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Study of mechanism of plasma surface modifications in Si by spectroscopic ellipsometry
Surface and Coatings Technology 173 – 174 (2003) 854–857
Study of mechanism of plasma surface modifications in Si by spectroscopic ellipsometry T. Yamada*, N. Harada, K. Kitahara, A. Moritani Department of Electronic and Control Systems Engineering, Shimane University, Matsue 690-8504, Shimane, Japan
Abstract Spectroscopic ellipsometry has been applied to study of plasma surface modifications in single-crystalline Si exposed to Ar plasma at room temperature. The complex dielectric functions of ion-bombarded layers were obtained from a new method for estimating the plasma-damage depth. Damage dependences of interband energy gaps and broadening parameters for the E1 optical transition were derived from the third-derivative spectra of dielectric functions. These parameters strongly suggest that Si surface exposed to Ar plasma has suffered from lattice expansion due to inclusions of Arq ions before lattice relaxation probably due to dislocation generation. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Plasma surface modification; Spectroscopic ellipsometry; Plasma damage; Si
1. Introduction The surface modification techniques in plasma and ion implantation processes have been widely applied in order to add superior functions to surfaces of metals and semiconductors. However, these techniques produce crystal damages at the surface. As for monitoring the surface modification of a solid exposed to plasma, spectroscopic ellipsometry is powerful since it is a nondestructive and highly sensitive technique and makes in situ measurement possible w1x. Therefore, it is of great importance to study plasma-damage process for understanding surface modification mechanism from the ellipsometric viewpoint. Several researchers have endeavored to characterize the damaged surface layer produced by ion implantation using the spectroscopic ellipsometry w2–5x. Aspnes et al. revealed that the damage depth of Arq ion implanted c-Ge obeys the ion bombarding energy E 2y3 dependence w2x, and Giri et al. evaluated the dielectric function spectra of two major optical interband transitions E1 and E2 in Arq ion implanted c-Si and indicated the existence of a threshold fluence in the process changing from crystalline to amorphous state w3x. *Corresponding author. Tel.: q81-852-328-902; fax: q81-852328-902. E-mail address: [email protected] (T. Yamada).
In this paper, we focus on the estimation of damage depth and strain-induced red shift in the bandgap energy at room temperature for understanding mechanism of plasma surface modifications in single-crystalline Si exposed to Ar plasma. 2. Experimental The ellipsometric measurements were made with a rotating-analyzer spectroscopic ellipsometer (Model M 88, J.A. Woollam Co., Inc.) in a photon energy range from 1.5 to 4.5 eV at angles of incidence of 75–798. Real part N´1M and imaginary part N´2M of pseudocomplex dielectric functions are derived from measured ellipsometric parameters psi and delta on the assumption that the sample is a homogeneous bulk-like solid. A single-crystalline Si (100) wafer with native oxide removed by dilute HF etching was directly set on the cathode plate in the plasma chamber. The dc glow discharge of high purity Ar gas was generated for 10 min at room temperature by 10 kHz rectangular-wave with a duty ratio of 0.5. The applied plasma voltage was set constant at 360 V and the Ar gas pressure was varied in a range from 6.7 to 53.0 Pa in order to prepare samples with various levels of ion bombardment. For the alternative condition the applied Ar gas pressure was set constant at 6.7 Pa and the plasma voltage was varied in a range from 240 to 560 V.
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00559-0
T. Yamada et al. / Surface and Coatings Technology 173 – 174 (2003) 854–857
Fig. 1. Dependence of imaginary part of pseudo-dielectric function N´2M spectra on Ar gas pressure.
3. Results and discussion Fig. 1 shows some typical gas pressure dependences of imaginary part of pseudo-complex dielectric spectra N´2M in Si measured at room temperature with spectroscopic ellipsometry. We assume here that the ion flux onto the surface increases with gas pressure though quantitative correlation between the two is unknown. It is observed in the spectra that both E1 and E2 peak decrease in height and shift to lower photon energy (red shift), and the spectral structures broaden gradually with
855
Fig. 2. Experimental and best-fitted spectra in model analysis for ionbombarded sample at a gas pressure of 27.0 Pa.
gas pressure. Here, the E2 peak is very sensitive to crystal damage as compared to the E1 due to the crystal symmetry w3x. The red shift arises probably from three fractions, which are oxidation, amorphization and expansion of crystal lattice due to the inclusion of Arq ions in the surface. Fig. 2 shows the experimental and best-fitted spectra at a gas pressure of 27.0 Pa obtained from analysis using the model consisting of SiO2 layer, mixed layer with crystal Siyamorphous Siyvoid for the ion-bombarded layer and Si substrate as shown in the inserted of this figure. It is recognized that the best-fitted spectra
Fig. 3. Calculated change in the E2 peak height as a function of the film thickness of ion-bombarded layer assuming a three-layer model shown in the inserted figure.
