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Evaluation of SIMS depth resolution using delta-doped multilayers and mixing–roughness-information depth model
Applied Surface Science 203±204 (2003) 294±297
Evaluation of SIMS depth resolution using delta-doped multilayers and mixing±roughness-information depth model A. Takanoa,*, Y. Hommab, Y. Higashic, H. Takenakaa, S. Hayashid, K. Gotoe, M. Inouef, R. Shimizug a
NTT Advanced Technology Corporation, Atsugi, Kanagawa 243-0124, Japan b NTT Basic Research Laboratories, Atsugi, Kanagawa 243-0198, Japan c NTT Life Style and Environmental Laboratories, Atsugi, Kanagawa 243-0198, Japan d Advanced Technology Research Labs, Nippon Steel Corporation, Futtsu, Chiba 293-8511, Japan e Faculty of Engineering, Nagoya Institute of Technology, Nagoya, Aichi 466-8555, Japan f Faculty of Electrical Engineering, Setsunan University, Neyagawa, Osaka 572-8508, Japan g Faculty of Information Science and Technology, Osaka Institute of Technology, Hirakata, Osaka 573-0196, Japan
Abstract Boron delta-doped multilayers are potential reference materials for the evaluation of depth resolution in secondary ion mass spectrometry (SIMS). In this work, we studied extraction of depth resolution parameters using a theoretical model, mixing± roughness-information (MRI) depth model from the measured pro®les under various O2 bombardment conditions. Specimens used were boron delta-doped multilayers in Si (period: 3±20 nm) which had been made by magnetron-sputtering deposition. For SIMS, information depth can be regarded to be very small, so we used only the two parameters concerning mixing and roughness. Measured B pro®les were ®tted well to the MRI model. The depth resolution parameters could be extracted even from a pro®le of multilayers with a short periodicity. The combination of short-period multilayers and MRI model analysis would be useful for evaluation of depth resolution in shallow depth pro®ling. # 2002 Elsevier Science B.V. All rights reserved. Keywords: SIMS; Delta; Boron; Silicon; Multilayers; MRI
1. Introduction Boron delta-doped multilayers are potential reference materials for the evaluation of depth resolution in secondary ion mass spectrometry (SIMS). They can give the depth-dependent depth resolution as well as the sputtered depth in a single depth pro®le. This is especially useful in shallow depth pro®ling. However, the method of resolution parameter evaluation using delta-doped multilayers has not been established. In * Corresponding author. Tel./fax: 81-46-250-1678. E-mail address: [email protected] (A. Takano).
this work, we studied extraction of depth resolution parameters using a theoretical model, the mixing± roughness-information (MRI) depth model [1], from the measured pro®les under various sputtering conditions. We also evaluated whether the MRI model is applicable to the specimen with a narrow interval of delta-doped layers. 2. Experimental Specimens used were boron nitride (BN) deltadoped multilayers prepared by magnetron-sputter
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 6 6 2 - 1
A. Takano et al. / Applied Surface Science 203±204 (2003) 294±297
deposition. The multilayers consisted of four Si spacer and BN delta pairs with a buffer Si layer on the Si(0 0 1) substrate. The BN delta layer thickness corresponded to 0.002 nm (0.007 monolayer coverage), and the Si spacer layer thickness was 9 or 20 nm. SIMS measurements were performed with O2 bombardment and positive ion detection using an ATOMIKA SIMS 4000 system. The primary ion energy was varied from 1.5 to 4 keV and the incident angles used were 98, 458 and 638 to the surface normal. In the MRI model, three parameters, information depth (l), mixing length w and roughness parameter (s) are used to describe a pro®le [1]. In SIMS, since most secondary ions are emitted from the ®rst layer of the surface [2], the information depth is much smaller than the other two parameters for general ion energies. Therefore, l was set to 0 in this work. In the analysis of the delta layers, ®tting of the trailing edge of the pro®le was ®rst carried out to obtain the w parameter, and subsequently, s was determined from ®tting of the peak region of the pro®le. The calculation was performed at 0.1 nm intervals. 3. Results and discussion Fig. 1 shows a boron pro®le measured using 1.5 keV O2 at 458 together with the ®tted curve by the MRI model. The delta-doped layers and background B concentrations assumed for this calculation
