Anal. Chem. 1999, 71, 5033-5036
Grazing-Exit Particle-Induced X-ray Emission Analysis with Extremely Low Background Kouichi Tsuji,*,† Zoya Spolnik,† Kazuaki Wagatsuma,‡ Rene´ E. Van Grieken,† and Ronald D. Vis§
Micro- and Trace Analysis Center, Department of Chemistry, University of Antwerp (UIA), B-2610 Antwerpen, Belgium, Institute for Materials Research, Tohoku University, Katahira-2-1-1, Aoba, Sendai, 980-8577 Japan, and Faculty of Physics and Astronomy, Free University, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands
The grazing-exit technique is applied to particle-induced X-ray emission (PIXE) analysis of thin-films and aerosols deposited on flat silicon wafers. PIXE is known as a traceanalytical method; however, Bremsstrahlung background is still important in the low-energy region. A 2.5 MeV proton beam bombarded the sample at an incident angle of 90°, and the emitted X-rays were detected at grazing exit angles by using an energy-dispersive detector. When the characteristic X-rays were measured at grazing exit angles, this background almost disappeared. The characteristic X-rays of Ca Kr and Zn Kr emitted from atmospheric particles could hardly be observed by PIXE in the normal arrangement; however, they were clearly observed with low background under grazing-exit conditions. The detection limit for Ca was improved by a factor of 7. It is also demonstrated that analysis of the topmost layer and depth analysis for double-layered samples are both possible by adjusting the exit angle. Consequently, grazing-exit PIXE is a promising method for surface-, micro-, and trace analysis. In particle-induced X-ray emission (PIXE), usually, a proton beam of several MeV energy bombards solid samples and excites the electrons of the inner shells of the atoms. In the process of deexcitation, X-rays are emitted from the sample. Proton-induced X-rays are normally measured by an energy-dispersive X-ray (EDX) detector. PIXE makes nondestructive analysis possible with high sensitivity. Furthermore, in microbeam-PIXE (micro-PIXE), the analysis with excellent spatial resolution of the order of 1 µm is possible. Therefore, PIXE is well recognized as a micro- and trace-analytical method.1,2 It has been applied in various fields such as biology, environmental chemistry, earth science, and archaeology.2-5 In the case of excitation with an electron beam, e.g., electron probe microanalysis (EPMA), a very large background, †
University of Antwerp. Tohoku University. § Free University of Amsterdam. * Corresponding author. E-mail:
[email protected]. (1) Johansson, T. B.; Akselsson, K. R.; Johansson, S. A. E. Nucl. Instrum. Methods 1970, 84, 141. (2) Johansson, S. A. E.; Campbell, J. L.; Malmqvist, K. G. Particle-Induced X-ray Emission Spectrometry (PIXE); John Wiley and Sons: New York, 1995. (3) Treiger, B.; Injuk, J.; Bondarenko, I.; Van Espen, P.; Van Grieken, R.; Breitenbach, L.; Wa¨tjen, U. Spectrochim. Acta, Part B 1994, 49, 345. (4) Injuk, J.; Van Grieken, R.; Klockenka¨mper, R.; von Bohlen, A.; Kump, P. Spectrochim. Acta, Part B 1997, 52, 977. ‡
10.1021/ac990568u CCC: $18.00 Published on Web 10/13/1999
© 1999 American Chemical Society
