- Email: [email protected]
Secondary ion measurements for oxygen cluster ion SIMS
Applied Surface Science 252 (2006) 7290–7292 www.elsevier.com/locate/apsusc
Secondary ion measurements for oxygen cluster ion SIMS Satoshi Ninomiya *, Takaaki Aoki, Toshio Seki, Jiro Matsuo Quantum Science and Engineering Center, Kyoto University, Uji, Kyoto 611-0011, Japan Available online 2 May 2006
Abstract We have proposed to use oxygen cluster ions, which are much larger than molecular ions, as primary ions for secondary ion mass spectrometry (SIMS). A high intensity gas cluster ion source with a current density of a few mAs/cm2 has been developed. Secondary ions emitted from Si have been investigated under O2 and Ar cluster ion bombardment. A large enhancement of the yield of secondary ions produced by large O2 and Ar cluster ions was found. The SIMS system utilizing large O2 cluster ions must give both excellent depth resolution and high secondary ion yield. # 2006 Elsevier B.V. All rights reserved. Keywords: Oxygen cluster ion; Secondary ion mass spectrometry; Low energy; Sputtering yield
1. Introduction SIMS has been widely used for depth profiling of semiconductors. Depth resolution in the nano meter range will soon be indispensable for this technique, because semiconductor devices are getting smaller and smaller. SIMS has improved the depth resolution by reducing incident primary ion energies [1]. However, further improvement of the depth resolution for the existing SIMS system has become extremely difficult, because the deterioration of surface morphology cannot be avoided during probe ion bombardment. There are two possible methods to solve the problem of surface morphology during probe ion bombardment. One such method consists in reducing projectile energy to below 1 keV. Ultra-low energy (<1 keV) atomic or molecular ions, such as oxygen and cesium, have been recently used as primary ions for SIMS [2– 4]. But ultra-low energy atomic or molecular ion SIMS often encounters serious problems that end up reducing both the sputtering and secondary ion yields. The other method consists in using cluster ions as probe ion (Cluster-SIMS). Small oxygen cluster ions such as O3+, could decrease primary ion energy per atom. However, small oxygen cluster beams provide a slow erosion rate and poor detection limits, because the incident ion current is very low even at high extraction energy [5]. On the other hand, cluster ions such as C60+ and Aun+ (n = 2,3) have
* Corresponding author. Tel.: +81 774 38 3977; fax: +81 774 38 3978. E-mail address: [email protected] (S. Ninomiya). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.138
lately begun to be used for SIMS depth profiling, because they can enhance both sputtering and secondary ion yields compared to atomic ions [6,7]. This report presents the results of secondary ion measurement under large gas cluster ion bombardment and suggests utilizing large gas cluster ions for Cluster-SIMS. Large gas cluster ions can give both high sputtering and high secondary ion yields compared to atomic ions at the same incident velocity. Moreover, large cluster ion beams can give a higher depth resolution, because they have the capability to smooth surfaces [8]. 2. Experimental Gas cluster ion beam (GCIB) techniques with a current density of a few mAs/cm2 have been developed at Kyoto University. The gas cluster ion formation and ionization techniques have been described elsewhere [8,9]. The experimental equipment for a gas Cluster-SIMS system is shown in Fig. 1. The equipment comprises a source chamber, an ionizing chamber and an analytical chamber with a mass spectrometer and XYZ sample manipulator. Neutral O2 and Ar clusters are formed by supersonic expansion of high-pressure gas (2000– 3000 Torr) through a nozzle (0.1 mm diameter) and are then introduced into the ionizing chamber. Electrons ejected from a hot filament are accelerated toward the neutral O2 and Ar clusters and ionize them. The ionized clusters are then extracted towards the target with an accelerating voltage up to 25 kV. Magnets installed between the ionizing and analytical
S. Ninomiya et al. / Applied Surface Science 252 (2006) 7290–7292
7291
Fig. 1. The experimental equipment for a gas Cluster-SIMS system.