856
T. Yamada et al. / Surface and Coatings Technology 173 – 174 (2003) 854–857
Fig. 4. Dependence of damage depth and E2 peak height determined from the model analysis on Ar gas pressure.
does not coincide completely with the experimental spectra particularly near the E1 and E2 peak position. This observation indicates that the ion-bombarded layer is not satisfactorily described with amorphous Si and voids, and introduction of strain probably due to ion inclusion in the surface is required to explain the red shift of the spectra. In order to estimate the damage depth from the model analysis of as-measured ellipsometric spectra, we propose a three-layer model consisting of SiO2 layeryionbombarded layerySi substrate as shown in the inset of Fig. 3. Here, the thickness of SiO2 layer is fixed at 10 ˚ because thickness of native oxide just after HF A, ˚ reproducibly. treatment has been observed to 10"2 A, The E2 peak height read from the ´2 spectra of ion-
Fig. 5. Dependence of Eg and G values determined from the third derivatives of dielectric functions in ion-bombarded layer on Ar gas pressure.
bombarded layer is plotted in Fig. 3 as a function of the assumed film thickness, where the ´2 spectra are obtained from multi-wavelength best-fit analysis using the three-layer model. The E2 peak height decreases drastically from the point indicated by an arrow with decrease in the assumed film thickness as observed in Fig. 3. Therefore, we may recognize the thickness corresponding to the peak value as being the actual damage depth. Fig. 4 shows the E2 peak height and Ar gas pressure dependences of the damage depth acquired from the method described above. With gas pressure increase, the damage depth is kept almost constant. It is noted that the damage depth is found to be dependent on the applied plasma voltage and show dependence of the ion bombarding energy E 2y3 w2x. For studying in more detail, third derivatives for the dielectric functions of ion-bombarded layer were obtained and the interband energy gap, Eg and broadening parameter, G for the E1 optical transition were calculated on the basis of the three-point method w6x. Fig. 5 shows the gas pressure dependences of Eg and G values in the ion-bombarded layer, where Eg decreased in the first step, and then it turned to increase slightly at approximately 33 Pa and G starts increasing from 20 Pa. The increase in G indicates deterioration of crystallinity due to the ion bombardment. The decrease in Eg (red shift) cannot be explained by only deviation from single crystallinity, but addition of strain in the Si crystal lattice probably due to the inclusion of Arq ions. Turning to increase in Eg suggests that a certain mechanism, such as dislocation generation andyor reduction of Ar in the ion-bombarded layer, might act on the lattice strain to relax. A further study with other evalu-
T. Yamada et al. / Surface and Coatings Technology 173 – 174 (2003) 854–857
ation methods is required to clarify the mechanism in the near future.
857
pressure indicated that strain in the crystal lattice may be produced by inclusion of ions and then relaxed in the ion-bombarded layer.
4. Conclusion References We have investigated damages at Si surface exposed to Ar plasma for the purpose of applying the spectroscopic ellipsometry to studies for plasma surface modifications in other materials. We could estimate the damage depth from the E2 peak height, which is sensitive to the damage generation. The Eg and G values for the E1 optical transition were determined from thirdderivative spectra of the dielectric functions in the ionbombarded layer. Dependence of these values on gas
w1x T. Yamada, N. Kubo, K. Kitahara, A. Moritani, Surf. Coat. Technol., in press. w2x D.E. Aspnes, A.A. Studna, Surf. Sci. 96 (1980) 294–306. w3x P.K. Giri, S. Tripurasundari, G. Raghavan, B.K. Panigrahi, P. Magudapathy, K.G.M. Nair, A.K. Tyagi, J. Appl. Phys. 90 (2001) 2. w4x M.M. Ibrahim, N.M. Bashara, Surf. Sci. 30 (1972) 632–640. w5x P.J. McMarr, K. Vedam, J. Appl. Phys. 59 (1986) 694–701. w6x D.E. Aspnes, J.E. Rowe, Phys. Rev. Lett. 27 (1971) 188–190.
