- Email: [email protected]
Round robin analyses of hydrogen isotope thin films standards
Nuclear Instruments and Methods in Physics Research B 219–220 (2004) 444–449 www.elsevier.com/locate/nimb
Round robin analyses of hydrogen isotope thin films standards q J.C. Banks a,*, J.F. Browning a, W.R. Wampler a, B.L. Doyle a, C.A. LaDuca a, J.R. Tesmer b, C.J. Wetteland b, Y.Q. Wang c,1 a
Sandia National Laboratories, P.O. Box 5800, MS-1056, Albuquerque, NM 87185-1056, USA b Los Alamos National Laboratory, Los Alamos, NM 87545, USA c University of Minnesota, Minneapolis, MN 55455, USA
Abstract Hydrogen isotope thin film standards have been manufactured at Sandia National Laboratories for use by the materials characterization community. Several considerations were taken into account during the manufacture of the ErHD standards, with accuracy and stability being the most important. The standards were fabricated by e-beam deposition of Er onto a Mo substrate and the film stoichiometrically loaded with hydrogen and deuterium. To determine the loading accuracy of the standards two random samples were measured by thermal desorption mass spectrometry and atomic absorption spectrometry techniques with a stated combined accuracy of 1.6% (1r). All the standards were then measured by high energy RBS/ERD and RBS/NRA with the accuracy of the techniques 5% (1r). The standards were then distributed to the IBA materials characterization community for analysis. This paper will discuss the suitability of the standards for use by the IBA community and compare measurement results to highlight the accuracy of the techniques used. 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen isotope standards; Round robin; Rutherford backscattering spectrometry; ERD; NRA
1. Introduction
q Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC0494AL85000. * Corresponding author. Present address: Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. Tel.: +1-505-844-8824; fax: +1505-844-7775. E-mail address: [email protected] (J.C. Banks). 1 Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
There is a need for highly accurate and stable thin film hydrogen isotope standards that can be used to improve the techniques used for measuring H-isotopes in thin and bulk materials. The need to characterize hydrogen in materials is very important to many areas, including the semiconductor, transportation and nuclear industries. This need will grow if there is a transition to a hydrogen economy in coming years. Although this transition will be complex, the benefits are promising. According to the National Hydrogen Energy
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.099
J.C. Banks et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 444–449
Roadmap the transition has the potential to resolve issues involving energy supplies and air pollution [1]. To meet the need for thin film H-isotope standards Sandia National Laboratories conducted a workshop to discuss their development [2] and has recently fabricated highly stable and accurate ErH2 , ErD2 and ErHD thin film standards for use by the materials characterization community. Fifteen of the ErHD standards were distributed to interested participants worldwide to determine their usefulness and to compare the accuracy of the techniques used to measure them. A third of the participants reported results.
2. Selection and fabrication of ErHD standards Several types of hydrogen isotope standards have been used in the past for calibrating H measurement techniques. Polymer films have been used extensively, as well as H implanted Si, SiNx Hy and a-C:D films on Si [3–5]. Some of the drawbacks of using these films are beam induced hydrogen loss and interfering Si(a, p) reactions at higher beam energies. These considerations were taken into account when choosing to manufacture ErHD standards, where accuracy and stability were of primary importance. The standards were fabricated by e-beam deposition of 0.5 lm (±5%) of Er onto a clean 12.7 mm diameter, 0.5 mm thick Mo substrate and the film stoichiometrically loaded with hydrogen and deuterium. The substrate was cleaned prior to deposition by hydrogen and vacuum firing. The Er was deposited using a substrate temperature of 450 C and deposition rate of 600 A/min in a 107 Torr vacuum. Loading was also done at 450 C at a pressure of 10–20 Torr for 10 min. The standards were stored in a N2 atmosphere to reduce interaction of the films with any contaminants. After fabrication the films were measured using high-energy He RBS, taking advantage of the resonance in the O cross-section at 10 MeV, to determine the thickness of the surface erbia (Er2 O3 ) layer (known to be self-limiting) and bulk oxygen content that may have been incorporated during processing. Large amounts of oxygen could affect the film integrity. The O profiles for the standards were all similar and a sim-
445
ulation of the profile for the ErHD#9 standard using SIMNRA software [6] and John’s O crosssections [7] gave a surface erbia thickness of 2.0 · 1016 Er2 O3 /cm2 or 4.8 nm and bulk O content of 3.8 at.%.
3. Experimental techniques The thermal desorption/mass spectrometry and atomic absorption spectrometry techniques were used to measure the load ratio (H+D/Er) of the standards. Both use commercially available instruments. Hydrogen isotope concentrations were measured by thermally desorbing the film and analyzing the evolved gas on a Finnigan MAT 271 hydrogen isotope mass spectrometer. Film areas were measured using a Nikon MM-11 Measure-Scope. The estimated uncertainty in both the mass spectrometric analysis and film area determination is 1.4%. The number of Er atoms in the film was determined by chemically etching the Er film from the Mo substrate and measuring the solution using a Perkin Elmer 5100 PC atomic absorption spectrometer. The overall uncertainty in this measurement is 0.6%. A summary of the analysis parameters used by the IBA techniques is shown in Table 1. The RBS/ ERD techniques used at Sandia National Laboratories (Sandia HDT) and Los Alamos National Laboratories (LANL) have been described in [8,9]. The RBS/ERD techniques employed by both labs use high-energy 4 He2þ beams to enable sampling at greater depth and to allow for the separation of the three hydrogen isotopes, as shown in Fig. 1. Measurements for the Sandia HDT system were made using first principal calculations, i.e. using the thin film yield equation and well known analysis parameters. A ErHD standard was used to monitor for any systematic change. The mean energy of the beam within the film was used to calculate both the Rutherford cross-sections for Er and the non-Rutherford recoil cross-sections for H and D. Sandia measured values for the H and D cross-sections were used. The values have been compared to those in the literature and found to be in good agreement [10]. The detector solid angles were determined using a NIST certified (1.3% 1r)
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Table 1 Analysis parameters used by the IBA techniques to determine the areal densities of Er, H and D in the Sandia ErHD standards Laboratory
Beam/ E1 =E2 a (MeV)
Detector angleb ()
Solid angle (msr)
Tilt angle ()
Total charge (plC)
Beam current (nA)
ERD range foil thickness (lm)
References
Overall uncertainty (%)
University of Minnesota LANL
a/5.0/5.0
165/30
0/75
5/5
10/10
28 Mylar
155/30
N/A
N/A
N/A
50 Al + 8 Mylar
a/10.0/ 10.0 3 He/2.0/ 2.0
165/30
75/75
1/1
10/10
109 Mylar
Bi–Si, d-PS/Si Au, ErD2 , ErH2 ErHDd
5
a/7.6/9.4
3.84/ 1.17 N/A
0
2
1.5
N/A
Aud
5
Sandia (HDT) Sandia
6.89/ 1.29 11
165
2.1c
5
a
E1 ¼ RBS and E2 ¼ ERD or NRA. LANL RBS at 7.6 MeV done at 165. c Deuterium only. d Used for calibration check. b
∆ Energy (Channel)
200
150
100 Tiritium
50
Deuterium Hydrogen
50
100
150
200
Energy (Channel) Fig. 1. Both the Sandia and LANL ERD analysis systems use a particle telescope to allow separation of all three hydrogen isotopes for analysis. The separation of H isotopes using a 10 MeV a-beam in the Sandia analysis system is shown. 238
Pu a-source. The total number of ions hitting a standard was determined using a beam current monitoring system that includes detection of backscattered He particle hitting a rotating Au coated blade intercepting the beam in front of the sample. The integrated charge collected into a Faraday cup during calibration runs prior to analyses was used to find the accumulated charge and calculate the total number of ions for each measurement. The charge collected during cali-
bration runs was measured with a digital current integrator that was calibrated by the Primary Standards Lab at Sandia. The uncertainty in this measurement is 2%. Both LANL and the University of Minnesota used other standards (references) for quantifying the ErHD standard constituents. LANL used a two-step process to analyze the ErHD#9 standard. First, the hydrogen isotope to Er ratio for the standard was determined by making simultaneous RBS and ERD measurements using a 9.4 MeV 4 He2þ analysis beam, first on the standard and then on a well characterized ErD2 reference and finally on an ErH2 reference. Both references are similar to the Sandia standards analyzed in this round robin. The ratio of H/Er in the ErH2 reference was not known, but taken to be 2:1. The geometry of the system used for these measurements is also not well known, but highly reproducible [9]. The ratio of H or D to Er is determined using the thin target yield equation, NHD =NEr ¼ bðYHD =YEr Þ=ðYHD =YEr Þref c ðNHD =NEr Þref . Second, the areal density for the Er film on the Sandia HD#9 standard was determined by RBS using a beam energy of 7.6 MeV at normal incidence. The amount of Er was determined by comparison with a Au reference (certified areal density ±1.2% 1r) [11] using the thin target yield equation, NEr ¼ NAu ðYEr =YAu ÞðrAu =rEr Þ and Rutherford cross-sections for Er and Au. The two quantities (H+D/Er and Er areal densities) were then multiplied
J.C. Banks et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 444–449
together to get the H and D areal densities. The uncertainty in the D measurement is 2.1%. The dominant uncertainty in the Er areal density measurement is the uncertainty in the Au standard. At the University of Minnesota the Er content was determined using a 5.0 MeV He beam at a sample tilt angle of 0 utilizing a Bi-implanted Si reference with an implant depth of 24 nm. A first principal calculation was made to cross check the Er areal density value and showed very good agreement. For the ERD measurement a partially deuterated polystyrene (d-PS) film coated on Si was used as a reference. The reference was made from premixed d-PS and h-PS powders and the film thickness determined from profilometry. The H content of the film was confirmed by using a Kapton reference with a 1nm Pt coating. In determining the areal densities for the ErHD#13 standard the mean energy was used to correct for the recoil cross-section change due to the energy loss in penetrating the film. The recoil cross-sections were taken from [12]. The ERD data shows that the H and D peaks exhibit overlap, therefore non-linear curve fitting was used to determine the individual yields of H and D, see Fig. 2. This
700
Yield (Counts/10µC)
H 600 500 400
D
300 200 100 0 200
300
400
500
600
700
800
900
1000
Channel Fig. 2. Non-linear peak fitting was used to obtain the H and D yields from 5 MeV 4 He2þ ERD measurements on the ErHD#13 standard at the University of Minnesota. The measured yield of H and D was 141 616 counts. The fitted H and D yields were 81 146 and 60 447 counts, respectively, for a total of 141 593.
447
overlap indicates that a beam energy P 5 MeV is needed to adequately analyze the Sandia standards by He ERD. The RBS/NRA technique at Sandia uses a 2 MeV 3 Heþ beam to simultaneously acquire both RBS and NRA data using the same detector. This technique cannot be used to measure H but has good sensitivity to D content. The detector depletion depth (300 lm) allows for separation of the highest energy backscattered particles and protons from the D(3 He,p)4 He reactions. Analysis of the RBS/NRA data uses the mean energy approximation with Rutherford scattering crosssections used for Er and NRA cross-sections from [13]. A Au reference was used to check the calibration of backscattering parameters before measuring the ErHD standards.
4. Results To determine the loading accuracy of the standards two random samples were measured by thermal desorption/mass spectrometry and atomic absorption spectrometry techniques with a stated combined accuracy of 1.6% (1r). The results of measurements on two random samples using the MS and AAS techniques can be seen in Table 2. Comparing these results with those obtained by the Sandia RBS/ERD and RBS/NRA techniques shows a 10% bias in the H and D areal density values for ErHD#3 and #15. At present we do not understand this disparity and the reason is being investigated. However, both the Sandia RBS/ERD and RBS/NRA techniques were used to analyze all the standards in this round robin. A comparison between these techniques showed that a ratio (NRA/ERD) of the average of the measured areal densities for D in the standards was 1.027 ± 3%. Good agreement was obtained between all the IBA techniques as seen in Table 3. The standard deviation divided by the average areal density for Er, H and D was 6 2.7%, 3.25% and 1.7%, respectively. The values are well within the stated uncertainties in each of the measurement techniques and indicate that the parameters and/or references used for analyzing the Sandia ErHD standards were well known.
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J.C. Banks et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 444–449
Table 2 Comparison of the areal densities for the ErHD round robin standards measured at Sandia National Laboratories is shown Areal densities (1E18)
Areal densities (1E18)
Sample ErHD#3
Sample ErHD#15
Laboratory
Techniques
Beam/ E1 =E2
Sample ID
Er
H
D
H+D/ Er
Er
H
D
H+D/ Er
Sandia (HDT) Sandia
RBS/ ERD RBS/ NRA MS/ AAS RBS/ ERD RBS/ ERD
a/10.0/ 10.0 3 He/2.0/ 2.0 N/A
All
1.61
1.76
1.43
1.98
1.62
1.73
1.41
1.94
All
1.57
N/A
1.39
1.57
N/A
1.39
HD#3, #15
1.59
1.58
1.30
1.57
1.59
1.28
HD#9
–
–
–
–
–
–
HD#13
–
–
–
–
–
–
Avg. S.D. (r) r/Avg. (%) NRA/MS ratio ERD/MS ratio
1.59 0.02 1.02
1.67 0.12 7.46
1.37 0.07 4.76 1.07
1.59 0.03 1.79
1.66 0.10 5.96
1.36 0.07 5.21 1.09
1.11
1.10
1.09
1.10
Sandia LANL University of Minnesota
a/7.6/ 9.4 a/5.0/ 5.0
1.81
1.83
A 10% bias exists between the ERD/NRA and MS techniques and the reason is under investigation. Table 3 Comparison of the areal densities as measured by IBA techniques at the University of Minnesota, LANL and Sandia Areal densities (1E18)
Areal densities (1E18)
Sample ErHD#9
Sample ErHD#13
Laboratory
Techniques
Beam/ E1 =E2
Sample ID
Er
H
D
H+D/ Er
Er
H
D
H+D/ Er
Sandia (HDT) Sandia
RBS/ ERD RBS/ NRA MS/ AAS RBS/ ERD RBS/ ERD
a/10.0/ 10.0 3 He/2.0/ 2.0 N/A
All
1.60
1.73
1.38
1.95
1.64
1.74
1.42
1.93
All
1.56
N/A
1.42
1.55
N/A
1.40
HD#3, #15
–
–
–
–
–
–
HD#9
1.59
1.73
1.42
–
–
–
HD#13
–
–
–
1.61
1.66
1.37
Avg. S.D. (r) r/Avg. (%)
1.58 0.02 1.19
1.73 0.00 0.04
1.41 0.02 1.54
1.60 0.04 2.65
1.70 0.06 3.25
1.40 0.02 1.67
Sandia LANL University of Minnesota
a/7.6/ 9.4 a/5.0/ 5.0
An evaluation of the stability for these standards was made by analyzing a sister ErHD standard that has been used repeatedly in IBA studies in the ERD system at Sandia. The standard has seen no measurable H or D loss after 50 measurements using a 10 Mev 4 He2þ beam and an
1.98
1.88
accumulated charge of 50 plC. Taking the average of five H areal density measurements on the standard at the beginning and again at the end of the 50 measurements gave a ratio of 1.010. Similarly, the same ratio for D was 1.002. These ratios (end/beginning) demonstrate both the sta-
J.C. Banks et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 444–449
bility of the standard and reproducibility of the Sandia HDT analysis system.
5. Conclusions Sandia has fabricated highly stable and accurate ErH2 , ErD2 and ErHD standards. IBA techniques were used to measure the areal density of Er, H and D in these standards and the results showed agreement in all measured values to within 3.3%, well within the stated uncertainties in each of the measurement techniques. Measurement of H and D loss in a sister sample has shown that these standards are not sensitive to beam effects using a 4 He2þ beam energy of 10 MeV and charge of 50 plC. The thickness (0.5 lm) of the standards restricts their use by IBA techniques employing low beam energies (<5 MeV). It is also believed that surface roughness may hinder their use. Therefore, thinner Er(H,D)2 standards may be fabricated in the future on highly polished Mo substrates or on Si with a Mo barrier layer. TiH2 on Si also may be evaluated for use by the semiconductor industry.
Acknowledgements The authors would like to thank Michael Lopez for fabricating the round robin standards and Michael Courtney for making the AAS measurements. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for
449
the US Department of Energy under contract No. DE-AC04-94AL85000.
References [1] National Hydrogen Energy Roadmap, US Department of Energy, November 2002. [2] Proceedings HDT Workshop, SAND Report 2000-1352, Sandia National Laboratories, April 2000. [3] J.R. Tesmer, M. Nastasi (Eds.), Handbook of Modern Ion Beam Materials Analysis, Materials Research Society, Pittsburgh, PA, 1995. [4] R.D. Verda, C.J. Maggiore, J.R. Tesmer, A. Misra, T. Hoechbauer, M. Nastasi, R.W. Bower, Nucl. Instr. and Meth. B 183 (2001) 401. [5] R. Behrisch, M. Mayer, W. Jacob, W. Assmann, G. Dollinger, A. Bergmaier, U. Kreissig, M. Friedrich, G.Y. Sun, D. Hildebrandt, M. Akbi, W. Schneider, D. Schleussner, W. Knapp, C. Edelmann, J. Nucl. Mater. 281 (2000) 42. [6] M. Mayer, SIMNRA User’s Guide, Report IPP 9/113, Max Plank-Institut f€ ur Plasmaphysik, Garching, Germany, 1997. [7] J. John, J.P. Aldridge, R.H. Davis, Phys. Rev. 181 (1969) 1455. [8] J.F. Browning, R.A. Langley, B.L. Doyle, J.C. Banks, W.R. Wampler, Nucl. Instr. and Meth. B 161–163 (2000) 211. [9] R.D. Verda, J.R. Tesmer, C.J. Maggiore, M. Nastasi, R.W. Bower, Nucl. Instr. and Meth. B 183 (2001) 391. [10] J.F. Browning, J.C. Banks, W.R. Wampler, B.L. Doyle, in press. [11] Reference supplied by Community Bureau of Reference, Commission of European Communities as Described in Certification Report BCR CRM 328. [12] C.S. Kim, S.K. Kim, H.D. Choi, Nucl. Instr. and Meth. B 155 (1999) 229. [13] W. Moller, F. Besenbacher, Nucl. Instr. and Meth. 168 (1980) 111.
Round robin analyses of hydrogen isotope thin films standards q J.C. Banks a,*, J.F. Browning a, W.R. Wampler a, B.L. Doyle a, C.A. LaDuca a, J.R. Tesmer b, C.J. Wetteland b, Y.Q. Wang c,1 a
Sandia National Laboratories, P.O. Box 5800, MS-1056, Albuquerque, NM 87185-1056, USA b Los Alamos National Laboratory, Los Alamos, NM 87545, USA c University of Minnesota, Minneapolis, MN 55455, USA
Abstract Hydrogen isotope thin film standards have been manufactured at Sandia National Laboratories for use by the materials characterization community. Several considerations were taken into account during the manufacture of the ErHD standards, with accuracy and stability being the most important. The standards were fabricated by e-beam deposition of Er onto a Mo substrate and the film stoichiometrically loaded with hydrogen and deuterium. To determine the loading accuracy of the standards two random samples were measured by thermal desorption mass spectrometry and atomic absorption spectrometry techniques with a stated combined accuracy of 1.6% (1r). All the standards were then measured by high energy RBS/ERD and RBS/NRA with the accuracy of the techniques 5% (1r). The standards were then distributed to the IBA materials characterization community for analysis. This paper will discuss the suitability of the standards for use by the IBA community and compare measurement results to highlight the accuracy of the techniques used. 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen isotope standards; Round robin; Rutherford backscattering spectrometry; ERD; NRA
1. Introduction
q Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC0494AL85000. * Corresponding author. Present address: Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. Tel.: +1-505-844-8824; fax: +1505-844-7775. E-mail address: [email protected] (J.C. Banks). 1 Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
There is a need for highly accurate and stable thin film hydrogen isotope standards that can be used to improve the techniques used for measuring H-isotopes in thin and bulk materials. The need to characterize hydrogen in materials is very important to many areas, including the semiconductor, transportation and nuclear industries. This need will grow if there is a transition to a hydrogen economy in coming years. Although this transition will be complex, the benefits are promising. According to the National Hydrogen Energy
0168-583X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.099
J.C. Banks et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 444–449
Roadmap the transition has the potential to resolve issues involving energy supplies and air pollution [1]. To meet the need for thin film H-isotope standards Sandia National Laboratories conducted a workshop to discuss their development [2] and has recently fabricated highly stable and accurate ErH2 , ErD2 and ErHD thin film standards for use by the materials characterization community. Fifteen of the ErHD standards were distributed to interested participants worldwide to determine their usefulness and to compare the accuracy of the techniques used to measure them. A third of the participants reported results.
2. Selection and fabrication of ErHD standards Several types of hydrogen isotope standards have been used in the past for calibrating H measurement techniques. Polymer films have been used extensively, as well as H implanted Si, SiNx Hy and a-C:D films on Si [3–5]. Some of the drawbacks of using these films are beam induced hydrogen loss and interfering Si(a, p) reactions at higher beam energies. These considerations were taken into account when choosing to manufacture ErHD standards, where accuracy and stability were of primary importance. The standards were fabricated by e-beam deposition of 0.5 lm (±5%) of Er onto a clean 12.7 mm diameter, 0.5 mm thick Mo substrate and the film stoichiometrically loaded with hydrogen and deuterium. The substrate was cleaned prior to deposition by hydrogen and vacuum firing. The Er was deposited using a substrate temperature of 450 C and deposition rate of 600 A/min in a 107 Torr vacuum. Loading was also done at 450 C at a pressure of 10–20 Torr for 10 min. The standards were stored in a N2 atmosphere to reduce interaction of the films with any contaminants. After fabrication the films were measured using high-energy He RBS, taking advantage of the resonance in the O cross-section at 10 MeV, to determine the thickness of the surface erbia (Er2 O3 ) layer (known to be self-limiting) and bulk oxygen content that may have been incorporated during processing. Large amounts of oxygen could affect the film integrity. The O profiles for the standards were all similar and a sim-
445
ulation of the profile for the ErHD#9 standard using SIMNRA software [6] and John’s O crosssections [7] gave a surface erbia thickness of 2.0 · 1016 Er2 O3 /cm2 or 4.8 nm and bulk O content of 3.8 at.%.
3. Experimental techniques The thermal desorption/mass spectrometry and atomic absorption spectrometry techniques were used to measure the load ratio (H+D/Er) of the standards. Both use commercially available instruments. Hydrogen isotope concentrations were measured by thermally desorbing the film and analyzing the evolved gas on a Finnigan MAT 271 hydrogen isotope mass spectrometer. Film areas were measured using a Nikon MM-11 Measure-Scope. The estimated uncertainty in both the mass spectrometric analysis and film area determination is 1.4%. The number of Er atoms in the film was determined by chemically etching the Er film from the Mo substrate and measuring the solution using a Perkin Elmer 5100 PC atomic absorption spectrometer. The overall uncertainty in this measurement is 0.6%. A summary of the analysis parameters used by the IBA techniques is shown in Table 1. The RBS/ ERD techniques used at Sandia National Laboratories (Sandia HDT) and Los Alamos National Laboratories (LANL) have been described in [8,9]. The RBS/ERD techniques employed by both labs use high-energy 4 He2þ beams to enable sampling at greater depth and to allow for the separation of the three hydrogen isotopes, as shown in Fig. 1. Measurements for the Sandia HDT system were made using first principal calculations, i.e. using the thin film yield equation and well known analysis parameters. A ErHD standard was used to monitor for any systematic change. The mean energy of the beam within the film was used to calculate both the Rutherford cross-sections for Er and the non-Rutherford recoil cross-sections for H and D. Sandia measured values for the H and D cross-sections were used. The values have been compared to those in the literature and found to be in good agreement [10]. The detector solid angles were determined using a NIST certified (1.3% 1r)
446
J.C. Banks et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 444–449
Table 1 Analysis parameters used by the IBA techniques to determine the areal densities of Er, H and D in the Sandia ErHD standards Laboratory
Beam/ E1 =E2 a (MeV)
Detector angleb ()
Solid angle (msr)
Tilt angle ()
Total charge (plC)
Beam current (nA)
ERD range foil thickness (lm)
References
Overall uncertainty (%)
University of Minnesota LANL
a/5.0/5.0
165/30
0/75
5/5
10/10
28 Mylar
155/30
N/A
N/A
N/A
50 Al + 8 Mylar
a/10.0/ 10.0 3 He/2.0/ 2.0
165/30
75/75
1/1
10/10
109 Mylar
Bi–Si, d-PS/Si Au, ErD2 , ErH2 ErHDd
5
a/7.6/9.4
3.84/ 1.17 N/A
0
2
1.5
N/A
Aud
5
Sandia (HDT) Sandia
6.89/ 1.29 11
165
2.1c
5
a
E1 ¼ RBS and E2 ¼ ERD or NRA. LANL RBS at 7.6 MeV done at 165. c Deuterium only. d Used for calibration check. b
∆ Energy (Channel)
200
150
100 Tiritium
50
Deuterium Hydrogen
50
100
150
200
Energy (Channel) Fig. 1. Both the Sandia and LANL ERD analysis systems use a particle telescope to allow separation of all three hydrogen isotopes for analysis. The separation of H isotopes using a 10 MeV a-beam in the Sandia analysis system is shown. 238
Pu a-source. The total number of ions hitting a standard was determined using a beam current monitoring system that includes detection of backscattered He particle hitting a rotating Au coated blade intercepting the beam in front of the sample. The integrated charge collected into a Faraday cup during calibration runs prior to analyses was used to find the accumulated charge and calculate the total number of ions for each measurement. The charge collected during cali-
bration runs was measured with a digital current integrator that was calibrated by the Primary Standards Lab at Sandia. The uncertainty in this measurement is 2%. Both LANL and the University of Minnesota used other standards (references) for quantifying the ErHD standard constituents. LANL used a two-step process to analyze the ErHD#9 standard. First, the hydrogen isotope to Er ratio for the standard was determined by making simultaneous RBS and ERD measurements using a 9.4 MeV 4 He2þ analysis beam, first on the standard and then on a well characterized ErD2 reference and finally on an ErH2 reference. Both references are similar to the Sandia standards analyzed in this round robin. The ratio of H/Er in the ErH2 reference was not known, but taken to be 2:1. The geometry of the system used for these measurements is also not well known, but highly reproducible [9]. The ratio of H or D to Er is determined using the thin target yield equation, NHD =NEr ¼ bðYHD =YEr Þ=ðYHD =YEr Þref c ðNHD =NEr Þref . Second, the areal density for the Er film on the Sandia HD#9 standard was determined by RBS using a beam energy of 7.6 MeV at normal incidence. The amount of Er was determined by comparison with a Au reference (certified areal density ±1.2% 1r) [11] using the thin target yield equation, NEr ¼ NAu ðYEr =YAu ÞðrAu =rEr Þ and Rutherford cross-sections for Er and Au. The two quantities (H+D/Er and Er areal densities) were then multiplied
J.C. Banks et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 444–449
together to get the H and D areal densities. The uncertainty in the D measurement is 2.1%. The dominant uncertainty in the Er areal density measurement is the uncertainty in the Au standard. At the University of Minnesota the Er content was determined using a 5.0 MeV He beam at a sample tilt angle of 0 utilizing a Bi-implanted Si reference with an implant depth of 24 nm. A first principal calculation was made to cross check the Er areal density value and showed very good agreement. For the ERD measurement a partially deuterated polystyrene (d-PS) film coated on Si was used as a reference. The reference was made from premixed d-PS and h-PS powders and the film thickness determined from profilometry. The H content of the film was confirmed by using a Kapton reference with a 1nm Pt coating. In determining the areal densities for the ErHD#13 standard the mean energy was used to correct for the recoil cross-section change due to the energy loss in penetrating the film. The recoil cross-sections were taken from [12]. The ERD data shows that the H and D peaks exhibit overlap, therefore non-linear curve fitting was used to determine the individual yields of H and D, see Fig. 2. This
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Yield (Counts/10µC)
H 600 500 400
D
300 200 100 0 200
300
400
500
600
700
800
900
1000
Channel Fig. 2. Non-linear peak fitting was used to obtain the H and D yields from 5 MeV 4 He2þ ERD measurements on the ErHD#13 standard at the University of Minnesota. The measured yield of H and D was 141 616 counts. The fitted H and D yields were 81 146 and 60 447 counts, respectively, for a total of 141 593.
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overlap indicates that a beam energy P 5 MeV is needed to adequately analyze the Sandia standards by He ERD. The RBS/NRA technique at Sandia uses a 2 MeV 3 Heþ beam to simultaneously acquire both RBS and NRA data using the same detector. This technique cannot be used to measure H but has good sensitivity to D content. The detector depletion depth (300 lm) allows for separation of the highest energy backscattered particles and protons from the D(3 He,p)4 He reactions. Analysis of the RBS/NRA data uses the mean energy approximation with Rutherford scattering crosssections used for Er and NRA cross-sections from [13]. A Au reference was used to check the calibration of backscattering parameters before measuring the ErHD standards.
4. Results To determine the loading accuracy of the standards two random samples were measured by thermal desorption/mass spectrometry and atomic absorption spectrometry techniques with a stated combined accuracy of 1.6% (1r). The results of measurements on two random samples using the MS and AAS techniques can be seen in Table 2. Comparing these results with those obtained by the Sandia RBS/ERD and RBS/NRA techniques shows a 10% bias in the H and D areal density values for ErHD#3 and #15. At present we do not understand this disparity and the reason is being investigated. However, both the Sandia RBS/ERD and RBS/NRA techniques were used to analyze all the standards in this round robin. A comparison between these techniques showed that a ratio (NRA/ERD) of the average of the measured areal densities for D in the standards was 1.027 ± 3%. Good agreement was obtained between all the IBA techniques as seen in Table 3. The standard deviation divided by the average areal density for Er, H and D was 6 2.7%, 3.25% and 1.7%, respectively. The values are well within the stated uncertainties in each of the measurement techniques and indicate that the parameters and/or references used for analyzing the Sandia ErHD standards were well known.
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Table 2 Comparison of the areal densities for the ErHD round robin standards measured at Sandia National Laboratories is shown Areal densities (1E18)
Areal densities (1E18)
Sample ErHD#3
Sample ErHD#15
Laboratory
Techniques
Beam/ E1 =E2
Sample ID
Er
H
D
H+D/ Er
Er
H
D
H+D/ Er
Sandia (HDT) Sandia
RBS/ ERD RBS/ NRA MS/ AAS RBS/ ERD RBS/ ERD
a/10.0/ 10.0 3 He/2.0/ 2.0 N/A
All
1.61
1.76
1.43
1.98
1.62
1.73
1.41
1.94
All
1.57
N/A
1.39
1.57
N/A
1.39
HD#3, #15
1.59
1.58
1.30
1.57
1.59
1.28
HD#9
–
–
–
–
–
–
HD#13
–
–
–
–
–
–
Avg. S.D. (r) r/Avg. (%) NRA/MS ratio ERD/MS ratio
1.59 0.02 1.02
1.67 0.12 7.46
1.37 0.07 4.76 1.07
1.59 0.03 1.79
1.66 0.10 5.96
1.36 0.07 5.21 1.09
1.11
1.10
1.09
1.10
Sandia LANL University of Minnesota
a/7.6/ 9.4 a/5.0/ 5.0
1.81
1.83
A 10% bias exists between the ERD/NRA and MS techniques and the reason is under investigation. Table 3 Comparison of the areal densities as measured by IBA techniques at the University of Minnesota, LANL and Sandia Areal densities (1E18)
Areal densities (1E18)
Sample ErHD#9
Sample ErHD#13
Laboratory
Techniques
Beam/ E1 =E2
Sample ID
Er
H
D
H+D/ Er
Er
H
D
H+D/ Er
Sandia (HDT) Sandia
RBS/ ERD RBS/ NRA MS/ AAS RBS/ ERD RBS/ ERD
a/10.0/ 10.0 3 He/2.0/ 2.0 N/A
All
1.60
1.73
1.38
1.95
1.64
1.74
1.42
1.93
All
1.56
N/A
1.42
1.55
N/A
1.40
HD#3, #15
–
–
–
–
–
–
HD#9
1.59
1.73
1.42
–
–
–
HD#13
–
–
–
1.61
1.66
1.37
Avg. S.D. (r) r/Avg. (%)
1.58 0.02 1.19
1.73 0.00 0.04
1.41 0.02 1.54
1.60 0.04 2.65
1.70 0.06 3.25
1.40 0.02 1.67
Sandia LANL University of Minnesota
a/7.6/ 9.4 a/5.0/ 5.0
An evaluation of the stability for these standards was made by analyzing a sister ErHD standard that has been used repeatedly in IBA studies in the ERD system at Sandia. The standard has seen no measurable H or D loss after 50 measurements using a 10 Mev 4 He2þ beam and an
1.98
1.88
accumulated charge of 50 plC. Taking the average of five H areal density measurements on the standard at the beginning and again at the end of the 50 measurements gave a ratio of 1.010. Similarly, the same ratio for D was 1.002. These ratios (end/beginning) demonstrate both the sta-
J.C. Banks et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 444–449
bility of the standard and reproducibility of the Sandia HDT analysis system.
5. Conclusions Sandia has fabricated highly stable and accurate ErH2 , ErD2 and ErHD standards. IBA techniques were used to measure the areal density of Er, H and D in these standards and the results showed agreement in all measured values to within 3.3%, well within the stated uncertainties in each of the measurement techniques. Measurement of H and D loss in a sister sample has shown that these standards are not sensitive to beam effects using a 4 He2þ beam energy of 10 MeV and charge of 50 plC. The thickness (0.5 lm) of the standards restricts their use by IBA techniques employing low beam energies (<5 MeV). It is also believed that surface roughness may hinder their use. Therefore, thinner Er(H,D)2 standards may be fabricated in the future on highly polished Mo substrates or on Si with a Mo barrier layer. TiH2 on Si also may be evaluated for use by the semiconductor industry.
Acknowledgements The authors would like to thank Michael Lopez for fabricating the round robin standards and Michael Courtney for making the AAS measurements. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for
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the US Department of Energy under contract No. DE-AC04-94AL85000.
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