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Surface roughening effect in sub-keV SIMS depth profiling
Applied Surface Science 203±204 (2003) 256±259
Surface roughening effect in sub-keV SIMS depth pro®ling R. Liu, C.M. Ng, A.T.S. Wee* Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore
Abstract Low energy secondary ion mass spectrometry (SIMS) is used for the purpose of achieving high depth resolution and reducing the surface transient. However, pro®ling using sub-keV primary ion energies is complicated by the early onset of surface roughening. A Cameca IMS 6f SIMS instrument is used to study the development of surface roughness in the sub-keV primary ion energy regime using B and SiGe delta doped Si standard samples. Energy dependent analyses using 0.5±2.0 keV O2 show that while higher depth resolution is achievable at 0.5 keV, the onset of roughening occurs earlier. The onset of surface roughness is also shown to occur earlier for smaller incidence angles (in the 46±698 range) using 0.5 keV primary ion energy. This angular dependence is explained using the heterogeneity layer model. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface roughening; SIMS
1. Introduction Low primary ion energy is often employed in low energy secondary ion mass spectrometry (SIMS) for achieving high depth resolution and reducing the surface transient. However, pro®ling using sub-keV primary ion energies is complicated by the early onset of surface roughening. Previous work suggests a direct relation between surface composition of the crater bottom and the development of surface roughness [1]. In this work, the development of surface roughness at low primary ion energies is investigated using B and SiGe delta-doped Si reference samples. The energy dependence was studied using 0.5±2.0 keV O2 . The onset of surface roughness is investigated as a function of ion incident angles between 46 and 698 at 0.5 keV. *
Corresponding author. Tel.: 65-68746362; fax: 65-67776126. E-mail address: [email protected] (A.T.S. Wee).
The angular dependence is discussed within the framework of the heterogeneity layer model [2]. 2. Experimental All depth pro®les were performed on a Cameca IMS 6f using an O2 beam at energies ranging from 0.5 to 2.0 keV. Due to the limitation of the instrumental geometry, the incidence angles were set between 46 and 698. For all analyses, the O2 beam was scanned over a raster area of 250 mm 250 mm. A secondary ion post-acceleration system was used to enhance electron multiplier (EM) signals from low energy secondary ions. No sample rotation or oxygen ¯ooding was used in this work. AFM measurements in tapping mode of the crater bottoms were performed on a Digital Instruments D3000 system. The B delta-doped (B-d) Si sample used in this work consists of ®ve 5 nm spaced delta layers on the
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 3 8 - 4
R. Liu et al. / Applied Surface Science 203±204 (2003) 256±259
257
comprises a 150 nm Si top layer followed by 10 Si0.7Ge0.3 delta layers of 0.4 nm thickness (nominally), grown by atmospheric-pressure chemical vapor deposition (APCVD) at 700 8C. The nominal depths of the deltas are at multiples of 11 nm. 3. Results and discussion
Fig. 1. Depth pro®le of B deltas obtained using 0.5 and 2 keV primary O2 beams.
top, followed by nine 15 nm spaced layers and ®nally another two layers separated by 10 and 5 nm, respectively. This sample was provided by SEMATECH for a round robin study on ultra shallow depth pro®ling of B deltas [3]. The Ge delta-doped (Ge-d) Si sample
Fig. 2.
11
B and
30
Fig. 1 compares the boron signals 11 B of the B delta-doped sample obtained using 0.5 and 2 keV O2 beam at a ®xed incidence angle of 468. The depth calibration was performed by assuming constant sputtering rate. The effect of primary ion energy on the resolution of the depth pro®le is evident by noting the differences in several aspects. First, the dynamic range of the depth pro®le is obviously larger at 0.5 keV for the ®rst seven deltas. Secondly, the very ®rst delta that is 5 nm beneath the surface cannot be resolved at all by 2 keV pro®ling but is clearly resolved at 0.5 keV. The improvement in the depth resolution using lower primary ion energy is well-known to be due to the shallower surface transient [4]. Finally, despite the higher resolution attained in the ®rst six deltas, the depth pro®le obtained at 0.5 keV begins to broaden progressively after a depth of about 50 nm, accompanied by a decrease in the signal dynamic range.
Si of B delta layers using 0.5 keV O2 at 46 and 568 incidences.