which is caused by Bremsstrahlung induced by the primary electrons, is a serious drawback which limits the detection power. In contrast to this, a low background is observed in PIXE spectra. Therefore, the detection limit in PIXE is about 100 times lower than in EPMA.2 However, the detection limits for light elements in PIXE analysis are not so low,6 because the background in the low-energy region is rather high because of secondary electrons. This limits the applications of PIXE, because light elements such as Na, Al, Si, and Ca, which have characteristic X-ray peaks at lower energies, are important components of aerosols or biological samples. We report here on the reduction of the background in PIXE spectra by applying, for the first time, a grazing-exit measurement technique. The grazing-exit technique has been applied to X-ray fluorescence analysis7-9 and EPMA,10 which are named as grazing-exit (or emission) XRF (GE-XRF) and grazing-exit EPMA (GE-EPMA), respectively. This method is closely related to grazing incidence measurement.11-13 In the grazing-exit approach, X-rays emitted from the sample are detected under very small exit angles, normally several mrad. Since the X-rays produced in the depth of the sample cannot be observed at grazing-takeoff angles, due to reflection and refraction effects at the interface,14,15 grazing-exit methods make a surface analysis (nanometer order) possible.16 In normal EPMA, the continuous X-rays that are produced in deep positions (to several µm) contribute to the background in a wide energy region. This background is reduced under grazing-exit conditions; therefore, GE-EPMA measurement is useful to improve the signal-to-background ratio.10 In addition to the reduction of (5) Del Carmmine, P.; Lucarelli, F.; Mando`, P. A.; Pecchioli, A. Nucl. Instrum. Methods Phys. Res. Sect. B 1993, 75, 480. (6) Maenhaut, W.; De Reu, L.; Vandenhaute, J. Nucl. Instrum. Methods Phys. Res., Sect. B 1984, 3, 135. (7) Sasaki, Y.; Hirokawa, K. Appl. Phys. A 1991, 50, 397. (8) Tsuji, K.; Sato, S.; Hirokawa, K. Rev. Sci. Instrum. 1995, 66, 4847. (9) Claes, M.; de Bokx, P. K.; Willard, N.; Veny, P.; Van Grieken, R. Spectrochim. Acta, Part B 1997, 52, 1063. (10) Tsuji, K.; Wagatsuma, K.; Nullens, R.; Van Grieken, R. Anal. Chem. 1999, 71, 2497. (11) Yoneda, Y.; Horiuchi, T. Rev. Sci. Instrum. 1971, 42, 106. (12) de Boer, D. K. G. Phys. Rev. B: Condens. Matter 1991, 44, 498. (13) Klockenka¨mper, R.; Knoth, J.; Prange, A.; Schwenke, H. Anal. Chem. 1992, 64, 1115A. (14) Becker, R. S.; Golovchenko, J. A.; Patel, J. R. Phys. Rev. Lett. 1983, 50, 153. (15) Hasegawa, S.; Ino, S.; Yamamoto, Y.; Daimon, H. Jpn. J. Appl. Phys. 1985, 24, L387. (16) Tsuji, K.; Sato, S.; Hirokawa, K. J. Appl. Phys. 1994, 76, 7860.
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Figure 1. Schematic diagram of the GE-PIXE experimental setup from a top view. The exit angle is defined as the angle between the detected X-ray beam and the surface of the sample carrier.
the background, enhancement of the characteristic X-ray intensity has also been reported for GE-XRF7-9 and GE-EPMA.17 In this paper, we perform grazing-exit PIXE (GE-PIXE) experiments for the first time, as far as we know, and expect a low background and a new feature of PIXE. EXPERIMENTAL SECTION To perform GE-PIXE experiments, accurate adjustment of the exit angle is necessary. The solid angle for the X-ray detection should be small, because the X-ray intensity is sensitive to the exit angle. We used a total reflection PIXE setup18,19 with a minor change for GE-PIXE experiments. As shown in Figure 1, a slit (1 mm in width, 10 mm in height) was attached between the sample and the X-ray detector parallel to the surface of the sample. The X-ray detector was also removed from the sample to the distance of 100 mm, to obtain an even smaller solid angle. The X-rays passed through an X-ray transparent window (Kapton thin film) attached on the sample chamber and then reached an EDX