295
are also shown. The MRI ®tting was carried out to reproduce the whole delta layer pro®le including the contaminated layer at the interface with the substrate. For the best ®tting, w and s were 1.7 and 1.4 nm, respectively, when the thickness of each delta layer was set to 0.4 nm. Under other measurement conditions, pro®les were analyzed in the same way and the ®tting parameters were determined. Fig. 2 shows the primary ion energy-dependence of w at each angle of incidence. The uncertainty of the w value is within 0.1 nm. The w value becomes smaller with decreasing primary ion energy. The data at 458 and 638 are comparable, while the 98 data are smaller than the others. Under the 98 irradiation condition, a SiO2 layer is formed due to oxygen accumulation on the surface, thus the surface composition is different from that under the other oblique irradiation conditions. Due to the increase of target atoms by oxygen atom incorporation into the SiO2 layer, the mixing range for the substrate Si atoms themselves is reduced. This may be the reason for the smaller w values at the 98 incidence. Fig. 3 shows the energy-dependence of s at each angle of incidence. The uncertainty of the s value is within 0.2 nm. The s value also decreases with decreasing the primary ion energy. Although the data at 98 and 458 exhibit similar energy-dependence as the w case, the data at 638 shows a less steep-dependence on ion energy compared to the other two angles. While w parameter is mostly dependent on the beam energy
Fig. 1. Boron pro®le measured by SIMS with 1.5 keV O2 at 458 and ®tted curve using the MRI model. The delta-doped layers used for the MRI calculation are also shown.
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A. Takano et al. / Applied Surface Science 203±204 (2003) 294±297
Fig. 2. Dependence of mixing parameter w on primary ion energy at different incident angles.
Fig. 3. Dependence of roughness parameter (s) on primary ion energy at different incident angles.
and the incident angle, s parameter is in¯uenced by the evenness of the crater bottom as well as surface roughness. The behavior at 638 might re¯ect such properties at the crater bottom. From these results, reduction of primary ion energy is effective to improve depth resolution, and it can be said that the effect of incident angle is rather small. However, present comparison is done among only three angles, 98, 458 and 638. The surface chemical state at 98 differs from those under 458 and 638
irradiation. It is expected that at around 278, which is the critical angle for SiO2 formation, depth resolution degrades considerably [3]. The validity of MRI analysis was examined when the dynamic range of a pro®le was small. Fig. 4 shows a boron pro®le of 9 nm-spacing delta layers measured using 2 keV O2 at 458, and a result of the MRI ®tting. The extracted parameters were w 2:3 nm and s 1:3 nm, which are in reasonable agreement with the data in Figs. 2 and 3. Using the MRI model, the depth
Fig. 4. Boron pro®le of 9 nm-spacing delta layers measured using 2 keV O2 at 458, and a result of the MRI ®tting.
A. Takano et al. / Applied Surface Science 203±204 (2003) 294±297
297
resolution parameters could be extracted even from a small dynamic range pro®le of multilayers with a short periodicity. The combination of short-period multilayers and MRI model analysis would be useful for evaluation of depth resolution in shallow depth pro®ling.
by the MRI model even for a pro®le with small amplitude. For general distributions, a true depth distribution can be theoretically extracted by using those parameters obtained from the standard reference materials.
4. Conclusions
Acknowledgements
The pro®les of delta-doped multilayers measured by SIMS were analyzed by the MRI model. The information depth (l) was set to 0, and depth resolution was evaluated using two parameters; mixing length w and surface roughness (s). The w and s values depended mainly on primary ion energy. Smaller w and s values were obtained at near normal incidence of the primary oxygen-ion beam. This is probably due to SiO2 formation. It was shown that measured delta-doped pro®les were well reproduced
This work was partially supported by Cooperation for Developing International Standards Project of Japanese Standards Association.
References [1] S. Hofmann, Surf. Interf. Anal. 21 (1994) 673. [2] M.T. Robinson, J. Appl. Phys. 54 (1983) 2650. [3] Y. Homma, T. Maruo, Surf. Interf. Anal. 14 (1989).
