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SiOF composite film deposition in a helicon plasma reactor
Thin Solid Films 341 (1999) 109±111
Low dielectric constant CF/SiOF composite ®lm deposition in a helicon plasma reactor Seok-Min Yun a,*, Hong-Young Chang a, Min-Sung Kang b, Chi-Kyu Choi b a
Department of Physics, Korea Advanced Institute of Science and Technology, Taejeon 305±701, South Korea b Department of Physics Cheju National University, Cheju 690-756, South Korea
Abstract Low dielectric constant CF/SiOF composite ®lms are deposited using tri-ethoxy-¯uorosilane (FTES) and O2 mixture in a helicon plasma reactor without intentional heating or biasing the substrate. Optical emission spectroscopy (OES) is used to study the relation between the relative densities of the radicals and the ®lm properties. The OES data imply that the FTES and O2 gases are greatly dissociated above the RF power of 900 W. Consequently, the deposition process of helicon plasma CVD where the source gases dissociated highly form CF/SiOF composite ®lm is different from thermal CVD where the gases react chemically on the substrate and make the SiOF ®lm. FTIR and XPS spectra show that the ®lm has Si±F, Si±O, and C±F bonds. The Si±F and C±F bonds may lower the dielectric constant greatly. As the O2/ FTES ratio decreases, the ¯uorine concentration increases and the dielectric constant decreases. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Low dielectric constant; CF/SiOF composite ®lm; Helicon plasma reactor
1. Introduction Present silicon dioxide (SiO2) ®lm as intermetal dielectric (IMD) layers will result in high parasitic capacitance and crosstalk interference in high density devices. Low dielectric materials such as ¯uorinated silicon oxide (SiOF) and ¯uoropolymer IMD layers have been tried to solve this problem [1±3]. The concept of a plasma processing apparatus with high density plasma at low pressure has received much attention for deposition because ®lms made in these plasma reactors have many advantages such as good ®lm quality and gap ®lling pro®le. The low contaminated [4] and good crosslinked [5] ®lm can be made due to high ion ¯ux with low ion energy in the high density plasma. A good gap ®lling pro®le can be obtained because the processing pressure is very low ( , 1.3 Pa) [6]. Especially, the helicon plasma reactor has attractive features for ®lm deposition because of its high density plasma production compared with other conventional types of plasma sources. Moreover, it is advantageous for dissociating source gases because high energy electrons exist in the plasma source region and these dissociated species can be expanded uniformly over large diameters by controlling the magnetic ®eld at the position of processing chamber. Thus, the helicon * Corresponding author. Tel.: 1 82-42-869-2566; fax: 1 82-42-8692510; e-mail: [email protected].
plasma source coupled to a reaction chamber have provided a new method for better high density plasma chemical vapor deposition (HDP-CVD).
2. Experimental In this paper, we present the results on the low dielectric constant CF/SiOF composite ®lm deposition using O2/FTES gas mixture in a helicon plasma reactor. High density plasma is generated in the conventional helicon plasma source with Nagoya type III antenna, 5±15 MHz and 1 kW RF power, 700 Gauss magnetic ®eld, and 0.2 Pa pressure. This helicon source is explained in detail in other papers [7,8]. The O2 and Ar gases are fed through a mass ¯ow controller system into the source chamber. The FTES liquid source is stored in a stainless steel container and vapor ¯ow rate is controlled by a mass ¯ow controller. No carrier gas is used due to low process pressure ( , 1.3 Pa). The discharge pressure is measured by a baratron gauge. The electron density and temperature of the O2/FTES discharge are measured by a Langmuir probe. The relative density of radicals are measured by optical emission spectroscopy (OES). OES is used to study the relation between the relative concentrations of the radicals (F, Si, O, C, and H) and the deposition mechanism. The OES is set up with a high gain diode array attached to a triple grating monochro-
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0257-897 2(98)00781-6
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S.M. Yun et al. / Thin Solid Films 341 (1999) 109±111
not intentionally heated. The deposition of the SiOF ®lm is carried out as functions of the O2/FTES ¯ow rate ratio and RF power. Film properties such as chemical bonding structure and dielectric constant are investigated. Chemical bonding structure is characterized using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The dielectric constant is measured using a metal insulator semiconductor (MIS) (Al/0.4 m-thick ®lm/pSi) structure at 1 MHz. Finally, we used the scotch tape test for adhesion of the deposited ®lms. 3. Results and discussion
Fig. 1. The normalized emission intensity (line with symbol) and deposition rate (line) versus RF power. The discharge condition is 0.2 Pa of O2/ FTES/Ar (3:1:1), 1 kW, 7 MHz RF power and Bo 700 Gauss.
