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Development of Miniaturized Electron Probe X-ray Microanalyzer Susumu Imashuku,* Akira Imanishi, and Jun Kawai Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan ABSTRACT: A miniaturized electron probe X-ray microanalyzer (EPMA) with a small chamber including the electron source and the sample stage was realized using a pyroelectric crystal as an electron source. The EPMA we propose is the smallest reported so far. Performance of the EPMA was evaluated by investigating energy of obtained continuous X-rays and lower detection limits of transition metals (titanium, iron, and nickel). End point energy (Duane-Hunt limit) of continuous X-rays of 45 keV was obtained. However, it is expected that the EPMA can analyze characteristic X-rays with energy less than 20 keV. The EPMA was able to measure titanium, iron, and nickel wires whose projected areas were more than 0.03 mm2.
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hen a temperature change under vacuum condition is applied to a pyroelectric crystal such as lithium tantarate (LiTaO3) and lithium niobate (LiNbO3), an electric field is produced due to uncompensated charge on the surface of the pyroelectric crystal. As a result, quite small amounts of suspended electrons and ions in the condition are accelerated by the electric field. This property first found application in X-ray generator by Brownridge.1 Then, other researchers applied this property to X-ray generator,2 7 X-ray fluorescence measurement,8,9 electron and ion beams,10 12 and the ion source for mass spectrometry.13 Pyroelectric crystals have not previously been utilized as an electron source for an electron probe X-ray microanalyzer (EPMA), which is an instrument to analyze characteristic X-rays emitted from a sample bombarded with electrons. In the present study, we present a novel EPMA with a palm-top size chamber including the electron source and the sample stage using a LiTaO3 crystal as an electron source. The palm-top EPMA we propose is the smallest reported so far.
’ EXPERIMENTAL SECTION The apparatus in the present study is shown in Figure 1. A single crystal of LiTaO3 (Shin-Etsu Chemical) was used as an electron source. The size of the LiTaO3 crystal was 3 mm 3 mm in the x y plane and 5 mm in the z-axis. The +z plane of the LiTaO3 crystal was attached on a Peltier device with silver paste. Silver paste was chosen as an adhesive agent due to its high thermal and electrical conductivity. The other face of the Peltier device was attached on a copper rod with silver paste. A copper wire was connected between the +z plane of the LiTaO3 crystal and the copper rod. The center of the copper rod had a hole to insert the wires of the Peltier device. An outer exit of the hole was closed with a low vapor pressure resin (Torr Seal, VARIAN). A sample stage was attached on another copper rod with silver paste. The copper rod was connected to an oil-sealed rotary pump through a vacuum joint. The sample stage had 45° gradients and consisted of brass. Samples were set on the sample stage with a carbon tape. A borosilicate glass tube whose outer diameter was 18 mm was put between the two copper rods. They were connected with detachable vacuum joints. The two r 2011 American Chemical Society
copper rods were connected with a copper wire in order to keep the electric potential of the sample stage the same as the +z plane of the LiTaO3 crystal. The center of the borosilicate glass tube had a through-hole whose diameter was 10 mm. A polyimide tape (Kapton tape) was put on the through-hole. Si-PIN (X-123, Amptek Inc.) or CdTe detector (XR-100T-CdTe, Amptek Inc.) was set toward the through-hole. The Peltier device was connected to a 3 V battery and heated the LiTaO3 crystal for 2 min. Then, the LiTaO3 was cooled by applying power to the Peltier device and measured the X-ray spectra for 90 s. The pressure of the sample chamber was monitored with a Pirani gauge with about 5 Pa during the measurement. Titanium (99.8%), iron (99.5%), and nickel (99%) wires, whose diameters were 0.05, 0.1, and 0.1 mm, respectively, were used as samples. Silver (99.98%) and copper (99.96%) plates, whose thicknesses were 0.05 and 0.1 mm, respectively, were also used as samples.
’ RESULTS AND DISCUSSION First, the energy of obtained continuous X-rays was evaluated with a CdTe detector. Silver and copper plates, whose sizes were 5 mm 10 mm, were used as samples. Figure 2 shows the EDX spectrum of the samples. Silver Lα and copper Kα lines were clearly obtained during 90 s measurement, which shows that our apparatus works as an EPMA. The end point energy (DuaneHunt limit14) of continuous X-rays was about 45 keV. However, the intensities of X-rays with energy more than 20 keV were weak because the silver Kα line (22.16 keV) was not detected. It is expected that the palm-top EPMA can analyze characteristic X-rays with energy less than 20 keV. Elemental analysis was carried out using metal wires of titanium, iron, and nickel with the palm-top EPMA. Projected areas of these metal wires were controlled by changing their lengths from 1 to 5 mm. Figure 3 shows the EDX spectra of titanium, iron, and nickel wires. Copper and zinc Kα lines were Received: July 29, 2011 Accepted: October 21, 2011 Published: October 21, 2011 8363
dx.doi.org/10.1021/ac201958d | Anal. Chem. 2011, 83, 8363–8365
Analytical Chemistry
Figure 1. (a) Pictures of the proposed palm-top EPMA. The picture on the right shows the overall photo of the palm-top EPMA except for a vacuum pump. The picture on the left shows the photo of the chamber including the electron source and the sample stage. The yellow tape put on a borosilicate glass is Kapton tape. (b) Schematic view of the palmtop EPMA. The palm-top EPMA is presented horizontally.
