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Molecular dynamics simulation analyses on injection angle dependence of SiO2 sputtering yields by fluorocarbon beams
Thin Solid Films 515 (2007) 4883 – 4886 www.elsevier.com/locate/tsf
Molecular dynamics simulation analyses on injection angle dependence of SiO2 sputtering yields by fluorocarbon beams Tomohito Kawase, Satoshi Hamaguchi ⁎ Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Available online 27 November 2006
Abstract Angular dependence of sputtering yields of SiO2 substrates subject to CF3 beam injections is evaluated with the use of molecular dynamics simulations. The obtained sputtering yield data are in reasonable agreement with experimental observations. Atomic compositions in the SiO2-CF mixing layer as well as kinetic energies and atomic compositions of sputtered species also exhibit strong dependence of the injection angle. © 2006 Elsevier B.V. All rights reserved. Keywords: Molecular dynamics simulations; Fluorocarbon; Plasma etching; Plasma surface interaction
1. Introduction Following the recent advancement of Very Large Scale Integrate (VLSI) circuit technologies, requirements for VLSI fabrication process technologies have become stricter than ever. For example, slight tapering at a trench edge in decanano-scale etching processes may violate process control requirements. Edge tapering in etching processes is known to be caused by the injection angle dependence of etch rates (i.e., sputtering yields) and therefore a better understanding of dependence of etch characteristics on the ion injection angle is expected to facilitate the development of more precise process control techniques. Since fluorocarbon is one of the most widely used feed gases for plasma processing, we focus on SiO2 selective etching processes by fluorocarbon plasmas in the present work and examine the SiO2 sputtering yield as a function of the beam injection angle and energy, using molecular dynamics (MD) simulations [1–7]. Recent development of beam–surface interaction experiments [8–10] can also allow one to test feasibility of such MD simulation studies. 2. MD simulation results The method of simulating beam–surface interaction used in this work is similar to that used in earlier studies [1–7], where ⁎ Corresponding author. E-mail address: [email protected] (S. Hamaguchi). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.10.024
atoms (or clusters of atoms) are injected into a substrate that consists of many (typically a few thousand) atoms and the equations of motion for all the particles are integrated for a sufficiently long duration of time after each injection. The classical interatomic potential functions used in the MD simulations presented here are the same as those used in Ref. [1]. Stillinger–Weber type potential functions for covalent bonds and Lennard–Jones type potential functions for Van der Waals interactions are employed in the simulation code. Using the MD simulations, we have evaluated sputtering yields of crystalline SiO2 surfaces that are subject to 200 eV or 300 eV CF3 beams. For vertical injections, as has been reported in Ref. [1], reactive etching is observed in the case of 300 eV CF3 injections whereas fluorocarbon film deposition is observed in the case of 200 eV CF3 injections. Fig. 1 shows atomistic-scale surface structures of the substrates after 2 × 1016 cm− 2 dose of (a) 300 eV and (b) 200 eV CF3 beam injections normal to the substrate surface (i.e., vertical injections). As can be seen, a layer of mixed Si, O, C, and F atoms (which we call a mixed layer) is formed on the top surface of the substrate during the etching process. The graphs on the both sides of each surface structure indicate atomic density profiles as functions of the depth. In addition to the vertical (i.e., 0°) injections, we have evaluated the sputtering yields at 15°, 30°, 45°, 60° and 75° beam injections from the MD simulations. The obtained sputtering yields are shown in Fig. 2, where yields of F, C, O, and Si are given as functions of the injection angle for CF3
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Fig. 1. Atomic surface structures after 2 × 1016 cm− 2 dose of (a) 300 eV and (b) 200 eV CF3 beam vertical injections. In (a), where reactive etching is taking place, a mixed layer of Si, O, C, and F atoms is formed. In (a) and (b), graphs on the both sides indicate atomic densities as functions of the depth.
