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Ion energy distribution in ionized dc sputtering measured by an energy-resolved mass spectrometer
Vaccum 53 (1999) 21—24
Ion energy distribution in ionized dc sputtering measured by an energy-resolved mass spectrometer E. Kusano*, T. Kobayashi, N. Kashiwagi, T. Saitoh, S. Saiki, H. Nanto, A. Kinbara Kanazawa Institute of Technology, AMS R & D Center, 3-1 Yatsukaho, Matto, Ishikawa 9240838, Japan
Abstract Ion energy distribution of sputtered Ti particles in ionized dc sputtering has been measured by an energy-resolved mass spectrometer. The energies of the Ti> and Ar> ions were measured by a Balzers PPM421 plasma monitor. The experimental results showed that the energy of sputtered Ti particles was enhanced from 2 or 3 to about 30 eV as a coil rf power increased from 0 to 200 W, for a constant magnetron cathode current of 0.3 A. On the other hand, when a cathode current increased from 0.1A to 0.5 A for a constant coil rf power of 200 W, the mean energy of Ti> decreased, as a result of the plasma quenching caused by the increase in the number of the sputtered Ti particles. In addition, it was found that the number of Ti> ions was saturated for cathode dc currents higher than 0.3 A. The results obtained in this study demonstrate that the ratio of the coil rf power to the magnetron cathode power or current is crucial to obtain a proper ionization ratio and energy of sputtered particles. 1999 Elsevier Science Ltd. All rights reserved. PACS: 81.15C; 52.75R Keywords: Ionized physical vapor deposition; Sputtering; Titanium
1. Introduction A deposition technique to use a high-frequency inductively coupled coil in conjunction with a conventional sputtering source is attractive to enhance ionization probability of sputtered metal particles [1—3]. The ionization occurs in a high-density rf plasma with an electron density of more than 10 cm\ and is influenced by the number of metal atoms in the plasma [4,5]. As a result of a high ionization ratio, the directionality of the particle can be controlled by applying a bias potential to the substrate [6]. In addition, the ionization and consequent energization occurring when the ions passing through the plasma sheath region toward the grounded substrate increase the incident energy to the substrate. In this respect, it is needed to examine energy distribution of ions arriving to the substrate. The authors have previously investigated energy distribution of Ar> and Ti>
ions by an energy-resolved mass analyzer for an rf cathode discharge and reported that the mean energies of Ar> and Ti> ions increased as the rf coil power increased and that they decreased as the cathode rf power increased [7]. These behaviors were explained by considering the relationship between the power applied to the rf plasma and the number of sputtered Ti passing through the coil rf plasma. For the dc cathode discharge, the effects of coil rf plasma power and dc cathode discharge current or power on ion energy distribution are expected to be similar to those investigated for the rf cathode discharge. In actual applications, especially for metallization, the dc cathode discharge is more important than the rf discharge. In this study, effects of coil rf power and cathode dc current in ionized dc sputtering on ion energy spectra of Ar> and Ti> ions have been studied.
2. Experimental
*Corresponding author. Tel.: 0081 76 274 9257; fax: 0081 76 274 9251; e-mail: [email protected]
The sputtering apparatus used in this experiment was the ULVAC ultra-high vacuum sputtering system. The cylindrical vacuum chamber of the apparatus is 350 mm
0042-207X/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 4 1 4 - X
22
E. Kusano et al. / Vacuum 53 (1999) 21—24
in diameter and 440 mm in height. The magnetron cathode (55 mm dia.) is mounted on the bottom plate of the chamber. The target was Ti (99.98%). The cathode discharge was generated by applying a constant dc current. The rf coil plasma was generated by applying a 13.56MHz rf power to the 60 mm dia. coil that is equipped in front of the cathode with a distance of about 25 mm. The working gas is Ar (99.9999% purity) and injected into the cathode region. The Balzers PPM-421 plasma monitor is mounted vertically on the top of the chamber. The distance between the cathode surface and the orifice is 200 mm. The diameter of the orifice is 0.10 mm. The mass of the particles was determined by a mass/charge ratio. The total ion power, the flux of ions injected into the orifice in a unit time, and the mean energy (the total ion power/the flux of ions) were also obtained from ion energy spectra. The schematic drawing of the sputtering apparatus, the mass analyzer, and the cathode system was presented elsewhere [7].