mator which detects the emission of the excited species in the plasma through a quartz window set on the reactor sidewall. The CF/SiOF composite ®lm is deposited on Si p(100) 5-inch silicon substrates with 80% of O2/FTES gas mixture and 20% of Ar gas. When the small amount of Ar is added, the O2/FTES gases are dissociated easily and ®lm quality is improved [7]. The wafers are left at ¯oating potential and
Fig. 2. FTIR spectrum (a) and C 1s XPS spectrum (b) of the CF/SiOF composite ®lm. The discharge condition is 0.2 Pa of O2/FTES/Ar (3:1:1), 1 kW, 7 MHz RF power and Bo 700 Gauss.
The normalized emission intensity of F (703.7 nm), Si (288.2 nm), O (777 nm), C (247.9 nm), and H (656.2 nm), and deposition rate with various RF power are shown in Fig. 1. The emission intensity by a neutral, X, (IX) can be expressed as follows Ix kex ne X
1
where kex is the rate coef®cient of the excitation process, ne is the electron density, and [X] is the density of the species (X F, Si, O, C, and H). kex is a function of the electron temperature which is nearly constant as RF power increases in the helicon plasma source. Hence, the emission intensities normalized for the electron density are representative of the relative density of each species. The rate of O2/FTES gas dissociation into Si, F, C, H, and O increases as power increases in the low-mode, and jumps up at the transition point from the low-mode to helicon mode. This shows that the O2/FTES gases are greatly dissociated above the transition point since there are many hot electrons (tail electrons) in the helicon mode [9±10]. Fig. 1 also shows that the deposition rate depends on the degree of dissociation of the source gases. The FTIR spectrum of the ®lm made with the discharge condition of 1 kW, 7 MHz RF power, 0.2 Pa pressure is shown in Fig. 2a. The spectrum is generally broad and overlapped due to the complex stoichiometry and the amorphous nature of the ®lm. The intense band near 1070 cm 21, called TO (transverse optical) mode, is attributed to the asymmetric stretching of the oxygen atoms along the direction parallel to Si±O±Si and the band near 800 cm 21 is due to the symmetric Si±O±Si stretching [11]. The band near 930 cm 21 is for Si±F bonds and the band near 1410 cm 21 is for C±C bonds. Small broad bands near 3500 cm 21 are due to Si±OH (3650 cm 21), H±O±H (3230, 3440 cm 21) bonds [12]. C±F bonds may exist in the ®lm but the band for C±F bond (near 1100 cm 21) is overlapped by the Si±O bands, so it is not shown in the FTIR data [13]. This FTIR spectrum is taken a month later from the ®lm deposition. We get the same spectrum right after the ®lm deposition. This means that the ®lm does not absorb any moisture in the air and the Si±OH and H±O±H bonds are formed during the deposition process because the O and H radicals exist in the
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111
dielectric constant. The lowest dielectric constant is 2.8 in this experiment. This value is lower than that of other SiOF ®lms ( . 3.0) but higher than that of ¯uoropolymers ( . 2.0). 4. Conclusions
Fig. 3. Fluorine emission intensity and dielectric constant versus O2/FTES ratio. The discharge condition is P 0:2 Pa, 1 kW, 7 MHz RF power and Bo 700 Gauss.
