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Influence of surface orientation on the formation of sputtering-induced ripple topography in silicon
Applied Surface Science 231–232 (2004) 678–683
Influence of surface orientation on the formation of sputtering-induced ripple topography in silicon B. Faresa,*, B. Gautiera, N. Babouxa, G. Prudona, P. Holligerb, J.C. Dupuya a
Laboratoire de Physique de la Matie`re, UMR CNRS 5511, INSA de lyon, 7 Avenue Capelle, F-69621 Villeurbanne Cedex, France b CEA-DRT—LETI/DST—CEA/GRE, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France Available online 25 May 2004
Abstract The oxygen ion-beam-induced surface roughening observed during SIMS depth profiling of both Si(1 0 0) and 108 disoriented Si(1 0 0) surface without flooding has been studied by atomic force microscopy (AFM). Facets have been created at the crater bottom and parameters such as RMS (root mean square) roughness, wavelength of the waves and orientation of the facets have been measured. The ripple amplitude increases with the depth of erosion from a critical depth of approximately 2.5 mm for O2þ (10–4.5 keV) bombardment. In this paper, we investigate the surface orientation dependence of the roughening in Si(1 0 0) and 108 disoriented-Si(1 0 0). The same transition phase of secondary ion intensities during monitoring of matrix secondary ions and a similar evolution of surface topography and RMS roughness have been observed on both kinds of samples. # 2004 Elsevier B.V. All rights reserved. Keywords: Oxygen ion bombardment; Silicon; Ripple topography; AFM; SIMS
1. Introduction Analysis by secondary ion mass spectrometry (SIMS) is limited by parasitic physical phenomena among which is included the development of roughness in the crater bottom. When the roughness appears, important parameters are affected: erosion rate, ionization yield and sputter yield. Moreover, the quantification of the SIMS analysis is not precise and the depth resolution is degraded. One of the earliest reports has been made by Stevie et al. [1], who have observed rather pronounced changes in secondary ion yields, correlated with the development of ridge crater bottom topography in *
Corresponding author. E-mail address: [email protected] (B. Fares).
silicon and GaAs. Elst et al. [2] investigated the ripples created during SIMS analysis using AFM as a function of sputtered depth, and also the effect of the introduction of gaseous oxygen near the sample. Based on their results, Elst et al. proposed a two-step model for the formation of the ripples on Si: the first step relates to the formation of small topographic features (seeds) caused by the heterogeneity of the internal layer; the second step relates to the rapid development of these seeds into the regular ripple structure. The driving force for the latter is the surface oxidation of the different faces of the ripples. The dependence of surface roughening on crystal orientation was inconsistent with some of the fundamentals of SIMS analysis because roughening is not
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.178
B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
included in the classical ion beam sputtering theory, and can be avoided by appropriate experimental conditions. Lewis et al. [3] have studied roughening of differently oriented Si by 6–8 keV inert ion bombardment and did not find any surface orientation dependence. In contrast, Liu and Alkemade [4] reached a different conclusion when the roughness was studied using O2þ ion bombardment on both Si(0 0 1) and Si(1 1 1) surfaces (528 incidence, impact energy of 1 keV) as a function of the polar and azimuthal incidence angle and primary beam current density. It was found here that there is a difference in sputter rate and onset of roughening between Si(0 0 1) and Si(1 1 1), suggesting a surface orientation dependence. Prenitzer et al. [5] show the formation of {1 1 1} and {1 0 0} facets on Si(0 0 1) and Si(1 1 1), respectively, at impact energy of 5.5 keV and incidence angle of 42.48. In this paper, the oxygen ion-beam-induced surface roughening observed during SIMS depth profiling of both Si(1 0 0) and 108 disoriented Si(1 0 0) surface has been studied by atomic force microscopy (AFM). Without flooding, facets have been created in the crater bottom and parameters such as the RMS (root mean square) roughness, wavelength of the waves and orientation of the facets have been measured. The purposes of this work were:
679
mode. Classical-doped silicon tips were used (300 kHz resonance frequency, 40 N/m stiffness). The typical imaged area was 5 mm 5 mm.