chambers remove small cluster and monomer ions included in the cluster ion beam. It is also possible that only monomer ions are incident on the target. In this study, the maximum current densities of O2 cluster, Ar cluster, O2 molecular (O2+) and Ar monomer (Ar+) beams were, respectively, 3, 10, 50 and 80 mA/ cm2. The primary ion beam was incident on a silicon target at an angle of 458 or 08 with respect to the surface normal. The base and working pressure in the analytical chamber were, respectively, 2 10 6 and 2 10 5 Pa. Positive secondary ions emitted from Si were measured using the quadrupole mass spectrometer (QMS; ANELVA AQA-360) with a mass range of 1–360 amu. This ion source produces large cluster ions consisting of more than several hundreds of atoms. Fig. 2 shows cluster size distributions of the primary ion beams measured using the timeof-flight technique [9]. In this study, the average sizes of O2 and Ar cluster ion beams were, respectively, 500 molecules/cluster and 1000 atoms/cluster. As the accelerating voltage was 20 kV, each oxygen atom in the cluster had an average energy of 20 eV. 3. Results and discussion Fig. 3 shows mass spectra of secondary ions emitted from Si measured with QMS. Primary ion beams were (a) 15 keV O2 cluster, (b) 20 keV O2+, (c) 25 keV Ar cluster and (d) 15 keV Ar+. The same mass spectra were obtained for both 08 and 458 incident angles. Many oxidized ions such as SiO+ and Si2O2+ were detected when O2 cluster ions were incident, whereas
Fig. 2. Cluster size distributions of the primary ion beams measured using the time-of-flight technique.
when Ar cluster ions were incident Si cluster ions such as Si2+ and Si3+ were detected. On the other hand, both oxidized ions and Si cluster ions were scarcely detected when O2 molecular and Ar monomer ions were incident, and nearly the same mass spectra were obtained. From these results, it is concluded that secondary ions were emitted from the oxidized surface of Si with O2 cluster irradiation. On the other hand, the Si surface was not oxidized when O2 molecular ions with energies ranging from 5 to 20 kV were incident. It can be understood that same mass spectra are obtained when O2 molecular and Ar monomer ions are incident, because secondary particle emission by atomic ions in this energy range is determined simply by physical sputtering [10]. Fig. 4(a) shows the relative secondary-ion yields as a function of accelerating voltage under O2 cluster ion bombardment. The yields of Si+, SiO+ and Si2O2+ had the same dependence on accelerating voltage, and their yields roughly kept constant when the accelerating voltage was increased between 10 and 25 kV. Fig. 4(b) depicts the accelerating voltage dependence of the Si+ yield under O2 cluster, O2 molecular, Ar cluster and Ar monomer ion bombardment. With O2 molecular ion and Ar monomer ion irradiation, the Si+ yields decreased gradually with increasing the accelerating voltage, and this dependence can be also
Fig. 3. Mass spectra of secondary ions emitted from Si under bombardment by (a) 15 keV O2 cluster, (b) 20 keV O2 molecular, (c) 25 keVAr cluster, and (d) 15 keV Ar monomer ions.
7292
S. Ninomiya et al. / Applied Surface Science 252 (2006) 7290–7292
were about 20–40 atoms/cluster at the energy of 20 keV [13]. The sputtering yield per incident atom under 20 keV O2 cluster ion bombardment is assumed to be 0.02–0.04 Si atoms per oxygen atom, and the yields of secondary ions produced by O2 cluster ions are much higher than those by Ar cluster ions, as is evident from Fig. 4(b). That is, both sputtering and secondary ion yields by large O2 cluster ions are quite high, although the mean energy per atom is only about 20 eV. Furthermore, surface morphology is kept smooth during O2 cluster ion irradiation because the large cluster beam has the capability to smooth surfaces [8]. If large O2 cluster ion beam would be available for SIMS depth profiling, it can be expected that the Cluster-SIMS improve significantly both resolution and sensitivity. 4. Summary
Fig. 4. Accelerating voltage dependence of the relative secondary ion yields. (a) Secondary ion yields under O2 cluster ion bombardment. (b) Secondary Si+ yields under O2 cluster, O2 molecular, Ar cluster and Ar monomer ion bombardment.