plasma. However, the H atoms are easily detached from the surface by ion bombardment which occurs very frequently in the helicon wave plasmas and only a small amount of H atoms remains in the ®lm [5]. To con®rm the existence of C±F bonds, we use XPS measurements, whose C 1s signal is shown in Fig. 2b. The peaks are deconvoluted into four components. The main peak is C±CFx at 287 eV and small peaks are C±F at 289 eV, and C±F2 at 292 eV [14]. C±C or C±CHn at 285 eV is very small. Referring to the above results we can summarize the deposition process as follows. First, the source gases are greatly dissociated into Si, C, F, H, and O radicals. There are also some species which are not dissociated and we can see that these species contain F±Si, Si±O, O±C, C±C, and C±H bonds considering the chemical structure of F±Si± (OC2H5)3. Si±F, Si±O, C±F, and C±O bonds can be made during the deposition process because F and O radicals easily react with Si and C. Also, Si±H, O±H, C±H, and H±F bonds can be made. However, because CO and H species on the ®lm are volatile and easily detachable by ion bombardment which occurs very frequently during the ®lm formation process, we can infer that the main remaining bonds in the ®lm are Si±F, Si±O, C±F (or C±CF). In the case of O2/FTES CVD methods, there are no C±F bonds because the FTES precursors such as F±Si(OC2H5)n21(OH)52n chemically react with O2 on the substrate and (OC2H5)n21(OH)52n is alternated with O [15]. Thus, the ®lm composition is different between CVD where SiOF ®lm is deposited and helicon plasma CVD where CF/SiOF composite ®lm is deposited. The normalized emission intensity of F radicals and the dielectric constant of the ®lm with various O2/FTES ratios are shown in Fig. 3. F emission intensity decreases and the dielectric constant increases as the O2/FTES ratio increases. The trends of C and H emission intensities with O2/FTES ratio are similar to the trend of F emission intensity. The trend of O emission intensity is similar to the trend of the
The CF/SiOF composite ®lm is deposited without intentional heating or biasing using O2/FTES gas mixture in the helicon plasma reactor. CF/SiOF composite ®lm is deposited only in the helicon mode where the source gases are highly dissociated. The Si, F, C, H, and O species monitored by OES increase greatly when the mode transits from lowmode to helicon mode. The ®lm has C±F bonds in addition to Si±F, Si±O bonds because the F and C radicals dissociated from the FTES gas can easily react and attend the ®lm formation. This deposition mechanism is different from the thermal O2/FTES CVD where the source gases chemically react on the substrate and make SiOF ®lm. As the O2/ FTES ratio decreases the dielectric constant decreases. The concentration of F may be the limiting factor in determining the dielectric constant. The ®lms absorb no moisture and passes the scotch tape test. These facts imply that the ®lm has good crosslinked structure due to high ion ¯ux with low ion energy in helicon plasma. The lowest dielectric constant is 2.8 in this experiment. Acknowledgements This work is supported in part by The Electronics and Telecommunications Research Institute. References [1] V. Shannon, M. Karim, Thin Solid Films 270 (1995) 498. [2] S. Mizuno, A. Verma, H. Tran, P. Lee, B. Nguyen, Thin Solid Films 283 (1996) 30. [3] M. Horie, J. Vac. Sci. Technol. A 13 (1995) 2490. [4] S. Gorbatkin, L. Berry, J. Vac. Sci. Technol. 10 (1992) 3104. [5] K. Seaward, J. Turner, K. Nauka, A. Nel, J. Vac. Sci. Technol. B 13 (1995) 118. [6] E. Korczynski, Solid State Technol (1996) 63. [7] J. Kim, S. Seo, S. Yun, H. Chang, K. Lee, C. Choi, J. Electrochem, Soc. 143 (1996) 2990. [8] J. Kim, H. Chang, Phys. Plasmas 3 (1996) 1462. [9] F. Chen, C. Decker, Plasma Phys. Contr. Fusion 34 (1992) 635. [10] P. Zhu, R. Boswell, Phys. Fluids B 3 (1991) 869. [11] D. Back, Physics of Thin Films, 18, Academic, New York, 1991 Chap. 1.. [12] H. Miyajima, R. Katsumata, Y. Nakasaki, Y. Nishiyama, N. Hayasaka, Jpn. J. Appl. Phys. 35 (1996) 6217. [13] D. Kim, Y. Lee, Appl. Phys. Lett. 69 (1996) 28. [14] K. Endo, T. Tatsumi, Appl. Phys. Lett. 78 (1995) 1370. [15] H. Kitoh, M. Muroyama, M. Sasaki, M. Iwasawa, H. Kimura, Jpn. J. Appl. Phys. 35 (1996) 1464.