3. Experimental results 3.1. Effects of the coil rf power Fig. 1 shows ion spectra of Ar> (a) and Ti> (b) as a function of the coil rf power. The spectra of Ar> without an rf coil discharge show a sharp peak at about 3 eV. When the coil rf plasma ignited, the spectra widely spread to a higher energy. As the coil rf power increases, the intensity of the Ar> peak increases and the position
Fig. 1. Energy spectra of Ar> (a) and Ti> (b) ions as a function of the coil rf power for a constant cathode current of 0.3 A.
of the peak shifts to a higher energy. The plateau was seen on the low-energy side in the Ar> ion energy distribution obtained for all rf coil powers. The Ti> spectra without the coil rf plasma show a widely spread distribution compared to those of Ar> for the dc discharge. As the coil rf power increases, the peak of Ti> spectra shifts to a higher energy. No shoulder peak is observed in the Ti> spectra at the energy where the peak is obtained for the dc discharge plasma. The number of detected ion flux (a), the total ion power (b), and the mean energy (c) of Ar> and Ti> are shown in Fig. 2. On increasing the coil rf power, the number and mean energies increase for both Ar> and Ti>, resulting in the increase in the total energies. While the number of Ar> is about two orders larger than that of Ti>, their mean energies are almost comparable for all coil powers. 3.2. Effects of cathode dc current Fig. 3 shows the energy spectra of Ar> (a) and Ti> (b) as a function of the cathode dc current for a fixed coil power of 200 W. The Ar> spectra obtained for a cathode current of 0.1 A spread to a higher energy and show a peak at about 41 eV. As the cathode dc current increases, the peak of Ar> spectra shifts to a lower energy. The shoulder observed at a few eV remains for the cathode dc current. The peak of Ti> ion spectra shifts to a lower energy and the intensity of the peak increases slightly with increasing the cathode dc current. The number of detected ion flux (a), the total ion power (b), and the mean energy (c) of Ar> and Ti> are shown as
Fig. 2. Number of detected ion flux (a), the total ion power (b), and the mean energy (c) of Ar> and Ti> ions as a function of the coil rf power for a constant cathode current of 0.3 A.
E. Kusano et al. / Vacuum 53 (1999) 21—24
23
ing cathode current. The Ti mass deposition rate and the number of Ti> (normalized for a value obtained at a cathode dc current of 0.1 A) are shown as a function of the cathode dc current in Fig. 5. While the mass deposition rate increases linearly as the cathode dc current increases, the number of Ti> saturates at a cathode dc current of 0.3 or 0.4 A.
4. Discussion
Fig. 3. Energy spectra of Ar> (a) and Ti> ions (b) as a function of the cathode dc current for a constant coil rf power of 200 W.
The ion energy distribution results demonstrate interesting features of incident Ar> and Ti> flux energies. The peak energy of 3 eV for the Ar> spectra without a coil rf plasma discharge is thought to be consistent with the plasma potential that was reported to be &5eV [8]. The plateau seen on the low-energy side in the Ar> ion flux energy distribution resulted from the ionization of Ar in the sheath region. The Ar> or Ti> ions ionized in the sheath region cannot obtain the energy equal to the plasma potential, producing a plateau in the low-energy side. The high-energy tail seen in the Ti> ion energy spectra is a result of the convolution of kinetic energy of sputtered Ti neutrals. The distribution of their kinetic energy is clearly shown in the Ti> ion energy spectra obtained for the dc cathode discharge. The high energy tail seen in the Ar> energy distribution might have originated from the kinetic energy of Ar neutrals. However, in the case of Ar>, most of the neutrals with high kinetic energies are produced by the reflection and neutralization of Ar> ions at the target surface. This is thought to be one reason for the difference in the energy distributions of Ar> and Ti> obtained for the dc discharge. The distribution of the ion energy spectra for both Ar> and Ti> is much different from those obtained for a combination of the rf-cathode and rf-coil discharges (both the power supplies were operated at 13.56 MHz with a slight shift in the phases); the peak at around 100 eV observed for the rf cathode discharge disappeared [7]. As a result of the difference in the distribution, the mean energy for
Fig. 4. Number of detected ion flux (a), the total ion power (b), and the mean energy (c) of Ar> and Ti> ions as a function of the cathode dc current for a constant coil rf power of 200 W.
a function of the cathode dc current in Fig. 4. As the cathode dc current increases, the number of Ti> ions increases while the number of Ar> decreases. In addition, the total ion power of Ar> decreases gradually with the cathode dc current while that of Ti> increases for a cathode dc current of 0.2 A and then remains almost constant. The mean ion energies decrease drastically with increas-
Fig. 5. Mass deposition rate and the number of Ti> ions as a function of the cathode dc current for a constant coil rf power of 200 W.
