- Email: [email protected]
Experimental verification of theoretical cross sections for FIB–PIXE
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 249 (2006) 92–94 www.elsevier.com/locate/nimb
Experimental verification of theoretical cross sections for FIB–PIXE Kenneth L. Streib b
a,*
, Terry L. Alford b, James W. Mayer
a,b
a Center for Solid State Science, Arizona State University, PSA 213, Tempe, AZ 85287-1704, United States Department of Chemical and Materials Engineering, Arizona State University, Tempe, AZ 85287-6006, United States
Available online 19 May 2006
Abstract X-ray production cross sections were found for films of Cr, Cu, Ge, Ag, W and Au, using incident H+ and Be+ ions at energies from 300 keV to 3.5 MeV. These experimental cross section results were compared with the cross section results obtained using software which calculates inner shell ionization and X-ray production cross sections. The software uses the ECPSSR-UA approach to finding X-ray production cross sections. This program was found to be useful for predicting cross sections for H+ and Be+ ions at the energies in this study. The software was then used to predict results for Li+, Be+ and B+ ions at 280 keV, energies available in the Arizona State University focused ion beam laboratory. Ó 2006 Elsevier B.V. All rights reserved. PACS: 32.30.Rj; 41.75.Ak; 32.80.Hd; 33.50.Dq Keywords: PIXE, Particle induced X-ray emission; Focused ion beam, FIB; FIB–PIXE; X-ray emission
1. Introduction In order to perform PIXE in a focused ion beam (FIB), it is necessary to use heavier ions than the H+ and He+ currently used in most ion beam systems. An added advantage of using heavier ions is that less bremsstrahlung radiation is expected [1] and less beam broadening might be expected with the heavier ions [2]. In addition, the NanoFab 150 FIB at ASU can have a spot size as small as 70 nm as opposed to the characteristic 100 nm of most modern microPIXE ion beam systems [3]. One drawback is that the NanoFab 150 is only capable of producing acceleration voltages of up to 150 kV. This limits the energies available to 300 keV with doubly charged ions. It has long been known that at lower energies, there is a decrease in X-ray production cross section with decreased energy [4]. This could limit analysis capabilities with this technique. In order to investigate the feasibility of performing PIXE in a FIB, it is necessary to find an accurate model *
Corresponding author. Fax: +1 480 965 9004. E-mail address: [email protected] (K.L. Streib).
0168-583X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.087
which will allow prediction of cross sections. The computer program Inner Shell Ionization Cross Sections (ISICS) is used for this [5]. This software uses a modification of the Born plane wave approximation known as the ECPSSR method developed by Brandt and Lapicki [6,7] and the united atom approximation [8]. The ISICS software is used to calculate X-ray production cross sections and to predict results for some X-ray lines. 2. Procedure Films of Cr, Cu, Ge, Ag, W and Au were deposited on Kapton by either evaporative deposition or sputtering. Kapton was chosen as Mylar decomposed due to heat. Rutherford backscattering spectrometry (RBS) was performed on these films using helium ions. Simulations were then performed using the Rutherford Universal Manipulation Program [9] and iterated until the simulation matched the RBS spectrum to infer film thickness. The films were found to be between 53 and 80 nm in thickness. PIXE was performed on these films with H+ and Be+ ions in order to determine the cross sections with two different
K.L. Streib et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 92–94
3. Results and discussion Figs. 1 and 2 show a sample of the cross section versus energy plots for Cr Ka, Cu Ka, Ge Ka, Ag Ka and Au L lines, plotted logarithmically using H+ ions. Fig. 3 shows total Au M and W M cross sections versus energy
Cross section (Barns)
1000
Cross section (Barns)
1000 100 10 1 Experimental Au L H+ ions Theoretical Au L H+ ions Experimental Cr Kα H+ ions Theoretical Cr Kα H+ ions Experimental Ag Kα H+ ions Theoretical Ag Kα H+ ions
0.1 0.01 0.001 0.0001 0
Experimental Cu Kα H+ ions Theoretical Cu Kα H+ ions
0.1
Experimental Ge Kα H+ ions Theoretical Ge Kα H+ ions
0.01 0
1
2 Energy (MeV)
3
4
Fig. 1. A logarithmic plot of Cu Ka and Ge Ka cross section results versus energy incident H+ ions. Note the excellent agreement for both series and that theoretical values underestimate experimental values.