258
R. Liu et al. / Applied Surface Science 203±204 (2003) 256±259
Moreover, the last two deltas merge into a broad peak. AFM measurements of the crater bottom reveal that this distortion in the pro®le at 0.5 keV is directly related to the onset of surface roughening [5]. The 11 B and 30 Si pro®les at 46 and 568 incidence using 0.5 keV O2 beam are shown in Fig. 2. The depth calibration was performed following procedures previously reported [6]. Note that the matrix 30 Si intensities for the two angles have considerably different pro®les. The typical surface spike and subsequent rise in matrix intensity in the very early stage of pro®ling appear in both conditions. For the case of 468 incidence, the matrix intensity reaches a maximum at about 20 nm and declines gradually for almost 40 nm. The pre-equilibrium state delays the steady pro®ling until a depth of 60 nm. In the case of 568 incidence, the matrix intensity stabilizes relatively earlier (within the ®rst 15 nm) but exhibits a distinct rise extending from 50 to 90 nm. This rise in matrix intensity is the signature of the onset of the surface roughening as previously observed in the case of 1 keV at 568 incidence with oxygen ¯ooding at intermediate partial oxygen pressures [1]. On this basis, we suggest that for the case of 568 incidence the rapid onset of surface roughening occurs over the depths from delta number 7 (50 nm) to number 9 (90 nm) after which the development of the sequence roughening become steady. On the other hand, a similar feature is not observed in the case of 468 incidence. However, the gradual decline in matrix intensity between delta number 4 (20 nm) to number 7 (50 nm) may indicate the onset of surface roughening as suggested by the decreasing dynamic range of the corresponding 11 B intensity. Beginning from delta number 7, the leading edges of the deltas at 468 incidence reveal shoulders followed by sharp peaks. This is in strong contrast to the rounded peaks observed at 568 incidence. However, careful examination of the deeper delta pro®les at 568 incidence reveals a similar shoulder on the leading edge of delta number 14. Dowsett et al. [7] pointed out that the appearance of the shoulder is an indication of the presence of macrotopograhy that is strongly related to the homogeneity of the primary ion beam. However, the possibility of having an inhomogeneous primary beam is ruled out as a similar feature (shoulder on the leading edge and rising background intensities) is not observed in the depth pro®les obtained using oxygen ¯ooding [5].
We reported that when oxygen ¯ooding was applied at intermediate partial oxygen pressures when pro®ling using 1 keV at 568 incidence, the onset of the surface roughness is related to the rapid change in surface chemical composition resulting in the formation of a layer of heterogeneous oxidation states with comparable proportions of stoichiometric dioxides and non-stoichiometric suboxides [1]. In the case of 0.5 keV incidence, surface roughening occurs even without oxygen ¯ooding. Since 0.5 keV O2 has a shallow penetration depth of about 1.1 nm for 568 incidence [8], it is believed that the shallower distribution of incorporated oxygen atoms favors the
Fig. 3. SIMS depth pro®les of 30 Si (a) and 70 Ge (b) secondary ions from the SiGe d-doped Si sample analyzed with a 500 eV O2 beam at 46, 56 and 698 incidence angles. The onset of surface roughening is different at 46, 56 and 698.