Study of mechanism of plasma surface modifications in Si by spectroscopic ellipsometry T. Yamada*, N. Harada, K. Kitahara, A. Moritani Department of Electronic and Control Systems Engineering, Shimane University, Matsue 690-8504, Shimane, Japan
Abstract Spectroscopic ellipsometry has been applied to study of plasma surface modifications in single-crystalline Si exposed to Ar plasma at room temperature. The complex dielectric functions of ion-bombarded layers were obtained from a new method for estimating the plasma-damage depth. Damage dependences of interband energy gaps and broadening parameters for the E1 optical transition were derived from the third-derivative spectra of dielectric functions. These parameters strongly suggest that Si surface exposed to Ar plasma has suffered from lattice expansion due to inclusions of Arq ions before lattice relaxation probably due to dislocation generation. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Plasma surface modification; Spectroscopic ellipsometry; Plasma damage; Si
1. Introduction The surface modification techniques in plasma and ion implantation processes have been widely applied in order to add superior functions to surfaces of metals and semiconductors. However, these techniques produce crystal damages at the surface. As for monitoring the surface modification of a solid exposed to plasma, spectroscopic ellipsometry is powerful since it is a nondestructive and highly sensitive technique and makes in situ measurement possible w1x. Therefore, it is of great importance to study plasma-damage process for understanding surface modification mechanism from the ellipsometric viewpoint. Several researchers have endeavored to characterize the damaged surface layer produced by ion implantation using the spectroscopic ellipsometry w2–5x. Aspnes et al. revealed that the damage depth of Arq ion implanted c-Ge obeys the ion bombarding energy E 2y3 dependence w2x, and Giri et al. evaluated the dielectric function spectra of two major optical interband transitions E1 and E2 in Arq ion implanted c-Si and indicated the existence of a threshold fluence in the process changing from crystalline to amorphous state w3x. *Corresponding author. Tel.: q81-852-328-902; fax: q81-852328-902. E-mail address: [email protected] (T. Yamada).
In this paper, we focus on the estimation of damage depth and strain-induced red shift in the bandgap energy at room temperature for understanding mechanism of plasma surface modifications in single-crystalline Si exposed to Ar plasma. 2. Experimental The ellipsometric measurements were made with a rotating-analyzer spectroscopic ellipsometer (Model M 88, J.A. Woollam Co., Inc.) in a photon energy range from 1.5 to 4.5 eV at angles of incidence of 75–798. Real part N´1M and imaginary part N´2M of pseudocomplex dielectric functions are derived from measured ellipsometric parameters psi and delta on the assumption that the sample is a homogeneous bulk-like solid. A single-crystalline Si (100) wafer with native oxide removed by dilute HF etching was directly set on the cathode plate in the plasma chamber. The dc glow discharge of high purity Ar gas was generated for 10 min at room temperature by 10 kHz rectangular-wave with a duty ratio of 0.5. The applied plasma voltage was set constant at 360 V and the Ar gas pressure was varied in a range from 6.7 to 53.0 Pa in order to prepare samples with various levels of ion bombardment. For the alternative condition the applied Ar gas pressure was set constant at 6.7 Pa and the plasma voltage was varied in a range from 240 to 560 V.
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0257-8972Ž03.00559-0
T. Yamada et al. / Surface and Coatings Technology 173 – 174 (2003) 854–857
Fig. 1. Dependence of imaginary part of pseudo-dielectric function N´2M spectra on Ar gas pressure.
3. Results and discussion Fig. 1 shows some typical gas pressure dependences of imaginary part of pseudo-complex dielectric spectra N´2M in Si measured at room temperature with spectroscopic ellipsometry. We assume here that the ion flux onto the surface increases with gas pressure though quantitative correlation between the two is unknown. It is observed in the spectra that both E1 and E2 peak decrease in height and shift to lower photon energy (red shift), and the spectral structures broaden gradually with
855
Fig. 2. Experimental and best-fitted spectra in model analysis for ionbombarded sample at a gas pressure of 27.0 Pa.
gas pressure. Here, the E2 peak is very sensitive to crystal damage as compared to the E1 due to the crystal symmetry w3x. The red shift arises probably from three fractions, which are oxidation, amorphization and expansion of crystal lattice due to the inclusion of Arq ions in the surface. Fig. 2 shows the experimental and best-fitted spectra at a gas pressure of 27.0 Pa obtained from analysis using the model consisting of SiO2 layer, mixed layer with crystal Siyamorphous Siyvoid for the ion-bombarded layer and Si substrate as shown in the inserted of this figure. It is recognized that the best-fitted spectra
Fig. 3. Calculated change in the E2 peak height as a function of the film thickness of ion-bombarded layer assuming a three-layer model shown in the inserted figure.
