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Plasma Etch Rates of Porous Silica Low-k Films with Different Dielectric Constants
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2006 Jpn. J. Appl. Phys. 45 8873 (http://iopscience.iop.org/1347-4065/45/11R/8873) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 139.133.11.2 This content was downloaded on 22/05/2015 at 06:38
Please note that terms and conditions apply.
Japanese Journal of Applied Physics Vol. 45, No. 11, 2006, pp. 8873–8875 #2006 The Japan Society of Applied Physics
Plasma Etch Rates of Porous Silica Low-k Films with Different Dielectric Constants Tetsuo O NO, Hideki T AKAHASHI, Keizo K INOSHITA, Nobutoshi FUJII, Nobuhiro H ATA1 and Takamaro K IKKAWA1;2 MIRAI Project, Association of Super-Advanced Electronics Technologies (ASET), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan 1 MIRAI Project, Advanced Semiconductor Research Center, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan 2 Research Center for Nanodevices and Systems, Hiroshima University, 1-4-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan (Received April 11, 2006; accepted August 3, 2006; published online November 8, 2006)
The reactive ion etch rates of porous silica films with different dielectric constants (k-values) or film densities were measured by varying wafer bias and gas ratio for Ar/C5 F8 /O2 plasma. Both the etch rates of porous silica films and the optical emission intensities from the etching products (SiF) increased with wafer bias power. Etch rate increased with decreasing k-value of porous silica, whereas SiF emission intensity was maintained constant regardless of k-value, indicating that the amount of etching products escaping from the porous silica surface to the gas phase remained unchanged. From this result it is concluded that mass etch rate, defined as the weight of a porous silica film etched from a unit area per unit time, is constant for Ar/C5 F8 / O2 plasma. [DOI: 10.1143/JJAP.45.8873] KEYWORDS: porous silica, low-k , plasma etching, density, SiF emission intensity
1.
Introduction
One of the key technologies for realizing high speed ultra large scale integrated circuits (ULSI) is Cu damascene interconnects with an ultra low-dielectric-constant (low-k) interlayer material. In fact, the dielectric constant of 2.1 is necessary for 45-nm-node interconnects. Porous dielectric materials can satisfy such a requirement because the introduction of pores reduces k-value. The dry etching of a low-k material to form vias and trenches for the Cu damascene induces two main problems: One is the plasmainduced damage in the low-k film and the other is the complexity in process optimization. Plasma-induced damage can be reduced using oxygen-free gas chemistry.1) In addition, a damage recovery method was proposed to restore both the hydrophobicity of the film and leakage current.2,3) For process optimization, it is important to determine the effect of pore introduction and material composition on etching characteristics, such as etch rate and selectivity. The etch rate of SiOCH was studied and a low hydrogen/carbon (H/C) ratio resulted in etch stop phenomena.4) The etch rate increase with porosity in an xerogel (SiOCH) film was reported.5) The effects of porosity on the diffusion of atoms into the bulk material and the coverage of a CF polymer on the surface of the film were also reported.5–8) The purpose of this work is to determine the etch rate dependence of porous silica films on their densities. Also, the intensity of optical emission from etch products (SiF) is measured and the relationship between etch rate and the amount of etch products escaping from the porous silica surface is clarified. 2.