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E. Kusano et al. / Vacuum 53 (1999) 21—24
the rf cathode discharge is higher than that for the dc cathode discharge, on comparing the mean energies for the same coils rf power and cathode power. The mass deposition rate of Ti for the dc cathode discharge was found to be about 3 times larger than that sputtered at the same cathode power for the rf cathode discharge. The decrease in the mass deposition rate may be caused by the loss in the rf power in the matching network or the difference in the average ion acceleration voltage between rf and dc cathode discharges. Judging from the result that the mass deposition rate is about 3 times larger in the dc discharge case, the difference in the ion energy distribution is thought to be a result of the difference in the number of Ti atoms passing through the coil rf plasma region. The influence of the difference in Ti density in the plasma is well interpreted by considering the results shown in Fig. 4. The results shown in Fig. 4 also are because of the increase in the number of sputtered Ti. The effect of the increase in the number of sputtered Ti is explained by discussing the two-step ionization and Penning ionization. Because the excitation energies of Ar, 11.5 eV, is large compared to the ionization energy of Ti, 6.8 eV, the Penning ionization process can occur in the plasma, as well as the electron-impact ionization process. The probability of the Penning ionization of Ti atoms by excited Ar, Ti#Ar*PTi>#Ar#e\,
(1)
increases as the number of Ti atoms in the coil rf plasma increases. On the other hand, the probability of the two-step ionization process of Ar via excited atoms e\#Ar*P2e\#Ar>,
(2)
decreases, resulting in the decrease in the number of Ar> with the cathode dc current. As a result of the increase in ionization of Ti that has a lower ionization energy, the electron temperature ¹ of the plasma decreases, result ing in lowering the plasma potential. Thus, the average energy of Ar> and Ti> ion fluxes decreases with the cathode dc current. The results observed and discussed in this study well-agree with the mechanisms discussed by Hopwood and Qian that the ¹ decreases as the Al atom density in the plasma increases [5]. The saturation of the number of Ti> for the cathode dc current of 0.4 and 0.5 A demonstrates that the increase in the coil energy or in the
number of Ar* ionizing Ti atoms is needed to enhance the ion fraction of Ti in a high Ti flux region. The above discussion suggests that the Penning ionization plays an important role in the coil rf plasma compared with the electron-impact ionization. The results obtained in this study emphasize that the rf plasma assistance to the conventional sputter deposition using an inductive rf coil is an attractive method to enhance incident energy to the grounded substrate. The control of the incident energy will be helpful to improve film properties such as film crystallinity, density, and adhesion to the substrate.
5. Conclusions Ion energy distribution of Ar> and Ti> ions has been investigated in ionized dc sputtering as a function of the coil rf power or cathode dc current. The mean and the total energies of Ti> increased as the coil rf power increased. On the contrary, the mean energy of Ti> decreased as the cathode dc current increased while the total ion power of Ti> remained almost constant. For a constant number of Ti atoms passing through the coil plasma, i.e., for a constant cathode dc current, the increase in the coil rf power resulted directly in the increase in the energy of Ti> ions. However, the increase in the number of Ti atoms in the coil rf plasma reduced the mean energy for a constant coil rf power as a result of the quenching of the coil rf plasma.
Acknowledgement The financial support from the Ministry of Education, Science and Culture of Japan is highly appreciated.
References [1] [2] [3] [4] [5] [6] [7]
Yamashita M. Japan J Appl Phys 1987;26:721. Yamashita M. J Vac Sci Technol 1989;A7:151. Rossnagel SM, Hopwood J. Appl Phys Lett 1993;63:3285. Rossnagel SM, Hopwood J. J Vac Sci Tecnol 1994;B12:449. Hopwood J, Qian F. J Appl Phys 1995;78:758. Dickson M, Qian F, Hopwood J. J Vac Sci Technol A 1997;15:340. Kusano E, Kashiwagi N, Kobayashi T, Nanto H, Kinbara A. Surface & Coatings Technol 1998;108—109:177. [8] Rossnagel SM, Kaufman HR. J Vac Sci Technol 1986;A4:1822.