3. Results and discussion Fig. 1(a) and (b) shows the secondary ion signal of the matric ions, where the transition indicating the development of the topography [6] is clearly visible for both kinds of samples. The topography of the crater bottom for both kinds of samples, imaged using AFM, is shown in Fig. 2(a) and (b). Each AFM image corresponds to a different
to investigate the surface orientation dependence of the roughening in Si(1 0 0) and 108 disoriented Si(1 0 0); to study the surface topography development as a function of depth. 2. Experimental The Si(1 0 0) and 108 disoriented Si(1 0 0) measurements were performed on Cameca IMS 3-4f. The experiments reported here were carried out with a O2þ primary beam at impact energy of 5.5 keV and incidence angle of 42.48 O2þ was rastered over a 250 mm 250 mm area with 400 nA current, and a beam diameter of 60 mm. The final crater shapes and depths were measured with a tensor a-step 300 surface profilometer. The topography of the crater bottoms was obtained using a digital instrument dimension 3100 atomic force microscopy (AFM), operated in the tapping
Fig. 1. (a) SIMS depth profile from Si(1 0 0) surface; (b) SIMS depth profile from Si(1 0 0) disoriented of 108 surface.
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B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
Fig. 2. (a) AFM images (5 mm 5 mm) of Si surface bombarded with 5.5 keV O2þ in different depth crater; (b) AFM images (5 mm 5 mm of Si surface disoriented 108 bombarded with 5.5 keV O2þ in different depth crater; (c) section analysis and image from AFM (5 mm 5 mm) of Si(1 0 0) for the depth of 6.5 mm.
sputtered depth and represents different phases of the evolution of the roughness. The ripple topography is periodic in the direction parallel to the beam but aligned in the direction perpendicular to the beam.
The cross-section of the topography in Fig. 2(c) reveals the presence of triangle-shaped patterns. The side facing the ion beam is slightly less steep than the other side.
B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
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Fig. 3. AFM RMS roughening vs. depth for Si(1 0 0) and Si(1 0 0) disoriented 108 surfaces at 42.48 incidence.
The RMS roughness is measured from the AFM images of the craters and is defined by: P 2 z RMS ¼ i i n
where zi is the height measurement of pixel i, and n the number of points. The AFM images from Fig. 2(a) and (b) and the plot of the evolution of the RMS roughness with the crater depth in Fig. 3 show that there is no difference in RMS
Fig. 4. Mean wavelength of the ripple structure vs. depth for Si(1 0 0) surfaces at 42.48 incidence.
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B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
roughness evolution between Si(1 0 0) and 108 disoriented Si(1 0 0) at 42.48 incidence. Ripples develop rapidly for sputtered depths near the transition depth zt where the secondary ion intensity changes occur [2]. For greater depths, the SIMS intensities are constant, although the morphology of the ripples continues to change (Fig. 2).
From Fig. 3, it can be seen that the ripple morphology is similar for both samples. Before transition, the roughness is not very pronounced (RMS ¼ 0:5 nm) and the ripples are relatively straight with a crosssection that appears to be sinusoidal. After transition, certain areas of the surface develop more quickly giving rise to isolated structures with a large height
Fig. 5. (a) Mean angles of the ripple sides for Si(1 0 0), angle 1 corresponds to the side facing the ion beam while angle 2 corresponds to the other side; (b) mean angles of the ripple sides for Si(1 0 0) disoriented 108.
