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Ultra-shallow arsenic implant depth profiling using low-energy nitrogen beams
Applied Surface Science 231–232 (2004) 645–648
Ultra-shallow arsenic implant depth profiling using low-energy nitrogen beams Sarah Fearn*, Richard Chater, David McPhail Department of Materials, Imperial College, London SW7 2AZ, UK Available online 30 April 2004
Abstract Sputtering of silicon by low-energy nitrogen primary ion beams has been studied by a number of authors to characterize the altered layer, ripple formation and the sputtered yields of secondary ions [Surf. Sci. 424 (1999) 299; Appl. Phys. A: Mater. Sci. Process 53 (1991) 179; Appl. Phys. Lett. 73 (1998) 1287]. This study examines the application of low-energy nitrogen primary ion beams for the possible depth profiling of ultra-shallow arsenic implants into silicon. The emphasis of this work is on the matrix silicon signals in the pre-equilibrium surface region that are used for dose calibration. Problems with these aspects of the concentration depth profiling can give significant inconsistencies well outside the error limits of the quoted dose for the arsenic implantation as independently verified by CV profiling. This occurs during depth profiling with either oxygen primary ion beams (with and without oxygen leaks) or cesium primary ion beams. # 2004 Elsevier B.V. All rights reserved. Keywords: Nitrogen; Silicon; Matrix signals; Depth profiling
1. Introduction Current demands of CMOS semiconductor technology require low-energy implants at high doses for shallow junction formation. Concentration depth profiling by SIMS of these structures has provided SIMS with the stimulus for sub-keV oxygen and cesium primary ions guns [1]. This trend not only increases the depth resolution by decay length reduction [2] but also reduces the surface transient region prior to steady state sputtering [3]. Most of the literature has focused on the sputtering of silicon with lowenergy oxygen ions and the measurement of low energy, high concentration boron dopant profiles. Less attention has been paid to the common n-type dopants, * Corresponding author. E-mail address: [email protected] (S. Fearn).
arsenic and phosphorus that are measured by cesium primary ion sputtering. Si(As) is usually concentration depth profiled at high incidence angles (50–808) for the highest detection sensitivities following negative matrix and dopant secondary ions (Si and AsSi). However, these conditions lead to ripple formation at angles above 608 and surface peaks for the matrix ions [4]. At most angles and for near surface features, the problem of negative depth scale offset [2] can be unacceptably large and variable [4]. This paper explores the use of nitrogen primary ion beams at sub-keV energies and normal incidence for profiling the low-energy arsenic implants. A number of authors have previously used low-energy nitrogen ion beams to characterise various features [5–7]. The work presented here mainly focuses on assessing the transient region width and fast stabilization of the matrix signals used for ‘getting the dose right’.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.132
646
S. Fearn et al. / Applied Surface Science 231–232 (2004) 645–648
At higher beam energies nitrogen ions have been shown to have characteristics that make it an attractive alternative to cesium. There is a similar chemical enhancement [8] and crater base ripples are absent for incidence angles less than 308 [9]. Effects of high concentration of oxygen on the matrix Si have been shown to be the least compared with Cs and Xe [10]. Fast stabilization prospects may well be improved because the nitride layer at the base of the crater becomes supersaturated with nitrogen due to the low mobility of nitrogen [7,11]. Also, the stoichiometry of the nitride is Si3N4 in contrast to oxygen where the sub-oxide forms before SiO2 [8]. By contrast, with oxygen beams, a supersaturation of oxygen in the oxide under depth profiling flux conditions is not possible because of the very high mobility of the oxygen in the oxide layer [11].
2. Experimental SIMS depth profiling has been carried out using a quadrupole SIMS tool [12] equipped with a floating low-energy primary ion gun [13,14]. A nitrogen primary ion beam with energies of 1 and 0.5 keV have been used with currents of 60 nA and raster of 222 mm 171 mm, and 70 nA rastered over an area of 221 mm 165 mm, respectively. The transient signals for Si, and Si2 and SiN were recorded.
