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Study on facing-target sputtering assisted by microwave plasma for enhanced ionization of sputtered atoms
ARTICLE IN PRESS
Vacuum 74 (2004) 521–524
Study on facing-target sputtering assisted by microwave plasma for enhanced ionization of sputtered atoms Akira Yonesu*, Suguru Watashi, Kiyotaka Kagawa, Yasumasa Yamashiro Faculty of Engineering, Ryukyu University, 1, Senbaru, Nishihara-cho, Okinawa 903-0213, Japan
Abstract Facing-target sputtering assisted by a microwave plasma to enhance ionization of sputtered atoms has been studied. With the assistance of microwave plasma, operation of the sputtering deposition has been achieved at pressure of the order of 105 Torr. Aluminum films were deposited in this apparatus at rates 30 nm/min at the low gas pressure of 0.06 mTorr. Optical emission spectroscopic measurements of aluminum ions and neutral species indicate that the ionization of sputtered atoms was enhanced by the assistance of the microwave plasma. r 2004 Elsevier Ltd. All rights reserved. Keywords: Ionized physical vapor deposition; Facing-target sputtering; Microwave plasma; Electron cyclotron resonance; Lowpressure sputtering
1. Introduction The facing targets sputtering apparatus is well known as one of the most effective systems for depositing thin films with high quality because the plasma is perfectly confined by the magnetic field between two targets and the thin films can be deposited in damage free condition[1,2]. In conventional sputtering deposition, metal atoms sputtered from targets are neutral and exhibit a cosine angular flux distribution that is unsuitable for the filling of high aspect ratio features. On the contrary, ionized physical vapor deposition (IPVD) is a process in which sputtered atoms from a target are ionized by a secondary plasma before depositing onto the substrate[3–5]. The *Corresponding author. Tel.: +81-98-895-8692; fax: +8198-895-8708. E-mail address: [email protected] (A. Yonesu).
directionality and 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 a secondary inductively coupled plasma (ICP) between the target and the substrate. But, immersed ICP coils have problems with flaking, contamination, and shadowing. In this study, therefore, we have developed an IPVD system in which the atoms sputtered by a facing-target sputtering system are ionized by a microwave plasma which can be produced without electrode.
2. Experimental apparatus and method A Schematic diagram of the experimental apparatus is shown in Fig. 1. The apparatus is constructed with a cylindrical stainless chamber, 16 cm in diameter and 150 cm long, which contains
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.01.047
ARTICLE IN PRESS 522
A. Yonesu et al. / Vacuum 74 (2004) 521–524
Fig. 2. Distribution of calculated magnetic field lines between two magnets.
Fig. 1. Schematic illustration of facing-target sputtering assisted by microwave plasma.
two disk-shaped targets with the same size and a substrate holder. The two targets are parallel and facing each other and magnetic field is applied to the targets surfaces with disk-shaped Sm–Co permanent magnets. These magnets have a flux density of approximately 3.8 kG at their surfaces. The distance between the two targets is 7 cm. The calculated magnetic field lines between two targets are illustrated in Fig. 2. The magnetic field lines produced by permanent magnets form a mirror magnetic field configuration. In our system, magnets placed behind targets have higher magnetic flux density than that of the magnets used in the conventional facing-target sputtering system, so that they provide resonant magnetic field for electron cyclotron resonance (ECR) absorption of the microwave power. When the microwave is introduced into the space between the facing
targets through a tapered waveguide, ECR microwave plasma is produced around the target surface. A three-stub tuner was installed for impedance matching between the plasma and microwave circuit. The two targets are aluminum with a diameter of 50 mm and a thickness of 2 mm. Electrostatic probe measurements were made to obtain spatial distributions of the plasma parameters. The deposition rate was measured with thickness monitor located at a distance of 4 cm from the targets. Optical emission spectra of the sputtered particle ions and neutral species were measured using a monochrometer with fiber optics for light collection. The emission from the plasma was observed through the side-view port of the chamber. Ar gas was used as the discharge gas.