Ion energy distribution in ionized dc sputtering measured by an energy-resolved mass spectrometer E. Kusano*, T. Kobayashi, N. Kashiwagi, T. Saitoh, S. Saiki, H. Nanto, A. Kinbara Kanazawa Institute of Technology, AMS R & D Center, 3-1 Yatsukaho, Matto, Ishikawa 9240838, Japan
Abstract Ion energy distribution of sputtered Ti particles in ionized dc sputtering has been measured by an energy-resolved mass spectrometer. The energies of the Ti> and Ar> ions were measured by a Balzers PPM421 plasma monitor. The experimental results showed that the energy of sputtered Ti particles was enhanced from 2 or 3 to about 30 eV as a coil rf power increased from 0 to 200 W, for a constant magnetron cathode current of 0.3 A. On the other hand, when a cathode current increased from 0.1A to 0.5 A for a constant coil rf power of 200 W, the mean energy of Ti> decreased, as a result of the plasma quenching caused by the increase in the number of the sputtered Ti particles. In addition, it was found that the number of Ti> ions was saturated for cathode dc currents higher than 0.3 A. The results obtained in this study demonstrate that the ratio of the coil rf power to the magnetron cathode power or current is crucial to obtain a proper ionization ratio and energy of sputtered particles. 1999 Elsevier Science Ltd. All rights reserved. PACS: 81.15C; 52.75R Keywords: Ionized physical vapor deposition; Sputtering; Titanium
1. Introduction A deposition technique to use a high-frequency inductively coupled coil in conjunction with a conventional sputtering source is attractive to enhance ionization probability of sputtered metal particles [1—3]. The ionization occurs in a high-density rf plasma with an electron density of more than 10 cm\ and is influenced by the number of metal atoms in the plasma [4,5]. As a result of a high ionization ratio, the directionality of the particle can be controlled by applying a bias potential to the substrate [6]. In addition, the ionization and consequent energization occurring when the ions passing through the plasma sheath region toward the grounded substrate increase the incident energy to the substrate. In this respect, it is needed to examine energy distribution of ions arriving to the substrate. The authors have previously investigated energy distribution of Ar> and Ti>
ions by an energy-resolved mass analyzer for an rf cathode discharge and reported that the mean energies of Ar> and Ti> ions increased as the rf coil power increased and that they decreased as the cathode rf power increased [7]. These behaviors were explained by considering the relationship between the power applied to the rf plasma and the number of sputtered Ti passing through the coil rf plasma. For the dc cathode discharge, the effects of coil rf plasma power and dc cathode discharge current or power on ion energy distribution are expected to be similar to those investigated for the rf cathode discharge. In actual applications, especially for metallization, the dc cathode discharge is more important than the rf discharge. In this study, effects of coil rf power and cathode dc current in ionized dc sputtering on ion energy spectra of Ar> and Ti> ions have been studied.
2. Experimental
*Corresponding author. Tel.: 0081 76 274 9257; fax: 0081 76 274 9251; e-mail: [email protected]
The sputtering apparatus used in this experiment was the ULVAC ultra-high vacuum sputtering system. The cylindrical vacuum chamber of the apparatus is 350 mm
0042-207X/99/$ — see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 9 8 ) 0 0 4 1 4 - X
22
E. Kusano et al. / Vacuum 53 (1999) 21—24
in diameter and 440 mm in height. The magnetron cathode (55 mm dia.) is mounted on the bottom plate of the chamber. The target was Ti (99.98%). The cathode discharge was generated by applying a constant dc current. The rf coil plasma was generated by applying a 13.56MHz rf power to the 60 mm dia. coil that is equipped in front of the cathode with a distance of about 25 mm. The working gas is Ar (99.9999% purity) and injected into the cathode region. The Balzers PPM-421 plasma monitor is mounted vertically on the top of the chamber. The distance between the cathode surface and the orifice is 200 mm. The diameter of the orifice is 0.10 mm. The mass of the particles was determined by a mass/charge ratio. The total ion power, the flux of ions injected into the orifice in a unit time, and the mean energy (the total ion power/the flux of ions) were also obtained from ion energy spectra. The schematic drawing of the sputtering apparatus, the mass analyzer, and the cathode system was presented elsewhere [7].