detector (HPGe, 100 mm2 sensitive area, PGT-107, PGT, Princeton, USA). Some air existed in the space between the chamber exit window and the X-ray detector; hence, the low-energy X-rays were strongly absorbed. The proton beam irradiated the sample at the normal incidence with an area of about 2 × 4 mm2. In this experimental arrangement, the solid angle divergence of X-rays in the plane of projection was about 0.57°. The proton beam (beam current: 100 nA) was accelerated by a 1.7 MV tandem NEC pelletron to an energy of 2.5 MeV. The sample chamber was evacuated by a turbo-molecular vacuum pump to less than 10-5 Torr. The angle between the proton beam and the detected X-ray beam was 90°. This arrangement is not the optimum for background reduction, because Bremsstrahlung X-rays excited by secondary electrons have a maximum at a 90° angle to the direction of the proton beam.2 Since we tentatively utilized the total-reflection PIXE setup, it was difficult to change this experimental arrangement in this work. The adjustment of the exit angle was carried out by using a rotating stage for the sample controlled by a stepping-motor (minimum step angle: 0.0053°; maximum angle variation: 5°). In this work, the exit angle was changed with a step angle of approximately 0.057° over an angular range of 5°. The slit and the X-ray detector were fixed during the measurements. (17) Tsuji, K.; Spolnik, Z.; Wagatsuma, K.; Zhang, J.; Van Grieken, R. Spectrochim. Acta, Part B 1999, 54, 1243. (18) Van Kan, J. A.; Vis, R. D. Nucl. Instrum. Methods B, Phys. Res., Sect. 1996, 109/110, 85. (19) Van Kan, J. A.; Vis, R. D. Spectrochim. Acta, Part B 1997, 52, 847.
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Figure 2. PIXE spectra obtained for a Au (50 nm)-Cu (500 nm)Si wafer sample at exit angles of 45° (a) and 0.5° (c). (b) The same X-ray spectrum as Figure 2a is shown with an expanded y-axis to show the existence of Bremsstrahlung background. The lifetime for EDX measurement was the same (120 s) for both the exit angles.
The thin film samples, a bilayer of Cu (10 nm)-Bi (6 nm) on a flat glass and of Au (50 nm)-Cu (500 nm) on a Si wafer, were prepared by vacuum evaporation. Atmospheric particles were collected at the campus of the University of Antwerp (UIA), Belgium. Air was sucked with a rotary vacuum pump into a multiple-orifice impactor (Berner-type, Hauke, Austria). Finally, spots (∼1 mm in diameter) of deposited atmospheric particles were obtained on a Si substrate. RESULTS AND DISCUSSION Thin-Film Analysis by GE-PIXE. Figure 2 shows GE-PIXE spectra measured at the different exit angles for a thin-film samplesAu (50 nm)-Cu (500 nm) on a flat Si substrate. At a large exit angle (45°), the Cu KR X-ray intensity, from the thicker Cu layer below the Au layer, is relatively high compared to the Au X-rays, as shown in Figure 2a. The same spectrum is shown with an expanded y axis in Figure 2b. It is clear that the characteristic X-rays of Cu K lines and Au L lines are superimposed on a background of continuous X-rays. The background intensity of continuous X-rays increases for low energy. This feature of the background in PIXE spectra has been theoretically well-understood.20 The background decreases below 3.0 keV (Figure 2b), because the soft X-rays are absorbed by air, by the Kapton window, and by the Be window of the X-ray detector. Figure 2c shows a typical X-ray spectrum taken for the same sample at a grazing exit angle of 0.5°. The y-axis scale of the X-ray (20) Ishii, K.; Morita, S. Nucl. Instrum. Methods Phys. Res., Sect. B 1987, 22, 68.