explained by the linear collision cascade theory of sputtering [10]. On the other hand, the Si+ yields under Ar cluster ion bombardment increased exponentially with accelerating voltage, but were lower than those under O2 cluster ion bombardment throughout this range. Thus, O2 cluster ions can give high secondary ion yields despite a low incident energy (10–25 eV/atom). Many researchers have investigated the enhancement of secondary ion yields. The positive secondary ion yields were enhanced when a target was sputtered by atomic ions with oxygen flooding [3,11]. In Ga focused ion beam SIMS analysis the positive secondary ion yields were enhanced due to the bombardment of ultra-low energy O2 ions [12]. In the last ten years, remarkable progress was achieved in ultra-low energy SIMS techniques. Jiang et al. reported that oblique O2+ beam below 200 eV at the incident angle of 458 gave high depth resolution without oxygen flooding for potential SIMS profiling [2]. According to the report, the sputtering yield of Si at 158 eV is 0.051 (Si atoms/oxygen atom). In former studies, sputtering yields of Si by O2 cluster and Ar cluster ion irradiation were almost the same at about the same total mass and energy, and sputtering yields by O2 cluster (mean size: 3000 molecules/ cluster) and Ar cluster (mean size: 3000 atoms/cluster) ions
Secondary ions emitted from Si irradiated by O2 cluster, O2 molecular, Ar cluster and Ar monomer ions with an acceleration voltage up to 25 kV were measured using the QMS. With O2 cluster ion irradiation, secondary ions were emitted from the oxidized surface of Si, and secondary ion yields were very high despite low energy of a few tens of eV/ atom. If O2 cluster ion beam were available for SIMS depth profiling, it can be expected that O2 Cluster-SIMS could improve significantly both resolution and sensitivity. Acknowledgement This work is supported by the New Energy and Industrial Technology Development Organization (NEDO). References [1] K. Wittmaack, J. Vac. Sci. Technol. B 16 (1998) 2776–2785. [2] Z.X. Jiang, J. Lerma, D. Sieloff, J.J. Lee, S. Backer, S. Bagchi, J. Conner, J. Vac. Sci. Technol. B 22 (2004) 630–635. [3] K. Kataoka, M. Shigeno, Y. Tada, K. Wittmaack, Appl. Surf. Sci. 203–204 (2003) 329–334. [4] J.B. Clegg, N.S. Smith, M.G. Dowsett, M.J.J. Theunissen, W.B. de Boer, J. Vac. Sci. Technol. A. 14 (1996) 2645–2650. [5] H. Yamazaki, Y. Mitani, Nucl. Instrum. Methods B 124 (1997) 91–94. [6] D. Weibel, S. Wong, N. Lockyer, P. Blenkinsopp, R. Hill, J.C. Vickerman, Anal. Chem. 75 (2003) 1754–1764. [7] N. Davies, D.E. Weibel, P. Blenkinsopp, N. Lockyer, R. Hill, J.C. Vickerman, Appl. Surf. Sci. 203–204 (2003) 223–227. [8] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng. R 34 (2001) 231–295. [9] T. Seki, J. Matsuo, G.H. Takaoka, I. Yamada, Nucl. Instrum. Methods B 206 (2003) 902–906. [10] P. Sigmund, in: R. Behrisch (Ed.), Sputtering by Particle Bombardment I, Springer-Verlag Berlin Heidelberg, New York, 1981, p. 9. [11] C. Tian, W. Vandervorst, J. Vac. Sci. Technol. A 15 (1997) 452–459. [12] J. Kikuma, H. Imai, Surf. Interface. Anal. 31 (2001) 901–904. [13] N. Toyoda, H. Kitani, J. Matsuo, I. Yamada, Nucl. Instrum. Methods B 121 (1997) 484–488.