Experimental Procedure
Nonperiodic porous silica films were spin-coated on 200 mm Si wafers. A precursor solution was prepared by adding both an nonionic surfactant (triblock copolymer) and an acidic silica sol derived from tetraethylorthosilicate (TEOS) and dimethyldiethoxysilane (DMDEOS) in ethanol diluted with water. k-value was controlled by the molar ratio of the surfactant and silica oligomer.9) The properties of the samples used in this experiment are listed in Table I. k-values were measured by capacitance-voltage analysis
Table I. k-values and densities of porous silica films. Porosity was calculated from density assuming that SiO2 density = 2.2 g cm3 . Sample
Relative surfactant ratio
k-value
Density (g cm3 )
Porosity
a
1
2.18
0.89
0.40
b
1/3
2.28
0.92
0.42
c
1/4.5
2.64
1.22
0.55
d
1/6
2.98
1.3
0.59
using an Hg probe in nitrogen atmosphere. Film density was determined by an X-ray reflection method. After the calcination of the spin-coated film it was annealed in 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) vapor to enhance mechanical strength and hydrophobicity.10) A UHF electron cyclotron resonance reactor was used for etching porous silica low-k films. A 450 MHz electromagnetic wave was introduced to produce plasma and a wafer bias power of 800 kHz was applied to accelerate ions towards the wafer. Magnetic coils were used to control the plasma density distribution in the reactor. Ar/C5 F8 /O2 chemistry was employed for etching and the pressure was 2.0 Pa. The selectivity for SiCN was compared with that for CF4 plasma. Etch rates were determined on the basis of the film thickness before and after etching. To estimate the amount of etch products in the plasma, the optical emission from SiF at 440 nm was measured using a photodiode array. 3.
Results and Discussion
The etch rate dependence on wafer bias power is shown in Fig. 1. The peak-to-peak voltage of wafer bias Vpp is also indicated in the figure. Since the motion of ions can follow the voltage change at 800 kHz, Vpp is a good measure of ion energy. Etch rate increased with wafer bias power because the increase in ion energy enhances the etching reaction. It is known that in a highly polymerized plasma or a Crich plasma (Cx Fy : x=y > 0:5), the etch rate of a porous lowk film is suppressed because ions cannot remove the polymer films that deposit on the subsurface of the pores and the etching reaction is avoided.7,8) The etch rate of a porous silica film is lower than that of SiO2 in C4 F6 plasma.7) In
8873
Jpn. J. Appl. Phys., Vol. 45, No. 11 (2006)
300
500
Vpp=940V k=2.18 2.28 750V 2.64
560V
Mass etch rate (x10-7 g cm-2 min-1 )
Etch rate (nm/min)
400
T. ONO et al.
2.98
200 370V
TEOS
100 200V
400
wafer bias 400 W
300
250 W 146 W
200
75 W
100 0
0 0
100 200 300 400 Wafer bias power (W)
0.5
500
Fig. 3. Mass etch rate dependence on density of porous silica film.
500 Mass etch rate (x10-7 g cm-2 min-1)
150
Wafer bias 400 W
Wafer bias 146 W 250 W
100
k=2.98 2.64 2.28 2.18 146 W
50 35 W
0 0
75 W
100
1.5
Density (g/cm )
200 k=2.28
1 3
Fig. 1. Etch rate dependence of porous silica films on wafer bias power.
SiF emission intensity (arb. unit)
35 W
O2 flow rate
400
0cc 20 cc 28 cc 32 cc 36 cc
300 200 100 0
200
300
0.5
400
1
1.5
Density (g/cm3 )
Etch Rate (nm/min)
Fig. 4. Mass etch rate dependence on O2 flow rate.
Fig. 2. Etch rate vs SiF emission intensity.