3. Experimental results and discussion The two-dimensional profile of the ion saturation current between the two targets is shown in Fig. 3, which is proportional to the plasma density. The gas pressure was 0.1 mTorr, microwave power was 220 W and target voltage was 200 V. It was found that the plasma was confined by mirror magnetic field (Fig. 2). Fig. 4 shows the radial electron density and electron temperature profiles at Z ¼ 0 cm for gas pressure 0.5 mTorr and
ARTICLE IN PRESS A. Yonesu et al. / Vacuum 74 (2004) 521–524
Fig. 3. Contour plots of ion saturation current between two targets. Microwave power is 220 W, and gas pressure is 0.1mTorr, and target voltage is 200 V.
Fig. 4. Radial profiles of electron density and electron temperature, measured at Z=0 cm. Microwave power is 220 W, and gas pressure is 0.5 mTorr, and target voltage is 200 V.
microwave power 220 W. The target voltage was 200 V. It is noted that the plasma density is high even at low gas pressure of 0.1 mTorr. Moreover, high electron temperature, which is a characteristic of ECR microwave plasma, was obtained in our apparatus. Fig. 5 shows the deposition rate and target current as a function of gas pressures. The microwave power, PIN, was 600 W and target voltage was 300 V. It appears that the operation
523
Fig. 5. Deposition rate and target current as a function of gas pressure. Microwave power is 600 W and target voltage is 300 V.
of the sputtering deposition has been achieved at pressure of the order of 105 Torr with the assistance of the ECR microwave plasma. Furthermore, it is found that the deposition rate increase with decreasing gas pressure, while the target current is almost constant over this gas pressure range. The sputtered atom may collide with background gas atoms on its way to the substrate at a rate which will decrease with decreasing the gas pressure. The result of the collision is to deflect the sputtered atom from the substrate. Therefore, the decrease in the gas pressure leads to the increase in the deposition rate. Aluminum films were deposited in this apparatus at rates 30 nm/ min at the low gas pressure of 0.06 mTorr. The lower operating pressure in sputtering deposition leads to a significant increase in the average kinetic energy and directionality of sputtered atoms. That is useful for obtaining high quality thin films at low substrate temperature. In order to investigate the influence of the microwave plasma on the sputtered atoms (aluminum), optical emission spectra were measured. Fig. 6 shows the optical emission intensity and intensity ratio as a function of microwave input power. Measured lines were Al ion line (Al II) at 390 nm and Al neutral line (Al I) at 394 nm. Here, the gas pressure was 0.4 mTorr and the target voltage was 300 V. This emission intensity ratio
ARTICLE IN PRESS 524
A. Yonesu et al. / Vacuum 74 (2004) 521–524
Fig. 7. Optical emission intensity ratio of AlII (390 nm) to AlI (394 nm) as a function of gas pressure. Microwave power is 720 W and target voltage is 300 V.
Fig. 6. Optical emission intensity of Al atoms line (AlI) at 390 nm and Al ions line (AlII) at 394 nm, and intensity ratio of AlII to AlI as a function of microwave power. Gas pressure is 0.4 mTorr and target voltage is 300 V.
the decrease in gas pressure results in the increase in the ionization rate of sputtered atoms.
4. Conclusion is considered to be qualitatively proportional to the ionization rate of sputtered atoms. It is found that the ionization rate of the sputtered Al atoms seems to increase with increasing microwave input power. Fig. 7 shows the emission intensity ratio of Al II to Al I as a function of gas pressure. Here, the microwave power was 720 W and the target voltage was 300 V. It can be seen that the ionization rate increases with decreasing the gas pressure. The sputtered atoms are ionized by the collision with electrons and excited gas atoms (Ar) which have higher energy than ionization potential of sputtered atoms. In an ECR microwave plasma, the electron temperature, i.e. average electron energy, increases drastically with decreasing the gas pressure, whereas the electron and the gas atom density decrease with the gas pressure. Thus,
A new facing-target sputtering system assisted by microwave plasma for enhanced ionization of sputtered atoms has been developed. The results of optical emission spectroscopy indicated that the ionization rate of the sputtered atoms was enhanced by the microwave plasma.