B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
amplitude (20–120 nm), and the structure is more faceted with a cross-section more triangular than sinusoidal. The ridges of the ripples which are perpendicular to the primary beam direction are more irregular. The SIMS depth profiles obtained using 5.5 keV impact energy and an incidence angle of 42.48 for the Si(1 0 0) and 108 disoriented Si(1 0 0) surfaces, and the crater depth dependence of the wavelength and root mean square (RMS) roughness are shown in Figs. 3 and 4. We note that the wavelength of the ripples is 300 nm at the critical depth and increases progressively with depth. A change in the intensities of the matrix ions is observed for both Si(1 0 0) and 108 disoriented Si(1 0 0), indicative of the surface roughening [6,7]. A transition in the matrix SIMS signal (SiO2þ, SiOþ, Siþ, etc.) is observed, the signals remain constant until the depth 2.5 mm for Si(1 0 0) and 2.3 mm for Si(1 0 0) disoriented 108 is reached. Then, they change dramatically (increase for SiO2, SiO, Si and decrease for SiO, Si2). After a transition phase, the signals stabilize again. An explanation for the intensity changes has been suggested by Wittmaack [8] who takes into account the local or microscopic angle of incidence which might not be the same (when roughness appears) as the macroscopic angle defined by the surface plane of the sample. The incidence angle of the primary ion is one of the most important factors in ripple formation. Wittmaack [8] reported that bombardment of Si by 10 keV ions causes ripples to form when the primary ion incident angle was 32–588 to the surface normal. Furthermore, the observed change in secondary ion intensity has been suggested to arise from a change in primary ion incident angle caused by the change in topography. The angles were determined at maximum inclination and with respect to the macroscopic surface. In our study, the mean angles of the ripple sides are almost the same for Si(1 0 0) and 108 disoriented Si(1 0 0), respectively, 358 for Si(1 0 0) and 288 for 108 disoriented Si (cf. Fig. 5(a) and (b). Consequently, the impact angle of the O2þ beam tends to approach
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near normal incidence (7–128) at the oxidized side and glazing incidence (72–778) at the other side.
4. Conclusion Our results from both Si(1 0 0) and 108 disoriented Si(1 0 0) surfaces do not show a strong surface orientation dependence on the development of roughening at impact energy of 5.5 keV and 42.48 incidence angle. The ripple formation is similar for both kinds of silicon. Before transition, the structure becomes slightly more pronounced (RMS roughness ¼ 0:5 nm) and the ripples are relatively straight with a crosssection that appears to be sinusoidal. After transition, some areas of the surface develop more quickly than others, giving rise to isolated structures with large roughness amplitude (20–120 nm RMS). The wavelength of the ripples is already 300 nm at the critical depth and increases progressively with depth. The mean angles of the ripple facets are almost the same for Si(1 0 0) and 108 disoriented Si(1 0 0), respectively, 358 for Si(1 0 0) and 288 for 108 disoriented Si. The impact angle of the O2þ beam tends to approach near normal incidence (7–128) at the oxidized side and grazing incidence (72–778) at the other side. References [1] F.A. Stevie, P.M. Kahora, D.S. Simons, P. Chi, J. Vac. Sci. Technol. A 6 (1988) 76–79. [2] K. Elst, W. Vandervorst, J. Alay, J. Vac. Sci. Technol. B 11 (1993) 1968–1993. [3] G.W. Lewis, M.J. Nobes, G. Carter, J.L. Whitton, Nucl. Instrum. Meth. 170 (1980) 363. [4] Z.X. Liu, F.A. Alkemade, in: A. Benninghoven, et al. (Eds.), Proceedings of the Secondary Ion Mass Spectrometry, SIMS XII, 1999, pp. 505–508. [5] B.I. Prenitzer, L.A. Giannuzzi, B.W. Kempshall, J.M. Mckinley, F.A. Stevie, in: A. Benninghoven, et al. (Eds.), Secondary Ion Mass Spectrometry, SIMS XII, 1999, pp. 77–80. [6] J.J. Vajo, R.E. Doty, E.-H. Cirlin, J. Vac. Sci. Technol. A 14 (5) (1996) 2709–2720. [7] G.S. Lau, E.S. Tok, Surf. Rev. Lett. 8 (2001) 453–457. [8] K. Wittmaack, J. Vac. Sci. Technol. A 8 (1999) 2246–2250.