R. Liu et al. / Applied Surface Science 203±204 (2003) 256±259
formation of a heterogeneous layer on the sputtered surface due to the higher oxygen content in the nearsurface regions. If the penetration of the primary ions is suf®ciently deep, the concentration of oxygen atoms in the near-surface region would be much lower and would probably not lead to the formation of localized SiO2 islands. This partly explains the absence of surface roughening during pro®ling at higher primary energies without oxygen ¯ooding. On the other hand, the concentration of the retained oxygen ions is determined by the sputtering yield during the pro®ling. It is established that oblique incidence always leads to high sputtering yield and the sputtering yield at 568 incidence is higher than that at 468 [9]. This results in a longer time for the oxygen concentration to achieve the critical level for the onset of roughening in the case of 568 incidence. On the same basis, it is therefore expected that the onset of the surface roughness would be further delayed when pro®ling at more oblique incidence angles. This hypothesis is supported by the depth pro®les shown in Fig. 3 obtained using 0.5 keV O2 beam at three different incidence angles (i.e., 46, 56 and 698) on a SiGe delta-doped sample. Signi®cant increases in the secondary ion intensities at 568 incidence occur at 60 nm depth, and for 698 incidence at 110 nm depth, indicating the onset of surface roughening. At 468 incidence, the roughening transition occurs at 15 nm, which is too shallow to observe a distinct change in the matrix signal since surface transient effects are dominant here. It can be seen that the Ge deltas are progressively broadened asymmetrically. The deltas broadening with depth is most severe at 468 incidence, intermediate at 568 incidence and smallest
259
at 698 incidence. These results show that the most oblique incidence (698) gives the best resolution in the angular range studied. 4. Conclusion Pro®ling using sub-keV primary ion energy is complicated by the accompanying surface roughening. The angular dependence of surface roughening during 0.5 keV pro®ling can be explained using the heterogeneous layer model. References [1] C.M. Ng, A.T.S. Wee, C.H.A. Huan, A. See, J. Vac. Sci. Technol. 19 (2001) 829. [2] K. Elst, W. Vandervorst, J. Alay, J. Snauwaert, L. Hellemans, J. Vac. Sci. Technol. 16 (1993) 1968. [3] J. Bennett, A. Diebold, in: A. Benninghoven, P. Bertrand, H.N. Migeon, H.W. Werner (Eds.), Secondary Ion Mass Spectrometry (SIMS),Vol. XII, Elsevier, Amsterdam, 2000, p. 541. [4] K. Wittmaack, W. Wach, Nucl. Instrum. Meth. 191 (1981) 327. [5] C.M. Ng, A.T.S. Wee, C.H.A. Huan, A. See, in: H.B. Harrison, A.T.S. Wee, S. Gupta (Eds.), Advanced Microelectronic Processing Techniques, Proc. SPIE 4227 (2000) 90. [6] C.M. Ng, Ph.D. Thesis, National University of Singapore, 2001. [7] M.G. Dowsett, R.D. Barlow, P.N. Allen, J. Vac. Sci. Technol. 12 (1994) 186. [8] J.C. Dupuy, G. Prudon, C. Dubois, P. Warren, D. Dutartre, Nucl. Instrum. Meth. 85 (1994) 379. [9] P.N.K. Deenapanray, M. Petravic, Surf. Interf. Anal. 27 (1999) 92.
Surface roughening effect in sub-keV SIMS depth pro®ling R. Liu, C.M. Ng, A.T.S. Wee* Department of Physics, National University of Singapore, Lower Kent Ridge Road, Singapore 119260, Singapore
Abstract Low energy secondary ion mass spectrometry (SIMS) is used for the purpose of achieving high depth resolution and reducing the surface transient. However, pro®ling using sub-keV primary ion energies is complicated by the early onset of surface roughening. A Cameca IMS 6f SIMS instrument is used to study the development of surface roughness in the sub-keV primary ion energy regime using B and SiGe delta doped Si standard samples. Energy dependent analyses using 0.5±2.0 keV O2 show that while higher depth resolution is achievable at 0.5 keV, the onset of roughening occurs earlier. The onset of surface roughness is also shown to occur earlier for smaller incidence angles (in the 46±698 range) using 0.5 keV primary ion energy. This angular dependence is explained using the heterogeneity layer model. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface roughening; SIMS
1. Introduction Low primary ion energy is often employed in low energy secondary ion mass spectrometry (SIMS) for achieving high depth resolution and reducing the surface transient. However, pro®ling using sub-keV primary ion energies is complicated by the early onset of surface roughening. Previous work suggests a direct relation between surface composition of the crater bottom and the development of surface roughness [1]. In this work, the development of surface roughness at low primary ion energies is investigated using B and SiGe delta-doped Si reference samples. The energy dependence was studied using 0.5±2.0 keV O2 . The onset of surface roughness is investigated as a function of ion incident angles between 46 and 698 at 0.5 keV. *
Corresponding author. Tel.: 65-68746362; fax: 65-67776126. E-mail address: [email protected] (A.T.S. Wee).
