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Micowave plasma-assisted ionization of sputtered aluminum atoms in DC magnetron sputtering
ARTICLE IN PRESS
Vacuum 80 (2006) 671–674 www.elsevier.com/locate/vacuum
Micowave plasma-assisted ionization of sputtered aluminum atoms in DC magnetron sputtering Akira Yonesu, Suguru Watashi, Mituaki Yoshimi, Yasumasa Yamashiro Faculty of Engineering, Ryukyu University, 1, Senbaru, Nishihara 903-0213, Japan
Abstract DC magnetron sputtering was carried out using a microwave plasma to enhance the ionization of sputtered aluminum atoms at a low gas pressure. The ionization fraction of sputtered aluminum atoms measured using a gridded thickness monitor was 40% at a low gas pressure of 0.05 Pa. The collision frequency for the ionization of sputtered aluminum atoms calculated theoretically under the assumption of electron-impact ionization was in good agreement with the experimental results. r 2005 Elsevier Ltd. All rights reserved. Keywords: Ionized physical vapor deposition; Microwave plasma; Low-pressure sputtering; DC magnetron sputtering; Ionization fraction of sputtered atoms; Electron-impact ionization
1. Introduction Ionized physical vapor deposition (IPVD) is a process in which atoms sputtered from a target are ionized by a secondary plasma before depositing onto a substrate. The directionality and kinetic energy of the ionized atoms can be controlled by applying a bias potential to the substrate. A typical IPVD system consists of a DC magnetron cathode and an inductively coupled plasma (ICP) between the target and the substrate [1–4]. The lower gas pressure operation in IPVD reduces the collision frequency between sputtered atoms and background gas atoms, thereby increasing the controllability of kinetic energy and directionality of the ionized sputtered atoms by means of the substrate bias potential. However, neither magnetron discharge nor ICP discharge can be achieved below the gas pressure of 0.1 Pa. In order to achieve high sputter deposition rates at low gas pressures, we have developed a new DC magnetron sputtering apparatus that uses a microwave plasma [5,6]. In general, microwave discharges under the electron cyclotron resonance (ECR) condition produce higher electron temperatures and plasma densities at lower gas pressures Corresponding author. Tel.: +81 98 895 8692; fax: +81 98 895 8708.
E-mail address: [email protected] (A. Yonesu). 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.023
(o0.1 Pa) as compared with other plasma generation techniques. Therefore, IPVD can be realized at low gas pressures with DC magnetron sputtering using a microwave plasma. In the present study, the ionization fraction of aluminum atoms sputtered onto the substrate is estimated during DC magnetron sputtering using a microwave plasma. The ionization mechanism of the sputtered aluminum atoms is also discussed.
2. Experimental apparatus and method Fig. 1 shows the schematic diagram of the experimental apparatus. The conventional configuration of a cylindrical multipolar magnetron was employed in this system. However, the magnetic flux density of the magnets placed in the cathode should be 3800 G, which is higher than that of the magnets used in the conventional magnetron system. Such strong magnets are required to form a resonant magnetic surface of 875 G at a distance of 10 mm from the target surface for the ECR absorption of microwaves. When microwaves are introduced into the chamber through a tapered waveguide, a microwave plasma is formed around the DC magnetron plasma. Sputtering deposition is carried out in this apparatus after the generation of the microwave plasma by applying a negative
ARTICLE IN PRESS A. Yonesu et al. / Vacuum 80 (2006) 671–674
672 Gas
Microwave Power 0.36 kW Target Voltage -350V
Gridded Thickness Monitor
DC Bias
Magnets
Pump
Microwaves
Deposition Rate [nm/min]
Magnetron Target
6
Deposition Rate 200
Target Current
4
100
Target Current [A]
300
2
Fig. 1. Schematic diagram of experimental setup.
3. Results and discussion 3.1. Deposition rate First, the deposition rate in the apparatus was measured using the thickness monitor without the grid. It was located at a distance of 80 mm from the target surface. Fig. 2 shows the deposition rate and target current as a function of gas pressure. The input microwave power was 0.36 kW, and the target voltage was 350 V. Using the microwave plasma, the DC magnetron discharge was achieved at pressures of the order of 102 Pa, and the minimum gas pressure for sustaining the magnetron sputtering was as low as 0.025 Pa. Moreover, the deposition rate at the minimum gas pressure was greater than 80 nm/min.
0
0.05
0.1 Pressure [Pa]
0.5
0
Fig. 2. Deposition rate as a function of gas pressure. Here, the microwave power was 0.36 kW and target voltage was 350 V.