Influence of surface orientation on the formation of sputtering-induced ripple topography in silicon B. Faresa,*, B. Gautiera, N. Babouxa, G. Prudona, P. Holligerb, J.C. Dupuya a
Laboratoire de Physique de la Matie`re, UMR CNRS 5511, INSA de lyon, 7 Avenue Capelle, F-69621 Villeurbanne Cedex, France b CEA-DRT—LETI/DST—CEA/GRE, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France Available online 25 May 2004
Abstract The oxygen ion-beam-induced surface roughening observed during SIMS depth profiling of both Si(1 0 0) and 108 disoriented Si(1 0 0) surface without flooding has been studied by atomic force microscopy (AFM). Facets have been created at the crater bottom and parameters such as RMS (root mean square) roughness, wavelength of the waves and orientation of the facets have been measured. The ripple amplitude increases with the depth of erosion from a critical depth of approximately 2.5 mm for O2þ (10–4.5 keV) bombardment. In this paper, we investigate the surface orientation dependence of the roughening in Si(1 0 0) and 108 disoriented-Si(1 0 0). The same transition phase of secondary ion intensities during monitoring of matrix secondary ions and a similar evolution of surface topography and RMS roughness have been observed on both kinds of samples. # 2004 Elsevier B.V. All rights reserved. Keywords: Oxygen ion bombardment; Silicon; Ripple topography; AFM; SIMS
1. Introduction Analysis by secondary ion mass spectrometry (SIMS) is limited by parasitic physical phenomena among which is included the development of roughness in the crater bottom. When the roughness appears, important parameters are affected: erosion rate, ionization yield and sputter yield. Moreover, the quantification of the SIMS analysis is not precise and the depth resolution is degraded. One of the earliest reports has been made by Stevie et al. [1], who have observed rather pronounced changes in secondary ion yields, correlated with the development of ridge crater bottom topography in *
Corresponding author. E-mail address: [email protected] (B. Fares).
silicon and GaAs. Elst et al. [2] investigated the ripples created during SIMS analysis using AFM as a function of sputtered depth, and also the effect of the introduction of gaseous oxygen near the sample. Based on their results, Elst et al. proposed a two-step model for the formation of the ripples on Si: the first step relates to the formation of small topographic features (seeds) caused by the heterogeneity of the internal layer; the second step relates to the rapid development of these seeds into the regular ripple structure. The driving force for the latter is the surface oxidation of the different faces of the ripples. The dependence of surface roughening on crystal orientation was inconsistent with some of the fundamentals of SIMS analysis because roughening is not
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.178
B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
included in the classical ion beam sputtering theory, and can be avoided by appropriate experimental conditions. Lewis et al. [3] have studied roughening of differently oriented Si by 6–8 keV inert ion bombardment and did not find any surface orientation dependence. In contrast, Liu and Alkemade [4] reached a different conclusion when the roughness was studied using O2þ ion bombardment on both Si(0 0 1) and Si(1 1 1) surfaces (528 incidence, impact energy of 1 keV) as a function of the polar and azimuthal incidence angle and primary beam current density. It was found here that there is a difference in sputter rate and onset of roughening between Si(0 0 1) and Si(1 1 1), suggesting a surface orientation dependence. Prenitzer et al. [5] show the formation of {1 1 1} and {1 0 0} facets on Si(0 0 1) and Si(1 1 1), respectively, at impact energy of 5.5 keV and incidence angle of 42.48. In this paper, the oxygen ion-beam-induced surface roughening observed during SIMS depth profiling of both Si(1 0 0) and 108 disoriented Si(1 0 0) surface has been studied by atomic force microscopy (AFM). Without flooding, facets have been created in the crater bottom and parameters such as the RMS (root mean square) roughness, wavelength of the waves and orientation of the facets have been measured. The purposes of this work were:
679
mode. Classical-doped silicon tips were used (300 kHz resonance frequency, 40 N/m stiffness). The typical imaged area was 5 mm 5 mm.