The angular dependence is discussed within the framework of the heterogeneity layer model [2]. 2. Experimental All depth pro®les were performed on a Cameca IMS 6f using an O2 beam at energies ranging from 0.5 to 2.0 keV. Due to the limitation of the instrumental geometry, the incidence angles were set between 46 and 698. For all analyses, the O2 beam was scanned over a raster area of 250 mm 250 mm. A secondary ion post-acceleration system was used to enhance electron multiplier (EM) signals from low energy secondary ions. No sample rotation or oxygen ¯ooding was used in this work. AFM measurements in tapping mode of the crater bottoms were performed on a Digital Instruments D3000 system. The B delta-doped (B-d) Si sample used in this work consists of ®ve 5 nm spaced delta layers on the
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 3 8 - 4
R. Liu et al. / Applied Surface Science 203±204 (2003) 256±259
257
comprises a 150 nm Si top layer followed by 10 Si0.7Ge0.3 delta layers of 0.4 nm thickness (nominally), grown by atmospheric-pressure chemical vapor deposition (APCVD) at 700 8C. The nominal depths of the deltas are at multiples of 11 nm. 3. Results and discussion
Fig. 1. Depth pro®le of B deltas obtained using 0.5 and 2 keV primary O2 beams.
top, followed by nine 15 nm spaced layers and ®nally another two layers separated by 10 and 5 nm, respectively. This sample was provided by SEMATECH for a round robin study on ultra shallow depth pro®ling of B deltas [3]. The Ge delta-doped (Ge-d) Si sample
Fig. 2.
11
B and
30
Fig. 1 compares the boron signals 11 B of the B delta-doped sample obtained using 0.5 and 2 keV O2 beam at a ®xed incidence angle of 468. The depth calibration was performed by assuming constant sputtering rate. The effect of primary ion energy on the resolution of the depth pro®le is evident by noting the differences in several aspects. First, the dynamic range of the depth pro®le is obviously larger at 0.5 keV for the ®rst seven deltas. Secondly, the very ®rst delta that is 5 nm beneath the surface cannot be resolved at all by 2 keV pro®ling but is clearly resolved at 0.5 keV. The improvement in the depth resolution using lower primary ion energy is well-known to be due to the shallower surface transient [4]. Finally, despite the higher resolution attained in the ®rst six deltas, the depth pro®le obtained at 0.5 keV begins to broaden progressively after a depth of about 50 nm, accompanied by a decrease in the signal dynamic range.
Si of B delta layers using 0.5 keV O2 at 46 and 568 incidences.
258
R. Liu et al. / Applied Surface Science 203±204 (2003) 256±259
Moreover, the last two deltas merge into a broad peak. AFM measurements of the crater bottom reveal that this distortion in the pro®le at 0.5 keV is directly related to the onset of surface roughening [5]. The 11 B and 30 Si pro®les at 46 and 568 incidence using 0.5 keV O2 beam are shown in Fig. 2. The depth calibration was performed following procedures previously reported [6]. Note that the matrix 30 Si intensities for the two angles have considerably different pro®les. The typical surface spike and subsequent rise in matrix intensity in the very early stage of pro®ling appear in both conditions. For the case of 468 incidence, the matrix intensity reaches a maximum at about 20 nm and declines gradually for almost 40 nm. The pre-equilibrium state delays the steady pro®ling until a depth of 60 nm. In the case of 568 incidence, the matrix intensity stabilizes relatively earlier (within the ®rst 15 nm) but exhibits a distinct rise extending from 50 to 90 nm. This rise in matrix intensity is the signature of the onset of the surface roughening as previously observed in the case of 1 keV at 568 incidence with oxygen ¯ooding at intermediate partial oxygen pressures [1]. On this basis, we suggest that for the case of 568 incidence the rapid onset of surface roughening occurs over the depths from delta number 7 (50 nm) to number 9 (90 nm) after which the development of the sequence roughening become steady. On the other hand, a similar feature is not observed in the case of 468 incidence. However, the gradual decline in matrix intensity between delta number 4 (20 nm) to number 7 (50 nm) may indicate the onset of surface roughening as suggested by the decreasing dynamic range of the corresponding 11 B intensity. Beginning from delta number 7, the leading edges of the deltas at 468 incidence reveal shoulders followed by sharp peaks. This is in strong contrast to the rounded peaks observed at 568 incidence. However, careful examination of the deeper delta pro®les at 568 incidence reveals a similar shoulder on the leading edge of delta number 14. Dowsett et al. [7] pointed out that the appearance of the shoulder is an indication of the presence of macrotopograhy that is strongly related to the homogeneity of the primary ion beam. However, the possibility of having an inhomogeneous primary beam is ruled out as a similar feature (shoulder on the leading edge and rising background intensities) is not observed in the depth pro®les obtained using oxygen ¯ooding [5].