Secondary ion measurements for oxygen cluster ion SIMS Satoshi Ninomiya *, Takaaki Aoki, Toshio Seki, Jiro Matsuo Quantum Science and Engineering Center, Kyoto University, Uji, Kyoto 611-0011, Japan Available online 2 May 2006
Abstract We have proposed to use oxygen cluster ions, which are much larger than molecular ions, as primary ions for secondary ion mass spectrometry (SIMS). A high intensity gas cluster ion source with a current density of a few mAs/cm2 has been developed. Secondary ions emitted from Si have been investigated under O2 and Ar cluster ion bombardment. A large enhancement of the yield of secondary ions produced by large O2 and Ar cluster ions was found. The SIMS system utilizing large O2 cluster ions must give both excellent depth resolution and high secondary ion yield. # 2006 Elsevier B.V. All rights reserved. Keywords: Oxygen cluster ion; Secondary ion mass spectrometry; Low energy; Sputtering yield
1. Introduction SIMS has been widely used for depth profiling of semiconductors. Depth resolution in the nano meter range will soon be indispensable for this technique, because semiconductor devices are getting smaller and smaller. SIMS has improved the depth resolution by reducing incident primary ion energies [1]. However, further improvement of the depth resolution for the existing SIMS system has become extremely difficult, because the deterioration of surface morphology cannot be avoided during probe ion bombardment. There are two possible methods to solve the problem of surface morphology during probe ion bombardment. One such method consists in reducing projectile energy to below 1 keV. Ultra-low energy (<1 keV) atomic or molecular ions, such as oxygen and cesium, have been recently used as primary ions for SIMS [2– 4]. But ultra-low energy atomic or molecular ion SIMS often encounters serious problems that end up reducing both the sputtering and secondary ion yields. The other method consists in using cluster ions as probe ion (Cluster-SIMS). Small oxygen cluster ions such as O3+, could decrease primary ion energy per atom. However, small oxygen cluster beams provide a slow erosion rate and poor detection limits, because the incident ion current is very low even at high extraction energy [5]. On the other hand, cluster ions such as C60+ and Aun+ (n = 2,3) have
* Corresponding author. Tel.: +81 774 38 3977; fax: +81 774 38 3978. E-mail address: [email protected] (S. Ninomiya). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.138
lately begun to be used for SIMS depth profiling, because they can enhance both sputtering and secondary ion yields compared to atomic ions [6,7]. This report presents the results of secondary ion measurement under large gas cluster ion bombardment and suggests utilizing large gas cluster ions for Cluster-SIMS. Large gas cluster ions can give both high sputtering and high secondary ion yields compared to atomic ions at the same incident velocity. Moreover, large cluster ion beams can give a higher depth resolution, because they have the capability to smooth surfaces [8]. 2. Experimental Gas cluster ion beam (GCIB) techniques with a current density of a few mAs/cm2 have been developed at Kyoto University. The gas cluster ion formation and ionization techniques have been described elsewhere [8,9]. The experimental equipment for a gas Cluster-SIMS system is shown in Fig. 1. The equipment comprises a source chamber, an ionizing chamber and an analytical chamber with a mass spectrometer and XYZ sample manipulator. Neutral O2 and Ar clusters are formed by supersonic expansion of high-pressure gas (2000– 3000 Torr) through a nozzle (0.1 mm diameter) and are then introduced into the ionizing chamber. Electrons ejected from a hot filament are accelerated toward the neutral O2 and Ar clusters and ionize them. The ionized clusters are then extracted towards the target with an accelerating voltage up to 25 kV. Magnets installed between the ionizing and analytical
S. Ninomiya et al. / Applied Surface Science 252 (2006) 7290–7292
7291
Fig. 1. The experimental equipment for a gas Cluster-SIMS system.