3. Results and discussion Fig. 1(a) and (b) shows the secondary ion signal of the matric ions, where the transition indicating the development of the topography [6] is clearly visible for both kinds of samples. The topography of the crater bottom for both kinds of samples, imaged using AFM, is shown in Fig. 2(a) and (b). Each AFM image corresponds to a different
to investigate the surface orientation dependence of the roughening in Si(1 0 0) and 108 disoriented Si(1 0 0); to study the surface topography development as a function of depth. 2. Experimental The Si(1 0 0) and 108 disoriented Si(1 0 0) measurements were performed on Cameca IMS 3-4f. The experiments reported here were carried out with a O2þ primary beam at impact energy of 5.5 keV and incidence angle of 42.48 O2þ was rastered over a 250 mm 250 mm area with 400 nA current, and a beam diameter of 60 mm. The final crater shapes and depths were measured with a tensor a-step 300 surface profilometer. The topography of the crater bottoms was obtained using a digital instrument dimension 3100 atomic force microscopy (AFM), operated in the tapping
Fig. 1. (a) SIMS depth profile from Si(1 0 0) surface; (b) SIMS depth profile from Si(1 0 0) disoriented of 108 surface.
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B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
Fig. 2. (a) AFM images (5 mm 5 mm) of Si surface bombarded with 5.5 keV O2þ in different depth crater; (b) AFM images (5 mm 5 mm of Si surface disoriented 108 bombarded with 5.5 keV O2þ in different depth crater; (c) section analysis and image from AFM (5 mm 5 mm) of Si(1 0 0) for the depth of 6.5 mm.
sputtered depth and represents different phases of the evolution of the roughness. The ripple topography is periodic in the direction parallel to the beam but aligned in the direction perpendicular to the beam.
The cross-section of the topography in Fig. 2(c) reveals the presence of triangle-shaped patterns. The side facing the ion beam is slightly less steep than the other side.
B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
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Fig. 3. AFM RMS roughening vs. depth for Si(1 0 0) and Si(1 0 0) disoriented 108 surfaces at 42.48 incidence.
The RMS roughness is measured from the AFM images of the craters and is defined by: P 2 z RMS ¼ i i n
where zi is the height measurement of pixel i, and n the number of points. The AFM images from Fig. 2(a) and (b) and the plot of the evolution of the RMS roughness with the crater depth in Fig. 3 show that there is no difference in RMS
Fig. 4. Mean wavelength of the ripple structure vs. depth for Si(1 0 0) surfaces at 42.48 incidence.
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B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
roughness evolution between Si(1 0 0) and 108 disoriented Si(1 0 0) at 42.48 incidence. Ripples develop rapidly for sputtered depths near the transition depth zt where the secondary ion intensity changes occur [2]. For greater depths, the SIMS intensities are constant, although the morphology of the ripples continues to change (Fig. 2).
From Fig. 3, it can be seen that the ripple morphology is similar for both samples. Before transition, the roughness is not very pronounced (RMS ¼ 0:5 nm) and the ripples are relatively straight with a crosssection that appears to be sinusoidal. After transition, certain areas of the surface develop more quickly giving rise to isolated structures with a large height
Fig. 5. (a) Mean angles of the ripple sides for Si(1 0 0), angle 1 corresponds to the side facing the ion beam while angle 2 corresponds to the other side; (b) mean angles of the ripple sides for Si(1 0 0) disoriented 108.
