering on the native oxygen film that covered a (111) Si wafer (Fig. 7). The TEM specimens were prepared by conventional cross-section techniques and the final thinning performed by Ar ion bombardment at liquid-nitrogen temperature (energy 6 keV, angle of incidence 8°). During the TEM experiments the specimens were mounted on a Gatan LN2-cooled specimen holder and cooled to liquid-nitrogen temperature in order to avoid contamination and to improve the measurement of the specimen thickness by EELS. A major problem of the EDS and EELS chemical analysis came from the rough surface that resulted from the ion milling. In order to better quantify the impurity content of the Ni and Ti layers, the specimen thickness in the electron beam direction was measured carefully by the EDS count rate method [4] and by EELS [5]. Figs. 8a and 8b show thickness profiles of the Ni/Ti film relative to the first Ni layer using the EDS count rate method for two different condensor 1 apertures (diameter 100 and 50/zm, respectively). The probe of 4 nm diameter (5 times smaller than the single layer thickness) was placed in the middle of the layers. The thickness ratio of the different materials (layers) was evaluated using the following formula [4]: Ni

I(Ti) ~ A x ( N i ) , Ax(Ti) =K(Xi, Ni) I(Ni) Ptot

(1)

with K(Ni, Ti) the Cliff-Lorimer K factor (measured using a NiTi alloy), I the peak intensity, p the film density, and Ax the film thickness in the electron beam direction. Fig. 8c shows the result of PEELS thickness measurements using the ratio of the intensities of the zero-loss peak and the inelastically scattered electrons [4]. Though the slope of curve 8c is steeper than that of curves 8a and 8b the decrease in thickness of the third Ti layer is well reproduced. This is a strong indication of preferential etching of the Ti layers during ion bombardment. This preferential etching has also been observed in AFM images of a bombarded Ni/Ti supermirror. The differences of the slopes might be due to supplementary inelastic scattering effects (i.e. secondary plasmons), when the layer thickness approaches the inelastic mean free path of the single plasmon scattering. Though preferential etching and instrumental background are real problems for accurate chemical analysis of the Ni/Ti multilayers, the specimen drift that practically always occurs at liquidnitrogen temperature, introduces further uncertainties. Figs. 9a and 9b show the Ti concentration in the Ni layers and the Ni concentration in the Ti layers measured using two different condenser 1 apertures (diameter 100 and 50 /zm, respectively). Using a 100/zm condenser 1 aperture the probe diameter was smaller than 3 nm and the concentration of Ni in Ti layers or Ti in Ni layers measured by EDS was in the range of 3-5 at%. Using the 50 /zm aperture, but a 4 times longer acquisition time (200 s), these con-

3-

.................................................................................

2.5-: 2-

EDS ap2......I

EDS ap3 [. . . . . . . . . . . . . . .

Fig. 8. Film thickness of several Ni and Ti layers measured by EDS and EELS. Two condensor 1 apertures (100 and 50/zm) where used during the EDS measurements.

P. Stadelmann et al. / Ultramicroscopy 58 (1995) 35-41

10At

i!i!c oct

~" 6AtO/ot....EDS..ap2 ~

PEELS

Fig. 9. Impurity concentration (Ti in Ni and Ni in Ti) measured by EDS and EELS. centrations increases to the range 5 - 9 at%. This is a strong indication that specimen drift starts to introduce supplementary errors in the EDS analysis. The EELS quantification gave always smaller concentrations of Ni in Ti and Ti in Ni (Fig. 9c). The error of the EDS measurements is purely statistical, as the accuracy of the K-factor for the N i / T i system is better than 1% ( N i / T i alloy, measured by EDS Ni49Ti51). The error of the PEELS measurements is due mainly to the background subtraction and to quantification intervals. We believe that the impurity concentration is overestimated by EDS due to high-angle scattering, which becomes important in thicker layers. Excluding the later from the EELS analysis an impurity concentration between 0.5 and 1.5 at% was finally measured.

4. Conclusion The combination on the very same T E M instrument of high current density, very small probe size, E D S spectroscopy, E E L S spectroscopy and slow-scan CCD camera with excellent high-resolution imaging capability opens new opportunities for the characterization of advanced materials. The studies of the structure of H V O F thermalsprayed coatings and N i / T i multilayers have

41

demonstrated that it is now possible to fully characterize these materials at a nano-scale level. The major remaining problems are the specimen preparation during which any source of possible contamination has to be controlled and the local m e a s u r e m e n t of the specimen thickness. As a matter of fact, chemical measurements become very sensitive to local film thickness variation and an accurate knowledge of the specimen thickness at the probe position is of uppermost importance. The preparation of clean specimens suitable for the analysis in a F E G - T E M is much more difficult and the use of a LNz-cooled specimen holder able to reach at least - 100°C is of prime importance when reproducibility of the measurements is required. Specimen drift as low as 1 n m / m i n has to be achieved when EDS analysis using a probe size of the order of a few nanometers has to be performed. Such a low drift rate is only achieved 2 h after the introduction of the LN zcooled specimen holder into the microscope. Furthermore monitoring the residual H 2 0 pressure in the specimen chamber becomes of crucial importance when it is necessary to avoid the formation of an icy layer on the specimen area that has to be analyzed.

References [1] C. Verdon, in preparation. [2] P. B6ni, I.S. Anderson, R. Hauert, P. Ruterana, K. Solt, B. Farnoux, G.J. Herdman, J. Penfold and O. Schaerpf, in: Proc. llth Meeting of the International Collaboration on Advanced Neutrons Sources, Tsukuba, 1990. [3] O. Elsenhans, P. B6ni, H.P. Friedli, H. Grimmer, P. Buffat, K. Leifer and I. Anderson, in: Thin Films for Neutron Optics, SPIE's 1992 International Symposiumon Optical Applied Science and Engineering, San-Diego, 1924 July 1992. [4] N.J. Zaluzec, in: Introduction to Analytical TEM, Eds. J.J. Hren, J.I. Goldstein and D.C. Joy (Plenum, New York, 1979). [5] R.F. Egerton, Electron Energy Loss Spectroscopy in the Electron Microscope (Plenum, New York, 1986).