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Non-Rutherford backscattering analysis of nitrogen content in titanium substrates
Nuclear Instruments and Methods in Physics Research B79 (1993) 454-456 North-Holland
Non-Rutherford backscattering in titanium substrates
NIONI B
Beam Interactions with Materials&Atoms
analysis of nitrogen content
L.A. Foster Nuclear Engineering Department,
Texas A&M University, College Station,
TX 77843, USA
J.R. Tesmer, T.R. Jervis and M. Nastasi Los Alamos National Laboratory,
Los Alamos,
New Mexico, 87544, USA
The cross section for 167” b$scattering of 4He from 14N in the energy range from 4.5 to 9.0 MeV was measured. The targets were made by evaporating 500 A of zirconium on a carbon substrate and implanting with 1.3X 10” N,/cm’ at 33 keV. A large scattering resonance, approximately 80 times Rutherford and FWHM > 200 keV, was found at 8.81 MeV. Titanium samples were irradiated using pulsed excimer laser radiation at 248 nm in nitrogen gas at 1 atm. The nitrogen content was measured using the 14N alpha scattering resonance at 8.81 MeV. For fluences greater than 500 J/cm’, concentrations of nitrogen approaching that of TiN were observed. The thickness of the nitride layer is on the order of 0.35 pm.
1. Introduction
2. Experiment and results
The Rutherford backscattering technique is routinely used for compositional depth profiling of surface layers. Because of the inverse 2’ dependence of the scattering cross section, the sensitivity for the detection of light elements in a heavy substrate is very poor. High-energy non-Rutherford scattering can be used to improve this sensitivity. Above a few MeV the cross section for He elastic scattering from light elements can exhibit drastic deviations from the Rutherford values. Narrow resonances and smooth plateaus have been found and used in conjunction with RBS analysis [l-3]. A large number of cross sections have been measured, but the database is not complete [l]. The 14N cross section has been previously measured in the energy range from 1.0 to 7.5 MeV [4-81. In addition, a measurement of the 14N(cx, a) cross section from 9.1 to 9.6 MeV has recently been reported at a scattering angle of 177” [9]. A very large resonance, about 190 times Rutherford and 200 keV wide, was observed near 9.3 MeV, and a resonance at 8.8 MeV approximately 60 keV wide was mentioned [lo]. In the present work the 14N(ol, u) scattering cross section was measured from 4.5 to 9.0 MeV. The measured cross section was used to determine the nitrogen content of titanium samples which were laser processed in a nitrogen atmosphere 1111.
Targets for the cross section measurement were prepared by implanting nitrogen into a thin zir$onium layer prepared by e-beam evaporation. A 500 A layer of Zr was evaporated onto a carbon substrate. The Zr layer was then implanted with 1.3 X 10” N,/cm’ at an energy of 33 keV. Rutherford backscattering analysis with 2.0 MeV He was used to determine the composition of the target. The surface layer contained 1.86 x 1Ol7 Zr/cm’. The ratio of N to Zr was 1.2: 1. Additionally, a significant amount of oxygen was incorporated into the Zr layer during the evaporation. The ratio of 0 to Zr was 0.5 : 1. The target was very stable during the 2.0 MeV analysis and during the subsequent high energy cross section measurements. No loss of nitrogen from the target was detected. The energy thickness of the ZrN layer was 14 keV at 5.0 MeV and 10 keV at 9.0 MeV. The He-ion beam for the cross section measurement was obtained from the 3 MV tandem accelerator at the Ion Beam Materials Laboratory at Los Alamos National Laboratory. The backscattering angle was 167”. Standard Si surface barrier detectors were used. The typical energy resolution was about 20 keV. The solid angle of the detector was approximately 2 msr. The target was biased at +300 V to suppress the ejection of secondary electrons. A pulser configured to
0168-583X/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved
LA. Foster et al. / Non-Rutherford
backscattering
analysis of nitrogen content
“N(a,a) k3,* =
40.0
1
Ratio
to Rutherford
167”
-
30.0
10.0 0.0
0
CrossSection
455
4.5
5.0
5.5
6.0
6.5
7.0
Lab Energy
7.5
6.0
6.5
9.0
9.5
W.OF....‘....‘....‘....‘....‘....’..”~ 6.65
6.70
6.75
6.60
Lab
(MN)
Fig. 1. Ratio of the measured 14N(a, (Y) scattering cross section to the Rutherford scattering cross section. The lahoratory scattering angle is 167”.