Low dielectric constant CF/SiOF composite ®lm deposition in a helicon plasma reactor Seok-Min Yun a,*, Hong-Young Chang a, Min-Sung Kang b, Chi-Kyu Choi b a
Department of Physics, Korea Advanced Institute of Science and Technology, Taejeon 305±701, South Korea b Department of Physics Cheju National University, Cheju 690-756, South Korea
Abstract Low dielectric constant CF/SiOF composite ®lms are deposited using tri-ethoxy-¯uorosilane (FTES) and O2 mixture in a helicon plasma reactor without intentional heating or biasing the substrate. Optical emission spectroscopy (OES) is used to study the relation between the relative densities of the radicals and the ®lm properties. The OES data imply that the FTES and O2 gases are greatly dissociated above the RF power of 900 W. Consequently, the deposition process of helicon plasma CVD where the source gases dissociated highly form CF/SiOF composite ®lm is different from thermal CVD where the gases react chemically on the substrate and make the SiOF ®lm. FTIR and XPS spectra show that the ®lm has Si±F, Si±O, and C±F bonds. The Si±F and C±F bonds may lower the dielectric constant greatly. As the O2/ FTES ratio decreases, the ¯uorine concentration increases and the dielectric constant decreases. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Low dielectric constant; CF/SiOF composite ®lm; Helicon plasma reactor
1. Introduction Present silicon dioxide (SiO2) ®lm as intermetal dielectric (IMD) layers will result in high parasitic capacitance and crosstalk interference in high density devices. Low dielectric materials such as ¯uorinated silicon oxide (SiOF) and ¯uoropolymer IMD layers have been tried to solve this problem [1±3]. The concept of a plasma processing apparatus with high density plasma at low pressure has received much attention for deposition because ®lms made in these plasma reactors have many advantages such as good ®lm quality and gap ®lling pro®le. The low contaminated [4] and good crosslinked [5] ®lm can be made due to high ion ¯ux with low ion energy in the high density plasma. A good gap ®lling pro®le can be obtained because the processing pressure is very low ( , 1.3 Pa) [6]. Especially, the helicon plasma reactor has attractive features for ®lm deposition because of its high density plasma production compared with other conventional types of plasma sources. Moreover, it is advantageous for dissociating source gases because high energy electrons exist in the plasma source region and these dissociated species can be expanded uniformly over large diameters by controlling the magnetic ®eld at the position of processing chamber. Thus, the helicon * Corresponding author. Tel.: 1 82-42-869-2566; fax: 1 82-42-8692510; e-mail: [email protected].
plasma source coupled to a reaction chamber have provided a new method for better high density plasma chemical vapor deposition (HDP-CVD).
2. Experimental In this paper, we present the results on the low dielectric constant CF/SiOF composite ®lm deposition using O2/FTES gas mixture in a helicon plasma reactor. High density plasma is generated in the conventional helicon plasma source with Nagoya type III antenna, 5±15 MHz and 1 kW RF power, 700 Gauss magnetic ®eld, and 0.2 Pa pressure. This helicon source is explained in detail in other papers [7,8]. The O2 and Ar gases are fed through a mass ¯ow controller system into the source chamber. The FTES liquid source is stored in a stainless steel container and vapor ¯ow rate is controlled by a mass ¯ow controller. No carrier gas is used due to low process pressure ( , 1.3 Pa). The discharge pressure is measured by a baratron gauge. The electron density and temperature of the O2/FTES discharge are measured by a Langmuir probe. The relative density of radicals are measured by optical emission spectroscopy (OES). OES is used to study the relation between the relative concentrations of the radicals (F, Si, O, C, and H) and the deposition mechanism. The OES is set up with a high gain diode array attached to a triple grating monochro-
0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0257-897 2(98)00781-6
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S.M. Yun et al. / Thin Solid Films 341 (1999) 109±111
not intentionally heated. The deposition of the SiOF ®lm is carried out as functions of the O2/FTES ¯ow rate ratio and RF power. Film properties such as chemical bonding structure and dielectric constant are investigated. Chemical bonding structure is characterized using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The dielectric constant is measured using a metal insulator semiconductor (MIS) (Al/0.4 m-thick ®lm/pSi) structure at 1 MHz. Finally, we used the scotch tape test for adhesion of the deposited ®lms. 3. Results and discussion
Fig. 1. The normalized emission intensity (line with symbol) and deposition rate (line) versus RF power. The discharge condition is 0.2 Pa of O2/ FTES/Ar (3:1:1), 1 kW, 7 MHz RF power and Bo 700 Gauss.