300 eV (denoted by •) and 200 eV (denoted by ×) injections. Since at low angle (less than about 15°) injections of 200 eV CF3, the process is essentially deposition and therefore the sputtering yields for O and Si are almost null whereas those of C and F are less than 1 and 3, which means that C and F are deposited during the process under these conditions. We have confirmed in MD simulations that under these conditions fluorocarbon layers were deposited on the top surface, as in the case of Fig. 1(b). It is interesting to note that at higher angle (larger than about 15°) injections of 200 eV CF3, the process is etching. In both 300 eV and 200 eV cases of CF3 injections, the peak sputtering yields for Si and O are obtained at about 60°. In Fig. 2, it is shown that the sputtering yields for F and C are about 3 and 1, i.e., the injected fluorocarbon species are essentially desorbed or reflected from the surface. The yield dependence on the injection angle as shown in Fig. 2 is in reasonable agreement with experimental observations [11]. Injected carbon is desorbed from the surface mainly as CO or CFx (x = 0–3) radicals [1]. Fig. 3 shows the yields (circle) and kinetic energies (squares) of (a) CO and (b) CF species desorbed
from the surface. In both figures, the black and grey marks represent those under 300 eV and 200 eV of CF3 injections. The solid and dotted curves in the figure are merely a guide to the eye. As shown in Fig. 3(a), yields of CO under 300 eV CF3 injections vary little with the injection angle between 0° and 50° and are a decreasing function of the injection angle for angles larger than about 50°. The kinetic energy of a CO molecule leaving the surface is typically small (of the order of 1 eV) at a smaller angle and an increasing function of the injection angle. These are typical properties for desorbed species (as opposed to reflected species). Note that desorbed CO molecules tend to gain more kinetic energies in horizontal directions from the injected species as the injection angle becomes larger. Nevertheless the kinetic energy gained by a desorbed species is typically much smaller than that by a reflected species, as we shall see below. As shown in Fig. 2, the total yield of O increases significantly as the injection angle increases from 30° to 60°. For such angles, in addition to the CO desorption shown in Fig. 3(a), more oxygen is observed to be removed as single atoms (i.e. O) from the surface by large-angle injection beams in our simulations. As to 200 eV injections, yields of CO at small angles (smaller than about 15°) are very small (less than 0.1) since the process is essentially deposition. Therefore, the angle dependence of the yield for CO desorption differs significantly between the angles smaller than about 15° and those larger than that, as shown in Fig. 3(a). For larger angles, the yields and kinetic energies of desorbed CO are similar for both cases of 200 eV and 300 eV injections. In Fig. 3(b), the yield of CF species is shown to be a steep increasing function of the injection angle larger than about 45°.
Fig. 2. Angle dependence of sputtering yields for atom species for different injection energies (•: CF3 300 eV, ×: CF3 200 eV). The lines and curves are a guide to the eye.
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Fig. 4. Yields of SiF4, SiF3, SiF2, and SiF for 300 eV (black circles) and 200 eV (grey circles) CF3 injections. The curves are the guide to the eye. SiFx are the major desorption species that contain Si atoms. The number of F atoms (i.e., the value x) of desorbed SiFx is shown to depend strongly on the injection angle.
Fig. 3. In (a) yields (circles) and kinetic energies (squares) of CO desorbed from the surface are shown as functions of the injection angles under for 300 eV (black) and 200 eV (grey) CF3 injections. Similarly, in (b), yields (circles) and kinetic energies (squares) of CF desorbed from the surface are shown as functions of the injection angles under for 300 eV (black) and 200 eV (grey) CF3 injections. Solid and dotted curves are a guide to the eye. Since fluorocarbon film deposition are essentially taking place at angles smaller than about 15° for 200 eV CF3 injections, yields of CO under these conditions are significantly smaller than those at larger injection angles.