Figure 3. The X-ray intensities of Au LR (O) and Cu KR (4) for the Au (50 nm)-Cu (500 nm)-Si wafer sample and the intensity ratio of Au LR/Cu KR (9) as a function of the exit angle. The amplitude of the calculated curves (dashed line for Au LR and dash-dotted line for Cu KR) was normalized to the measured intensities. The calculations are based on the following refractive indexes: 1 - (1.63 - 0.121 i) × 10-6 for Au LR-Au layer, 1 - (1.89 - 0.234 i) × 10-6 for Au LR-Cu layer, 1 - (2.41 - 0.238 i) × 10-6 for Cu KR-Au layer, and 1 - (2.727 - 0.060 i) × 10-6 for Cu KR-Cu layer (i ) imaginary unit).
intensity is almost the same as that in Figure 2b. First, it should be noted that the background in the low-energy region almost completely disappears. The X-rays produced at deep positions in the sample cannot be observed at extremely small angles, because these X-rays are reflected or refracted at the sample-vacuum interface.15 This is the reason for the extremely low background in Figure 2c. In other words, this result suggests that the background, which is caused mainly by secondary-electron Bremsstrahlung in the case of PIXE, is mostly produced at a certain depth. The grazing exit measurements in PIXE would also be useful for understanding the mechanism of secondary-electron Bremsstrahlung, because the observed depth can be controlled by the takeoff angle.16 Second, it is also a significant feature of Figure 2c that the X-ray intensities of Au LR and Lβ are stronger than the Cu KR intensity. The change of the relative intensities of Au and Cu X-rays is clearly recognized when Figure 2c is compared with Figure 2a. The angle-dependencies of Au LR and Cu KR are shown in Figure 3. The intensity of the characteristic X-rays of Au LR steeply increases at an exit angle of about 0.5°, and less significantly at larger angles, while Cu KR X-rays gradually increase above 0.7° and continue to increase with increasing exit angle. Au LR X-rays can be observed above 0.5°, which is near the critical exit angle (θc) for external total reflection of Au LR on Cu substrate: θc (degrees) ≈ 1.65/E(ZF/A)1/2 , where Ε(keV) is the energy of characteristic X-rays, Z is the atomic number, A is the atomic weight, and F (g/cm3) is the density of the substrate.21 Becker et al.14 performed grazing-incidence and grazing-exit X-ray fluorescence measurements and reported that both results were identical. This can be explained by the principle of microscopic reversibility.22 In fact, the exit-angle dependency of the characteristic X-ray intensity is similar to the incident-angle dependency. In the case of grazing incidence, the X-ray intensities drastically increase at the critical angle for total reflection of the primary X-rays. In the case of GE-PIXE, although they are actually detected and there
is no external total reflection, they increase at the critical angle for total reflection of the observed X-rays. The grazing-exit X-ray intensity was calculated with a stratified media model.23,24 This calculation is based on the Lorentz reciprocity theorem22 and Parratt’s formula.25 A good agreement is obtained except for Cu KR intensity at higher angles, as shown in Figure 3. This indicates that GE-PIXE results can be explained by reflection and refraction at the interface of two different media. The intensity ratio of Au LR/Cu KR is also shown in Figure 3. It has a maximum at 0.5° that is near θc for Au LR on a Cu substrate. This confirms that a surface-sensitive analysis is possible at the critical angle. Depth-Selective Analysis of Layered Thin Films by GEPIXE. Figure 4 shows the PIXE spectra taken for the Cu (10 nm)-Bi (6 nm)-glass substrate at exit angles of 0.4°, 1.0°, and 4.4°, respectively. Obviously, the X-ray spectrum changes depending on the exit angle. At an exit angle of 0.4°, only the X-ray peak of Cu KR, produced by the top Cu layer, is observed. In Figure 4b, Bi L lines from the second layer begin to be observed. Finally, in Figure 4c, Ca KR X-rays from the glass substrate are also seen. We can thus nondestructively analyze the structure of the layered material by GE-PIXE at different exit angles.
(21) Klockenka¨mper, R. Total-reflection X-ray Fluorescence Analysis; John Wiley: New York, 1997. (22) Born, M.; Wolf, E. Principles of Optics; Pergamon: Oxford, 1991.
(23) Noma, T.; Miyata, H.; Ino, S. Jpn. J. Appl. Phys. 1992, 31, L900. (24) Tsuji, K.; Hirokawa, K. J. Appl. Phys. 1994, 75, 7189. (25) Parratt, L. G. Phys. Rev. 1954, 95, 359.