output a given number of counts per unit charge collected was used for dead time correction. Typical dead time corrections were 3-7%. The error in the beam energy at 9.0 MeV is estimated to be f50 keV. Neglecting the energy loss in the ZrN layer, the ratio of the cross section at an energy E to the theoretical Rutherford cross section can be written as
(1) where A, and A, are the areas for the 14N and the Zr peaks and E, is a reference energy where Zr and N are both in the Rutherford scattering regime. Rutherford scattering is assumed for the Zr up to the highest energy of interest. In this work 2.0 MeV was used as the reference energy. The i4N scattering cross section from 4.5 to 9.0 MeV is shown in fig. 1. The cross section was measured in steps of 20 keV except near sharp resonances where steps of 10 keV were used. The cross section was not measured in the region from 6.6 to 7.1 MeV due to interference from an (OL,p) reaction. The large resonance at 8.81 MeV is shown in fig. 2. The cross section at the resonance maximum is approximately 80 times the Rutherford value. The resonance has a full width at half maximum (PWHM) of about 220 keV. The resonance at 8.81 MeV was used to measure the nitrogen content of laser treated titanium. The Ti samples, Annealed Commercial Pure Ti, were laser processed using pulsed excimer laser radiation at 248 nm from a KrF laser. A multielement refractive homogenizer provided a square spot approximately 0.5 cm on a side with uniform energy density on the target. The samples were translated in front of the beam at a calibrated rate to provide a given number of laser pulses per position on the sample. The laser energy density was 972 mJ/cm* per pulse and the pulse
6.85
Energy
6.90
6.95
9.00
(WV)
Fig. 2. Ratio of the measured 14N(a, a) scattering cross section to the Rutherford scattering cross section showing the large scattering resonance at 8.81 MeV.
frequency was 2.5 Hz. The samples were irradiated in a N, filled chamber at a pressure of 1 atm. The chamber was evacuated with a turbo pump and flushed with N, before being backfilled for the laser treatment. Samples were processed with 25, 50, 100, 400, 800, and 1200 pulses per position. The laser treated samples were analyzed using 8.81 MeV He ion beams. The ZrN/C target was used to adjust the beam energy to the resonance maximum before taking data. Typical RBS spectra of the laser treated samples are shown in fig. 3. Unprocessed Ti samples and samples treated in vacuum were also analyzed. No nitrogen was observed on these samples. Analysis of very thin nitride layers, less than 1000 A, is very straight forward using this resonance. Because the resonance is very broad, the cross section can be assumed to be constant in the nitride layer. In the present work a first approximation to the N content was obtained by taking the ratio of the 14N peak areas from the laser treated samples to the ZrN/C target
. . . . 25 pulses per
-
-
step
400 pulses per step 1200 pulses per step
OF.““‘.““..“‘.‘.11...~ 200
300
400 Channel
500
600
700
Number
Fig. 3. Typical RBS spectra of 8.81 MeV He on laser processed Ti samples. The energy density of the laser was 972 mJ per pulse. Samples with 25, 400, and 1200 pulses per position are shown. VII. PIXE, MICROPROBES,
...