3. Results and discussion For both the 1 and 0.5 keV nitrogen beam the transient signals show the same behavior. In the case of the SiN and Si2 signals they initially rise rapidly from a minimum point to a ‘knee’ which marks the start of gradual rise to steady state. The Sisignal on the other hand shows a small surface spike at both probing energies. The signal then drops to a minimum, which then rises again to a steady state. Under nitrogen bombardment transient signals behave different to that which has been observed when silicon is analysed using an oxygen beam with energies less than or equal to 1.5 keV [3]. In this case as the beam energy was lowered to energies of 350 and 750 eV the initial surface silicon spike was found to disappear, and a profile shape similar to the SiN and Si2 signals was observed.
In order to accurately identify the dose at which the transient signals reach steady state, they have been plotted on a linear scale and are shown in Fig. 1. Fig. 1a–c shows the transient regions for the SiN, Si2 and Si signals, respectively, with the very start of the profile highlighted by the insert. In all of the plots shown in Fig. 1 it can be seen that there is a rapid rise in the signals up to a nitrogen dose of approximately 5 1016 atoms/cm2. After this point, all the profiles show various differences. For the SiN and Si2 signals, both the 1 and 0.5 keV profiles are of a comparable shape. In both instances, the 1 keV transient signals appear to reach a steady state before the 0.5 keV transient signals, with the Si2 signal appearing to reach equilibrium before the SiN signal. For both sets of transient signals, however, the nitrogen dose required for equilibrium to be reached is much higher than that recorded for stabilization of transient signals under oxygen bombardment. The greatest difference is observed for the Si transient signal. After an approximate dose of 5 1016 atoms/cm2, the gradient of the profiles change so that the signal increases very gradually. This is most pronounced for the 1 keV transient Si signal, which only appears to stabilize after a dose of 300 1016 atoms/cm2. On the smaller insert, however, the two signals appear flat. This indicates that unless the analyses are carried out for longer periods of time, the gradually changing profile shape cannot be seen. The shape of a profile can also be hidden when plotted on a logarithmic scale. One reason for the changing slope observed on the Si transient signal could be voltage variations. This cannot be the case, however, as similar slope variation would have been observed on the molecular transient signals, which are more sensitive to this phenomenon. The surface of the silicon will include a thin native oxide layer (20 A), estimated to be thinner than the (range þ straggle) of the nitrogen ions in silicon. (50 A) [Trim98]. At the start of the profile the altered surface layer is expected to become an amorphous oxy-nitride with a high concentration of oxygen initially. Both nitrogen and oxygen are highly mobile in radiation damaged silicon oxy-nitride and the oxygen rapidly becomes dilute. The nitrogen implanted deeper amorphises the silicon and reacts to grow a nitride-rich layer nucleated on the oxynitride layer [15].
S. Fearn et al. / Applied Surface Science 231–232 (2004) 645–648
647
Fig. 1. The transient signals as a function of beam dose for a 1 and 0.5 keV nitrogen beam: (a) SiN transient signal; (b) Si transient signal; (c) Si2 transient signal.
The changeover from an amorphous nature of the silicon oxy-nitride to the crystalline nitride means different sputter-rates at the start of the profile, completed after a dose of 2 1016 cm2. Thus, Si surface yield reduction by a factor of 3 can be a combination of sputter rate change and surface stoichiometry. Nitrogen mobility in the silicon nitride is much less than the oxygen mobility in the silicon oxide so that the nitride layer will become nitrogen rich [15]. The 16 O profiles recorded at the same time as the silicon
ions do not show stabilization until doses comparable to than required for 28 Si, approximately 3 1018 cm2 at 1 keV. This is believed to reflect the strength of the Si–O bond being approximately double that for Si–N. Oxygen remaining as the nitride layer is formed will tend to migrate to the altered layer/substrate interface. The oxygen/silicon ratio stabilizes to a non-zerovalue, which depends on the beam current density. The dose at which the matrix signals stabilize may be dependent on the collision rate of oxygen from the residual vacuum.
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S. Fearn et al. / Applied Surface Science 231–232 (2004) 645–648
4. Conclusion For accurate dose quantification of shallow implanted arsenic it is essential to have a stable matrix signal. From the work carried out here it can be seen that over short periods of time that the matrix signals may appear to have reached a steady stated whereas longer profiles indicate this not to be the case. This is particularly true for the 28 Si signal. The stabilization of the signal appears to occur at beam doses well beyond those required for the profiling of shallow implants. Presently nitrogen does not seem to offer an alternative solution for depth profiling of arsenic, but this early work does suggest possibilities. Further work must be carried out, and a comparison with Cs depth profiling essential.