B. Fares et al. / Applied Surface Science 231–232 (2004) 678–683
amplitude (20–120 nm), and the structure is more faceted with a cross-section more triangular than sinusoidal. The ridges of the ripples which are perpendicular to the primary beam direction are more irregular. The SIMS depth profiles obtained using 5.5 keV impact energy and an incidence angle of 42.48 for the Si(1 0 0) and 108 disoriented Si(1 0 0) surfaces, and the crater depth dependence of the wavelength and root mean square (RMS) roughness are shown in Figs. 3 and 4. We note that the wavelength of the ripples is 300 nm at the critical depth and increases progressively with depth. A change in the intensities of the matrix ions is observed for both Si(1 0 0) and 108 disoriented Si(1 0 0), indicative of the surface roughening [6,7]. A transition in the matrix SIMS signal (SiO2þ, SiOþ, Siþ, etc.) is observed, the signals remain constant until the depth 2.5 mm for Si(1 0 0) and 2.3 mm for Si(1 0 0) disoriented 108 is reached. Then, they change dramatically (increase for SiO2, SiO, Si and decrease for SiO, Si2). After a transition phase, the signals stabilize again. An explanation for the intensity changes has been suggested by Wittmaack [8] who takes into account the local or microscopic angle of incidence which might not be the same (when roughness appears) as the macroscopic angle defined by the surface plane of the sample. The incidence angle of the primary ion is one of the most important factors in ripple formation. Wittmaack [8] reported that bombardment of Si by 10 keV ions causes ripples to form when the primary ion incident angle was 32–588 to the surface normal. Furthermore, the observed change in secondary ion intensity has been suggested to arise from a change in primary ion incident angle caused by the change in topography. The angles were determined at maximum inclination and with respect to the macroscopic surface. In our study, the mean angles of the ripple sides are almost the same for Si(1 0 0) and 108 disoriented Si(1 0 0), respectively, 358 for Si(1 0 0) and 288 for 108 disoriented Si (cf. Fig. 5(a) and (b). Consequently, the impact angle of the O2þ beam tends to approach
683
near normal incidence (7–128) at the oxidized side and glazing incidence (72–778) at the other side.
4. Conclusion Our results from both Si(1 0 0) and 108 disoriented Si(1 0 0) surfaces do not show a strong surface orientation dependence on the development of roughening at impact energy of 5.5 keV and 42.48 incidence angle. The ripple formation is similar for both kinds of silicon. Before transition, the structure becomes slightly more pronounced (RMS roughness ¼ 0:5 nm) and the ripples are relatively straight with a crosssection that appears to be sinusoidal. After transition, some areas of the surface develop more quickly than others, giving rise to isolated structures with large roughness amplitude (20–120 nm RMS). The wavelength of the ripples is already 300 nm at the critical depth and increases progressively with depth. The mean angles of the ripple facets are almost the same for Si(1 0 0) and 108 disoriented Si(1 0 0), respectively, 358 for Si(1 0 0) and 288 for 108 disoriented Si. The impact angle of the O2þ beam tends to approach near normal incidence (7–128) at the oxidized side and grazing incidence (72–778) at the other side. References [1] F.A. Stevie, P.M. Kahora, D.S. Simons, P. Chi, J. Vac. Sci. Technol. A 6 (1988) 76–79. [2] K. Elst, W. Vandervorst, J. Alay, J. Vac. Sci. Technol. B 11 (1993) 1968–1993. [3] G.W. Lewis, M.J. Nobes, G. Carter, J.L. Whitton, Nucl. Instrum. Meth. 170 (1980) 363. [4] Z.X. Liu, F.A. Alkemade, in: A. Benninghoven, et al. (Eds.), Proceedings of the Secondary Ion Mass Spectrometry, SIMS XII, 1999, pp. 505–508. [5] B.I. Prenitzer, L.A. Giannuzzi, B.W. Kempshall, J.M. Mckinley, F.A. Stevie, in: A. Benninghoven, et al. (Eds.), Secondary Ion Mass Spectrometry, SIMS XII, 1999, pp. 77–80. [6] J.J. Vajo, R.E. Doty, E.-H. Cirlin, J. Vac. Sci. Technol. A 14 (5) (1996) 2709–2720. [7] G.S. Lau, E.S. Tok, Surf. Rev. Lett. 8 (2001) 453–457. [8] K. Wittmaack, J. Vac. Sci. Technol. A 8 (1999) 2246–2250.