Mass etch rate (x10-7 g cm -2 min-1)
500
contrast, the etch rate of a porous silica film for Ar/C5 F8 /O2 plasma is higher than that of TEOS in spite of a high C ratio (C/F ¼ 0:625) as shown in Fig. 1. This is due to the O2 that can decompose the polymer films deposited in the pore. The overall etching reaction of SiO2 with Ar/C5 F8 /O2 plasma is given by11,12) 2SiO2 þ C5 F8 ! 2SiF4 þ 5COx¼1;2 :
9 cc 15 cc 21 cc
300 200 100 0
ð1Þ
SiF4 is decomposed in plasma into SiFx¼1;3 . Decomposition rate depends on electron temperature. Figure 2 shows the relationship between the etch rate and emission intensity of SiF. When k-value was kept constant and wafer bias was varied, SiF emission intensity increased with etch rate, indicating an increase in the amount of etching reaction products in the plasma. On the other hand, SiF emission intensity remained unchanged when etch rate increased with decreasing k-value at a wafer bias of 146 W. Thus, the amount of etching products escaping from the porous silica films was kept constant regardless of k-value. Correspondingly, mass etch rate, defined as the weight of a porous silica material etched from a unit surface area per unit time (g cm2 min1 ), was kept constant regardless of film density, as shown in Fig. 3. When the amount of pores introduced into a SiO2 film is increased, an apparent increase in etch rate occurs. However, the mass of the film etched per unit time does not change with porosity. This result is consistent with that in the case of F-rich plasma (CF4 or C3 F8 ).7) Figure 4 shows the effects of O2 addition on mass etch rate. Mass etch rate reached its maximum at an O2 flow rate of approximately 20 sccm. The O atoms decompose CF polymer on the porous silica surface during etching, thereby increasing etch rate. On the other hand, O atoms oxidize the
C5F8 flow rate
400
0.5
1
1.5 3
Density (g/cm ) Fig. 5. Mass etch rate dependence on C5 F8 flow rate.
surface and suppress the etching reaction. Thus, etch rate reaches its maximum value at an optimum amount of O2 . The amount of F radicals generated in CF4 plasma depends on the amount of O2 addition because the density of F radicals reaches its maximum value at a certain amount of O2 .11) This is another possible reason for the etch rate dependence on O2 addition. Mass etch rate increased with C5 F8 flow rate up to 20 sccm, as shown in Fig. 5, simply because of the increase in the amount of F radicals. As shown in Figs. 4 and 5, mass etch rates were almost constant with decreasing film density. The merit of Ar/C5 F8 /O2 chemistry is its high selectivity for SiCN films. The selectivity of a porous silica film for SiCN is 2.9-fold higher than that of CF4 (F-rich gas) plasma as shown in Table II. A SiCN film is often used as an etch stop layer for the dual damascene structure of low-k materials. Thus, high selectivity between porous silica and SiCN is necessary to etch the dual damascene structure without punching through the SiCN film. The disadvantage
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Jpn. J. Appl. Phys., Vol. 45, No. 11 (2006)
T. ONO et al.
Table II. Selectivity of porous silica for SiCN film. Wafer biases are 400 W for Ar 1000 sccm/C5 F8 15 sccm/O2 28 sccm 2 Pa plasma and 100 W for CF4 200 sccm 1 Pa plasma. Etch rate (nm/min)
Gas
Selectivity
Porous silica (k ¼ 2:18)
SiCN
Ar/C5 F8 /O2
364
26
CF4
389
80
14 4.9
of Ar/C5 F8 /O2 chemistry is the oxygen-induced damage in porous silica films. However, the plasma-induced damage can be suppressed by organosiloxane vapor annealing.2,3) 4.
Conclusions
The dependence of etch rate on low-k film density for Ar/ C5 F8 /O2 plasma was examined. The optical emission intensity of SiF indicated that the amount of etching products in the plasma is kept constant even though the etch rate of the porous films increased with decreasing density. This is because mass etch rate, defined as the weight of a porous silica material etched from a unit surface area per unit time, g cm2 min1 , was constant regardless of the change in film density.