References [1] Y. Kataoka, K. Wittmaack, Surf. Sci. 424 (1999) 299.
[2] D. Bouchier, A. Bosseboeuf, Appl. Phys. A: Mater. Sci. Process 53 (1991) 179. [3] M. Petravic, J.S. Williams, M. Conway, P.N.K. Deenapanray, Appl. Phys. Lett. 73 (1998) 1287. [4] M.G. Dowsett, in: Proceedings of the SIMS X, 1997, p. 367. [5] K. Wittmaack, in: Proceedings of the SIMS II, 1981. [6] M.G. Dowsett, in: Proceedings of the SIMS XI, 1999. [7] K. Wittmaack, Appl. Surf. Sci. 203 (4) (2003) 329. [8] K. Mann, M.L. Yu, in: Proceedings of the SIMS V, 1987. [9] V.I. Bachurin, P.A. Lepshin, V.K. Smirnov, Vacuum 56 (2000) 241. [10] S.B. Patel, J.A. Kilner, in: Proceedings of the SIMS VIII, p. 107. [11] R.J. Chater, J.A. Kilner, P.L.F. Hemment, K.J. Reeson, R.F. Peart, Electrochem. Soc. Proc. 86 (4) (1986). [12] Atomika 6500 SIMS, FEI Deutschland GmbH, Oberschleissheim/Munich, Germany. [13] M.G. Dowsett, N.S. Smith, D.R. Bridgeland, A.C. Lovejoy, P. Pendrick, in: Proceedings of the SIMS XI, 1997. [14] Ionoptika FLIG. [15] K.J. Reeson, P.L.F. Hemment, C.D. Meekison, C.D. Marsh, G.R. Booker, R.J. Chater, J.A. Kilner, J.R. Davis, Nucl. Instrum. Meth. Phys. Res. B 32 (1988) 427.
Ultra-shallow arsenic implant depth profiling using low-energy nitrogen beams Sarah Fearn*, Richard Chater, David McPhail Department of Materials, Imperial College, London SW7 2AZ, UK Available online 30 April 2004
Abstract Sputtering of silicon by low-energy nitrogen primary ion beams has been studied by a number of authors to characterize the altered layer, ripple formation and the sputtered yields of secondary ions [Surf. Sci. 424 (1999) 299; Appl. Phys. A: Mater. Sci. Process 53 (1991) 179; Appl. Phys. Lett. 73 (1998) 1287]. This study examines the application of low-energy nitrogen primary ion beams for the possible depth profiling of ultra-shallow arsenic implants into silicon. The emphasis of this work is on the matrix silicon signals in the pre-equilibrium surface region that are used for dose calibration. Problems with these aspects of the concentration depth profiling can give significant inconsistencies well outside the error limits of the quoted dose for the arsenic implantation as independently verified by CV profiling. This occurs during depth profiling with either oxygen primary ion beams (with and without oxygen leaks) or cesium primary ion beams. # 2004 Elsevier B.V. All rights reserved. Keywords: Nitrogen; Silicon; Matrix signals; Depth profiling
1. Introduction Current demands of CMOS semiconductor technology require low-energy implants at high doses for shallow junction formation. Concentration depth profiling by SIMS of these structures has provided SIMS with the stimulus for sub-keV oxygen and cesium primary ions guns [1]. This trend not only increases the depth resolution by decay length reduction [2] but also reduces the surface transient region prior to steady state sputtering [3]. Most of the literature has focused on the sputtering of silicon with lowenergy oxygen ions and the measurement of low energy, high concentration boron dopant profiles. Less attention has been paid to the common n-type dopants, * Corresponding author. E-mail address: [email protected] (S. Fearn).