Kinetic energies of the CF species leaving the surface are shown to be large (of the order of 10–100 eV) compared with desorbed CO species shown in Fig. 3(a). These are typical characteristics of reflected fragments of the injected CF3 radicals. Since the loss of horizontal momenta due to the collision with the surface species are typically smaller than that of vertical momentum, the kinetic energy of a reflected species is usually larger for a larger injection angle. Note that the yield of CF is higher at the smaller injection energy (i.e., 200 eV). This is because, with a larger injection energy, the injected CF3 radicals are more likely to be fragmented into smaller pieces. In addition, the fragmented species are more likely to be implanted into the substrate with
higher vertical momenta (unless sputtering of the substrate dominates over the C deposition). The latter also accounts for the increase of CF sputtering yield as a function of injection angle, shown in Fig. 3(b). As in the case of vertical injections [1], nearly half of oxygen atoms in the SiO2 substrates are removed as CO radicals during the etching process with relatively small injection angles (less than about 50°) under the conditions considered in this work, as shown in Figs. 2 and 3 for 300 eV CF3 injections. (For example, for vertical injections, the sputtering yield of CO molecules is about 0.7 from Fig. 3 whereas the total sputtering yield of oxygen atoms, whether they are removed as CO or any other forms, is about 1.6 from Fig. 2.) For larger angles, more atomic oxygen is sputtered from the surface, as mentioned earlier. Similarly, approximately half of Si atoms are removed as SiFx radicals during the etching process with relatively small injection angles under the conditions considered in this work. Fig. 4 shows the dependence of yields of SiF, SiF2 , SiF3 and SiF4 desorption species on the injection angle for 300 eV (black circles) and 200 eV (grey circles) CF3 injections. As shown in the case of 300 eV injections in Fig. 4, where an etching process is taking place for all injection angles, the peak position of the yield for SiFx desorption species as functions of the injection angle is smaller for larger x values. In other words, Table 1 Ratios of the numbers of Si and F atoms to the total number of atoms in the mixed layer as functions of the injection angle for 300 eV injections of CF3 Angle (°)
Si
F
0 15 30 45 60 75
0.23 0.24 0.24 0.28 0.29 0.31
0.34 0.31 0.31 0.19 0.17 0.10
It is seen that the larger the injection angle is, the less fluorinated the surface is.
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at smaller injection angles, larger SiFx desorption species (i.e., those with larger x) are dominant whereas, at larger injection angles, smaller SiFx are dominant. The same tendency is seen for the etching processes with relatively larger angles of 200 eV CF3 injections. For smaller angles (less than about 15°) under these conditions, yields of SiFx are small since fluorocarbon deposition takes place, as mentioned earlier. Table 1 shows the ratios of the numbers of Si and F atoms to the total number of atoms in the mixed layer as functions of the injection angle for 300 eV CF3 injections. The depth of the mixed layer is defined as a typical depth that injected atoms penetrate through and it is a decreasing function of the injection angle. It is seen that the larger the injection angle is, the fewer F atoms are found in the mixed layer. This is because, with larger injection angles, the injected species are more likely to be specularly reflected and fewer F atoms of injected CF3 are adsorbed on the substrate surface. In the etching process with a mixed layer of fewer F atoms (as in the cases of larger injection angles given in Table 1), the silicon fluoride radicals SiFx desorbed from the surface are expected to be those of fewer F atoms due to the scarcity of F in the layer, which agrees with the observation shown in Fig. 4. 3. Conclusions In the present work, MD simulations have been used to analyze injection angle dependence of sputtering yields and other etching characteristics of SiO2 substrates for CF3 radical beam injections. For vertical (i.e., the injection angle of 0°) injections, it has been observed that the SiO2 surface is etched at 300 eV injections whereas it is hardly etched and significant deposition of fluorocarbon films are observed at 200 eV injections, as reported earlier [1]. What we have found interesting is that, for the same 200 eV CF3 radical beam injections, the SiO2 surface is substantially etched at larger injection angles. As shown in Fig. 2, at the 60° injection angle of 200 eV CF3 injections, the sputtering yield of Si of the SiO2 surface is about unity, which is comparable to the Si sputtering yield of vertical injections of 300 eV CF3 beams. In other words, under the same beam conditions, fluorocarbon deposition and SiO2 sputtering can take place simultaneously, depending on the slope angles of submicron structures on the substrate surface.