1) N. Ohashi, K. Misawa, S. Sone, H. J. Shin, K. Inukai, E. Soda, S. Kondo, A. Furuya, H. Okamura, S. Ogawa and N. Kobayashi: IEDM Tech. Dig., 2003, p. 857. 2) Y. Oku, N. Fujii, Y. Seino, Y. Takasu, H. Takahashi, Y. Sonoda, T. Goto, H. Miyoshi, S. Takada and T. Kikkawa: Ext. Abstr. Solid State Devices and Materials, Tokyo, 2004, p. 46. 3) T. Ono, K. Kinoshita, T. Goto, H. Takahashi, N. Fujii, Y. Sonoda, Y. Oku, K. Kohmura, N. Hata and T. Kikkawa: Dry Process Symp., 2004, p. 229. 4) T. Tatsumi, K. Nagahata, K. Urata, T. Saito, M. Minami, Y. Nogami, S. Iseda and Y. Morita: Dry Process Symp., 2002, p. 9. 5) T. E. F. M. Standaert, E. A. Joseph, G. S. Oehrlein, A. Jain, W. N. Gill, P. C. Wayner, Jr. and J. L. Plawsky: J. Vac. Sci. Technol. A 18 (2000) 2742. 6) P. Lazzeri, X. Hua, G. S. Oehrlein, M. Barozzi, E. Iacob and M. Anderle: J. Vac. Sci. Technol. B 23 (2005) 1491. 7) Y. Morikawa, N. Mizutani, M. Ozawa, T. Hayashi, W. Chen and T. Uchida: J. Vac. Sci. Technol. B 21 (2003) 1344. 8) X. Hua, C. Stolz, G. S. Oehrlein, P. Lazzeri, N. Coghe, M. Anderle, C. K. Inoki, T. S. Kuan and P. Jiang: J. Vac. Sci. Technol. A 23 (2005) 151. 9) N. Fujii, K. Yamada, Y. Oku, N. Hata, Y. Seino, C. Negoro and T. Kikkawa: Mater. Res. Soc. Symp. Proc. 812 (2004) F.4.10. 10) K. Kohmura, S. Oike, M. Murakami, H. Tanaka, S. Takada, Y. Seino and T. Kikkawa: Mater. Res. Soc. Symp. Proc. 812 (2004) F.6.2. 11) C. J. Mogab, A. C. Adams and D. L. Flamm: J. Appl. Phys. 49 (1978) 3796. 12) R. J. Holland and S. L. Bernasek: J. Appl. Phys. 60 (1986) 2553.
8875
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My IOPscience
Plasma Etch Rates of Porous Silica Low-k Films with Different Dielectric Constants
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2006 Jpn. J. Appl. Phys. 45 8873 (http://iopscience.iop.org/1347-4065/45/11R/8873) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 139.133.11.2 This content was downloaded on 22/05/2015 at 06:38
Please note that terms and conditions apply.
Japanese Journal of Applied Physics Vol. 45, No. 11, 2006, pp. 8873–8875 #2006 The Japan Society of Applied Physics
Plasma Etch Rates of Porous Silica Low-k Films with Different Dielectric Constants Tetsuo O NO, Hideki T AKAHASHI, Keizo K INOSHITA, Nobutoshi FUJII, Nobuhiro H ATA1 and Takamaro K IKKAWA1;2 MIRAI Project, Association of Super-Advanced Electronics Technologies (ASET), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan 1 MIRAI Project, Advanced Semiconductor Research Center, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan 2 Research Center for Nanodevices and Systems, Hiroshima University, 1-4-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8527, Japan (Received April 11, 2006; accepted August 3, 2006; published online November 8, 2006)
The reactive ion etch rates of porous silica films with different dielectric constants (k-values) or film densities were measured by varying wafer bias and gas ratio for Ar/C5 F8 /O2 plasma. Both the etch rates of porous silica films and the optical emission intensities from the etching products (SiF) increased with wafer bias power. Etch rate increased with decreasing k-value of porous silica, whereas SiF emission intensity was maintained constant regardless of k-value, indicating that the amount of etching products escaping from the porous silica surface to the gas phase remained unchanged. From this result it is concluded that mass etch rate, defined as the weight of a porous silica film etched from a unit area per unit time, is constant for Ar/C5 F8 / O2 plasma. [DOI: 10.1143/JJAP.45.8873] KEYWORDS: porous silica, low-k , plasma etching, density, SiF emission intensity
1.
