390 ELECTRIC FIELD INFLUENCE ON EMISSION OF CHARACTERISTIC X-RAY FROM Al 2 O 3 TARGETS BOMBARDED BY SLOW Xe + IONS J. C. Rao 1, 2 *, M. Song 2, K. Mitsuishi 2, M. Takeguchi 2, K. Furuya 2 1 Department of Materials Science, Harbin Institute of Technology, Harbin 150001, CHINA 2 High Voltage Electron Microscopy Station, National Institute for Materials Science, 3-13, Sakura, Tsukuba, Ibaraki, 305-0003, JAPAN E-mail: jcrao@hit.edu.cn, RAO.Jiancun@nims.go.jp ABSTRACT Low energy characteristic X-ray emission from Al 2 O 3 monocrystalline specimens is measured under bombardment of 100 kev Xe + ions. The electric field influence on emission of the X-rays of constitute elements in the specimens was investigated. The energy dispersive X-ray spectroscopy (EDS) spectra show that the characteristic X-ray of Al-Kα seems to be depressed by the applied direct current (DC) voltages, while the peak intensity of O-Kα was not notably influenced. The O-Kα peaks were broadened and the total counts increased as a higher DC bias was applied. It is possible that a DC electric field parallel to the target surface may influence the X-ray emission from it under ion bombardment. INTRODUCTION Some insulating samples under ion beam bombardment show important charging effects: a large bremsstrahlung background in the X-ray spectra and enhancement of characteristic X-ray yields of component elements [1-9]. When these insulating samples were bombarded by ion beams, such as protons and alpha particles, they produce a very large bremsstrahlung background in the X-ray spectra. Also, the yields of the X-ray lines of component elements are largely enhanced in comparison to those obtained from the same elemental concentrations in conductive samples or samples simultaneously neutralised by an electron gun. There is compelling evidence connecting the enhancement with the charging up of the samples and some researchers [1-9] attributed the effect to accelerated electrons as the cause of the X-ray yield enhancement. Recently, low energy characteristic X-ray emission was observed, by our group, as a result of bombardment by positive slow ions onto insulating materials. However, the mechanism that leads to the charging of the samples, with consequent buildup of high potentials, is not known. Theory or models concerning the charging and breakdown of insulating samples bombarded by ion
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391 beams are scarce. There is a study of the charging process by He + ion beams [10] and another [11] relative to the charging by some kev electron bombardment and subsequent surface breakdown. It is considered that some ideas of that model can be applied to the charging up of insulators under proton bombardment. The evidence both from our previous work and from the reports seems to confirm that the charging process is responsible for the X-ray yield enhancement in some insulating samples under slow ion bombardment. In the present work, the influence of an electric field on the characteristic X-ray yield was studied. It was concluded that application of a DC bias has an influence on low energy characteristic X-ray emission from insulator targets under ion bombardment. EXPERIMENTAL Pure Cu film was deposited as electrodes on one polished clean side of an Al 2 O 3 monocrystalline substrate with the normal of <1 1 02>. The electrodes were 2.0 mm apart from each other. The substrate size is 7 3 mm (long wide). The thickness of Al 2 O 3 substrate is 0.2 mm while that of Cu electrode films of 2 μm. The DC voltage was applied to generate an electric field on the polished surface (finished by 0.1 micron Al 2 O 3 polishing solution) of Al 2 O 3 monocrystalline substrate through the deposited Cu electrodes and a special specimen holder. The EDS spectra were detected by EDAX DX-4 analysis system with an energy resolution of 136 ev. The collecting live time was set as 100 seconds. The EDS were collected from the center part of the Al 2 O 3 area in the electric field built up by three different DC biases (0, 100 and 200V) under the ion bombardment in the vacuum of about 4 10-6 Pa at room temperature. 3 mm Cu 7 mm 2.0 mm Cu 2 μm thick Ion EDS Detector A 0.2 mm Al 2 O 3 Fig. 1 Top view and side view of the diagram of the sample and the circuit for measuring both the characteristic X-ray and sample current.