1.5
2
2.5
3
3.5
4
100 10 1 0.1 Experimental W M Be+ ions Theoretical W M Be+ ions"
0.01
0.001 0.25
Experimental Au M Be+ ions Theoretical Au M Be+ ions
0.3
0.35
0.4
0.45
0.5
0.55
Energy (MeV) Fig. 3. A logarithmic plot of total Au M and total W M cross section results versus energy incident Be+ ions. Note that theoretical values underestimate experimental values.
for Be+ ions. The ISICS calculations are labeled ‘‘Theoretical’’. Table 1 contains number of counts for some selected spectral lines that would be expected if spectra were taken
Table 1 Spectral counts expected with windowless SDD detector for films of the thickness in this investigation, irradiated with 25 pA of ion beam current for 10 min, at 280 keV X-ray line
1
1
Fig. 2. A logarithmic plot of Cr Ka, Ag Ka and total Au L cross section results versus energy incident H+ ions. Note the agreement is as good as in Fig. 1.
100 10
0.5
Energy (MeV)
Cross sections (Barns)
ion masses. Data was taken at 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 MeV. At higher energies a lithium drifted detector manufactured by Princeton Gamma Tech (PGT) was used. However, at lower energies (300 and 500 keV), it was discovered that the noise from a silicon drifted detector (SDD) manufactured by PGT was lower and this was installed temporarily on the Tandetron. Due to limited beam time, data for all combinations of the above ions and energies could not be taken. Enough data was taken, however, to compare with the ISICS program [5]. A Microsoft Excel template was used to determine cross sections from this data using the variables of total incident charge, film thickness, detector solid angle, Mylar filter thickness, beryllium window thickness and detector efficiency at the energy of the X-ray line in question. Detector efficiency was found from data supplied by PGT for both detectors used. The absorption of the Mylar filter and the beryllium window, where applicable was taken into account using the mass absorption coefficients for Mylar and beryllium at the energies of the X-ray line in question. For this calculation, the Excel template used a look up table with mass absorption coefficient values for beryllium and Mylar. This look up table was constructed using WinXCOM written by Gerward et al. [10]. This program was a modification of XCOM, by Berger et al. [11]. These cross sections were then plotted along with theoretical calculations performed using ISICS for the same energies and ions. After the data was plotted, ISICS was used to make predictions for the number of counts that would be expected in the NanoFab focused ion beam, for films of the thickness mentioned. This was done assuming an ion energy of 280 keV and a current of 25 pA for 10 min for lithium, Be and B ions.
93
Al Ka Cr Ka Cu L Ge L Mo L Ag M Ag L WM WL Au M Au L Cr L
Incident ion Li
Be
B
7 0 1661 189 11 9880 1 26 3 3 6 3294
1 0 547 50 2 3890 0 3 0 0 0 1213
0 0 147 13 0 1294 0 0 0 0 0 422
M and L shell contributions are total for all the spectral lines for that shell.
94
K.L. Streib et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 92–94
in the NanoFab for 10 min at a current of 25 pA and an energy of 280 keV for the incident ions listed. It includes not only the effect of cross section, but that of SDD detector efficiency, operating with no beryllium window. 4. Conclusion One can see from the spectral data that the ISICS model can be quite close to experimental cross section data. In the cases where it is not very close, it is within 60% of the experimental value and a conservative estimate, yielding values that are lower than the experimental cross sections. Therefore, in the focused ion beam, spectral intensities would be expected to exceed any values predicted by ISICS. In Table 1, it can be seen that some elements might be detected extremely easily, for example chromium L lines. Others would be very hard to detect. One such case would be gold as even the M lines do not have sufficient cross section to be detected. However, operating windowless, many of the transitions for the lower energy lines have generally high enough cross section to be detected. Given the improvements in detector technology, PIXE in a focused ion beam could be developed into a useful technique.