LA. Foster et al. / Non-Rutherford backscattering analysis of nitrogen content
456
and multiplying by the nitrogen content of the ZrN/C target. The absolute nitrogen content of the ZrN/C target was determined from the 2.0 MeV He RBS spectra using RUMP with the C substrate as the reference. This simple approach underestimates the N content of thicker nitride layers since the scattering cross section is decreasing with increasing depth. Since the resonance width is significantly larger than the energy straggling for shallow penetration depths, the resonance can be treated as a slowly varying cross section. The number of counts in a given channel, Ni, was weighted with the inverse of the scattering cross section for the average collision energy of that channel. The N concentration at a depth xi is given by [12], =N,/(QRg(Ei)Axi),
(2) where Q is the charge collected, 0 is the solid angle, and Axi is the width of the slice of the target corresponding to the ith channel. Numerical integration of C,(x) gives the total N content. Bragg’s rule was used to calculate a new stopping power for the layer and the profile was recalculated. The oxygen content was neglected in the depth scale calculation. The profile converged after three iterations or less in each case. The results are shown in fig. 4. For the shallow profiles, 25 and 50 pulses, the two methods agree very well, < 3%. The correction increases as the nitrogen penetration depth increases. The correction was 16% for the deepest profile, 1200 pulses per position. The depth of the nitride layer on the 1200 pulse sample is on the order of 0.35 pm and the N concentration at the surface is estimated to be 4.2 X lo** N/cm3. C,(ni)
1..
*
I
“E z 1o'8 7
8
1.
I,
-.
I
-
*
-
I
-.
-
I..
.
?
i f
H
.
8
b Z =
-
10"
!
FS :f Eachlaserpulse=lJ/cm2
f IO"
..r'.,I'...'...'.,,'.,.'.., 0 200 400 600 600
1000
1200
1400
Number of Laser Pulses
Fig. 4. Nitrogen area1 density (atoms/cm’) as a function of the number of laser pulses per step measured using 8.81 MeV He resonant scattering.
3. Conclusion The r4N(a, a) scattering cross section was measured from 4.5 to 9.0 MeV. A wide resonance useful for analyzing thin nitride layers with a cross section 80 times the Rutherford value was found at 8.81 MeV. The nitrogen measurement technique was demonstrated by measuring the N content of Ti which was laser processed in a nitrogen atmosphere. The detection sensitivity for a nitride surface layer on Ti is estimated to be 1 x 1016 N/cm*. When bombarding substrates with highly energetic light ions, large numbers of neutrons and gamma rays can be produced. Radiation levels should be carefully monitored.
Acknowledgements
We would like to thank Mark Hollander and Caleb Evans of the Los Alamos Ion Beam Materials Laboratory for their help and technical assistance. This research was performed under appointment to the Nuclear Engineering and Health Physics Fellowship program administered by Oak Ridge Associated Universities for the U.S. Department of Energy.
ill J.A. Leavitt and L.C. McIntyre Jr., Nucl. Instr. and Meth. B56/57 (1991) 734.
El V. Hnatowicz, H. Macholdt and F.-W. Richter, Nucl. Instr. and Meth. B62 (1991) 247.
t31 C.R. Gossett, Nucl. Instr. and Meth. B40/41 (1989) 813. [41 E.A. Silverstein, S.R. Salisbury, G. Hardie and L.D. Oppliger, Phys. Rev. 124 (1961) 868. [51 D.F. Herring, R. Chiba, B.R. Gasten and H.T. Richards, Phys. Rev. 112 (1958) 1210. [61 E. Kashy, P.D. Miller and J.R. Risser, Phys. Rev. 112 (1958) 547. [71 D.F. Herring, Phys. Rev. 112 (1958) 1217. k31 H. Artigalas, A. Chevarier, N. Chevarier, M. El Bovanani, E. Gerlic, N. Moncoffre, B. Row, M. Stern and .I. Tousset, Nucl. Instr. and Meth. B66 (1992) 237. 191Yuanxun Qiu, A.P. Rice and T.A. Tombrello, Nucl. Instr. and Meth. B71 (1992) 324. [lOI L. Pan, Y. Qiu, Z. Zhou, 0. Zhao, J. Tang and F. Yang, to be published. [ill E. D’Anna, G. Leggieri, A. Luches, M. Martino, A.V. Drigo, I.N. Mihailescu and S. Ganatsios, J. Appl. Phys. 69 (1991) 1687. WI W.-K. Chu, M.-A. Nicolet and J.M. Mayer, Backscattering Spectrometry (Academic Press, New York, 1977).