We reported that when oxygen ¯ooding was applied at intermediate partial oxygen pressures when pro®ling using 1 keV at 568 incidence, the onset of the surface roughness is related to the rapid change in surface chemical composition resulting in the formation of a layer of heterogeneous oxidation states with comparable proportions of stoichiometric dioxides and non-stoichiometric suboxides [1]. In the case of 0.5 keV incidence, surface roughening occurs even without oxygen ¯ooding. Since 0.5 keV O2 has a shallow penetration depth of about 1.1 nm for 568 incidence [8], it is believed that the shallower distribution of incorporated oxygen atoms favors the
Fig. 3. SIMS depth pro®les of 30 Si (a) and 70 Ge (b) secondary ions from the SiGe d-doped Si sample analyzed with a 500 eV O2 beam at 46, 56 and 698 incidence angles. The onset of surface roughening is different at 46, 56 and 698.
R. Liu et al. / Applied Surface Science 203±204 (2003) 256±259
formation of a heterogeneous layer on the sputtered surface due to the higher oxygen content in the nearsurface regions. If the penetration of the primary ions is suf®ciently deep, the concentration of oxygen atoms in the near-surface region would be much lower and would probably not lead to the formation of localized SiO2 islands. This partly explains the absence of surface roughening during pro®ling at higher primary energies without oxygen ¯ooding. On the other hand, the concentration of the retained oxygen ions is determined by the sputtering yield during the pro®ling. It is established that oblique incidence always leads to high sputtering yield and the sputtering yield at 568 incidence is higher than that at 468 [9]. This results in a longer time for the oxygen concentration to achieve the critical level for the onset of roughening in the case of 568 incidence. On the same basis, it is therefore expected that the onset of the surface roughness would be further delayed when pro®ling at more oblique incidence angles. This hypothesis is supported by the depth pro®les shown in Fig. 3 obtained using 0.5 keV O2 beam at three different incidence angles (i.e., 46, 56 and 698) on a SiGe delta-doped sample. Signi®cant increases in the secondary ion intensities at 568 incidence occur at 60 nm depth, and for 698 incidence at 110 nm depth, indicating the onset of surface roughening. At 468 incidence, the roughening transition occurs at 15 nm, which is too shallow to observe a distinct change in the matrix signal since surface transient effects are dominant here. It can be seen that the Ge deltas are progressively broadened asymmetrically. The deltas broadening with depth is most severe at 468 incidence, intermediate at 568 incidence and smallest
259
at 698 incidence. These results show that the most oblique incidence (698) gives the best resolution in the angular range studied. 4. Conclusion Pro®ling using sub-keV primary ion energy is complicated by the accompanying surface roughening. The angular dependence of surface roughening during 0.5 keV pro®ling can be explained using the heterogeneous layer model. References [1] C.M. Ng, A.T.S. Wee, C.H.A. Huan, A. See, J. Vac. Sci. Technol. 19 (2001) 829. [2] K. Elst, W. Vandervorst, J. Alay, J. Snauwaert, L. Hellemans, J. Vac. Sci. Technol. 16 (1993) 1968. [3] J. Bennett, A. Diebold, in: A. Benninghoven, P. Bertrand, H.N. Migeon, H.W. Werner (Eds.), Secondary Ion Mass Spectrometry (SIMS),Vol. XII, Elsevier, Amsterdam, 2000, p. 541. [4] K. Wittmaack, W. Wach, Nucl. Instrum. Meth. 191 (1981) 327. [5] C.M. Ng, A.T.S. Wee, C.H.A. Huan, A. See, in: H.B. Harrison, A.T.S. Wee, S. Gupta (Eds.), Advanced Microelectronic Processing Techniques, Proc. SPIE 4227 (2000) 90. [6] C.M. Ng, Ph.D. Thesis, National University of Singapore, 2001. [7] M.G. Dowsett, R.D. Barlow, P.N. Allen, J. Vac. Sci. Technol. 12 (1994) 186. [8] J.C. Dupuy, G. Prudon, C. Dubois, P. Warren, D. Dutartre, Nucl. Instrum. Meth. 85 (1994) 379. [9] P.N.K. Deenapanray, M. Petravic, Surf. Interf. Anal. 27 (1999) 92.