3. Experimental results 3.1. Effects of the coil rf power Fig. 1 shows ion spectra of Ar> (a) and Ti> (b) as a function of the coil rf power. The spectra of Ar> without an rf coil discharge show a sharp peak at about 3 eV. When the coil rf plasma ignited, the spectra widely spread to a higher energy. As the coil rf power increases, the intensity of the Ar> peak increases and the position
Fig. 1. Energy spectra of Ar> (a) and Ti> (b) ions as a function of the coil rf power for a constant cathode current of 0.3 A.
of the peak shifts to a higher energy. The plateau was seen on the low-energy side in the Ar> ion energy distribution obtained for all rf coil powers. The Ti> spectra without the coil rf plasma show a widely spread distribution compared to those of Ar> for the dc discharge. As the coil rf power increases, the peak of Ti> spectra shifts to a higher energy. No shoulder peak is observed in the Ti> spectra at the energy where the peak is obtained for the dc discharge plasma. The number of detected ion flux (a), the total ion power (b), and the mean energy (c) of Ar> and Ti> are shown in Fig. 2. On increasing the coil rf power, the number and mean energies increase for both Ar> and Ti>, resulting in the increase in the total energies. While the number of Ar> is about two orders larger than that of Ti>, their mean energies are almost comparable for all coil powers. 3.2. Effects of cathode dc current Fig. 3 shows the energy spectra of Ar> (a) and Ti> (b) as a function of the cathode dc current for a fixed coil power of 200 W. The Ar> spectra obtained for a cathode current of 0.1 A spread to a higher energy and show a peak at about 41 eV. As the cathode dc current increases, the peak of Ar> spectra shifts to a lower energy. The shoulder observed at a few eV remains for the cathode dc current. The peak of Ti> ion spectra shifts to a lower energy and the intensity of the peak increases slightly with increasing the cathode dc current. The number of detected ion flux (a), the total ion power (b), and the mean energy (c) of Ar> and Ti> are shown as
Fig. 2. Number of detected ion flux (a), the total ion power (b), and the mean energy (c) of Ar> and Ti> ions as a function of the coil rf power for a constant cathode current of 0.3 A.
E. Kusano et al. / Vacuum 53 (1999) 21—24
23
ing cathode current. The Ti mass deposition rate and the number of Ti> (normalized for a value obtained at a cathode dc current of 0.1 A) are shown as a function of the cathode dc current in Fig. 5. While the mass deposition rate increases linearly as the cathode dc current increases, the number of Ti> saturates at a cathode dc current of 0.3 or 0.4 A.
4. Discussion
Fig. 3. Energy spectra of Ar> (a) and Ti> ions (b) as a function of the cathode dc current for a constant coil rf power of 200 W.
The ion energy distribution results demonstrate interesting features of incident Ar> and Ti> flux energies. The peak energy of 3 eV for the Ar> spectra without a coil rf plasma discharge is thought to be consistent with the plasma potential that was reported to be &5eV [8]. The plateau seen on the low-energy side in the Ar> ion flux energy distribution resulted from the ionization of Ar in the sheath region. The Ar> or Ti> ions ionized in the sheath region cannot obtain the energy equal to the plasma potential, producing a plateau in the low-energy side. The high-energy tail seen in the Ti> ion energy spectra is a result of the convolution of kinetic energy of sputtered Ti neutrals. The distribution of their kinetic energy is clearly shown in the Ti> ion energy spectra obtained for the dc cathode discharge. The high energy tail seen in the Ar> energy distribution might have originated from the kinetic energy of Ar neutrals. However, in the case of Ar>, most of the neutrals with high kinetic energies are produced by the reflection and neutralization of Ar> ions at the target surface. This is thought to be one reason for the difference in the energy distributions of Ar> and Ti> obtained for the dc discharge. The distribution of the ion energy spectra for both Ar> and Ti> is much different from those obtained for a combination of the rf-cathode and rf-coil discharges (both the power supplies were operated at 13.56 MHz with a slight shift in the phases); the peak at around 100 eV observed for the rf cathode discharge disappeared [7]. As a result of the difference in the distribution, the mean energy for
Fig. 4. Number of detected ion flux (a), the total ion power (b), and the mean energy (c) of Ar> and Ti> ions as a function of the cathode dc current for a constant coil rf power of 200 W.
a function of the cathode dc current in Fig. 4. As the cathode dc current increases, the number of Ti> ions increases while the number of Ar> decreases. In addition, the total ion power of Ar> decreases gradually with the cathode dc current while that of Ti> increases for a cathode dc current of 0.2 A and then remains almost constant. The mean ion energies decrease drastically with increas-
Fig. 5. Mass deposition rate and the number of Ti> ions as a function of the cathode dc current for a constant coil rf power of 200 W.