As to 300 eV CF3 injections, etching of the SiO2 surface is observed for all injection angles. In both 200 eV and 300 eV cases, the maximum sputtering yields are obtained at about 60°. This angular dependence of sputtering yields obtained from our MD simulation results is consistent with unpublished data obtained from mass-selected ion beam experiments [11]. We have also examined desorbed or reflected species from the substrate surface under the beam bombardment. The major desorbed species containing oxygen and silicon are found to be CO and silicon fluoride (SiFx) radicals for relatively small injection angles, which is consistent with earlier simulation results [1]. As the injection angle increases, more atomic oxygen and smaller SiFx radicals (i.e., SiFx with lower x) also become important desorption species for oxygen and silicon. Such data provide useful insight into the analyses of submicronscale surface profiles obtained in SiO2 selective etching processes. Acknowledgements This work is partially supported by a Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology of Japan and also by Sony Corporation. References [1] H. Ohta and S. Hamaguchi, J. Vac. Sci. Technol. A (submitted for publication). [2] V.V. Smirnov, A.V. Stengach, K.G. Gaynullin, V.A. Pavlovsky, S. Rauf, P.J. Stout, P.L.G. Ventzek, J. Appl. Phys. 97 (2005) 093302. [3] V.V. Smirnov, A.V. Stengach, K.G. Gaynullin, V.A. Pavlovsky, S. Rauf, P.J. Stout, P.L.G. Ventzek, J. Appl. Phys. 97 (2005) 093303. [4] H. Yamada, S. Hamaguchi, J. Appl. Phys. 96 (2004) 6147. [5] H. Ohta, S. Hamaguchi, J. Vac. Sci. Technol. A19 (2001) 2373. [6] C.F. Abrams, D.B. Graves, J. Vac. Sci. Technol. A19 (2001) 175. [7] J. Tanaka, C.F. Abrams, D.B. Graves, J. Vac. Sci. Technol., A 18 (2000) 938. [8] K. Ishikawa, K. Karahashi, H. Tsuboi, K. Yanai, M. Nakamura, J. Vac. Sci. Technol. A21 (2003) L1. [9] K. Karahashi, K. Yanai, K. Ishikawa, K. Kurihara, M. Nakamura, J. Vac. Sci. Technol., A 22 (2004) 1166. [10] H. Toyoda, H. Morishima, R. Fukute, Y. Hori, I. Murakami, H. Sugai, J. Appl. Phys. 95 (2004) 5172. [11] K. Karahashi, private communication.
Molecular dynamics simulation analyses on injection angle dependence of SiO2 sputtering yields by fluorocarbon beams Tomohito Kawase, Satoshi Hamaguchi ⁎ Center for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Available online 27 November 2006
Abstract Angular dependence of sputtering yields of SiO2 substrates subject to CF3 beam injections is evaluated with the use of molecular dynamics simulations. The obtained sputtering yield data are in reasonable agreement with experimental observations. Atomic compositions in the SiO2-CF mixing layer as well as kinetic energies and atomic compositions of sputtered species also exhibit strong dependence of the injection angle. © 2006 Elsevier B.V. All rights reserved. Keywords: Molecular dynamics simulations; Fluorocarbon; Plasma etching; Plasma surface interaction
1. Introduction Following the recent advancement of Very Large Scale Integrate (VLSI) circuit technologies, requirements for VLSI fabrication process technologies have become stricter than ever. For example, slight tapering at a trench edge in decanano-scale etching processes may violate process control requirements. Edge tapering in etching processes is known to be caused by the injection angle dependence of etch rates (i.e., sputtering yields) and therefore a better understanding of dependence of etch characteristics on the ion injection angle is expected to facilitate the development of more precise process control techniques. Since fluorocarbon is one of the most widely used feed gases for plasma processing, we focus on SiO2 selective etching processes by fluorocarbon plasmas in the present work and examine the SiO2 sputtering yield as a function of the beam injection angle and energy, using molecular dynamics (MD) simulations [1–7]. Recent development of beam–surface interaction experiments [8–10] can also allow one to test feasibility of such MD simulation studies. 2. MD simulation results The method of simulating beam–surface interaction used in this work is similar to that used in earlier studies [1–7], where ⁎ Corresponding author. E-mail address: [email protected] (S. Hamaguchi). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.10.024