Introduction
One of the key technologies for realizing high speed ultra large scale integrated circuits (ULSI) is Cu damascene interconnects with an ultra low-dielectric-constant (low-k) interlayer material. In fact, the dielectric constant of 2.1 is necessary for 45-nm-node interconnects. Porous dielectric materials can satisfy such a requirement because the introduction of pores reduces k-value. The dry etching of a low-k material to form vias and trenches for the Cu damascene induces two main problems: One is the plasmainduced damage in the low-k film and the other is the complexity in process optimization. Plasma-induced damage can be reduced using oxygen-free gas chemistry.1) In addition, a damage recovery method was proposed to restore both the hydrophobicity of the film and leakage current.2,3) For process optimization, it is important to determine the effect of pore introduction and material composition on etching characteristics, such as etch rate and selectivity. The etch rate of SiOCH was studied and a low hydrogen/carbon (H/C) ratio resulted in etch stop phenomena.4) The etch rate increase with porosity in an xerogel (SiOCH) film was reported.5) The effects of porosity on the diffusion of atoms into the bulk material and the coverage of a CF polymer on the surface of the film were also reported.5–8) The purpose of this work is to determine the etch rate dependence of porous silica films on their densities. Also, the intensity of optical emission from etch products (SiF) is measured and the relationship between etch rate and the amount of etch products escaping from the porous silica surface is clarified. 2.
Experimental Procedure
Nonperiodic porous silica films were spin-coated on 200 mm Si wafers. A precursor solution was prepared by adding both an nonionic surfactant (triblock copolymer) and an acidic silica sol derived from tetraethylorthosilicate (TEOS) and dimethyldiethoxysilane (DMDEOS) in ethanol diluted with water. k-value was controlled by the molar ratio of the surfactant and silica oligomer.9) The properties of the samples used in this experiment are listed in Table I. k-values were measured by capacitance-voltage analysis
Table I. k-values and densities of porous silica films. Porosity was calculated from density assuming that SiO2 density = 2.2 g cm3 . Sample
Relative surfactant ratio
k-value
Density (g cm3 )
Porosity
a
1
2.18
0.89
0.40
b
1/3
2.28
0.92
0.42
c
1/4.5
2.64
1.22
0.55
d
1/6
2.98
1.3
0.59
using an Hg probe in nitrogen atmosphere. Film density was determined by an X-ray reflection method. After the calcination of the spin-coated film it was annealed in 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) vapor to enhance mechanical strength and hydrophobicity.10) A UHF electron cyclotron resonance reactor was used for etching porous silica low-k films. A 450 MHz electromagnetic wave was introduced to produce plasma and a wafer bias power of 800 kHz was applied to accelerate ions towards the wafer. Magnetic coils were used to control the plasma density distribution in the reactor. Ar/C5 F8 /O2 chemistry was employed for etching and the pressure was 2.0 Pa. The selectivity for SiCN was compared with that for CF4 plasma. Etch rates were determined on the basis of the film thickness before and after etching. To estimate the amount of etch products in the plasma, the optical emission from SiF at 440 nm was measured using a photodiode array. 3.
Results and Discussion
The etch rate dependence on wafer bias power is shown in Fig. 1. The peak-to-peak voltage of wafer bias Vpp is also indicated in the figure. Since the motion of ions can follow the voltage change at 800 kHz, Vpp is a good measure of ion energy. Etch rate increased with wafer bias power because the increase in ion energy enhances the etching reaction. It is known that in a highly polymerized plasma or a Crich plasma (Cx Fy : x=y > 0:5), the etch rate of a porous lowk film is suppressed because ions cannot remove the polymer films that deposit on the subsurface of the pores and the etching reaction is avoided.7,8) The etch rate of a porous silica film is lower than that of SiO2 in C4 F6 plasma.7) In
8873
Jpn. J. Appl. Phys., Vol. 45, No. 11 (2006)
300
500
Vpp=940V k=2.18 2.28 750V 2.64
560V
Mass etch rate (x10-7 g cm-2 min-1 )
Etch rate (nm/min)
400
T. ONO et al.
2.98
200 370V
TEOS
100 200V
400
wafer bias 400 W
300
250 W 146 W
200
75 W
100 0
0 0
100 200 300 400 Wafer bias power (W)
0.5
500
Fig. 3. Mass etch rate dependence on density of porous silica film.
