A COMPACT X-RAY SPECTROMETER WITH MULTI-CAPILLARY X-RAY LENS AND FLAT CRYSTALS

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1 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol A COMPACT X-RAY SPECTROMETER WITH MULTI-CAPILLARY X-RAY LENS AND FLAT CRYSTALS Hiroyoshi SOEJIMA and T&hi NARUSAWA* I Nishinokyo-Kuwabaracho, Shimadzu Scientific Research, Nakagyou-ku, Kyoto JAPAN *Department of Electronics and Photonics System Engineering, Kochi University of Technology, Tosa-Yamada, Kochi JAPAN ABSTRACT We present a new type of wavelength dispersive x-ray spectrometer with a multi-capillary x-ray lens. This spectrometer has a simple and compact construction with good performance for small area analysis. A multi-capillary x-ray lens, a device that collects fluorescent x-rays from a selected sample area of diameter 0.1 mm or less and collimates them into a nearly-parallel beam form, was combined with conventional flat crystals. The flat crystal and an x-ray detector were set on a 8-B simple goniometer. The result of [Cu K-emission source / LiF flat crystal / Fe Ka line from stainless sheet sample] is as follows: the Fe KQ! intensity from a,o.l mm area is 130 times higher than a conventional XKF spectrometer and the intensity from a 0.05 mm is 270 times higher. The FWHM of the FeKd line is A. These results indicate solid feasibility of a very small, simple-construction, easy-to-handle and high-sensitivity spectrometer. In addition, this spectrometer has several merits; 1)the number of simultaneously detectable elements is increased, 2) the influence of the surface roughness on measurements is very small because all elements are detected under the same conditions with respect to the x-ray take off angle, 3) it should be easy to adapt the spectrometer to PXKF, EPMA and SEM. 1. INTRODUCTION A new type of wavelength dispersive x-ray spectrometer (WDX) is developed. This spectrometer is consisted of a multi-capillary (or monolithic poly capillary) x-ray lens (a product of XOS Inc.) and a flat crystal, having a simple and compact construction with practically good enough performance for small area analyses Before the main topic in this paper, we look back to some historical aspects of capillary lens development. In 1986 one of the authors (H.Soejima) /l/ applied a Japanese patent as shown in Fig. 1 (a), where a new idea of bending and focusing of x-rays by utilizing the total reflection phenomena by smooth inner wall surfaces of fine capillaries was clearly described. Soon after this first patent, Soejima has applied another Japanese patent as shown in Fig.l(b) /2/, where he has given several ideas to actually fabricate such a bunch of capillaries, however, to our best knowledge, only a few people in Fig.1 Old Japanese patents where new ideas to use fine capillaries for collimation and/or focussing of x-rays are given. (a) in 1986 and (b) in (4

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website -

3 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Japan tried to materialize them, resulting in a less sufficient success to attract many people s attention. Although Soejima s patents are effective through 2006, in the mean time, a new company named X-ray Optical Systems (XOS) Inc. came out in US in 1991 with quite a similar idea but with much more sophisticated fabrication technologies, and has already commercialized various kinds of x-my and neutron optics. These optics prevail worldwide and new applications of x-ray fluorescence (XRF) and diffraction (XRD) are now being developed 13-Y. Fig.2 An example of capillary optics applications in XRF. As an example of applications of capillary optics, we have demonstrated a dual-optic XRF system a couple years ago /6/. As shown in Fig.2, a focusing optic and a collimating optic were arranged so that both optics focus on one point within a sample, thus giving a certain depth resolution to XRF which used to be acknowledged to have no depth resolution. Alignment of both optics was not an easy task, but when aligned best, the system showed a depth resolution of several tens of mm and it was useful for analyses of bonding and packaging problems of Si-ICs. Flat Crystal Curved Cry&I Selected Area Pipe 60 %enter 09 of Rowland CircI Fig.3 Conventional WDX arrangements. (a): Flat crystals with Soller slits, and (b): Curved crystals with complicated detector driving mechanisms. Conventional arrangements of WDX include either flat or curved crystals as shown in Fig.3(a) and (b). Figure 3(a) is the most common arrangement, where the fluorescent x-rays are first collimated by one Soller slits, and analyzed by a flat crystal, and then again collimated by another Soller slits before being counted by a detector such as a scintillation counter. This is of course because of getting enough wavelength resolution. When we need to restrict the analysis sample area in this arrangement, an area selecting pipe or aperture must be put just in front of the sample surface as shown in the figure. Consequently the total system results in a rather complex configuration. The situation is even more complex in Fig.3(b). This kind of spectrometers is mainly used to detect electron beam excited x-rays as Electron Probe Micro Analyzer (EPMA). In this case, the analysis sample area can be made very small by focusing the primary electron beam size. However, in order to collect sufficiently strong x-ray intensity, or in other words, in order to increase the sensitivity of analysis, spherically curved crystals are often used. One important point here to obtain quantitatively meaningful spectra is that, we need to move the crystal linearly with respect to the sample to keep the so-called take-off angle of x-rays at a