arsenic and phosphorus that are measured by cesium primary ion sputtering. Si(As) is usually concentration depth profiled at high incidence angles (50–808) for the highest detection sensitivities following negative matrix and dopant secondary ions (Si and AsSi). However, these conditions lead to ripple formation at angles above 608 and surface peaks for the matrix ions [4]. At most angles and for near surface features, the problem of negative depth scale offset [2] can be unacceptably large and variable [4]. This paper explores the use of nitrogen primary ion beams at sub-keV energies and normal incidence for profiling the low-energy arsenic implants. A number of authors have previously used low-energy nitrogen ion beams to characterise various features [5–7]. The work presented here mainly focuses on assessing the transient region width and fast stabilization of the matrix signals used for ‘getting the dose right’.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.132
646
S. Fearn et al. / Applied Surface Science 231–232 (2004) 645–648
At higher beam energies nitrogen ions have been shown to have characteristics that make it an attractive alternative to cesium. There is a similar chemical enhancement [8] and crater base ripples are absent for incidence angles less than 308 [9]. Effects of high concentration of oxygen on the matrix Si have been shown to be the least compared with Cs and Xe [10]. Fast stabilization prospects may well be improved because the nitride layer at the base of the crater becomes supersaturated with nitrogen due to the low mobility of nitrogen [7,11]. Also, the stoichiometry of the nitride is Si3N4 in contrast to oxygen where the sub-oxide forms before SiO2 [8]. By contrast, with oxygen beams, a supersaturation of oxygen in the oxide under depth profiling flux conditions is not possible because of the very high mobility of the oxygen in the oxide layer [11].
2. Experimental SIMS depth profiling has been carried out using a quadrupole SIMS tool [12] equipped with a floating low-energy primary ion gun [13,14]. A nitrogen primary ion beam with energies of 1 and 0.5 keV have been used with currents of 60 nA and raster of 222 mm 171 mm, and 70 nA rastered over an area of 221 mm 165 mm, respectively. The transient signals for Si, and Si2 and SiN were recorded.
3. Results and discussion For both the 1 and 0.5 keV nitrogen beam the transient signals show the same behavior. In the case of the SiN and Si2 signals they initially rise rapidly from a minimum point to a ‘knee’ which marks the start of gradual rise to steady state. The Sisignal on the other hand shows a small surface spike at both probing energies. The signal then drops to a minimum, which then rises again to a steady state. Under nitrogen bombardment transient signals behave different to that which has been observed when silicon is analysed using an oxygen beam with energies less than or equal to 1.5 keV [3]. In this case as the beam energy was lowered to energies of 350 and 750 eV the initial surface silicon spike was found to disappear, and a profile shape similar to the SiN and Si2 signals was observed.
In order to accurately identify the dose at which the transient signals reach steady state, they have been plotted on a linear scale and are shown in Fig. 1. Fig. 1a–c shows the transient regions for the SiN, Si2 and Si signals, respectively, with the very start of the profile highlighted by the insert. In all of the plots shown in Fig. 1 it can be seen that there is a rapid rise in the signals up to a nitrogen dose of approximately 5 1016 atoms/cm2. After this point, all the profiles show various differences. For the SiN and Si2 signals, both the 1 and 0.5 keV profiles are of a comparable shape. In both instances, the 1 keV transient signals appear to reach a steady state before the 0.5 keV transient signals, with the Si2 signal appearing to reach equilibrium before the SiN signal. For both sets of transient signals, however, the nitrogen dose required for equilibrium to be reached is much higher than that recorded for stabilization of transient signals under oxygen bombardment. The greatest difference is observed for the Si transient signal. After an approximate dose of 5 1016 atoms/cm2, the gradient of the profiles change so that the signal increases very gradually. This is most pronounced for the 1 keV transient Si signal, which only appears to stabilize after a dose of 300 1016 atoms/cm2. On the smaller insert, however, the two signals appear flat. This indicates that unless the analyses are carried out for longer periods of time, the gradually changing profile shape cannot be seen. The shape of a profile can also be hidden when plotted on a logarithmic scale. One reason for the changing slope observed on the Si transient signal could be voltage variations. This cannot be the case, however, as similar slope variation would have been observed on the molecular transient signals, which are more sensitive to this phenomenon. The surface of the silicon will include a thin native oxide layer (20 A), estimated to be thinner than the (range þ straggle) of the nitrogen ions in silicon. (50 A) [Trim98]. At the start of the profile the altered surface layer is expected to become an amorphous oxy-nitride with a high concentration of oxygen initially. Both nitrogen and oxygen are highly mobile in radiation damaged silicon oxy-nitride and the oxygen rapidly becomes dilute. The nitrogen implanted deeper amorphises the silicon and reacts to grow a nitride-rich layer nucleated on the oxynitride layer [15].