mator which detects the emission of the excited species in the plasma through a quartz window set on the reactor sidewall. The CF/SiOF composite ®lm is deposited on Si p(100) 5-inch silicon substrates with 80% of O2/FTES gas mixture and 20% of Ar gas. When the small amount of Ar is added, the O2/FTES gases are dissociated easily and ®lm quality is improved [7]. The wafers are left at ¯oating potential and
Fig. 2. FTIR spectrum (a) and C 1s XPS spectrum (b) of the CF/SiOF composite ®lm. The discharge condition is 0.2 Pa of O2/FTES/Ar (3:1:1), 1 kW, 7 MHz RF power and Bo 700 Gauss.
The normalized emission intensity of F (703.7 nm), Si (288.2 nm), O (777 nm), C (247.9 nm), and H (656.2 nm), and deposition rate with various RF power are shown in Fig. 1. The emission intensity by a neutral, X, (IX) can be expressed as follows Ix kex ne X
1
where kex is the rate coef®cient of the excitation process, ne is the electron density, and [X] is the density of the species (X F, Si, O, C, and H). kex is a function of the electron temperature which is nearly constant as RF power increases in the helicon plasma source. Hence, the emission intensities normalized for the electron density are representative of the relative density of each species. The rate of O2/FTES gas dissociation into Si, F, C, H, and O increases as power increases in the low-mode, and jumps up at the transition point from the low-mode to helicon mode. This shows that the O2/FTES gases are greatly dissociated above the transition point since there are many hot electrons (tail electrons) in the helicon mode [9±10]. Fig. 1 also shows that the deposition rate depends on the degree of dissociation of the source gases. The FTIR spectrum of the ®lm made with the discharge condition of 1 kW, 7 MHz RF power, 0.2 Pa pressure is shown in Fig. 2a. The spectrum is generally broad and overlapped due to the complex stoichiometry and the amorphous nature of the ®lm. The intense band near 1070 cm 21, called TO (transverse optical) mode, is attributed to the asymmetric stretching of the oxygen atoms along the direction parallel to Si±O±Si and the band near 800 cm 21 is due to the symmetric Si±O±Si stretching [11]. The band near 930 cm 21 is for Si±F bonds and the band near 1410 cm 21 is for C±C bonds. Small broad bands near 3500 cm 21 are due to Si±OH (3650 cm 21), H±O±H (3230, 3440 cm 21) bonds [12]. C±F bonds may exist in the ®lm but the band for C±F bond (near 1100 cm 21) is overlapped by the Si±O bands, so it is not shown in the FTIR data [13]. This FTIR spectrum is taken a month later from the ®lm deposition. We get the same spectrum right after the ®lm deposition. This means that the ®lm does not absorb any moisture in the air and the Si±OH and H±O±H bonds are formed during the deposition process because the O and H radicals exist in the
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dielectric constant. The lowest dielectric constant is 2.8 in this experiment. This value is lower than that of other SiOF ®lms ( . 3.0) but higher than that of ¯uoropolymers ( . 2.0). 4. Conclusions
Fig. 3. Fluorine emission intensity and dielectric constant versus O2/FTES ratio. The discharge condition is P 0:2 Pa, 1 kW, 7 MHz RF power and Bo 700 Gauss.