Acknowledgements The authors wish to acknowledge The National Science Foundation (L. Hess grant # DMR0308127) as well as partial support through the Galvin Chair at Arizona State University. The authors also wish to acknowledge Barry Wilkens for his effort in setting up beryllium ion PIXE in the Arizona State University Tandetron. References [1] L.C. Feldman, J.W. Mayer, Fundamentals of Surface and Thin Film Analysis, North-Holland, NY, 1986, p. 148, 248. [2] S.A.E. Johanssen, J.L. Campbell, PIXE: A Novel Technique for Elemental Analysis, John Wiley and Sons, NY, 1988, p. 287. [3] S.A.E. Johansson, J.L. Campbell, K.G. Malmqvist, Particle Induced X-ray Emission Spectrometry (PIXE), John Wiley and Sons, NY, 1995, p. 101. [4] T.A. Cahill, Ann. Rev. Nucl. Part. Sci. 30 (1980) 211. [5] Z. Liu, S.J. Cipolla, Comput. Phys. Commun. 97 (1996) 315. [6] W. Brandt, G. Lapicki, Phys. Rev. A 20 (2) (1979) 465. [7] W. Brandt, G. Lapicki, Phys. Rev. A 23 (4) (1981) 1717. [8] L. Sarkadi, T. Mukoyama, Nucl. Instr. and Meth. B 61 (1991) 167. [9] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [10] L. Gerward, N. Guillbert, K. Jensen, Lervig H. Bjorn, Radiat. Phys. Chem. 60 (2001) 23. [11] M.J. Berger, J.H. Hubbell, S.M. Seltzer, J.S. Coursey, D.S. Zucker, NBSIR 87-3597, 1987 and 1999. Available from:.
Nuclear Instruments and Methods in Physics Research B 249 (2006) 92–94 www.elsevier.com/locate/nimb
Experimental verification of theoretical cross sections for FIB–PIXE Kenneth L. Streib b
a,*
, Terry L. Alford b, James W. Mayer
a,b
a Center for Solid State Science, Arizona State University, PSA 213, Tempe, AZ 85287-1704, United States Department of Chemical and Materials Engineering, Arizona State University, Tempe, AZ 85287-6006, United States
Available online 19 May 2006
Abstract X-ray production cross sections were found for films of Cr, Cu, Ge, Ag, W and Au, using incident H+ and Be+ ions at energies from 300 keV to 3.5 MeV. These experimental cross section results were compared with the cross section results obtained using software which calculates inner shell ionization and X-ray production cross sections. The software uses the ECPSSR-UA approach to finding X-ray production cross sections. This program was found to be useful for predicting cross sections for H+ and Be+ ions at the energies in this study. The software was then used to predict results for Li+, Be+ and B+ ions at 280 keV, energies available in the Arizona State University focused ion beam laboratory. Ó 2006 Elsevier B.V. All rights reserved. PACS: 32.30.Rj; 41.75.Ak; 32.80.Hd; 33.50.Dq Keywords: PIXE, Particle induced X-ray emission; Focused ion beam, FIB; FIB–PIXE; X-ray emission
1. Introduction In order to perform PIXE in a focused ion beam (FIB), it is necessary to use heavier ions than the H+ and He+ currently used in most ion beam systems. An added advantage of using heavier ions is that less bremsstrahlung radiation is expected [1] and less beam broadening might be expected with the heavier ions [2]. In addition, the NanoFab 150 FIB at ASU can have a spot size as small as 70 nm as opposed to the characteristic 100 nm of most modern microPIXE ion beam systems [3]. One drawback is that the NanoFab 150 is only capable of producing acceleration voltages of up to 150 kV. This limits the energies available to 300 keV with doubly charged ions. It has long been known that at lower energies, there is a decrease in X-ray production cross section with decreased energy [4]. This could limit analysis capabilities with this technique. In order to investigate the feasibility of performing PIXE in a FIB, it is necessary to find an accurate model *
Corresponding author. Fax: +1 480 965 9004. E-mail address: [email protected] (K.L. Streib).