atoms (or clusters of atoms) are injected into a substrate that consists of many (typically a few thousand) atoms and the equations of motion for all the particles are integrated for a sufficiently long duration of time after each injection. The classical interatomic potential functions used in the MD simulations presented here are the same as those used in Ref. [1]. Stillinger–Weber type potential functions for covalent bonds and Lennard–Jones type potential functions for Van der Waals interactions are employed in the simulation code. Using the MD simulations, we have evaluated sputtering yields of crystalline SiO2 surfaces that are subject to 200 eV or 300 eV CF3 beams. For vertical injections, as has been reported in Ref. [1], reactive etching is observed in the case of 300 eV CF3 injections whereas fluorocarbon film deposition is observed in the case of 200 eV CF3 injections. Fig. 1 shows atomistic-scale surface structures of the substrates after 2 × 1016 cm− 2 dose of (a) 300 eV and (b) 200 eV CF3 beam injections normal to the substrate surface (i.e., vertical injections). As can be seen, a layer of mixed Si, O, C, and F atoms (which we call a mixed layer) is formed on the top surface of the substrate during the etching process. The graphs on the both sides of each surface structure indicate atomic density profiles as functions of the depth. In addition to the vertical (i.e., 0°) injections, we have evaluated the sputtering yields at 15°, 30°, 45°, 60° and 75° beam injections from the MD simulations. The obtained sputtering yields are shown in Fig. 2, where yields of F, C, O, and Si are given as functions of the injection angle for CF3
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Fig. 1. Atomic surface structures after 2 × 1016 cm− 2 dose of (a) 300 eV and (b) 200 eV CF3 beam vertical injections. In (a), where reactive etching is taking place, a mixed layer of Si, O, C, and F atoms is formed. In (a) and (b), graphs on the both sides indicate atomic densities as functions of the depth.
300 eV (denoted by •) and 200 eV (denoted by ×) injections. Since at low angle (less than about 15°) injections of 200 eV CF3, the process is essentially deposition and therefore the sputtering yields for O and Si are almost null whereas those of C and F are less than 1 and 3, which means that C and F are deposited during the process under these conditions. We have confirmed in MD simulations that under these conditions fluorocarbon layers were deposited on the top surface, as in the case of Fig. 1(b). It is interesting to note that at higher angle (larger than about 15°) injections of 200 eV CF3, the process is etching. In both 300 eV and 200 eV cases of CF3 injections, the peak sputtering yields for Si and O are obtained at about 60°. In Fig. 2, it is shown that the sputtering yields for F and C are about 3 and 1, i.e., the injected fluorocarbon species are essentially desorbed or reflected from the surface. The yield dependence on the injection angle as shown in Fig. 2 is in reasonable agreement with experimental observations [11]. Injected carbon is desorbed from the surface mainly as CO or CFx (x = 0–3) radicals [1]. Fig. 3 shows the yields (circle) and kinetic energies (squares) of (a) CO and (b) CF species desorbed
from the surface. In both figures, the black and grey marks represent those under 300 eV and 200 eV of CF3 injections. The solid and dotted curves in the figure are merely a guide to the eye. As shown in Fig. 3(a), yields of CO under 300 eV CF3 injections vary little with the injection angle between 0° and 50° and are a decreasing function of the injection angle for angles larger than about 50°. The kinetic energy of a CO molecule leaving the surface is typically small (of the order of 1 eV) at a smaller angle and an increasing function of the injection angle. These are typical properties for desorbed species (as opposed to reflected species). Note that desorbed CO molecules tend to gain more kinetic energies in horizontal directions from the injected species as the injection angle becomes larger. Nevertheless the kinetic energy gained by a desorbed species is typically much smaller than that by a reflected species, as we shall see below. As shown in Fig. 2, the total yield of O increases significantly as the injection angle increases from 30° to 60°. For such angles, in addition to the CO desorption shown in Fig. 3(a), more oxygen is observed to be removed as single atoms (i.e. O) from the surface by large-angle injection beams in our simulations. As to 200 eV injections, yields of CO at small angles (smaller than about 15°) are very small (less than 0.1) since the process is essentially deposition. Therefore, the angle dependence of the yield for CO desorption differs significantly between the angles smaller than about 15° and those larger than that, as shown in Fig. 3(a). For larger angles, the yields and kinetic energies of desorbed CO are similar for both cases of 200 eV and 300 eV injections. In Fig. 3(b), the yield of CF species is shown to be a steep increasing function of the injection angle larger than about 45°.