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Figure 3. EDX spectra of various lengths of (a) titanium, (b) iron, and (c) nickel wires with the palm-top EPMA. The Si-PIN detector was used for the measurements. The diameter and length of the wires are written in the graphs.
of detection of titanium, iron, and nickel were 0.03 mm2 as projected areas. From the results, we can say that the palm-top EPMA is able to detect these metals whose total projected area was more than 170 μm2, which is 4 orders of magnitude wider than the minimum area a conventional EPMA can analyze.
Figure 2. EDX spectrum of silver and copper plates with the palm-top EPMA. The CdTe detector was used for the measurement.
detected in all samples. These elements came from the brass sample stage. Intensities of titanium, iron, and nickel Kα lines were reduced with the decrease of their lengths (projected areas). The measurements were repeated at least three times for each sample. There was a variation of the intensities of the strongest peaks for the same sample, but a similar shape of the spectra was obtained by normalizing with respect to the strongest peak. Lower limits of detection of these metals were calculated from EDX spectra in Figure 3. In the calculation, the lower limit of detection is assumed to be equal to 3 times the standard counting error of the background intensity.15,16 The calculated lower limits
’ CONCLUSIONS A miniaturized electron probe X-ray microanalyzer (EPMA) was realized using a palm-top size chamber including the electron source and the sample stage with a pyroelectric crystal. The palmtop EPMA is the smallest reported so far. Peaks of metals were clearly obtained in 90 s measurements. Duane-Hunt limit of continuous X-rays was about 45 keV. However, the palm-top EPMA can analyze characteristic X-rays with energy less than 20 keV. The palm-top EPMA was able to measure titanium, iron, and nickel wires whose projected areas were more than 0.03 mm2. Although the minimum projected area the palm-top EPMA can analyze is 4 orders of magnitude wider than that conventional EPMA can analyze, we expect that the palm-top EPMA will be widely used in chemical analysis for its small size. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: +81-75753-5483. Fax: +81-75-753-5436. 8364
dx.doi.org/10.1021/ac201958d |Anal. Chem. 2011, 83, 8363–8365
Analytical Chemistry
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’ ACKNOWLEDGMENT S.I. expresses his gratitude to the Murata Science Foundation for their support. J.K. thanks the Asahi Glass Foundation for their support. We also express our deep thanks to Shin-Etsu Chemical Co., Ltd. for the supply of single crystal of lithium tantalite. ’ REFERENCES (1) Brownridge, J. D. Nature 1992, 358, 287–288. (2) Kawai, J; Inada, N.; Maeda, K. Adv. X-ray Chem. Anal., Japan 1997, 29, 203–222. (3) Brownridge, J. D.; Reboy, S. J. Appl. Phys. 1999, 86, 640–647. (4) Brownridge, J. D. Appl. Phys. Lett. 2004, 85, 1298–1300. (5) Tornow, W.; Shafroth, S. M.; Brownridge, J. D. J. Appl. Phys. 2008, 104, 034905. (6) Geuther, J. A.; Danon, Y. J. Appl. Phys. 2005, 97, 104916. (7) Hiro, E.; Yamamoto, T.; Kawai, J. Adv. X-ray Chem. Anal., Japan 2010, 41, 195–200. (8) Kawai, J.; Yamada, T.; Fujimura, H. Bunseki Kagaku 2004, 53, 183–186. (9) Kawai, J.; Ida, H.; Koyama, T. X-ray Spectrom. 2005, 34, 521–524. (10) Brownridge, J. D.; Shafroth, S. M. J. Electrost. 2005, 63, 249–259. (11) Rosenblum, B.; Braulich, P.; Carrico, J. P. Appl. Phys. Lett. 1974, 25, 17–19. (12) Brownridge, J. D.; Shafroth, S. M. J. Appl. Phys. 2001, 79, 3364–3366. (13) Neidholdt, E. L.; Beauchamp, J. L. Anal. Chem. 2007, 79, 3945–3948. (14) Goldstein, L.; Newbury, D.; Joy, D.; Lyman, C.; Echlin, P.; Lifshin, E.; Sawyer, L.; Michael, J. Scanning Electron Microscopy and X-Ray Microanalysis, 3rd ed.; Kluwer Academic/Plenum Publishers: New York, 2003; Chapter 6, p 273. (15) Bertin, E. P. Principles and Practice of X-ray Spectrometric Analysis, 2nd ed.; Plenum Press: New York, 1975; Chapter 13, pp 529 532. (16) Rousseau, R. M. Rigaku J. 2001, 18, 33–47.
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