Non-Rutherford backscattering in titanium substrates
NIONI B
Beam Interactions with Materials&Atoms
analysis of nitrogen content
L.A. Foster Nuclear Engineering Department,
Texas A&M University, College Station,
TX 77843, USA
J.R. Tesmer, T.R. Jervis and M. Nastasi Los Alamos National Laboratory,
Los Alamos,
New Mexico, 87544, USA
The cross section for 167” b$scattering of 4He from 14N in the energy range from 4.5 to 9.0 MeV was measured. The targets were made by evaporating 500 A of zirconium on a carbon substrate and implanting with 1.3X 10” N,/cm’ at 33 keV. A large scattering resonance, approximately 80 times Rutherford and FWHM > 200 keV, was found at 8.81 MeV. Titanium samples were irradiated using pulsed excimer laser radiation at 248 nm in nitrogen gas at 1 atm. The nitrogen content was measured using the 14N alpha scattering resonance at 8.81 MeV. For fluences greater than 500 J/cm’, concentrations of nitrogen approaching that of TiN were observed. The thickness of the nitride layer is on the order of 0.35 pm.
1. Introduction
2. Experiment and results
The Rutherford backscattering technique is routinely used for compositional depth profiling of surface layers. Because of the inverse 2’ dependence of the scattering cross section, the sensitivity for the detection of light elements in a heavy substrate is very poor. High-energy non-Rutherford scattering can be used to improve this sensitivity. Above a few MeV the cross section for He elastic scattering from light elements can exhibit drastic deviations from the Rutherford values. Narrow resonances and smooth plateaus have been found and used in conjunction with RBS analysis [l-3]. A large number of cross sections have been measured, but the database is not complete [l]. The 14N cross section has been previously measured in the energy range from 1.0 to 7.5 MeV [4-81. In addition, a measurement of the 14N(cx, a) cross section from 9.1 to 9.6 MeV has recently been reported at a scattering angle of 177” [9]. A very large resonance, about 190 times Rutherford and 200 keV wide, was observed near 9.3 MeV, and a resonance at 8.8 MeV approximately 60 keV wide was mentioned [lo]. In the present work the 14N(ol, u) scattering cross section was measured from 4.5 to 9.0 MeV. The measured cross section was used to determine the nitrogen content of titanium samples which were laser processed in a nitrogen atmosphere 1111.
Targets for the cross section measurement were prepared by implanting nitrogen into a thin zir$onium layer prepared by e-beam evaporation. A 500 A layer of Zr was evaporated onto a carbon substrate. The Zr layer was then implanted with 1.3 X 10” N,/cm’ at an energy of 33 keV. Rutherford backscattering analysis with 2.0 MeV He was used to determine the composition of the target. The surface layer contained 1.86 x 1Ol7 Zr/cm’. The ratio of N to Zr was 1.2: 1. Additionally, a significant amount of oxygen was incorporated into the Zr layer during the evaporation. The ratio of 0 to Zr was 0.5 : 1. The target was very stable during the 2.0 MeV analysis and during the subsequent high energy cross section measurements. No loss of nitrogen from the target was detected. The energy thickness of the ZrN layer was 14 keV at 5.0 MeV and 10 keV at 9.0 MeV. The He-ion beam for the cross section measurement was obtained from the 3 MV tandem accelerator at the Ion Beam Materials Laboratory at Los Alamos National Laboratory. The backscattering angle was 167”. Standard Si surface barrier detectors were used. The typical energy resolution was about 20 keV. The solid angle of the detector was approximately 2 msr. The target was biased at +300 V to suppress the ejection of secondary electrons. A pulser configured to
0168-583X/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved
LA. Foster et al. / Non-Rutherford
backscattering
analysis of nitrogen content
“N(a,a) k3,* =
40.0
1
Ratio
to Rutherford
167”
-
30.0
10.0 0.0
0
CrossSection
455
4.5
5.0
5.5
6.0
6.5
7.0
Lab Energy
7.5
6.0
6.5
9.0
9.5
W.OF....‘....‘....‘....‘....‘....’..”~ 6.65
6.70
6.75
6.60
Lab
(MN)
Fig. 1. Ratio of the measured 14N(a, (Y) scattering cross section to the Rutherford scattering cross section. The lahoratory scattering angle is 167”.