0168-583X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2006.03.087
which will allow prediction of cross sections. The computer program Inner Shell Ionization Cross Sections (ISICS) is used for this [5]. This software uses a modification of the Born plane wave approximation known as the ECPSSR method developed by Brandt and Lapicki [6,7] and the united atom approximation [8]. The ISICS software is used to calculate X-ray production cross sections and to predict results for some X-ray lines. 2. Procedure Films of Cr, Cu, Ge, Ag, W and Au were deposited on Kapton by either evaporative deposition or sputtering. Kapton was chosen as Mylar decomposed due to heat. Rutherford backscattering spectrometry (RBS) was performed on these films using helium ions. Simulations were then performed using the Rutherford Universal Manipulation Program [9] and iterated until the simulation matched the RBS spectrum to infer film thickness. The films were found to be between 53 and 80 nm in thickness. PIXE was performed on these films with H+ and Be+ ions in order to determine the cross sections with two different
K.L. Streib et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 92–94
3. Results and discussion Figs. 1 and 2 show a sample of the cross section versus energy plots for Cr Ka, Cu Ka, Ge Ka, Ag Ka and Au L lines, plotted logarithmically using H+ ions. Fig. 3 shows total Au M and W M cross sections versus energy
Cross section (Barns)
1000
Cross section (Barns)
1000 100 10 1 Experimental Au L H+ ions Theoretical Au L H+ ions Experimental Cr Kα H+ ions Theoretical Cr Kα H+ ions Experimental Ag Kα H+ ions Theoretical Ag Kα H+ ions
0.1 0.01 0.001 0.0001 0
Experimental Cu Kα H+ ions Theoretical Cu Kα H+ ions
0.1
Experimental Ge Kα H+ ions Theoretical Ge Kα H+ ions
0.01 0
1
2 Energy (MeV)
3
4
Fig. 1. A logarithmic plot of Cu Ka and Ge Ka cross section results versus energy incident H+ ions. Note the excellent agreement for both series and that theoretical values underestimate experimental values.
1.5
2
2.5
3
3.5
4
100 10 1 0.1 Experimental W M Be+ ions Theoretical W M Be+ ions"
0.01
0.001 0.25
Experimental Au M Be+ ions Theoretical Au M Be+ ions
0.3
0.35
0.4
0.45
0.5
0.55
Energy (MeV) Fig. 3. A logarithmic plot of total Au M and total W M cross section results versus energy incident Be+ ions. Note that theoretical values underestimate experimental values.
for Be+ ions. The ISICS calculations are labeled ‘‘Theoretical’’. Table 1 contains number of counts for some selected spectral lines that would be expected if spectra were taken
Table 1 Spectral counts expected with windowless SDD detector for films of the thickness in this investigation, irradiated with 25 pA of ion beam current for 10 min, at 280 keV X-ray line
1
1
Fig. 2. A logarithmic plot of Cr Ka, Ag Ka and total Au L cross section results versus energy incident H+ ions. Note the agreement is as good as in Fig. 1.