100 Film Thickness [nm]
DC voltage to the target cathode. The target material used in this study was aluminum. The retarding potential method and a gridded thickness monitor were employed to estimate the ionization fraction of sputtered atoms incident onto the substrate. Two stainless grids of 300 meshes were used to isolate the detector from the plasma and admit or repel the ionized sputtered atoms. Electric probe measurements were performed to determine the electron density and electron temperature. The probe lead was insulated by a ceramic tube. A hollow gap of 0.5 mm between the electrode and the ceramic tube avoided any increase in the collection area due to the possible deposition of an additional conductive layer on the insulating part of the probes. Ar gas was used as the discharge gas.
50
Gas Pressure 0.05 Pa Microwave Power 0.36 kW Target Voltage -200 V
0
20
40 60 Grid Voltage [V]
80
100
Fig. 3. Deposition rate as a function of grid voltage. Here, the microwave power was 0.36 kW, gas pressure was 0.05 Pa and target voltage was 200 V.
3.2. Ionization fraction In order to estimate the ionization fraction of sputtered aluminum atoms, we measured the deposition rate using the gridded thickness monitor. Fig. 3 shows the dependence of the deposition rate on grid voltage. The gas pressure was 0.05 Pa; input microwave power, 0.36 kW; and target voltage, 200 V. Below 70 V, the deposition rate decreased with an increase in the grid voltage. At 70 V and above, the deposition rate was almost constant. This result
indicates that at voltages of 70 V and above, all the ionized sputtered aluminum atoms were repelled by the biased grid and only neutral aluminum atoms were deposited. Thus, the ionization fraction of sputtered aluminum atoms was estimated by comparing the deposition rates at the grid voltages of 0 and 100 V. In this case, the ionization fraction was 35%. Similarly, the ionization fraction of sputtered aluminum atoms was measured as a function of the input microwave power at various gas pressures. The result is
ARTICLE IN PRESS A. Yonesu et al. / Vacuum 80 (2006) 671–674
673
10
40
30
20
8
6
1
4 Electron Density
0.5
Electron Temperature 10
0.05 Pa
2
Gas Pressure 0.05 Pa
0.13 Pa
0
0
0.1
0.2 0.3 Microwave power [kW]
0.4
Electron Temperature [eV]
Electron Density [× 1011cm-3]
Ionization Fraction [%]
1.5
0.5
0
0.1
0.2 0.3 0.4 Microwave Power [kW]
0.5
0
Fig. 5. Microwave power dependence of electron density and electron temperature at gas pressure of 0.05 Pa.
Fig. 4. Ionization fraction of sputtered aluminum atoms as a function of microwave power for gas pressures of 0.05 and 0.13 Pa. Target voltage was 200 V.
3.3. Discussion on the ionization mechanism There are two possible mechanisms by which to ionize the sputtered atoms in the plasma: electron impact ionization and Penning ionization. Penning ionization is considered to be the dominant ionization mechanism in DC magnetron sputtering assisted by RF plasma [8]. The probability of Penning ionization increases with gas pressure due to an increase in the probability of collisions between sputtered metal atoms and excited Ar atoms. However, in this study, the ionization fraction of sputtered atoms increased with a decrease in gas pressure. Therefore, the dominant ionization mechanism in this study seems to be electron impact ionization. In order to understand the ionization mechanism, we measured the electron temperature and electron density in the ECR region (at a distance of 10 mm from the target surface) by employing the Langmuir probe method. The electrons in the ECR region were directly heated by the injected microwaves. These resonant electrons were confined in local mirror fields near the target surface. The gas pressure was 0.05 Pa, and the target voltage was 200 V. The result is shown in Fig. 5. A high electron density and temperature, which are the
1 Collision frequency [× 104 Hz]
shown in Fig. 4. It is observed that the ionization fraction increases with the input microwave power at all pressures. It is also shown that the ionization fraction is high at the low gas pressure of 0.05 Pa. The maximum ionization fraction measured was approximately 40% at the gas pressure of 0.05 Pa and an input microwave power of 0.47 kW, which is higher than the maximum value achieved by the conventional DC magnetron sputtering [7].