output a given number of counts per unit charge collected was used for dead time correction. Typical dead time corrections were 3-7%. The error in the beam energy at 9.0 MeV is estimated to be f50 keV. Neglecting the energy loss in the ZrN layer, the ratio of the cross section at an energy E to the theoretical Rutherford cross section can be written as
(1) where A, and A, are the areas for the 14N and the Zr peaks and E, is a reference energy where Zr and N are both in the Rutherford scattering regime. Rutherford scattering is assumed for the Zr up to the highest energy of interest. In this work 2.0 MeV was used as the reference energy. The i4N scattering cross section from 4.5 to 9.0 MeV is shown in fig. 1. The cross section was measured in steps of 20 keV except near sharp resonances where steps of 10 keV were used. The cross section was not measured in the region from 6.6 to 7.1 MeV due to interference from an (OL,p) reaction. The large resonance at 8.81 MeV is shown in fig. 2. The cross section at the resonance maximum is approximately 80 times the Rutherford value. The resonance has a full width at half maximum (PWHM) of about 220 keV. The resonance at 8.81 MeV was used to measure the nitrogen content of laser treated titanium. The Ti samples, Annealed Commercial Pure Ti, were laser processed using pulsed excimer laser radiation at 248 nm from a KrF laser. A multielement refractive homogenizer provided a square spot approximately 0.5 cm on a side with uniform energy density on the target. The samples were translated in front of the beam at a calibrated rate to provide a given number of laser pulses per position on the sample. The laser energy density was 972 mJ/cm* per pulse and the pulse
6.85
Energy
6.90
6.95
9.00
(WV)
Fig. 2. Ratio of the measured 14N(a, a) scattering cross section to the Rutherford scattering cross section showing the large scattering resonance at 8.81 MeV.
frequency was 2.5 Hz. The samples were irradiated in a N, filled chamber at a pressure of 1 atm. The chamber was evacuated with a turbo pump and flushed with N, before being backfilled for the laser treatment. Samples were processed with 25, 50, 100, 400, 800, and 1200 pulses per position. The laser treated samples were analyzed using 8.81 MeV He ion beams. The ZrN/C target was used to adjust the beam energy to the resonance maximum before taking data. Typical RBS spectra of the laser treated samples are shown in fig. 3. Unprocessed Ti samples and samples treated in vacuum were also analyzed. No nitrogen was observed on these samples. Analysis of very thin nitride layers, less than 1000 A, is very straight forward using this resonance. Because the resonance is very broad, the cross section can be assumed to be constant in the nitride layer. In the present work a first approximation to the N content was obtained by taking the ratio of the 14N peak areas from the laser treated samples to the ZrN/C target
. . . . 25 pulses per
-
-
step
400 pulses per step 1200 pulses per step
OF.““‘.““..“‘.‘.11...~ 200
300
400 Channel
500
600
700
Number
Fig. 3. Typical RBS spectra of 8.81 MeV He on laser processed Ti samples. The energy density of the laser was 972 mJ per pulse. Samples with 25, 400, and 1200 pulses per position are shown. VII. PIXE, MICROPROBES,
...