392 100 kev Xe + was used as the incident ion source. The ion current was measured to be about 0.6 na and using a beam size about 1.0 mm in diameter using a special Faraday cup. Figure 1 gives the diagram of the sample and the circuit for measuring both the characteristic X-ray and sample current. There is an angle of 15 between the normal of the sample and the incident ion beam. RESULTS AND DISCUSSION Low energy characteristic X-ray emission is observed under the bombardment of positive slow ions onto Al 2 O 3 monocrystalline specimens. Figure 2 shows the relationship between low energy characteristic X-ray emission and electric field under slow ion bombardment. Fig. 2(a) gives the EDS spectra over an energy range from 0.4 to 2.0 kev from the Al 2 O 3 specimens with different DC bias (0 V, 100 V and 200 V) under 100 kev Xe + ion bombardment. The normalized counts of spectra for DC 100 V and DC 200 V spectra were shifted upward by adding 0.05 and 0.10, respectively, for clarity in the figure. The X-ray counts were normalized to 10 9 counts per Coulomb ions. It is obvious that the characteristic X-ray of Al-Kα was depressed by the applied direct current voltages while the peak intensity of O-Kα was not influenced very much. On the other hand, the O-Kα peaks were broadened when the DC bias was increase. Fig. 2(b) gives the total counts of O-Kα peaks in range from 0.46 to 0.60 kev under different DC bias. It shows clearly that the total counts increased linearly as the higher DC bias applied to the Al 2 O 3 substrate under ion bombardment. The X-ray emission from the insulating materials is related with electrical charge buildup and discharge on the surface due to the incident ions. The applied DC bias has obvious effect on the X-ray emission. The spiked secondary electrons in the insulator were accelerated by the applied electric field. These electrons were regarded to contribute to generate very intense X-rays. The interaction for ion bombarding on surface of bulk materials is dominated by the role of the potential energy carried by the incident ion beam toward the surface while the influence of the kinetic projectile energy remains much less important. The interaction at and below the surface is quite different for metal and insulator surfaces because of comparably much lower electron density and higher electron binding energy of the latter. As mentioned above, the influence of electric field on the low energy characteristic X-ray yield is obvious though the DC bias used in
the present work is not very high. Unfortunately, our experimental results are the indirect observation of this interaction process. Much work remains to be done before a theoretical description of the interaction of slow ions bombarding on the insulator surface can be made. To this goal, the experiment can be improved by the following aspects: collecting more data from different kinds of insulator, much higher DC bias and optimum detect parameters. a /C) 9 X-ray Counts (x10 0.6 0.4 0.2 0.0 O-K Al2O3 100 kev Xe + Current: 0.6 na Size: 1.0 mm Live time: 100 s DC 200V+0.10 DC 100V+0.05 DC 0V 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 Engergy (kev) Al-K b X-ray Counts (x10 9 /C) 5.0 4.5 4.0 3.5 3.0 0 100 O-Kα Peak Counts (0.46-0.60 kev) DC (V) 200 393 Fig. 2 Relationship between low energy characteristic X-ray emission and electric field under 100 kev Xe + bombardment. The characteristic X-ray of Al-Kα seems to be depressed by the applied direct current (DC) voltages while the peak intensity of O-Kα was not influenced obviously (a). The O-Kα peaks were broadened with the total counts in energy range from 0.46 to 0.60 kev increased as the higher DC bias applied (b). CONCLUSIONS (1) An electric field can influence the emission of low energy characteristic X-rays of constitute elements from an Al 2 O 3 monocrystalline specimen under bombardment of 100 kev Xe + ions. (2) The characteristic X-rays of Al-Kα seems to be decreased by the applied electric field, while the peak intensity of O-Kα was not measurably influenced. (3) The O-Kα peaks were broadened with the total counts increased as the higher DC bias applied. REFERENCES [1] Peisach M.; Pillay A. E.; Pineda C. A., Nucl. Instr. and Meth. B, 1993, 75, 14-16. [2] Peisach M.; Pineda C. A.; Pillay A. E.; Springhorn K. A., Nucl. Instr. and Meth. B, 1994, 94,
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