100 10
0.5
Energy (MeV)
Cross sections (Barns)
ion masses. Data was taken at 0.3, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 MeV. At higher energies a lithium drifted detector manufactured by Princeton Gamma Tech (PGT) was used. However, at lower energies (300 and 500 keV), it was discovered that the noise from a silicon drifted detector (SDD) manufactured by PGT was lower and this was installed temporarily on the Tandetron. Due to limited beam time, data for all combinations of the above ions and energies could not be taken. Enough data was taken, however, to compare with the ISICS program [5]. A Microsoft Excel template was used to determine cross sections from this data using the variables of total incident charge, film thickness, detector solid angle, Mylar filter thickness, beryllium window thickness and detector efficiency at the energy of the X-ray line in question. Detector efficiency was found from data supplied by PGT for both detectors used. The absorption of the Mylar filter and the beryllium window, where applicable was taken into account using the mass absorption coefficients for Mylar and beryllium at the energies of the X-ray line in question. For this calculation, the Excel template used a look up table with mass absorption coefficient values for beryllium and Mylar. This look up table was constructed using WinXCOM written by Gerward et al. [10]. This program was a modification of XCOM, by Berger et al. [11]. These cross sections were then plotted along with theoretical calculations performed using ISICS for the same energies and ions. After the data was plotted, ISICS was used to make predictions for the number of counts that would be expected in the NanoFab focused ion beam, for films of the thickness mentioned. This was done assuming an ion energy of 280 keV and a current of 25 pA for 10 min for lithium, Be and B ions.
93
Al Ka Cr Ka Cu L Ge L Mo L Ag M Ag L WM WL Au M Au L Cr L
Incident ion Li
Be
B
7 0 1661 189 11 9880 1 26 3 3 6 3294
1 0 547 50 2 3890 0 3 0 0 0 1213
0 0 147 13 0 1294 0 0 0 0 0 422
M and L shell contributions are total for all the spectral lines for that shell.
94
K.L. Streib et al. / Nucl. Instr. and Meth. in Phys. Res. B 249 (2006) 92–94
in the NanoFab for 10 min at a current of 25 pA and an energy of 280 keV for the incident ions listed. It includes not only the effect of cross section, but that of SDD detector efficiency, operating with no beryllium window. 4. Conclusion One can see from the spectral data that the ISICS model can be quite close to experimental cross section data. In the cases where it is not very close, it is within 60% of the experimental value and a conservative estimate, yielding values that are lower than the experimental cross sections. Therefore, in the focused ion beam, spectral intensities would be expected to exceed any values predicted by ISICS. In Table 1, it can be seen that some elements might be detected extremely easily, for example chromium L lines. Others would be very hard to detect. One such case would be gold as even the M lines do not have sufficient cross section to be detected. However, operating windowless, many of the transitions for the lower energy lines have generally high enough cross section to be detected. Given the improvements in detector technology, PIXE in a focused ion beam could be developed into a useful technique.
Acknowledgements The authors wish to acknowledge The National Science Foundation (L. Hess grant # DMR0308127) as well as partial support through the Galvin Chair at Arizona State University. The authors also wish to acknowledge Barry Wilkens for his effort in setting up beryllium ion PIXE in the Arizona State University Tandetron. References [1] L.C. Feldman, J.W. Mayer, Fundamentals of Surface and Thin Film Analysis, North-Holland, NY, 1986, p. 148, 248. [2] S.A.E. Johanssen, J.L. Campbell, PIXE: A Novel Technique for Elemental Analysis, John Wiley and Sons, NY, 1988, p. 287. [3] S.A.E. Johansson, J.L. Campbell, K.G. Malmqvist, Particle Induced X-ray Emission Spectrometry (PIXE), John Wiley and Sons, NY, 1995, p. 101. [4] T.A. Cahill, Ann. Rev. Nucl. Part. Sci. 30 (1980) 211. [5] Z. Liu, S.J. Cipolla, Comput. Phys. Commun. 97 (1996) 315. [6] W. Brandt, G. Lapicki, Phys. Rev. A 20 (2) (1979) 465. [7] W. Brandt, G. Lapicki, Phys. Rev. A 23 (4) (1981) 1717. [8] L. Sarkadi, T. Mukoyama, Nucl. Instr. and Meth. B 61 (1991) 167. [9] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [10] L. Gerward, N. Guillbert, K. Jensen, Lervig H. Bjorn, Radiat. Phys. Chem. 60 (2001) 23. [11] M.J. Berger, J.H. Hubbell, S.M. Seltzer, J.S. Coursey, D.S. Zucker, NBSIR 87-3597, 1987 and 1999. Available from: