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Surface metal standards produced by ion implantation through a removable layer
Applied Surface Science 203±204 (2003) 847±850
Surface metal standards produced by ion implantation through a removable layer B.W. Schuelera,*, C.N. Grangerb, L. McCaigc, J.M. McKinleyb, J. Metzc, I. Mowatc, D.F. Reicha, S. Smithc, F.A. Stevieb, M.H. Yangc a
Physical Electronics, 810 Kifer Road, Sunnyvale, CA 94086, USA b Agere Systems, 9333 S. John Pky., Orlando, FL 32819, USA c Charles Evans & Associates, 810 Kifer Road, Sunnyvale, CA 94086, USA
Abstract Surface metal concentration standards were produced by ion implantation and investigated for their suitability to calibrate surface metal measurements by secondary ion mass spectrometry (SIMS). Single isotope implants were made through a 100 nm oxide layer on silicon. The implant energies were chosen to place the peak of the implanted species at a depth of 100 nm. Subsequent removal of the oxide layer was used to expose the implant peak and to produce controlled surface metal concentrations. Surface metal concentration measurements by time-of-¯ight SIMS (TOF-SIMS) with an analysis depth of 1 nm agreed with the expected surface concentrations of the implant standards with a relative mean standard deviation of 20%. Since the TOF-SIMS relative sensitivity factors (RSFs) were originally derived from surface metal measurements of surface contaminated silicon wafers, the agreement implies that the implant standards can be used to measure RSF values. The homogeneity of the surface metal concentration was typically <10%. The dopant dose remaining in silicon after oxide removal was measured using the surface-SIMS protocol. The measured implant dose agreed with the expected dose with a mean relative standard deviation of 25%. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface metal standards; Surface metal concentration; Implant dose; TOF-SIMS; Surface-SIMS
1. Introduction Shrinking device geometries with junction depths of <20 nm present new analytical challenges since major portions of the implants are located in the immediate vicinity of the surface. It has thus become necessary to determine the concentration of the dopants, as well as the surface concentration of contaminants. Surface metal quanti®cation not only *
Corresponding author. Tel.: 1-408-530-3788; fax: 1-408-530-3601. E-mail address: [email protected] (B.W. Schueler).
requires appropriate analytical techniques and protocols, but also reliable calibration standards. One method to produce surface metal standards is by ion implantation through a removable oxide layer [1,2]. The implantation into a substrate will, however, position the implant peak concentration at some distance below the surface. Ion implantation through the SiO2 layer and subsequent removal of the oxide layer should result in a well-de®ned surface concentration of the remaining layer. Earlier experiments [2] have shown that surface metal measurements on the implant standards by TOF-SIMS are reproducible with a mean deviation <15% over a period of 5 months, using
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 8 2 0 - 6
848
B.W. Schueler et al. / Applied Surface Science 203±204 (2003) 847±850
different instruments. The aim of this paper is to con®rm the suitability of the implant standards for common surface metal measurement techniques and to verify that the predicted surface metal concentrations are re¯ected in the results of standard surface metal analysis techniques and protocols.
and/or VPD/AAS (vapor phase decomposition/atomic absorption spectroscopy). A peak concentration of 1019 atoms/cm3 corresponds to an areal surface concentration of 1012 atoms/cm2 for an analysis depth of 1 nm. Dynamic SIMS depth pro®les were performed using CAMECA IMS 4f and IMS 6f instruments. Electropositive species in the implanted bare Si wafers and the silicon wafers with 100 nm oxide layer were analyzed using O2 primary ions (3±5.5 keV) with and without O2 leak. The 31 P and 75 As doses were measured using 10.5 keV Cs bombardment. For the surface-SIMS [3] measurements of the implants after SiO2 layer removal, positive ion depth pro®les using 3 and 5.5 keV O2 primary ions with O2 leak at sputter rates of 0.2 and 0.4 nm/s were acquired. The areal implant dose remaining in the Si substrate after removal of the SiO2 layer was determined by applying archival RSF values to normalized analyte signals.
2. Experimental Single isotope implants were made through a 100 nm oxide layer on a silicon substrate. SRIM 2000 was used to calculate the ion energies required to place the implant peak at a depth of 100 nm in SiO2, using an implantation angle of 78 (see Table 1). The elements were implanted with dose of 1014 atoms/ cm2, which should result in a peak concentration of the implants 1019 atoms/cm3 at 100 nm depth. For an implant into a continuous substrate, the implant concentration is expected to be constant to 5% at a distance of 10 nm from the peak concentration. Control implants into bare Si were made in the same implant batch. The 100 nm oxide layer was removed by etching for 3 min in a fresh bath of buffered HF solution (15:1 deionized water:HF). The etch step was followed by a rinse in deionized rinse. Surface metal measurements on the implanted samples after oxide removal were performed on PHI TRIFT II/III TOF-SIMS instruments. A bunched Ga ion beam of 12 keV impact energy was used to analyze a sample area of 40 mm 40 mm and an estimated analysis depth of 1 nm. Positive secondary ion mass spectra were recorded and the integrated counts of the implants species were normalized to the integrated 30 Si counts. The surface concentration was evaluated applying archival RSFs (relative sensitivity factors) to these ratios. The RSF values were obtained by cross-correlation measurements between TOFSIMS and TXRF (total re¯ection X-ray ¯uorescence)
3. Results and discussion SIMS depth pro®les (no O2 leak) of the implants into bare Si were used to verify the depth of the peak concentration (Table 1). SRIM predicts a 4% larger ion range in SiO2 than in Si. In addition, depth pro®les using a Ga primary ion beam (no O2 leak) showed that peak concentrations of the implants into bare Si were located within 10 nm of each other. Depth pro®les of the implants through the 100 nm SiO2 layer (before oxide removal) qualitatively con®rmed that the implant peak was located near the SiO2/Si interface. The TOF-SIMS surface concentration measurement of the implants after removal of the 100 nm SiO2 layer sampled a surface region of 1 nm thickness. The ``expected'' surface concentration was estimated using the concentration of the implant into bare Si measured in the dynamic SIMS depth pro®le at 100 nm depth
Table 1 Ion implant energies (E, keV) used for the different isotope species to place the implant peak concentration at a depth of 100 nma 11
E (keV) d (nm) a
B
26 105
31
P
75 97
75
As
160 103
24
Mg
56 102
Al
39
62 100
96 103
K
58
Ni
141 120
Na
40
52 102
100 93
Ca
59
Co
137 104
64
Zn
152 95
48
Ti
110 106
56
Fe
131 108
The implant depth (d, nm) as measured by dynamic SIMS depth pro®les (no O2 leak) of the implants into bare silicon.
63
Cu
147 110
B.W. Schueler et al. / Applied Surface Science 203±204 (2003) 847±850
(C100), multiplied by the sampling depth of 1 nm. The actual surface concentration of the sample may differ from the expected value as de®ned above for two obvious reasons. First, the exact location of the implant with respect to the SiO2/Si interface is dif®cult to measure in an SIMS depth pro®le. Assuming a Gaussian distribution and a peak location error of 15 nm, the uncertainty in the peak concentration at the interface is 10%. Secondly, the implant through the SiO2 layer may have a discontinuity in the concentration pro®le at the SiO2/Si interface and deviate from the distribution of an implant into homogeneous substrate. For example, SRIM predicts that a 25 keV B implant through 100 nm SiO2 into Si has a concentration change of 10% at the interface. Selective dopant etching at the top surface during oxide removal may also occur. Keeping in mind potential SRIM simulation artifacts, the ``expected'' surface concentration has an estimated accuracy of 10±20%. The results of the TOF-SIMS surface concentration measurements of the implants after removal of the 100 nm SiO2 layer are summarized in Table 2. Each value in the table represents the average of 10 measurements. The surface concentrations measured by TOF-SIMS are in good agreement with the expected surface concentrations (C100 10 7 cm). The mean deviation of the measured from the expected surface
849
concentrations is <20% and the maximum deviation is 40% for the Cu measurement. Because the RSF values for TOF-SIMS surface metal measurements were generally derived from contaminated (e.g. by spincoating) native SiO2 layers, the results imply that RSF values for surface metals can be derived from the implant standards and applied in the analysis of contaminated surfaces. The TOF-SIMS measurements were performed on at least 10 different spots of the wafer pieces to evaluate the lateral variation of the surface concentration. For most elements, the relative standard deviation (RSD) of the measured surface concentration was <10%. One Ca measurement yielded three times the average value, indicating potential particle contamination due to sample handling. Without this data point, the RSD for Ca was 9%. The results of the surface-SIMS dose measurements of the implant standards after removal of the 100 nm SiO2 layer are shown in Table 3. The expected dose remaining in Si is about 5 1013 atoms/cm2, with the exact value depending on the actual implant peak depth and pro®le shape. In general, the surface-SIMS measurements have a mean deviation of 25% from the expected dose, which is within the accuracy of the measurement. For Ti, the larger dose value obtained for measurement set (a) is due to a higher numerical
Table 2 Surface concentration (1010 atoms/cm2) of the implanted elements after oxide removala Element
TOF-SIMS Expected
K
Al
Mg
Ni
B
P
Ti
Fe
Cu
Ca
Co
Zn
117 96
69 77
86 69
56 70
75 79
98 105
63 78
89 86
119 83
68 91
72 91
163 120
a Surface concentrations measured by TOF-SIMS were evaluated using archival RSF values. The expected surface concentration is the concentration in the SIMS depth pro®le of the implants into Si at 100 nm depth, assuming an analysis depth of 1 nm: C100 10 7 cm.
Table 3 Dose (1013 atoms/cm2) in Si of the implanted elements after oxide removal as measured by surface-SIMS (3 keV O2, O2 leak, sputter rate 0.4 nm/s)a Element
Surface-SIMS (a) Surface-SIMS (b)
K
Al
Mg
Ni
B
P
As
Ti
Fe
Cu
Na
Ca
Co
Zn
3.5
4.7
3.8
3.5
5.3
3.9
5.0
9.4 5.7
4.1 6.0
4.5 7.2
3.5 3.5
5.1 4.5
6.5 5.9
7.5 10.0
a The expected dose is about 5 1013 atoms/cm2. Measurement set (a) quanti®ed using archival RSFs, set (b) was quanti®ed using RSFs obtained concurrently from the implanted bare Si samples.
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B.W. Schueler et al. / Applied Surface Science 203±204 (2003) 847±850
Table 4 TXRF surface concentration (1012 atoms/cm2) measurement of the implants after removal of the 100 nm SiO2 layera Element
TXRF Expected a
K
Ni
As
Ti
Fe
Cu
Ca
Co
Zn
8.5 3.8
9.0 2.8
3.0 4.0
4.0 3.1
4.0 3.4
3.7 3.3
30 3.6
2.0 3.6
2.0 4.8
Surface concentration expected for 5 nm analysis depth.
value of the archival RSF for this element as compared with the RSF determined from the control sample. The RSF difference could be explained if the actual implanted dose of the Ti is somewhat higher than the nominal 1 1014 cm 2. The high Zn value for set (b) is problematic, but may represent poor measurement precision for this dif®cult-to-ionize element. Table 4 gives a summary of the TXRF measurements of the implant standards after oxide removal. The surface concentration measured by TXRF appears to be consistent with the assumption of a sampling depth of 5 nm. Except for Ca, the measured surface concentrations agree with the expected values with a mean error of 40%. The large deviation of the Ca surface concentration is most likely due to particle contamination of the sample. 4. Summary Surface metal concentration standards were produced by ion implantation through a SiO2 layer and
subsequent removal of the oxide layer. Dose and depth of the implanted elements were con®rmed by SIMS depth pro®le measurements. The surface metal measurements of the implant standards by TOF-SIMS and surface-SIMS are, within the accuracy of the respective protocols, consistent with the expected surface metal levels of the implant standards. RSFs used in TOF-SIMS surface metal measurements of contaminated native oxide layers also apply to the implant structures. The surface uniformity of the implant standards was typically <10%, but random surface contamination was observed for Ca. References [1] F.A. Stevie, R. Roberts, J.M. McKinley, M. Decker, C. Granger, in: Proceedings of the USJ Conference, NC, 1999. [2] D.F. Reich, B.W. Schueler, F.A. Stevie, J.M. McKinley, C.N. Granger, in: Proceedings of the SIMS XII, Wiley, New York, 1999, p. 425. [3] S.P. Smith, in: Proceedings of the SIMS X, Wiley, New York, 1999, p. 485.
Surface metal standards produced by ion implantation through a removable layer B.W. Schuelera,*, C.N. Grangerb, L. McCaigc, J.M. McKinleyb, J. Metzc, I. Mowatc, D.F. Reicha, S. Smithc, F.A. Stevieb, M.H. Yangc a
Physical Electronics, 810 Kifer Road, Sunnyvale, CA 94086, USA b Agere Systems, 9333 S. John Pky., Orlando, FL 32819, USA c Charles Evans & Associates, 810 Kifer Road, Sunnyvale, CA 94086, USA
Abstract Surface metal concentration standards were produced by ion implantation and investigated for their suitability to calibrate surface metal measurements by secondary ion mass spectrometry (SIMS). Single isotope implants were made through a 100 nm oxide layer on silicon. The implant energies were chosen to place the peak of the implanted species at a depth of 100 nm. Subsequent removal of the oxide layer was used to expose the implant peak and to produce controlled surface metal concentrations. Surface metal concentration measurements by time-of-¯ight SIMS (TOF-SIMS) with an analysis depth of 1 nm agreed with the expected surface concentrations of the implant standards with a relative mean standard deviation of 20%. Since the TOF-SIMS relative sensitivity factors (RSFs) were originally derived from surface metal measurements of surface contaminated silicon wafers, the agreement implies that the implant standards can be used to measure RSF values. The homogeneity of the surface metal concentration was typically <10%. The dopant dose remaining in silicon after oxide removal was measured using the surface-SIMS protocol. The measured implant dose agreed with the expected dose with a mean relative standard deviation of 25%. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface metal standards; Surface metal concentration; Implant dose; TOF-SIMS; Surface-SIMS
1. Introduction Shrinking device geometries with junction depths of <20 nm present new analytical challenges since major portions of the implants are located in the immediate vicinity of the surface. It has thus become necessary to determine the concentration of the dopants, as well as the surface concentration of contaminants. Surface metal quanti®cation not only *
Corresponding author. Tel.: 1-408-530-3788; fax: 1-408-530-3601. E-mail address: [email protected] (B.W. Schueler).
requires appropriate analytical techniques and protocols, but also reliable calibration standards. One method to produce surface metal standards is by ion implantation through a removable oxide layer [1,2]. The implantation into a substrate will, however, position the implant peak concentration at some distance below the surface. Ion implantation through the SiO2 layer and subsequent removal of the oxide layer should result in a well-de®ned surface concentration of the remaining layer. Earlier experiments [2] have shown that surface metal measurements on the implant standards by TOF-SIMS are reproducible with a mean deviation <15% over a period of 5 months, using
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 8 2 0 - 6
848
B.W. Schueler et al. / Applied Surface Science 203±204 (2003) 847±850
different instruments. The aim of this paper is to con®rm the suitability of the implant standards for common surface metal measurement techniques and to verify that the predicted surface metal concentrations are re¯ected in the results of standard surface metal analysis techniques and protocols.
and/or VPD/AAS (vapor phase decomposition/atomic absorption spectroscopy). A peak concentration of 1019 atoms/cm3 corresponds to an areal surface concentration of 1012 atoms/cm2 for an analysis depth of 1 nm. Dynamic SIMS depth pro®les were performed using CAMECA IMS 4f and IMS 6f instruments. Electropositive species in the implanted bare Si wafers and the silicon wafers with 100 nm oxide layer were analyzed using O2 primary ions (3±5.5 keV) with and without O2 leak. The 31 P and 75 As doses were measured using 10.5 keV Cs bombardment. For the surface-SIMS [3] measurements of the implants after SiO2 layer removal, positive ion depth pro®les using 3 and 5.5 keV O2 primary ions with O2 leak at sputter rates of 0.2 and 0.4 nm/s were acquired. The areal implant dose remaining in the Si substrate after removal of the SiO2 layer was determined by applying archival RSF values to normalized analyte signals.
2. Experimental Single isotope implants were made through a 100 nm oxide layer on a silicon substrate. SRIM 2000 was used to calculate the ion energies required to place the implant peak at a depth of 100 nm in SiO2, using an implantation angle of 78 (see Table 1). The elements were implanted with dose of 1014 atoms/ cm2, which should result in a peak concentration of the implants 1019 atoms/cm3 at 100 nm depth. For an implant into a continuous substrate, the implant concentration is expected to be constant to 5% at a distance of 10 nm from the peak concentration. Control implants into bare Si were made in the same implant batch. The 100 nm oxide layer was removed by etching for 3 min in a fresh bath of buffered HF solution (15:1 deionized water:HF). The etch step was followed by a rinse in deionized rinse. Surface metal measurements on the implanted samples after oxide removal were performed on PHI TRIFT II/III TOF-SIMS instruments. A bunched Ga ion beam of 12 keV impact energy was used to analyze a sample area of 40 mm 40 mm and an estimated analysis depth of 1 nm. Positive secondary ion mass spectra were recorded and the integrated counts of the implants species were normalized to the integrated 30 Si counts. The surface concentration was evaluated applying archival RSFs (relative sensitivity factors) to these ratios. The RSF values were obtained by cross-correlation measurements between TOFSIMS and TXRF (total re¯ection X-ray ¯uorescence)
3. Results and discussion SIMS depth pro®les (no O2 leak) of the implants into bare Si were used to verify the depth of the peak concentration (Table 1). SRIM predicts a 4% larger ion range in SiO2 than in Si. In addition, depth pro®les using a Ga primary ion beam (no O2 leak) showed that peak concentrations of the implants into bare Si were located within 10 nm of each other. Depth pro®les of the implants through the 100 nm SiO2 layer (before oxide removal) qualitatively con®rmed that the implant peak was located near the SiO2/Si interface. The TOF-SIMS surface concentration measurement of the implants after removal of the 100 nm SiO2 layer sampled a surface region of 1 nm thickness. The ``expected'' surface concentration was estimated using the concentration of the implant into bare Si measured in the dynamic SIMS depth pro®le at 100 nm depth
Table 1 Ion implant energies (E, keV) used for the different isotope species to place the implant peak concentration at a depth of 100 nma 11
E (keV) d (nm) a
B
26 105
31
P
75 97
75
As
160 103
24
Mg
56 102
Al
39
62 100
96 103
K
58
Ni
141 120
Na
40
52 102
100 93
Ca
59
Co
137 104
64
Zn
152 95
48
Ti
110 106
56
Fe
131 108
The implant depth (d, nm) as measured by dynamic SIMS depth pro®les (no O2 leak) of the implants into bare silicon.
63
Cu
147 110
B.W. Schueler et al. / Applied Surface Science 203±204 (2003) 847±850
(C100), multiplied by the sampling depth of 1 nm. The actual surface concentration of the sample may differ from the expected value as de®ned above for two obvious reasons. First, the exact location of the implant with respect to the SiO2/Si interface is dif®cult to measure in an SIMS depth pro®le. Assuming a Gaussian distribution and a peak location error of 15 nm, the uncertainty in the peak concentration at the interface is 10%. Secondly, the implant through the SiO2 layer may have a discontinuity in the concentration pro®le at the SiO2/Si interface and deviate from the distribution of an implant into homogeneous substrate. For example, SRIM predicts that a 25 keV B implant through 100 nm SiO2 into Si has a concentration change of 10% at the interface. Selective dopant etching at the top surface during oxide removal may also occur. Keeping in mind potential SRIM simulation artifacts, the ``expected'' surface concentration has an estimated accuracy of 10±20%. The results of the TOF-SIMS surface concentration measurements of the implants after removal of the 100 nm SiO2 layer are summarized in Table 2. Each value in the table represents the average of 10 measurements. The surface concentrations measured by TOF-SIMS are in good agreement with the expected surface concentrations (C100 10 7 cm). The mean deviation of the measured from the expected surface
849
concentrations is <20% and the maximum deviation is 40% for the Cu measurement. Because the RSF values for TOF-SIMS surface metal measurements were generally derived from contaminated (e.g. by spincoating) native SiO2 layers, the results imply that RSF values for surface metals can be derived from the implant standards and applied in the analysis of contaminated surfaces. The TOF-SIMS measurements were performed on at least 10 different spots of the wafer pieces to evaluate the lateral variation of the surface concentration. For most elements, the relative standard deviation (RSD) of the measured surface concentration was <10%. One Ca measurement yielded three times the average value, indicating potential particle contamination due to sample handling. Without this data point, the RSD for Ca was 9%. The results of the surface-SIMS dose measurements of the implant standards after removal of the 100 nm SiO2 layer are shown in Table 3. The expected dose remaining in Si is about 5 1013 atoms/cm2, with the exact value depending on the actual implant peak depth and pro®le shape. In general, the surface-SIMS measurements have a mean deviation of 25% from the expected dose, which is within the accuracy of the measurement. For Ti, the larger dose value obtained for measurement set (a) is due to a higher numerical
Table 2 Surface concentration (1010 atoms/cm2) of the implanted elements after oxide removala Element
TOF-SIMS Expected
K
Al
Mg
Ni
B
P
Ti
Fe
Cu
Ca
Co
Zn
117 96
69 77
86 69
56 70
75 79
98 105
63 78
89 86
119 83
68 91
72 91
163 120
a Surface concentrations measured by TOF-SIMS were evaluated using archival RSF values. The expected surface concentration is the concentration in the SIMS depth pro®le of the implants into Si at 100 nm depth, assuming an analysis depth of 1 nm: C100 10 7 cm.
Table 3 Dose (1013 atoms/cm2) in Si of the implanted elements after oxide removal as measured by surface-SIMS (3 keV O2, O2 leak, sputter rate 0.4 nm/s)a Element
Surface-SIMS (a) Surface-SIMS (b)
K
Al
Mg
Ni
B
P
As
Ti
Fe
Cu
Na
Ca
Co
Zn
3.5
4.7
3.8
3.5
5.3
3.9
5.0
9.4 5.7
4.1 6.0
4.5 7.2
3.5 3.5
5.1 4.5
6.5 5.9
7.5 10.0
a The expected dose is about 5 1013 atoms/cm2. Measurement set (a) quanti®ed using archival RSFs, set (b) was quanti®ed using RSFs obtained concurrently from the implanted bare Si samples.
850
B.W. Schueler et al. / Applied Surface Science 203±204 (2003) 847±850
Table 4 TXRF surface concentration (1012 atoms/cm2) measurement of the implants after removal of the 100 nm SiO2 layera Element
TXRF Expected a
K
Ni
As
Ti
Fe
Cu
Ca
Co
Zn
8.5 3.8
9.0 2.8
3.0 4.0
4.0 3.1
4.0 3.4
3.7 3.3
30 3.6
2.0 3.6
2.0 4.8
Surface concentration expected for 5 nm analysis depth.
value of the archival RSF for this element as compared with the RSF determined from the control sample. The RSF difference could be explained if the actual implanted dose of the Ti is somewhat higher than the nominal 1 1014 cm 2. The high Zn value for set (b) is problematic, but may represent poor measurement precision for this dif®cult-to-ionize element. Table 4 gives a summary of the TXRF measurements of the implant standards after oxide removal. The surface concentration measured by TXRF appears to be consistent with the assumption of a sampling depth of 5 nm. Except for Ca, the measured surface concentrations agree with the expected values with a mean error of 40%. The large deviation of the Ca surface concentration is most likely due to particle contamination of the sample. 4. Summary Surface metal concentration standards were produced by ion implantation through a SiO2 layer and
subsequent removal of the oxide layer. Dose and depth of the implanted elements were con®rmed by SIMS depth pro®le measurements. The surface metal measurements of the implant standards by TOF-SIMS and surface-SIMS are, within the accuracy of the respective protocols, consistent with the expected surface metal levels of the implant standards. RSFs used in TOF-SIMS surface metal measurements of contaminated native oxide layers also apply to the implant structures. The surface uniformity of the implant standards was typically <10%, but random surface contamination was observed for Ca. References [1] F.A. Stevie, R. Roberts, J.M. McKinley, M. Decker, C. Granger, in: Proceedings of the USJ Conference, NC, 1999. [2] D.F. Reich, B.W. Schueler, F.A. Stevie, J.M. McKinley, C.N. Granger, in: Proceedings of the SIMS XII, Wiley, New York, 1999, p. 425. [3] S.P. Smith, in: Proceedings of the SIMS X, Wiley, New York, 1999, p. 485.