Evaluation of SIMS depth resolution using delta-doped multilayers and mixing±roughness-information depth model A. Takanoa,*, Y. Hommab, Y. Higashic, H. Takenakaa, S. Hayashid, K. Gotoe, M. Inouef, R. Shimizug a
NTT Advanced Technology Corporation, Atsugi, Kanagawa 243-0124, Japan b NTT Basic Research Laboratories, Atsugi, Kanagawa 243-0198, Japan c NTT Life Style and Environmental Laboratories, Atsugi, Kanagawa 243-0198, Japan d Advanced Technology Research Labs, Nippon Steel Corporation, Futtsu, Chiba 293-8511, Japan e Faculty of Engineering, Nagoya Institute of Technology, Nagoya, Aichi 466-8555, Japan f Faculty of Electrical Engineering, Setsunan University, Neyagawa, Osaka 572-8508, Japan g Faculty of Information Science and Technology, Osaka Institute of Technology, Hirakata, Osaka 573-0196, Japan
Abstract Boron delta-doped multilayers are potential reference materials for the evaluation of depth resolution in secondary ion mass spectrometry (SIMS). In this work, we studied extraction of depth resolution parameters using a theoretical model, mixing± roughness-information (MRI) depth model from the measured pro®les under various O2 bombardment conditions. Specimens used were boron delta-doped multilayers in Si (period: 3±20 nm) which had been made by magnetron-sputtering deposition. For SIMS, information depth can be regarded to be very small, so we used only the two parameters concerning mixing and roughness. Measured B pro®les were ®tted well to the MRI model. The depth resolution parameters could be extracted even from a pro®le of multilayers with a short periodicity. The combination of short-period multilayers and MRI model analysis would be useful for evaluation of depth resolution in shallow depth pro®ling. # 2002 Elsevier Science B.V. All rights reserved. Keywords: SIMS; Delta; Boron; Silicon; Multilayers; MRI
1. Introduction Boron delta-doped multilayers are potential reference materials for the evaluation of depth resolution in secondary ion mass spectrometry (SIMS). They can give the depth-dependent depth resolution as well as the sputtered depth in a single depth pro®le. This is especially useful in shallow depth pro®ling. However, the method of resolution parameter evaluation using delta-doped multilayers has not been established. In * Corresponding author. Tel./fax: 81-46-250-1678. E-mail address: [email protected] (A. Takano).
this work, we studied extraction of depth resolution parameters using a theoretical model, the mixing± roughness-information (MRI) depth model [1], from the measured pro®les under various sputtering conditions. We also evaluated whether the MRI model is applicable to the specimen with a narrow interval of delta-doped layers. 2. Experimental Specimens used were boron nitride (BN) deltadoped multilayers prepared by magnetron-sputter
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 6 6 2 - 1
A. Takano et al. / Applied Surface Science 203±204 (2003) 294±297
deposition. The multilayers consisted of four Si spacer and BN delta pairs with a buffer Si layer on the Si(0 0 1) substrate. The BN delta layer thickness corresponded to 0.002 nm (0.007 monolayer coverage), and the Si spacer layer thickness was 9 or 20 nm. SIMS measurements were performed with O2 bombardment and positive ion detection using an ATOMIKA SIMS 4000 system. The primary ion energy was varied from 1.5 to 4 keV and the incident angles used were 98, 458 and 638 to the surface normal. In the MRI model, three parameters, information depth (l), mixing length w and roughness parameter (s) are used to describe a pro®le [1]. In SIMS, since most secondary ions are emitted from the ®rst layer of the surface [2], the information depth is much smaller than the other two parameters for general ion energies. Therefore, l was set to 0 in this work. In the analysis of the delta layers, ®tting of the trailing edge of the pro®le was ®rst carried out to obtain the w parameter, and subsequently, s was determined from ®tting of the peak region of the pro®le. The calculation was performed at 0.1 nm intervals. 3. Results and discussion Fig. 1 shows a boron pro®le measured using 1.5 keV O2 at 458 together with the ®tted curve by the MRI model. The delta-doped layers and background B concentrations assumed for this calculation
295