LA. Foster et al. / Non-Rutherford backscattering analysis of nitrogen content
456
and multiplying by the nitrogen content of the ZrN/C target. The absolute nitrogen content of the ZrN/C target was determined from the 2.0 MeV He RBS spectra using RUMP with the C substrate as the reference. This simple approach underestimates the N content of thicker nitride layers since the scattering cross section is decreasing with increasing depth. Since the resonance width is significantly larger than the energy straggling for shallow penetration depths, the resonance can be treated as a slowly varying cross section. The number of counts in a given channel, Ni, was weighted with the inverse of the scattering cross section for the average collision energy of that channel. The N concentration at a depth xi is given by [12], =N,/(QRg(Ei)Axi),
(2) where Q is the charge collected, 0 is the solid angle, and Axi is the width of the slice of the target corresponding to the ith channel. Numerical integration of C,(x) gives the total N content. Bragg’s rule was used to calculate a new stopping power for the layer and the profile was recalculated. The oxygen content was neglected in the depth scale calculation. The profile converged after three iterations or less in each case. The results are shown in fig. 4. For the shallow profiles, 25 and 50 pulses, the two methods agree very well, < 3%. The correction increases as the nitrogen penetration depth increases. The correction was 16% for the deepest profile, 1200 pulses per position. The depth of the nitride layer on the 1200 pulse sample is on the order of 0.35 pm and the N concentration at the surface is estimated to be 4.2 X lo** N/cm3. C,(ni)
1..
*
I
“E z 1o'8 7
8
1.
I,
-.
I
-
*
-
I
-.
-
I..
.
?
i f
H
.
8
b Z =
-
10"
!
FS :f Eachlaserpulse=lJ/cm2
f IO"
..r'.,I'...'...'.,,'.,.'.., 0 200 400 600 600
1000
1200
1400
Number of Laser Pulses
Fig. 4. Nitrogen area1 density (atoms/cm’) as a function of the number of laser pulses per step measured using 8.81 MeV He resonant scattering.
3. Conclusion The r4N(a, a) scattering cross section was measured from 4.5 to 9.0 MeV. A wide resonance useful for analyzing thin nitride layers with a cross section 80 times the Rutherford value was found at 8.81 MeV. The nitrogen measurement technique was demonstrated by measuring the N content of Ti which was laser processed in a nitrogen atmosphere. The detection sensitivity for a nitride surface layer on Ti is estimated to be 1 x 1016 N/cm*. When bombarding substrates with highly energetic light ions, large numbers of neutrons and gamma rays can be produced. Radiation levels should be carefully monitored.
Acknowledgements
We would like to thank Mark Hollander and Caleb Evans of the Los Alamos Ion Beam Materials Laboratory for their help and technical assistance. This research was performed under appointment to the Nuclear Engineering and Health Physics Fellowship program administered by Oak Ridge Associated Universities for the U.S. Department of Energy.
ill J.A. Leavitt and L.C. McIntyre Jr., Nucl. Instr. and Meth. B56/57 (1991) 734.
El V. Hnatowicz, H. Macholdt and F.-W. Richter, Nucl. Instr. and Meth. B62 (1991) 247.
t31 C.R. Gossett, Nucl. Instr. and Meth. B40/41 (1989) 813. [41 E.A. Silverstein, S.R. Salisbury, G. Hardie and L.D. Oppliger, Phys. Rev. 124 (1961) 868. [51 D.F. Herring, R. Chiba, B.R. Gasten and H.T. Richards, Phys. Rev. 112 (1958) 1210. [61 E. Kashy, P.D. Miller and J.R. Risser, Phys. Rev. 112 (1958) 547. [71 D.F. Herring, Phys. Rev. 112 (1958) 1217. k31 H. Artigalas, A. Chevarier, N. Chevarier, M. El Bovanani, E. Gerlic, N. Moncoffre, B. Row, M. Stern and .I. Tousset, Nucl. Instr. and Meth. B66 (1992) 237. 191Yuanxun Qiu, A.P. Rice and T.A. Tombrello, Nucl. Instr. and Meth. B71 (1992) 324. [lOI L. Pan, Y. Qiu, Z. Zhou, 0. Zhao, J. Tang and F. Yang, to be published. [ill E. D’Anna, G. Leggieri, A. Luches, M. Martino, A.V. Drigo, I.N. Mihailescu and S. Ganatsios, J. Appl. Phys. 69 (1991) 1687. WI W.-K. Chu, M.-A. Nicolet and J.M. Mayer, Backscattering Spectrometry (Academic Press, New York, 1977).