4 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol fixed value, otherwise the roughness or artificial structures on the sample surface affects the resultant analysis data. This goal is achieved by extremely complex mechanical driving system of the detector. Details of such mechanics and control- ling systems are beyond the scope of present paper, however, if we add some other words about this spectrometer, it is an established spectrometer and still holds the top ranking with regard to the wavelength resolution. 2. EXPERIMENTAL PROCEDURES Figure 4 illustrates our arrangement to test the feasibility of a combination of the multi-capillary x-ray (MCX) optic and a flat LiF crystal as a WDX spectrometer. As clearly shown in the figure, the basic construction is quite simple, i.e. one collimating optic plus a flat crystal and a detector mounted on a 6-20 goniometer, that s it and nothing else. The primary x-ray from Cu-target x-ray tube (40 kv, 40 ma max.) hits the rear surface of the stainless steel foil sample. The fluorescent x-rays are collected by the MCX optic which is misoriented by 25 from the incidence direction to avoid the transmitted primary x-ray directly hitting the optic. The MCX collimator has a focal length of -8 mm and its focal point size is -0.1 mm. Thin Film Sample Fig.4 Brand new simple arrangement for WDX. The insertion shows details around the test sample The insertion in Fig.4 shows details around the sample. The thickness of SS foil was carefully chosen to be 10 pm so that forward emitting fluorescent x-rays intensity should be maximized. The foil sample is pasted on a thick SS plate that has a small aperture in its central region, and its aperture size is varied to limit the fluorescence emitting area. In the present feasibility test, since we have used a commercial XRD goniometer and an x-ray source, the thin foil sample is excited from its rear surface as shown in Fig.4. When we analyze realistic samples such as bulk samples or thick films, we of course put the x-ray source in the direction indicated by the broken line and arrow in Fig.4. Measurements of fluorescence spectra were done simply by rotating the LiF flat crystal and scintillation detector with respect to the sample. Results of measurements were compared between the arrangement of Fig.3(a) and Fig RESULTS AND DISCUSSION Typical spectrum obtained in the arrangement of Fig.4 is shown in Fig.5 where the detector count rates (cps) are plotted against the detector angle (28). The prominent peak at corresponds to FeKcr line, and its line width (FWHM) is This line width corresponds to the wave- length width of , at A, or energy-wise -40 ev at 6.4 kev. This resolution is not surprisingly excellent but good enough for practical use. Clear evidence of usefulness is given below as a computer simulation of the spectral region around Assuming above-mentioned resolution and sufficient accumulation of data, we calculated the wavelength region of Crwand MnKo lines, which is sometimes referred to the resolution check. The result of calculation is shown in Fig.6 indicating fairly well separation of both peaks. If the original spectrum looks like in Fig.6, it would be easy to deconvolute each peak from another. This result provides certain evidence that the present arrangement is practically useful enough for most XRF applications in spite of its extreme compactness.

5 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol _i gl Fig.5 Typical spectrum from stainless steel foil sample. Note fairly good resolution in spite of the simple arrangement of spectrometer. Fig.6 Computer calculation of spectrum in wavelength region of CrK,8 and MnKc lines at wavelength resolution of Fig5 Separation of both peaks is fairly well and practically useful. Arialyzed Area (@mm) Existing Flat Crystal Spectrometer MCX Spectrometer (cps) Table 1 Changes of count rates with decreasing analyzed area, comparison between existing flat crystal spectrometer and MCX spectrometer. Note drastic contrast particularly when the analyzed area is 0.1 mm dia. or less. At last and most important, Table 1 shows the intensity comparison between the arrangements of Fig.3 (a) and Fig.4. In existing XRF spectrometer such as in Fig.3 (a), the resolution is normally made variable. We can choose better or worse resolution modes, but for the sake of comparison, we fixed the resolution of existing spectrometer to be When the aperture size or the analyzed area is decreased from 1.O mm dia. to 0.05 mm dia. step by step, the count rates of existing spectrometer naturally decreases in proportion to the analyzed area. On the other hand, the MCX spectrometer gives roughly equal count rates down to 0.1 mm dia. though there is some statistical fluctuation, and then the count rate decreases significantly at 0.05 mm dia.. This result agrees with the fact that the focal point size of our MCX collimator optic is -0.1 mm, even in case the aperture size is larger than -0.1 mm our collimator collects fluorescent x-rays originating from within this area. These results clearly indicate the area-selective sensitivity and the effective collimation of fluorescence by the MCX optic. 4. SUMMARY AND ACKNOWLEDGEMENT We have shown an effective use of MCX optics for wavelength dispersive x-ray spectra-meter. The spectral resolution is , for FeKa line, and its intensity is higher than conventional spectrometers. Particularly when the sample size is -0.1 mm dia. or less the enhancement is more than two orders. These results are evidence that a practically useful WDX, that is extremely compact and easy-to handle, is surely feasible. Although the present WDX spectrometer is not very

6 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol compact since we use a conventional diffraction goniometer, a compact goniometer of -10 cm size, that is within easy reach of our hands, would really shrink the total system. In case a very compact XRF system is realized as described in this paper, there is no doubt that great merits will be brought about in small area XRF analyses by p-xrf, EPMA and SEM. Also its sensitivity to small area will make possible to analyze very subtle region of solids such as, for instance, semiconductor-metal interfaces. One of the authors (T.N.) acknowledges that this work is partly supported by Japanese Grant-in- Aid for Scientific Research (B). REFERENCES /l/ H.Soejima; Japan Patent (1986). /2/ H.Soejima; Japan Patent (1988). /3/ W.M.Gibson and C.A.MacDonald; Capillary Optics for X-rays and Neutrons An MRS Tutorial, November 30, /4/ K.M.Matney, M.Wormington, D.K.Bowen, Q.F.Xiao; Denver X-ray Conference (1997). /5/ N.Gao, I.Y.Ponomarev, Q.F.Xiao, W.M.Gibson and D.A.Carpenter; Appl.Phys.Lett.71 (23), 3441(1997). /6/ M.Yamamoto, I. Klotzuko, Q.F.Xiao and T.Narusawa; Proc. of 4th Symp. on Microjoining and Assembly Technol. in Electronics, 131 (1998) (in Japanese).