Fig. 2. Angle dependence of sputtering yields for atom species for different injection energies (•: CF3 300 eV, ×: CF3 200 eV). The lines and curves are a guide to the eye.
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Fig. 4. Yields of SiF4, SiF3, SiF2, and SiF for 300 eV (black circles) and 200 eV (grey circles) CF3 injections. The curves are the guide to the eye. SiFx are the major desorption species that contain Si atoms. The number of F atoms (i.e., the value x) of desorbed SiFx is shown to depend strongly on the injection angle.
Fig. 3. In (a) yields (circles) and kinetic energies (squares) of CO desorbed from the surface are shown as functions of the injection angles under for 300 eV (black) and 200 eV (grey) CF3 injections. Similarly, in (b), yields (circles) and kinetic energies (squares) of CF desorbed from the surface are shown as functions of the injection angles under for 300 eV (black) and 200 eV (grey) CF3 injections. Solid and dotted curves are a guide to the eye. Since fluorocarbon film deposition are essentially taking place at angles smaller than about 15° for 200 eV CF3 injections, yields of CO under these conditions are significantly smaller than those at larger injection angles.
Kinetic energies of the CF species leaving the surface are shown to be large (of the order of 10–100 eV) compared with desorbed CO species shown in Fig. 3(a). These are typical characteristics of reflected fragments of the injected CF3 radicals. Since the loss of horizontal momenta due to the collision with the surface species are typically smaller than that of vertical momentum, the kinetic energy of a reflected species is usually larger for a larger injection angle. Note that the yield of CF is higher at the smaller injection energy (i.e., 200 eV). This is because, with a larger injection energy, the injected CF3 radicals are more likely to be fragmented into smaller pieces. In addition, the fragmented species are more likely to be implanted into the substrate with
higher vertical momenta (unless sputtering of the substrate dominates over the C deposition). The latter also accounts for the increase of CF sputtering yield as a function of injection angle, shown in Fig. 3(b). As in the case of vertical injections [1], nearly half of oxygen atoms in the SiO2 substrates are removed as CO radicals during the etching process with relatively small injection angles (less than about 50°) under the conditions considered in this work, as shown in Figs. 2 and 3 for 300 eV CF3 injections. (For example, for vertical injections, the sputtering yield of CO molecules is about 0.7 from Fig. 3 whereas the total sputtering yield of oxygen atoms, whether they are removed as CO or any other forms, is about 1.6 from Fig. 2.) For larger angles, more atomic oxygen is sputtered from the surface, as mentioned earlier. Similarly, approximately half of Si atoms are removed as SiFx radicals during the etching process with relatively small injection angles under the conditions considered in this work. Fig. 4 shows the dependence of yields of SiF, SiF2 , SiF3 and SiF4 desorption species on the injection angle for 300 eV (black circles) and 200 eV (grey circles) CF3 injections. As shown in the case of 300 eV injections in Fig. 4, where an etching process is taking place for all injection angles, the peak position of the yield for SiFx desorption species as functions of the injection angle is smaller for larger x values. In other words, Table 1 Ratios of the numbers of Si and F atoms to the total number of atoms in the mixed layer as functions of the injection angle for 300 eV injections of CF3 Angle (°)
Si
F
0 15 30 45 60 75
0.23 0.24 0.24 0.28 0.29 0.31
0.34 0.31 0.31 0.19 0.17 0.10
It is seen that the larger the injection angle is, the less fluorinated the surface is.