500 Mass etch rate (x10-7 g cm-2 min-1)
150
Wafer bias 400 W
Wafer bias 146 W 250 W
100
k=2.98 2.64 2.28 2.18 146 W
50 35 W
0 0
75 W
100
1.5
Density (g/cm )
200 k=2.28
1 3
Fig. 1. Etch rate dependence of porous silica films on wafer bias power.
SiF emission intensity (arb. unit)
35 W
O2 flow rate
400
0cc 20 cc 28 cc 32 cc 36 cc
300 200 100 0
200
300
0.5
400
1
1.5
Density (g/cm3 )
Etch Rate (nm/min)
Fig. 4. Mass etch rate dependence on O2 flow rate.
Fig. 2. Etch rate vs SiF emission intensity.
Mass etch rate (x10-7 g cm -2 min-1)
500
contrast, the etch rate of a porous silica film for Ar/C5 F8 /O2 plasma is higher than that of TEOS in spite of a high C ratio (C/F ¼ 0:625) as shown in Fig. 1. This is due to the O2 that can decompose the polymer films deposited in the pore. The overall etching reaction of SiO2 with Ar/C5 F8 /O2 plasma is given by11,12) 2SiO2 þ C5 F8 ! 2SiF4 þ 5COx¼1;2 :
9 cc 15 cc 21 cc
300 200 100 0
ð1Þ
SiF4 is decomposed in plasma into SiFx¼1;3 . Decomposition rate depends on electron temperature. Figure 2 shows the relationship between the etch rate and emission intensity of SiF. When k-value was kept constant and wafer bias was varied, SiF emission intensity increased with etch rate, indicating an increase in the amount of etching reaction products in the plasma. On the other hand, SiF emission intensity remained unchanged when etch rate increased with decreasing k-value at a wafer bias of 146 W. Thus, the amount of etching products escaping from the porous silica films was kept constant regardless of k-value. Correspondingly, mass etch rate, defined as the weight of a porous silica material etched from a unit surface area per unit time (g cm2 min1 ), was kept constant regardless of film density, as shown in Fig. 3. When the amount of pores introduced into a SiO2 film is increased, an apparent increase in etch rate occurs. However, the mass of the film etched per unit time does not change with porosity. This result is consistent with that in the case of F-rich plasma (CF4 or C3 F8 ).7) Figure 4 shows the effects of O2 addition on mass etch rate. Mass etch rate reached its maximum at an O2 flow rate of approximately 20 sccm. The O atoms decompose CF polymer on the porous silica surface during etching, thereby increasing etch rate. On the other hand, O atoms oxidize the
C5F8 flow rate
400
0.5
1
1.5 3
Density (g/cm ) Fig. 5. Mass etch rate dependence on C5 F8 flow rate.
surface and suppress the etching reaction. Thus, etch rate reaches its maximum value at an optimum amount of O2 . The amount of F radicals generated in CF4 plasma depends on the amount of O2 addition because the density of F radicals reaches its maximum value at a certain amount of O2 .11) This is another possible reason for the etch rate dependence on O2 addition. Mass etch rate increased with C5 F8 flow rate up to 20 sccm, as shown in Fig. 5, simply because of the increase in the amount of F radicals. As shown in Figs. 4 and 5, mass etch rates were almost constant with decreasing film density. The merit of Ar/C5 F8 /O2 chemistry is its high selectivity for SiCN films. The selectivity of a porous silica film for SiCN is 2.9-fold higher than that of CF4 (F-rich gas) plasma as shown in Table II. A SiCN film is often used as an etch stop layer for the dual damascene structure of low-k materials. Thus, high selectivity between porous silica and SiCN is necessary to etch the dual damascene structure without punching through the SiCN film. The disadvantage
8874
Jpn. J. Appl. Phys., Vol. 45, No. 11 (2006)