Figure 4. Typical PIXE spectra obtained for a Cu (10 nm)-Bi (6 nm)-glass sample at different exit angles of 0.4° (a), 1.0° (b), and 4.4° (c).
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in Figure 5a, are also detected now. The merit of GE-PIXE is effectively demonstrated in Figure 5b: it is possible to reduce the continuous X-ray background in the low-energy region under grazing-exit conditions. The background reduction leads to low detection limits. The detection limit (DL) in X-ray analysis is normally evaluated by the relation DL ) (3 CxIback)/Inet, where C is the concentration of analyte, Inet is the peak net intensity, and Iback is the background intensity under the peak. The net intensity of Ca KR peak was evaluated by subtracting the background described with a linear line. The same aerosol particles were measured by normal PIXE (Figure 5a) and GE-PIXE (Figure 5b). As a result, it is found that the detection limit for Ca KR is improved by a factor of 7 under the grazing-exit conditions.
Figure 5. PIXE spectra measured for aerosols collected on a Si substrate at different exit angles of 4.4° (a) and 0.4° (b). The lifetime for EDX measurements was 180 s.
These results can be explained by the change of the observation depth in GE-PIXE. Simply, the observation depth increases with increasing exit angle; however, it does not monotonically increase. It should suddenly increase at the critical angle for total reflection of the observed characteristic X-rays.16 Therefore, strict control of the exit angle is necessary for this measurement. GE-PIXE Analysis of Atmospheric Particles. The GE-PIXE method would be especially useful for the analysis of particles, which are normally deposited on a sample carrier. Characteristic X-rays are isotropically emitted from particles, that is, there is no clear angle dependence of the characteristic X-ray intensity. However, X-rays emitted from the sample carrier show a strong angular dependence, and their intensity becomes extremely small below θc for the energy of the observed characteristic X-rays. Therefore, particle analysis with low detection limits is possible in the GE-PIXE mode. Figure 5a shows a typical PIXE spectrum obtained at an exit angle of 4.4° for atmospheric particles deposited on a Si substrate. The characteristic X-rays from Fe, which is one of the main components of aerosols, were observed. However, it is not easy to recognize the characteristic X-rays of Ca KR because of a large continuous X-ray background. In contrast to this, the X-ray peak of Ca KR is clearly observed with a low background in the PIXE spectrum taken at 0.4°, as shown in Figure 5b. Besides Fe K lines and Ca KR, Ti KR, and Zn KR, which are not recognized
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CONCLUSIONS PIXE measurements for thin-film samples and particles deposited on a flat substrate were performed under grazing-exit conditions. As a result, PIXE spectra were obtained with extremely low background. The grazing-exit technique can reduce the Bremsstrahlung background that is normally found in conventional PIXE spectra. Therefore, GE-PIXE is a very promising method for trace analysis of contaminations or particles, like aerosols on a flat substrate. Another advantage of the grazing-exit arrangement, in comparison with grazing incidence, is the possibility of microanalysis. We have applied the GE-EPMA for single particle analysis;10,17 however, the damage of organic samples due to electron irradiation is a serious problem. Micro-PIXE combined with the grazingexit technique will be a powerful analytical method. ACKNOWLEDGMENT K. Tsuji was financially supported by Japan Society for the Promotion of Science (JSPS). Z. Spolnik was supported by EU research project ENV4-CT 95-0104 and by the Belgium Office for Scientific, Technical, and Cultural Affairs under contract MN/ 10/01. The authors thank Dr. Wiederspahn (Free University of Amsterdam) and Prof. J. Zhang (The Ocean University of Qingtao, China) for sample preparation of thin films and aerosols and also thank Prof. P. Van Espen (University of Antwerp) for quantitative analysis of PIXE data. Received for review May 27, 1999. Accepted August 27, 1999. AC990568U
