ANALYTICAL MICRO X-RAY FLUORESCENCE SPECTROMETER

Transcription

1 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol ANALYTICAL MICRO X-RAY FLUORESCENCE SPECTROMETER ABSTRACT William Chang, Jonathan Kerner, and Edward Franc0 ARACOR 425 Lakeside Drive Sunnyvale, CA An analytical micro-spectrometer has been developed for materials characterization using X-ray fluorescence with a spatial resolution between 1 and 25 microns. The key component of this system is a monocapillary X-ray optic that has been optimized for intensity throughput. INTRODUCTION Monocapillary X-ray optics are a promising technology for producing intense microbeams of X rays. When coupled with laboratory X-ray sources, these optics are an enabling technology for the implementation of X-ray-based micro-spectrometer with spatial resolutions ranging from 1 to 25 microns. These instruments use X-ray fluorescence to determine the composition and thickness of materials with a spatial resolution that is comparable to the beam size. This paper concentrates on the measured performance of these optics and their incorporation into a generalpurpose micro-spectrometer. SYSTEM OVERVIEW An overview of the micro-spectrometer is shown in Figure 1. The primary components are 1) a high-magnification microscope to image the sample; 2) a precision translation stage; 3) a low power X-ray tube and capillary optic; 4) an energy dispersive X-ray detector; and, 5) a computer to control the data acquisition and stage position. The X-ray tube, with a tungsten anode and a focal spot size of about 30 microns, can be operated up to a maximum voltage of 80 kv and at 50 W.2 A significant advantage of this source is the extremely close coupling (less than 5 mm) that can be achieved between the X-ray focal spot and the entrance of the capillary optic. The X-ray detector is a Si PIN diode with an energy resolution of about 240 ev.3 The user interface for acquiring the spectra was adapted from commercially available software. X-RAY OPTICS Tapered monocapillary optics are used to concentrate the X-ray beam and produce a small spot. These optics are all standardized to a common length which allows us to rapidly interchange the optics to produce a range of beam sizes, depending on the sample requirements. The optics were all fabricated at ARACOR using an advanced capillary puller that can be programmed to reproducibly fabricate optics with almost any desired shape. We pull the glass dynamically, moving the location of the heat zone, to shape the glass over a total length of about 30 cm. For this application, we fabricated conical optics that are optimized for close-coupled anode geometry of the X-ray tube. Typical entrance diameters for this application range from 50 to 250 microns, with an exit diameter between 1 and 100 microns, over a lo-cm length of glass.

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 f Capillary Stage Low Power X-ray Tube Video Microscope Monocapillary Image Plane Y 1 Precision Stage Figure 1. Schematic of the micro-spectrometer. Figure 2 shows a representative capillary optic and a standardized capillary holder. The capillary, which was designed for a different application, is about 15 cm long. The capillary holder is designed to locate the tapered optic in a reproducible manner so that a common optical axis exists for a wide variety of optic geometries. It also contains shielding to reduce the leakage of radiation that passes through the glass walls of the optic. Prior to their installation into the system, these optics are characterized on a laboratory bench to determine their efficiency, throughput, beam size and divergence. We have the choice of using copper, molybdenum, or tungsten X-ray tubes. Data using molybdenum and tungsten tubes indicate that the range of absolute gains in intensity (over a pinhole located at the exit) are from 40 to 120X. The variability in the gain is due to differences in the optic design that affect the entrance solid angle and the exit aperture. Figure 2. Tapered monocapillary optic and holder. Typical beam size data are shown in Figure 3, obtained by scanning a smooth wire through the beam and recording the resulting attenuation as a function of distance from the tip of the capillary. The beam from the capillary appears to be well behaved for distances from tip to sample of about 10 mm or less. This can be seen from the smooth fall-off in the intensity as a function of the wire position. Also, the width of the beam is comparable when the X-ray tube is operated at 20 or 45 kv, indicating that the beam size does not significantly depend on the

4 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol energy of the incident X-ray beam. This is a consequence of the fact that we have achieved an essentially straight centerline and that energy-dependent leakage is not a significant factor. Finally, the divergence of the beam is approximately 4 milliradians, which means that the beam grows by approximately 4 microns per mm of separation between the exit of the capillary and the sample. Thus at a distance of 1 mm, 90 percent of the beam is contained within 15 microns, while at a distance of 6 mm, 90 percent of the beam is contained within 35 microns g Position (urn) Distance from tip (mm) Figure 3. Measured beam profiles and sizes (595%) as a function of distance from the tip. Once these capillaries are installed into the system, an edge is scanned through the beam while recording the resulting X-ray fluorescence intensity as a function of position. This indicates that the sample is correctly positioned within the instrument and provides a direct measurement of the actual X-ray beam size used in the examination. PERFORMANCE The signal levels from various bulk samples were examined to obtain an indication of the performance of the system. Typical results for a potential of 45 kv and a measured beam size of about 10 microns were obtained with the micro-spectrometer. A 13 mm2 Si PIN detector (300 microns thick) located at a distance of 15 mm from the sample with a take-off angle of 45 degrees, was used to record the resulting spectra. Under these conditions, we measured a Cu-Ka count rate of 120 cps/w, a MO-Ka count rate of 8 cps/w, and a Sn-Ka count rate of 3 cps/w. The background, due to scatter of the incident X-ray beam, is generally two orders of magnitude lower than the signal. The Cu count rate is significantly higher than the MO and Sn count rates. This is due to the efficient excitation of the Cu-Ka X rays by the characteristic W-L lines, produced in the W anode, and by the almost 100% detection efficiency of the Si PIN detector for

5 Copyright(c)JCPDS-International Centre for Diffraction Data 2001,Advances in X-ray Analysis,Vol Cu Ka X rays. In contrast, only bremsstrahlung radiation above the Sn-K edge (at kev) can produce characteristic Sn X rays, and their detection efficiency is only about 10%. Figure 4 shows representative images, obtained in l-3 hours, that illustrate the performance and usefulness of the micro-spectrometer. The diameter of each copper wire in the grid is 60 microns. The other two images are representative of applications in mineralogy and in contamination analysis. The mineral sample depicts an iron silicate species that is on the order of 50 microns in diameter contained in a fluorapatite sample. The image of the indium bump (60 microns square) shows a Zn-containing particle, estimated to be about 10 microns in diameter, on the alignment cross that was introduced during deposition. The high spatial resolution achieved with the capillary optic allowed the identification of the composition of this particle. Copper grid Mineral sample Figure 4. Example elemental images. Indium bump SUMMARY We have shown that resolution in the range of 10 microns is achievable for elemental mapping with a monocapillary optic and a laboratory X-ray source. These optics produce a gain in beam intensity, and can produce a beam size that is normally not feasible for pinhole geometries. The X-ray micro-spectrometer is complementary to other microbeam technologies. ACKNOWLEDGEMENTS This effort was partially supported by the Department of Energy under DE-FG03-96ER82133 and the National Institutes of Health under 2R42GM We would like to thank Don Bilderback, CHESS, and Brian Cross, METARA, for many helpful discussions. REFERENCES 1) Bilderback D. H. and E. D. France, Total Reflection Optics: Single Capillaries, in Handbook of Optics Vol. III, Optical Society of America (2001), pages ) XTG UltraBright Microfocus x-ray source from Oxford Instruments, Scotts Valley, CA. 3) Si-PIN photodiode detector XR-1 OOCR from Amptek, Bedford, MA.