T. ONO et al.
Table II. Selectivity of porous silica for SiCN film. Wafer biases are 400 W for Ar 1000 sccm/C5 F8 15 sccm/O2 28 sccm 2 Pa plasma and 100 W for CF4 200 sccm 1 Pa plasma. Etch rate (nm/min)
Gas
Selectivity
Porous silica (k ¼ 2:18)
SiCN
Ar/C5 F8 /O2
364
26
CF4
389
80
14 4.9
of Ar/C5 F8 /O2 chemistry is the oxygen-induced damage in porous silica films. However, the plasma-induced damage can be suppressed by organosiloxane vapor annealing.2,3) 4.
Conclusions
The dependence of etch rate on low-k film density for Ar/ C5 F8 /O2 plasma was examined. The optical emission intensity of SiF indicated that the amount of etching products in the plasma is kept constant even though the etch rate of the porous films increased with decreasing density. This is because mass etch rate, defined as the weight of a porous silica material etched from a unit surface area per unit time, g cm2 min1 , was constant regardless of the change in film density.
1) N. Ohashi, K. Misawa, S. Sone, H. J. Shin, K. Inukai, E. Soda, S. Kondo, A. Furuya, H. Okamura, S. Ogawa and N. Kobayashi: IEDM Tech. Dig., 2003, p. 857. 2) Y. Oku, N. Fujii, Y. Seino, Y. Takasu, H. Takahashi, Y. Sonoda, T. Goto, H. Miyoshi, S. Takada and T. Kikkawa: Ext. Abstr. Solid State Devices and Materials, Tokyo, 2004, p. 46. 3) T. Ono, K. Kinoshita, T. Goto, H. Takahashi, N. Fujii, Y. Sonoda, Y. Oku, K. Kohmura, N. Hata and T. Kikkawa: Dry Process Symp., 2004, p. 229. 4) T. Tatsumi, K. Nagahata, K. Urata, T. Saito, M. Minami, Y. Nogami, S. Iseda and Y. Morita: Dry Process Symp., 2002, p. 9. 5) T. E. F. M. Standaert, E. A. Joseph, G. S. Oehrlein, A. Jain, W. N. Gill, P. C. Wayner, Jr. and J. L. Plawsky: J. Vac. Sci. Technol. A 18 (2000) 2742. 6) P. Lazzeri, X. Hua, G. S. Oehrlein, M. Barozzi, E. Iacob and M. Anderle: J. Vac. Sci. Technol. B 23 (2005) 1491. 7) Y. Morikawa, N. Mizutani, M. Ozawa, T. Hayashi, W. Chen and T. Uchida: J. Vac. Sci. Technol. B 21 (2003) 1344. 8) X. Hua, C. Stolz, G. S. Oehrlein, P. Lazzeri, N. Coghe, M. Anderle, C. K. Inoki, T. S. Kuan and P. Jiang: J. Vac. Sci. Technol. A 23 (2005) 151. 9) N. Fujii, K. Yamada, Y. Oku, N. Hata, Y. Seino, C. Negoro and T. Kikkawa: Mater. Res. Soc. Symp. Proc. 812 (2004) F.4.10. 10) K. Kohmura, S. Oike, M. Murakami, H. Tanaka, S. Takada, Y. Seino and T. Kikkawa: Mater. Res. Soc. Symp. Proc. 812 (2004) F.6.2. 11) C. J. Mogab, A. C. Adams and D. L. Flamm: J. Appl. Phys. 49 (1978) 3796. 12) R. J. Holland and S. L. Bernasek: J. Appl. Phys. 60 (1986) 2553.
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