0.5
Gas Pressure 0.05 Pa
0
0
0.1
0.2 0.3 Microwave Power [kW]
0.4
0.5
Fig. 6. Calculated collision frequency for ionization of sputtered aluminum atoms as a function of microwave power at gas pressure of 0.05 Pa.
characteristics of ECR microwave plasmas, were achieved at the low gas pressure of 0.05 Pa. We calculated the collision frequency for the ionization of sputtered aluminum atoms in the ECR region under the assumption of resonant-electron-impact ionization. The collision frequency for electron-impact ionization is given by v ¼ ne hsve i, where ne is the electron density, s the cross section between an aluminum atom and an electron for impact ionization, and ve the electron velocity. Brackets indicate that the value
ARTICLE IN PRESS 674
A. Yonesu et al. / Vacuum 80 (2006) 671–674
is averaged over a Maxwellian electron energy distribution. An empirical cross section for ionization reported by Lotz [9] and the electron temperature and density shown in Fig. 5 were considered. The calculated collision frequency as a function of the microwave power is shown in Fig. 6. This microwave power dependence of collision frequency is similar to that of the ionization fraction obtained experimentally, as shown in Fig. 4. Therefore, it is concluded that aluminum atoms sputtered from the target surface are ionized by electron impact in the microwave plasma near the target. 4. Conclusion We measured the ionization fraction of sputtered aluminum atoms using a DC magnetron sputtering apparatus that generates a microwave ECR plasma by means of the retarding potential method. Its value was 40% at the low gas pressure of 0.05 Pa. Moreover, it was observed that the ionization fraction increased with the input microwave power and with a decrease in gas
pressure. We calculated the collision frequency for the ionization of sputtered aluminum atoms under the assumption that they were ionized by electron impact in the microwave plasma, which has a high electron temperature and density. The calculated value was in good agreement with the experimental results. Reference [1] Rossangel SM, Hopwood J. J Vac Sci Technol B 1994;12:449. [2] Kusano Y, Chiristou C, Barber ZH, Events JE, Huchings IM. Thin Solid Films 1999;355:117. [3] Nouvellon C, Konstantinidis S, Dauchot JP, Wautelet M, Jouan PY, Ricard A, et al. J Appl Phys 2002;92(1):32. [4] Nikiforov SA, Urm KW, Kim GH, Rim GH, Lee SH. Surf Coat Technol 2003;171:106. [5] Yonesu A, Kato T, Takemoto H, Nishimura N, Yamashiro Y. Jpn J Appl Phys 1999;38:4326. [6] Yonesu A, Takemoto H, Hirata M, Yamashiro Y. Vacuum 2002;66:275. [7] Christou C, Barber ZH. J Vac Sci Technol A 2000;18:2987. [8] Hopwood J, Qian F. J Appl Phys 1995;78:758. [9] Lotz W. Z Phys 1968;216:241.
Vacuum 80 (2006) 671–674 www.elsevier.com/locate/vacuum
Micowave plasma-assisted ionization of sputtered aluminum atoms in DC magnetron sputtering Akira Yonesu, Suguru Watashi, Mituaki Yoshimi, Yasumasa Yamashiro Faculty of Engineering, Ryukyu University, 1, Senbaru, Nishihara 903-0213, Japan
Abstract DC magnetron sputtering was carried out using a microwave plasma to enhance the ionization of sputtered aluminum atoms at a low gas pressure. The ionization fraction of sputtered aluminum atoms measured using a gridded thickness monitor was 40% at a low gas pressure of 0.05 Pa. The collision frequency for the ionization of sputtered aluminum atoms calculated theoretically under the assumption of electron-impact ionization was in good agreement with the experimental results. r 2005 Elsevier Ltd. All rights reserved. Keywords: Ionized physical vapor deposition; Microwave plasma; Low-pressure sputtering; DC magnetron sputtering; Ionization fraction of sputtered atoms; Electron-impact ionization
1. Introduction Ionized physical vapor deposition (IPVD) is a process in which atoms sputtered from a target are ionized by a secondary plasma before depositing onto a substrate. The directionality and kinetic energy of the ionized atoms can be controlled by applying a bias potential to the substrate. A typical IPVD system consists of a DC magnetron cathode and an inductively coupled plasma (ICP) between the target and the substrate [1–4]. The lower gas pressure operation in IPVD reduces the collision frequency between sputtered atoms and background gas atoms, thereby increasing the controllability of kinetic energy and directionality of the ionized sputtered atoms by means of the substrate bias potential. However, neither magnetron discharge nor ICP discharge can be achieved below the gas pressure of 0.1 Pa. In order to achieve high sputter deposition rates at low gas pressures, we have developed a new DC magnetron sputtering apparatus that uses a microwave plasma [5,6]. In general, microwave discharges under the electron cyclotron resonance (ECR) condition produce higher electron temperatures and plasma densities at lower gas pressures Corresponding author. Tel.: +81 98 895 8692; fax: +81 98 895 8708.