S. Fearn et al. / Applied Surface Science 231–232 (2004) 645–648
647
Fig. 1. The transient signals as a function of beam dose for a 1 and 0.5 keV nitrogen beam: (a) SiN transient signal; (b) Si transient signal; (c) Si2 transient signal.
The changeover from an amorphous nature of the silicon oxy-nitride to the crystalline nitride means different sputter-rates at the start of the profile, completed after a dose of 2 1016 cm2. Thus, Si surface yield reduction by a factor of 3 can be a combination of sputter rate change and surface stoichiometry. Nitrogen mobility in the silicon nitride is much less than the oxygen mobility in the silicon oxide so that the nitride layer will become nitrogen rich [15]. The 16 O profiles recorded at the same time as the silicon
ions do not show stabilization until doses comparable to than required for 28 Si, approximately 3 1018 cm2 at 1 keV. This is believed to reflect the strength of the Si–O bond being approximately double that for Si–N. Oxygen remaining as the nitride layer is formed will tend to migrate to the altered layer/substrate interface. The oxygen/silicon ratio stabilizes to a non-zerovalue, which depends on the beam current density. The dose at which the matrix signals stabilize may be dependent on the collision rate of oxygen from the residual vacuum.
648
S. Fearn et al. / Applied Surface Science 231–232 (2004) 645–648
4. Conclusion For accurate dose quantification of shallow implanted arsenic it is essential to have a stable matrix signal. From the work carried out here it can be seen that over short periods of time that the matrix signals may appear to have reached a steady stated whereas longer profiles indicate this not to be the case. This is particularly true for the 28 Si signal. The stabilization of the signal appears to occur at beam doses well beyond those required for the profiling of shallow implants. Presently nitrogen does not seem to offer an alternative solution for depth profiling of arsenic, but this early work does suggest possibilities. Further work must be carried out, and a comparison with Cs depth profiling essential.
References [1] Y. Kataoka, K. Wittmaack, Surf. Sci. 424 (1999) 299.
[2] D. Bouchier, A. Bosseboeuf, Appl. Phys. A: Mater. Sci. Process 53 (1991) 179. [3] M. Petravic, J.S. Williams, M. Conway, P.N.K. Deenapanray, Appl. Phys. Lett. 73 (1998) 1287. [4] M.G. Dowsett, in: Proceedings of the SIMS X, 1997, p. 367. [5] K. Wittmaack, in: Proceedings of the SIMS II, 1981. [6] M.G. Dowsett, in: Proceedings of the SIMS XI, 1999. [7] K. Wittmaack, Appl. Surf. Sci. 203 (4) (2003) 329. [8] K. Mann, M.L. Yu, in: Proceedings of the SIMS V, 1987. [9] V.I. Bachurin, P.A. Lepshin, V.K. Smirnov, Vacuum 56 (2000) 241. [10] S.B. Patel, J.A. Kilner, in: Proceedings of the SIMS VIII, p. 107. [11] R.J. Chater, J.A. Kilner, P.L.F. Hemment, K.J. Reeson, R.F. Peart, Electrochem. Soc. Proc. 86 (4) (1986). [12] Atomika 6500 SIMS, FEI Deutschland GmbH, Oberschleissheim/Munich, Germany. [13] M.G. Dowsett, N.S. Smith, D.R. Bridgeland, A.C. Lovejoy, P. Pendrick, in: Proceedings of the SIMS XI, 1997. [14] Ionoptika FLIG. [15] K.J. Reeson, P.L.F. Hemment, C.D. Meekison, C.D. Marsh, G.R. Booker, R.J. Chater, J.A. Kilner, J.R. Davis, Nucl. Instrum. Meth. Phys. Res. B 32 (1988) 427.