chambers remove small cluster and monomer ions included in the cluster ion beam. It is also possible that only monomer ions are incident on the target. In this study, the maximum current densities of O2 cluster, Ar cluster, O2 molecular (O2+) and Ar monomer (Ar+) beams were, respectively, 3, 10, 50 and 80 mA/ cm2. The primary ion beam was incident on a silicon target at an angle of 458 or 08 with respect to the surface normal. The base and working pressure in the analytical chamber were, respectively, 2 10 6 and 2 10 5 Pa. Positive secondary ions emitted from Si were measured using the quadrupole mass spectrometer (QMS; ANELVA AQA-360) with a mass range of 1–360 amu. This ion source produces large cluster ions consisting of more than several hundreds of atoms. Fig. 2 shows cluster size distributions of the primary ion beams measured using the timeof-flight technique [9]. In this study, the average sizes of O2 and Ar cluster ion beams were, respectively, 500 molecules/cluster and 1000 atoms/cluster. As the accelerating voltage was 20 kV, each oxygen atom in the cluster had an average energy of 20 eV. 3. Results and discussion Fig. 3 shows mass spectra of secondary ions emitted from Si measured with QMS. Primary ion beams were (a) 15 keV O2 cluster, (b) 20 keV O2+, (c) 25 keV Ar cluster and (d) 15 keV Ar+. The same mass spectra were obtained for both 08 and 458 incident angles. Many oxidized ions such as SiO+ and Si2O2+ were detected when O2 cluster ions were incident, whereas
Fig. 2. Cluster size distributions of the primary ion beams measured using the time-of-flight technique.
when Ar cluster ions were incident Si cluster ions such as Si2+ and Si3+ were detected. On the other hand, both oxidized ions and Si cluster ions were scarcely detected when O2 molecular and Ar monomer ions were incident, and nearly the same mass spectra were obtained. From these results, it is concluded that secondary ions were emitted from the oxidized surface of Si with O2 cluster irradiation. On the other hand, the Si surface was not oxidized when O2 molecular ions with energies ranging from 5 to 20 kV were incident. It can be understood that same mass spectra are obtained when O2 molecular and Ar monomer ions are incident, because secondary particle emission by atomic ions in this energy range is determined simply by physical sputtering [10]. Fig. 4(a) shows the relative secondary-ion yields as a function of accelerating voltage under O2 cluster ion bombardment. The yields of Si+, SiO+ and Si2O2+ had the same dependence on accelerating voltage, and their yields roughly kept constant when the accelerating voltage was increased between 10 and 25 kV. Fig. 4(b) depicts the accelerating voltage dependence of the Si+ yield under O2 cluster, O2 molecular, Ar cluster and Ar monomer ion bombardment. With O2 molecular ion and Ar monomer ion irradiation, the Si+ yields decreased gradually with increasing the accelerating voltage, and this dependence can be also
Fig. 3. Mass spectra of secondary ions emitted from Si under bombardment by (a) 15 keV O2 cluster, (b) 20 keV O2 molecular, (c) 25 keVAr cluster, and (d) 15 keV Ar monomer ions.
7292
S. Ninomiya et al. / Applied Surface Science 252 (2006) 7290–7292
were about 20–40 atoms/cluster at the energy of 20 keV [13]. The sputtering yield per incident atom under 20 keV O2 cluster ion bombardment is assumed to be 0.02–0.04 Si atoms per oxygen atom, and the yields of secondary ions produced by O2 cluster ions are much higher than those by Ar cluster ions, as is evident from Fig. 4(b). That is, both sputtering and secondary ion yields by large O2 cluster ions are quite high, although the mean energy per atom is only about 20 eV. Furthermore, surface morphology is kept smooth during O2 cluster ion irradiation because the large cluster beam has the capability to smooth surfaces [8]. If large O2 cluster ion beam would be available for SIMS depth profiling, it can be expected that the Cluster-SIMS improve significantly both resolution and sensitivity. 4. Summary
Fig. 4. Accelerating voltage dependence of the relative secondary ion yields. (a) Secondary ion yields under O2 cluster ion bombardment. (b) Secondary Si+ yields under O2 cluster, O2 molecular, Ar cluster and Ar monomer ion bombardment.