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E. Kusano et al. / Vacuum 53 (1999) 21—24
the rf cathode discharge is higher than that for the dc cathode discharge, on comparing the mean energies for the same coils rf power and cathode power. The mass deposition rate of Ti for the dc cathode discharge was found to be about 3 times larger than that sputtered at the same cathode power for the rf cathode discharge. The decrease in the mass deposition rate may be caused by the loss in the rf power in the matching network or the difference in the average ion acceleration voltage between rf and dc cathode discharges. Judging from the result that the mass deposition rate is about 3 times larger in the dc discharge case, the difference in the ion energy distribution is thought to be a result of the difference in the number of Ti atoms passing through the coil rf plasma region. The influence of the difference in Ti density in the plasma is well interpreted by considering the results shown in Fig. 4. The results shown in Fig. 4 also are because of the increase in the number of sputtered Ti. The effect of the increase in the number of sputtered Ti is explained by discussing the two-step ionization and Penning ionization. Because the excitation energies of Ar, 11.5 eV, is large compared to the ionization energy of Ti, 6.8 eV, the Penning ionization process can occur in the plasma, as well as the electron-impact ionization process. The probability of the Penning ionization of Ti atoms by excited Ar, Ti#Ar*PTi>#Ar#e\,
(1)
increases as the number of Ti atoms in the coil rf plasma increases. On the other hand, the probability of the two-step ionization process of Ar via excited atoms e\#Ar*P2e\#Ar>,
(2)
decreases, resulting in the decrease in the number of Ar> with the cathode dc current. As a result of the increase in ionization of Ti that has a lower ionization energy, the electron temperature ¹ of the plasma decreases, result ing in lowering the plasma potential. Thus, the average energy of Ar> and Ti> ion fluxes decreases with the cathode dc current. The results observed and discussed in this study well-agree with the mechanisms discussed by Hopwood and Qian that the ¹ decreases as the Al atom density in the plasma increases [5]. The saturation of the number of Ti> for the cathode dc current of 0.4 and 0.5 A demonstrates that the increase in the coil energy or in the
number of Ar* ionizing Ti atoms is needed to enhance the ion fraction of Ti in a high Ti flux region. The above discussion suggests that the Penning ionization plays an important role in the coil rf plasma compared with the electron-impact ionization. The results obtained in this study emphasize that the rf plasma assistance to the conventional sputter deposition using an inductive rf coil is an attractive method to enhance incident energy to the grounded substrate. The control of the incident energy will be helpful to improve film properties such as film crystallinity, density, and adhesion to the substrate.
5. Conclusions Ion energy distribution of Ar> and Ti> ions has been investigated in ionized dc sputtering as a function of the coil rf power or cathode dc current. The mean and the total energies of Ti> increased as the coil rf power increased. On the contrary, the mean energy of Ti> decreased as the cathode dc current increased while the total ion power of Ti> remained almost constant. For a constant number of Ti atoms passing through the coil plasma, i.e., for a constant cathode dc current, the increase in the coil rf power resulted directly in the increase in the energy of Ti> ions. However, the increase in the number of Ti atoms in the coil rf plasma reduced the mean energy for a constant coil rf power as a result of the quenching of the coil rf plasma.
Acknowledgement The financial support from the Ministry of Education, Science and Culture of Japan is highly appreciated.
References [1] [2] [3] [4] [5] [6] [7]
Yamashita M. Japan J Appl Phys 1987;26:721. Yamashita M. J Vac Sci Technol 1989;A7:151. Rossnagel SM, Hopwood J. Appl Phys Lett 1993;63:3285. Rossnagel SM, Hopwood J. J Vac Sci Tecnol 1994;B12:449. Hopwood J, Qian F. J Appl Phys 1995;78:758. Dickson M, Qian F, Hopwood J. J Vac Sci Technol A 1997;15:340. Kusano E, Kashiwagi N, Kobayashi T, Nanto H, Kinbara A. Surface & Coatings Technol 1998;108—109:177. [8] Rossnagel SM, Kaufman HR. J Vac Sci Technol 1986;A4:1822.