E-mail address: [email protected] (A. Yonesu). 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.023
(o0.1 Pa) as compared with other plasma generation techniques. Therefore, IPVD can be realized at low gas pressures with DC magnetron sputtering using a microwave plasma. In the present study, the ionization fraction of aluminum atoms sputtered onto the substrate is estimated during DC magnetron sputtering using a microwave plasma. The ionization mechanism of the sputtered aluminum atoms is also discussed.
2. Experimental apparatus and method Fig. 1 shows the schematic diagram of the experimental apparatus. The conventional configuration of a cylindrical multipolar magnetron was employed in this system. However, the magnetic flux density of the magnets placed in the cathode should be 3800 G, which is higher than that of the magnets used in the conventional magnetron system. Such strong magnets are required to form a resonant magnetic surface of 875 G at a distance of 10 mm from the target surface for the ECR absorption of microwaves. When microwaves are introduced into the chamber through a tapered waveguide, a microwave plasma is formed around the DC magnetron plasma. Sputtering deposition is carried out in this apparatus after the generation of the microwave plasma by applying a negative
ARTICLE IN PRESS A. Yonesu et al. / Vacuum 80 (2006) 671–674
672 Gas
Microwave Power 0.36 kW Target Voltage -350V
Gridded Thickness Monitor
DC Bias
Magnets
Pump
Microwaves
Deposition Rate [nm/min]
Magnetron Target
6
Deposition Rate 200
Target Current
4
100
Target Current [A]
300
2
Fig. 1. Schematic diagram of experimental setup.
3. Results and discussion 3.1. Deposition rate First, the deposition rate in the apparatus was measured using the thickness monitor without the grid. It was located at a distance of 80 mm from the target surface. Fig. 2 shows the deposition rate and target current as a function of gas pressure. The input microwave power was 0.36 kW, and the target voltage was 350 V. Using the microwave plasma, the DC magnetron discharge was achieved at pressures of the order of 102 Pa, and the minimum gas pressure for sustaining the magnetron sputtering was as low as 0.025 Pa. Moreover, the deposition rate at the minimum gas pressure was greater than 80 nm/min.
0
0.05
0.1 Pressure [Pa]
0.5
0
Fig. 2. Deposition rate as a function of gas pressure. Here, the microwave power was 0.36 kW and target voltage was 350 V.
100 Film Thickness [nm]
DC voltage to the target cathode. The target material used in this study was aluminum. The retarding potential method and a gridded thickness monitor were employed to estimate the ionization fraction of sputtered atoms incident onto the substrate. Two stainless grids of 300 meshes were used to isolate the detector from the plasma and admit or repel the ionized sputtered atoms. Electric probe measurements were performed to determine the electron density and electron temperature. The probe lead was insulated by a ceramic tube. A hollow gap of 0.5 mm between the electrode and the ceramic tube avoided any increase in the collection area due to the possible deposition of an additional conductive layer on the insulating part of the probes. Ar gas was used as the discharge gas.
50
Gas Pressure 0.05 Pa Microwave Power 0.36 kW Target Voltage -200 V
0
20
40 60 Grid Voltage [V]
80
100
Fig. 3. Deposition rate as a function of grid voltage. Here, the microwave power was 0.36 kW, gas pressure was 0.05 Pa and target voltage was 200 V.
3.2. Ionization fraction In order to estimate the ionization fraction of sputtered aluminum atoms, we measured the deposition rate using the gridded thickness monitor. Fig. 3 shows the dependence of the deposition rate on grid voltage. The gas pressure was 0.05 Pa; input microwave power, 0.36 kW; and target voltage, 200 V. Below 70 V, the deposition rate decreased with an increase in the grid voltage. At 70 V and above, the deposition rate was almost constant. This result
indicates that at voltages of 70 V and above, all the ionized sputtered aluminum atoms were repelled by the biased grid and only neutral aluminum atoms were deposited. Thus, the ionization fraction of sputtered aluminum atoms was estimated by comparing the deposition rates at the grid voltages of 0 and 100 V. In this case, the ionization fraction was 35%. Similarly, the ionization fraction of sputtered aluminum atoms was measured as a function of the input microwave power at various gas pressures. The result is
ARTICLE IN PRESS A. Yonesu et al. / Vacuum 80 (2006) 671–674
673
10
40
30
20
8
6
1
4 Electron Density
0.5
Electron Temperature 10
0.05 Pa
2
Gas Pressure 0.05 Pa
0.13 Pa
0
0
0.1
0.2 0.3 Microwave power [kW]
0.4
Electron Temperature [eV]
Electron Density [× 1011cm-3]
Ionization Fraction [%]
1.5
0.5
0
0.1
0.2 0.3 0.4 Microwave Power [kW]
0.5
0
Fig. 5. Microwave power dependence of electron density and electron temperature at gas pressure of 0.05 Pa.