plasma. However, the H atoms are easily detached from the surface by ion bombardment which occurs very frequently in the helicon wave plasmas and only a small amount of H atoms remains in the ®lm [5]. To con®rm the existence of C±F bonds, we use XPS measurements, whose C 1s signal is shown in Fig. 2b. The peaks are deconvoluted into four components. The main peak is C±CFx at 287 eV and small peaks are C±F at 289 eV, and C±F2 at 292 eV [14]. C±C or C±CHn at 285 eV is very small. Referring to the above results we can summarize the deposition process as follows. First, the source gases are greatly dissociated into Si, C, F, H, and O radicals. There are also some species which are not dissociated and we can see that these species contain F±Si, Si±O, O±C, C±C, and C±H bonds considering the chemical structure of F±Si± (OC2H5)3. Si±F, Si±O, C±F, and C±O bonds can be made during the deposition process because F and O radicals easily react with Si and C. Also, Si±H, O±H, C±H, and H±F bonds can be made. However, because CO and H species on the ®lm are volatile and easily detachable by ion bombardment which occurs very frequently during the ®lm formation process, we can infer that the main remaining bonds in the ®lm are Si±F, Si±O, C±F (or C±CF). In the case of O2/FTES CVD methods, there are no C±F bonds because the FTES precursors such as F±Si(OC2H5)n21(OH)52n chemically react with O2 on the substrate and (OC2H5)n21(OH)52n is alternated with O [15]. Thus, the ®lm composition is different between CVD where SiOF ®lm is deposited and helicon plasma CVD where CF/SiOF composite ®lm is deposited. The normalized emission intensity of F radicals and the dielectric constant of the ®lm with various O2/FTES ratios are shown in Fig. 3. F emission intensity decreases and the dielectric constant increases as the O2/FTES ratio increases. The trends of C and H emission intensities with O2/FTES ratio are similar to the trend of F emission intensity. The trend of O emission intensity is similar to the trend of the
The CF/SiOF composite ®lm is deposited without intentional heating or biasing using O2/FTES gas mixture in the helicon plasma reactor. CF/SiOF composite ®lm is deposited only in the helicon mode where the source gases are highly dissociated. The Si, F, C, H, and O species monitored by OES increase greatly when the mode transits from lowmode to helicon mode. The ®lm has C±F bonds in addition to Si±F, Si±O bonds because the F and C radicals dissociated from the FTES gas can easily react and attend the ®lm formation. This deposition mechanism is different from the thermal O2/FTES CVD where the source gases chemically react on the substrate and make SiOF ®lm. As the O2/ FTES ratio decreases the dielectric constant decreases. The concentration of F may be the limiting factor in determining the dielectric constant. The ®lms absorb no moisture and passes the scotch tape test. These facts imply that the ®lm has good crosslinked structure due to high ion ¯ux with low ion energy in helicon plasma. The lowest dielectric constant is 2.8 in this experiment. Acknowledgements This work is supported in part by The Electronics and Telecommunications Research Institute. References [1] V. Shannon, M. Karim, Thin Solid Films 270 (1995) 498. [2] S. Mizuno, A. Verma, H. Tran, P. Lee, B. Nguyen, Thin Solid Films 283 (1996) 30. [3] M. Horie, J. Vac. Sci. Technol. A 13 (1995) 2490. [4] S. Gorbatkin, L. Berry, J. Vac. Sci. Technol. 10 (1992) 3104. [5] K. Seaward, J. Turner, K. Nauka, A. Nel, J. Vac. Sci. Technol. B 13 (1995) 118. [6] E. Korczynski, Solid State Technol (1996) 63. [7] J. Kim, S. Seo, S. Yun, H. Chang, K. Lee, C. Choi, J. Electrochem, Soc. 143 (1996) 2990. [8] J. Kim, H. Chang, Phys. Plasmas 3 (1996) 1462. [9] F. Chen, C. Decker, Plasma Phys. Contr. Fusion 34 (1992) 635. [10] P. Zhu, R. Boswell, Phys. Fluids B 3 (1991) 869. [11] D. Back, Physics of Thin Films, 18, Academic, New York, 1991 Chap. 1.. [12] H. Miyajima, R. Katsumata, Y. Nakasaki, Y. Nishiyama, N. Hayasaka, Jpn. J. Appl. Phys. 35 (1996) 6217. [13] D. Kim, Y. Lee, Appl. Phys. Lett. 69 (1996) 28. [14] K. Endo, T. Tatsumi, Appl. Phys. Lett. 78 (1995) 1370. [15] H. Kitoh, M. Muroyama, M. Sasaki, M. Iwasawa, H. Kimura, Jpn. J. Appl. Phys. 35 (1996) 1464.