explained by the linear collision cascade theory of sputtering [10]. On the other hand, the Si+ yields under Ar cluster ion bombardment increased exponentially with accelerating voltage, but were lower than those under O2 cluster ion bombardment throughout this range. Thus, O2 cluster ions can give high secondary ion yields despite a low incident energy (10–25 eV/atom). Many researchers have investigated the enhancement of secondary ion yields. The positive secondary ion yields were enhanced when a target was sputtered by atomic ions with oxygen flooding [3,11]. In Ga focused ion beam SIMS analysis the positive secondary ion yields were enhanced due to the bombardment of ultra-low energy O2 ions [12]. In the last ten years, remarkable progress was achieved in ultra-low energy SIMS techniques. Jiang et al. reported that oblique O2+ beam below 200 eV at the incident angle of 458 gave high depth resolution without oxygen flooding for potential SIMS profiling [2]. According to the report, the sputtering yield of Si at 158 eV is 0.051 (Si atoms/oxygen atom). In former studies, sputtering yields of Si by O2 cluster and Ar cluster ion irradiation were almost the same at about the same total mass and energy, and sputtering yields by O2 cluster (mean size: 3000 molecules/ cluster) and Ar cluster (mean size: 3000 atoms/cluster) ions
Secondary ions emitted from Si irradiated by O2 cluster, O2 molecular, Ar cluster and Ar monomer ions with an acceleration voltage up to 25 kV were measured using the QMS. With O2 cluster ion irradiation, secondary ions were emitted from the oxidized surface of Si, and secondary ion yields were very high despite low energy of a few tens of eV/ atom. If O2 cluster ion beam were available for SIMS depth profiling, it can be expected that O2 Cluster-SIMS could improve significantly both resolution and sensitivity. Acknowledgement This work is supported by the New Energy and Industrial Technology Development Organization (NEDO). References [1] K. Wittmaack, J. Vac. Sci. Technol. B 16 (1998) 2776–2785. [2] Z.X. Jiang, J. Lerma, D. Sieloff, J.J. Lee, S. Backer, S. Bagchi, J. Conner, J. Vac. Sci. Technol. B 22 (2004) 630–635. [3] K. Kataoka, M. Shigeno, Y. Tada, K. Wittmaack, Appl. Surf. Sci. 203–204 (2003) 329–334. [4] J.B. Clegg, N.S. Smith, M.G. Dowsett, M.J.J. Theunissen, W.B. de Boer, J. Vac. Sci. Technol. A. 14 (1996) 2645–2650. [5] H. Yamazaki, Y. Mitani, Nucl. Instrum. Methods B 124 (1997) 91–94. [6] D. Weibel, S. Wong, N. Lockyer, P. Blenkinsopp, R. Hill, J.C. Vickerman, Anal. Chem. 75 (2003) 1754–1764. [7] N. Davies, D.E. Weibel, P. Blenkinsopp, N. Lockyer, R. Hill, J.C. Vickerman, Appl. Surf. Sci. 203–204 (2003) 223–227. [8] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng. R 34 (2001) 231–295. [9] T. Seki, J. Matsuo, G.H. Takaoka, I. Yamada, Nucl. Instrum. Methods B 206 (2003) 902–906. [10] P. Sigmund, in: R. Behrisch (Ed.), Sputtering by Particle Bombardment I, Springer-Verlag Berlin Heidelberg, New York, 1981, p. 9. [11] C. Tian, W. Vandervorst, J. Vac. Sci. Technol. A 15 (1997) 452–459. [12] J. Kikuma, H. Imai, Surf. Interface. Anal. 31 (2001) 901–904. [13] N. Toyoda, H. Kitani, J. Matsuo, I. Yamada, Nucl. Instrum. Methods B 121 (1997) 484–488.