Fig. 4. Ionization fraction of sputtered aluminum atoms as a function of microwave power for gas pressures of 0.05 and 0.13 Pa. Target voltage was 200 V.
3.3. Discussion on the ionization mechanism There are two possible mechanisms by which to ionize the sputtered atoms in the plasma: electron impact ionization and Penning ionization. Penning ionization is considered to be the dominant ionization mechanism in DC magnetron sputtering assisted by RF plasma [8]. The probability of Penning ionization increases with gas pressure due to an increase in the probability of collisions between sputtered metal atoms and excited Ar atoms. However, in this study, the ionization fraction of sputtered atoms increased with a decrease in gas pressure. Therefore, the dominant ionization mechanism in this study seems to be electron impact ionization. In order to understand the ionization mechanism, we measured the electron temperature and electron density in the ECR region (at a distance of 10 mm from the target surface) by employing the Langmuir probe method. The electrons in the ECR region were directly heated by the injected microwaves. These resonant electrons were confined in local mirror fields near the target surface. The gas pressure was 0.05 Pa, and the target voltage was 200 V. The result is shown in Fig. 5. A high electron density and temperature, which are the
1 Collision frequency [× 104 Hz]
shown in Fig. 4. It is observed that the ionization fraction increases with the input microwave power at all pressures. It is also shown that the ionization fraction is high at the low gas pressure of 0.05 Pa. The maximum ionization fraction measured was approximately 40% at the gas pressure of 0.05 Pa and an input microwave power of 0.47 kW, which is higher than the maximum value achieved by the conventional DC magnetron sputtering [7].
0.5
Gas Pressure 0.05 Pa
0
0
0.1
0.2 0.3 Microwave Power [kW]
0.4
0.5
Fig. 6. Calculated collision frequency for ionization of sputtered aluminum atoms as a function of microwave power at gas pressure of 0.05 Pa.
characteristics of ECR microwave plasmas, were achieved at the low gas pressure of 0.05 Pa. We calculated the collision frequency for the ionization of sputtered aluminum atoms in the ECR region under the assumption of resonant-electron-impact ionization. The collision frequency for electron-impact ionization is given by v ¼ ne hsve i, where ne is the electron density, s the cross section between an aluminum atom and an electron for impact ionization, and ve the electron velocity. Brackets indicate that the value
ARTICLE IN PRESS 674
A. Yonesu et al. / Vacuum 80 (2006) 671–674
is averaged over a Maxwellian electron energy distribution. An empirical cross section for ionization reported by Lotz [9] and the electron temperature and density shown in Fig. 5 were considered. The calculated collision frequency as a function of the microwave power is shown in Fig. 6. This microwave power dependence of collision frequency is similar to that of the ionization fraction obtained experimentally, as shown in Fig. 4. Therefore, it is concluded that aluminum atoms sputtered from the target surface are ionized by electron impact in the microwave plasma near the target. 4. Conclusion We measured the ionization fraction of sputtered aluminum atoms using a DC magnetron sputtering apparatus that generates a microwave ECR plasma by means of the retarding potential method. Its value was 40% at the low gas pressure of 0.05 Pa. Moreover, it was observed that the ionization fraction increased with the input microwave power and with a decrease in gas
pressure. We calculated the collision frequency for the ionization of sputtered aluminum atoms under the assumption that they were ionized by electron impact in the microwave plasma, which has a high electron temperature and density. The calculated value was in good agreement with the experimental results. Reference [1] Rossangel SM, Hopwood J. J Vac Sci Technol B 1994;12:449. [2] Kusano Y, Chiristou C, Barber ZH, Events JE, Huchings IM. Thin Solid Films 1999;355:117. [3] Nouvellon C, Konstantinidis S, Dauchot JP, Wautelet M, Jouan PY, Ricard A, et al. J Appl Phys 2002;92(1):32. [4] Nikiforov SA, Urm KW, Kim GH, Rim GH, Lee SH. Surf Coat Technol 2003;171:106. [5] Yonesu A, Kato T, Takemoto H, Nishimura N, Yamashiro Y. Jpn J Appl Phys 1999;38:4326. [6] Yonesu A, Takemoto H, Hirata M, Yamashiro Y. Vacuum 2002;66:275. [7] Christou C, Barber ZH. J Vac Sci Technol A 2000;18:2987. [8] Hopwood J, Qian F. J Appl Phys 1995;78:758. [9] Lotz W. Z Phys 1968;216:241.