are also shown. The MRI ®tting was carried out to reproduce the whole delta layer pro®le including the contaminated layer at the interface with the substrate. For the best ®tting, w and s were 1.7 and 1.4 nm, respectively, when the thickness of each delta layer was set to 0.4 nm. Under other measurement conditions, pro®les were analyzed in the same way and the ®tting parameters were determined. Fig. 2 shows the primary ion energy-dependence of w at each angle of incidence. The uncertainty of the w value is within 0.1 nm. The w value becomes smaller with decreasing primary ion energy. The data at 458 and 638 are comparable, while the 98 data are smaller than the others. Under the 98 irradiation condition, a SiO2 layer is formed due to oxygen accumulation on the surface, thus the surface composition is different from that under the other oblique irradiation conditions. Due to the increase of target atoms by oxygen atom incorporation into the SiO2 layer, the mixing range for the substrate Si atoms themselves is reduced. This may be the reason for the smaller w values at the 98 incidence. Fig. 3 shows the energy-dependence of s at each angle of incidence. The uncertainty of the s value is within 0.2 nm. The s value also decreases with decreasing the primary ion energy. Although the data at 98 and 458 exhibit similar energy-dependence as the w case, the data at 638 shows a less steep-dependence on ion energy compared to the other two angles. While w parameter is mostly dependent on the beam energy
Fig. 1. Boron pro®le measured by SIMS with 1.5 keV O2 at 458 and ®tted curve using the MRI model. The delta-doped layers used for the MRI calculation are also shown.
296
A. Takano et al. / Applied Surface Science 203±204 (2003) 294±297
Fig. 2. Dependence of mixing parameter w on primary ion energy at different incident angles.
Fig. 3. Dependence of roughness parameter (s) on primary ion energy at different incident angles.
and the incident angle, s parameter is in¯uenced by the evenness of the crater bottom as well as surface roughness. The behavior at 638 might re¯ect such properties at the crater bottom. From these results, reduction of primary ion energy is effective to improve depth resolution, and it can be said that the effect of incident angle is rather small. However, present comparison is done among only three angles, 98, 458 and 638. The surface chemical state at 98 differs from those under 458 and 638
irradiation. It is expected that at around 278, which is the critical angle for SiO2 formation, depth resolution degrades considerably [3]. The validity of MRI analysis was examined when the dynamic range of a pro®le was small. Fig. 4 shows a boron pro®le of 9 nm-spacing delta layers measured using 2 keV O2 at 458, and a result of the MRI ®tting. The extracted parameters were w 2:3 nm and s 1:3 nm, which are in reasonable agreement with the data in Figs. 2 and 3. Using the MRI model, the depth
Fig. 4. Boron pro®le of 9 nm-spacing delta layers measured using 2 keV O2 at 458, and a result of the MRI ®tting.
A. Takano et al. / Applied Surface Science 203±204 (2003) 294±297
297
resolution parameters could be extracted even from a small dynamic range pro®le of multilayers with a short periodicity. The combination of short-period multilayers and MRI model analysis would be useful for evaluation of depth resolution in shallow depth pro®ling.
by the MRI model even for a pro®le with small amplitude. For general distributions, a true depth distribution can be theoretically extracted by using those parameters obtained from the standard reference materials.
4. Conclusions
Acknowledgements
The pro®les of delta-doped multilayers measured by SIMS were analyzed by the MRI model. The information depth (l) was set to 0, and depth resolution was evaluated using two parameters; mixing length w and surface roughness (s). The w and s values depended mainly on primary ion energy. Smaller w and s values were obtained at near normal incidence of the primary oxygen-ion beam. This is probably due to SiO2 formation. It was shown that measured delta-doped pro®les were well reproduced
This work was partially supported by Cooperation for Developing International Standards Project of Japanese Standards Association.
References [1] S. Hofmann, Surf. Interf. Anal. 21 (1994) 673. [2] M.T. Robinson, J. Appl. Phys. 54 (1983) 2650. [3] Y. Homma, T. Maruo, Surf. Interf. Anal. 14 (1989).