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at smaller injection angles, larger SiFx desorption species (i.e., those with larger x) are dominant whereas, at larger injection angles, smaller SiFx are dominant. The same tendency is seen for the etching processes with relatively larger angles of 200 eV CF3 injections. For smaller angles (less than about 15°) under these conditions, yields of SiFx are small since fluorocarbon deposition takes place, as mentioned earlier. Table 1 shows the ratios of the numbers of Si and F atoms to the total number of atoms in the mixed layer as functions of the injection angle for 300 eV CF3 injections. The depth of the mixed layer is defined as a typical depth that injected atoms penetrate through and it is a decreasing function of the injection angle. It is seen that the larger the injection angle is, the fewer F atoms are found in the mixed layer. This is because, with larger injection angles, the injected species are more likely to be specularly reflected and fewer F atoms of injected CF3 are adsorbed on the substrate surface. In the etching process with a mixed layer of fewer F atoms (as in the cases of larger injection angles given in Table 1), the silicon fluoride radicals SiFx desorbed from the surface are expected to be those of fewer F atoms due to the scarcity of F in the layer, which agrees with the observation shown in Fig. 4. 3. Conclusions In the present work, MD simulations have been used to analyze injection angle dependence of sputtering yields and other etching characteristics of SiO2 substrates for CF3 radical beam injections. For vertical (i.e., the injection angle of 0°) injections, it has been observed that the SiO2 surface is etched at 300 eV injections whereas it is hardly etched and significant deposition of fluorocarbon films are observed at 200 eV injections, as reported earlier [1]. What we have found interesting is that, for the same 200 eV CF3 radical beam injections, the SiO2 surface is substantially etched at larger injection angles. As shown in Fig. 2, at the 60° injection angle of 200 eV CF3 injections, the sputtering yield of Si of the SiO2 surface is about unity, which is comparable to the Si sputtering yield of vertical injections of 300 eV CF3 beams. In other words, under the same beam conditions, fluorocarbon deposition and SiO2 sputtering can take place simultaneously, depending on the slope angles of submicron structures on the substrate surface.
As to 300 eV CF3 injections, etching of the SiO2 surface is observed for all injection angles. In both 200 eV and 300 eV cases, the maximum sputtering yields are obtained at about 60°. This angular dependence of sputtering yields obtained from our MD simulation results is consistent with unpublished data obtained from mass-selected ion beam experiments [11]. We have also examined desorbed or reflected species from the substrate surface under the beam bombardment. The major desorbed species containing oxygen and silicon are found to be CO and silicon fluoride (SiFx) radicals for relatively small injection angles, which is consistent with earlier simulation results [1]. As the injection angle increases, more atomic oxygen and smaller SiFx radicals (i.e., SiFx with lower x) also become important desorption species for oxygen and silicon. Such data provide useful insight into the analyses of submicronscale surface profiles obtained in SiO2 selective etching processes. Acknowledgements This work is partially supported by a Grant-in-Aid for Scientific Research of the Ministry of Education, Culture, Sports, Science and Technology of Japan and also by Sony Corporation. References [1] H. Ohta and S. Hamaguchi, J. Vac. Sci. Technol. A (submitted for publication). [2] V.V. Smirnov, A.V. Stengach, K.G. Gaynullin, V.A. Pavlovsky, S. Rauf, P.J. Stout, P.L.G. Ventzek, J. Appl. Phys. 97 (2005) 093302. [3] V.V. Smirnov, A.V. Stengach, K.G. Gaynullin, V.A. Pavlovsky, S. Rauf, P.J. Stout, P.L.G. Ventzek, J. Appl. Phys. 97 (2005) 093303. [4] H. Yamada, S. Hamaguchi, J. Appl. Phys. 96 (2004) 6147. [5] H. Ohta, S. Hamaguchi, J. Vac. Sci. Technol. A19 (2001) 2373. [6] C.F. Abrams, D.B. Graves, J. Vac. Sci. Technol. A19 (2001) 175. [7] J. Tanaka, C.F. Abrams, D.B. Graves, J. Vac. Sci. Technol., A 18 (2000) 938. [8] K. Ishikawa, K. Karahashi, H. Tsuboi, K. Yanai, M. Nakamura, J. Vac. Sci. Technol. A21 (2003) L1. [9] K. Karahashi, K. Yanai, K. Ishikawa, K. Kurihara, M. Nakamura, J. Vac. Sci. Technol., A 22 (2004) 1166. [10] H. Toyoda, H. Morishima, R. Fukute, Y. Hori, I. Murakami, H. Sugai, J. Appl. Phys. 95 (2004) 5172. [11] K. Karahashi, private communication.