856
T. Yamada et al. / Surface and Coatings Technology 173 – 174 (2003) 854–857
Fig. 4. Dependence of damage depth and E2 peak height determined from the model analysis on Ar gas pressure.
does not coincide completely with the experimental spectra particularly near the E1 and E2 peak position. This observation indicates that the ion-bombarded layer is not satisfactorily described with amorphous Si and voids, and introduction of strain probably due to ion inclusion in the surface is required to explain the red shift of the spectra. In order to estimate the damage depth from the model analysis of as-measured ellipsometric spectra, we propose a three-layer model consisting of SiO2 layeryionbombarded layerySi substrate as shown in the inset of Fig. 3. Here, the thickness of SiO2 layer is fixed at 10 ˚ because thickness of native oxide just after HF A, ˚ reproducibly. treatment has been observed to 10"2 A, The E2 peak height read from the ´2 spectra of ion-
Fig. 5. Dependence of Eg and G values determined from the third derivatives of dielectric functions in ion-bombarded layer on Ar gas pressure.
bombarded layer is plotted in Fig. 3 as a function of the assumed film thickness, where the ´2 spectra are obtained from multi-wavelength best-fit analysis using the three-layer model. The E2 peak height decreases drastically from the point indicated by an arrow with decrease in the assumed film thickness as observed in Fig. 3. Therefore, we may recognize the thickness corresponding to the peak value as being the actual damage depth. Fig. 4 shows the E2 peak height and Ar gas pressure dependences of the damage depth acquired from the method described above. With gas pressure increase, the damage depth is kept almost constant. It is noted that the damage depth is found to be dependent on the applied plasma voltage and show dependence of the ion bombarding energy E 2y3 w2x. For studying in more detail, third derivatives for the dielectric functions of ion-bombarded layer were obtained and the interband energy gap, Eg and broadening parameter, G for the E1 optical transition were calculated on the basis of the three-point method w6x. Fig. 5 shows the gas pressure dependences of Eg and G values in the ion-bombarded layer, where Eg decreased in the first step, and then it turned to increase slightly at approximately 33 Pa and G starts increasing from 20 Pa. The increase in G indicates deterioration of crystallinity due to the ion bombardment. The decrease in Eg (red shift) cannot be explained by only deviation from single crystallinity, but addition of strain in the Si crystal lattice probably due to the inclusion of Arq ions. Turning to increase in Eg suggests that a certain mechanism, such as dislocation generation andyor reduction of Ar in the ion-bombarded layer, might act on the lattice strain to relax. A further study with other evalu-
T. Yamada et al. / Surface and Coatings Technology 173 – 174 (2003) 854–857
ation methods is required to clarify the mechanism in the near future.
857
pressure indicated that strain in the crystal lattice may be produced by inclusion of ions and then relaxed in the ion-bombarded layer.
4. Conclusion References We have investigated damages at Si surface exposed to Ar plasma for the purpose of applying the spectroscopic ellipsometry to studies for plasma surface modifications in other materials. We could estimate the damage depth from the E2 peak height, which is sensitive to the damage generation. The Eg and G values for the E1 optical transition were determined from thirdderivative spectra of the dielectric functions in the ionbombarded layer. Dependence of these values on gas
w1x T. Yamada, N. Kubo, K. Kitahara, A. Moritani, Surf. Coat. Technol., in press. w2x D.E. Aspnes, A.A. Studna, Surf. Sci. 96 (1980) 294–306. w3x P.K. Giri, S. Tripurasundari, G. Raghavan, B.K. Panigrahi, P. Magudapathy, K.G.M. Nair, A.K. Tyagi, J. Appl. Phys. 90 (2001) 2. w4x M.M. Ibrahim, N.M. Bashara, Surf. Sci. 30 (1972) 632–640. w5x P.J. McMarr, K. Vedam, J. Appl. Phys. 59 (1986) 694–701. w6x D.E. Aspnes, J.E. Rowe, Phys. Rev. Lett. 27 (1971) 188–190.