References [1] Naoe M, Yamanaka S, Hoshi Y. IEEE Trans Magn 1980;16:646. [2] Shi JR, Wang JP. Thin Solid Films 2002;420:172. [3] Hayden DB, Juliano DR, Neumann MN, Allain MC, Ruzic DN. Surf Coat Technol 1999;53:21. [4] Kusano Y, Christou C, Barber ZH, Evetts JE, Hutchings IM. Thin Solid Films 1999;355:117. [5] Chiu KF, Barder ZH. Thin Solid Films 2000;358:264.
Vacuum 74 (2004) 521–524
Study on facing-target sputtering assisted by microwave plasma for enhanced ionization of sputtered atoms Akira Yonesu*, Suguru Watashi, Kiyotaka Kagawa, Yasumasa Yamashiro Faculty of Engineering, Ryukyu University, 1, Senbaru, Nishihara-cho, Okinawa 903-0213, Japan
Abstract Facing-target sputtering assisted by a microwave plasma to enhance ionization of sputtered atoms has been studied. With the assistance of microwave plasma, operation of the sputtering deposition has been achieved at pressure of the order of 105 Torr. Aluminum films were deposited in this apparatus at rates 30 nm/min at the low gas pressure of 0.06 mTorr. Optical emission spectroscopic measurements of aluminum ions and neutral species indicate that the ionization of sputtered atoms was enhanced by the assistance of the microwave plasma. r 2004 Elsevier Ltd. All rights reserved. Keywords: Ionized physical vapor deposition; Facing-target sputtering; Microwave plasma; Electron cyclotron resonance; Lowpressure sputtering
1. Introduction The facing targets sputtering apparatus is well known as one of the most effective systems for depositing thin films with high quality because the plasma is perfectly confined by the magnetic field between two targets and the thin films can be deposited in damage free condition[1,2]. In conventional sputtering deposition, metal atoms sputtered from targets are neutral and exhibit a cosine angular flux distribution that is unsuitable for the filling of high aspect ratio features. On the contrary, ionized physical vapor deposition (IPVD) is a process in which sputtered atoms from a target are ionized by a secondary plasma before depositing onto the substrate[3–5]. The *Corresponding author. Tel.: +81-98-895-8692; fax: +8198-895-8708. E-mail address: [email protected] (A. Yonesu).
directionality and 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 a secondary inductively coupled plasma (ICP) between the target and the substrate. But, immersed ICP coils have problems with flaking, contamination, and shadowing. In this study, therefore, we have developed an IPVD system in which the atoms sputtered by a facing-target sputtering system are ionized by a microwave plasma which can be produced without electrode.
2. Experimental apparatus and method A Schematic diagram of the experimental apparatus is shown in Fig. 1. The apparatus is constructed with a cylindrical stainless chamber, 16 cm in diameter and 150 cm long, which contains
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.01.047
ARTICLE IN PRESS 522
A. Yonesu et al. / Vacuum 74 (2004) 521–524
Fig. 2. Distribution of calculated magnetic field lines between two magnets.
Fig. 1. Schematic illustration of facing-target sputtering assisted by microwave plasma.
two disk-shaped targets with the same size and a substrate holder. The two targets are parallel and facing each other and magnetic field is applied to the targets surfaces with disk-shaped Sm–Co permanent magnets. These magnets have a flux density of approximately 3.8 kG at their surfaces. The distance between the two targets is 7 cm. The calculated magnetic field lines between two targets are illustrated in Fig. 2. The magnetic field lines produced by permanent magnets form a mirror magnetic field configuration. In our system, magnets placed behind targets have higher magnetic flux density than that of the magnets used in the conventional facing-target sputtering system, so that they provide resonant magnetic field for electron cyclotron resonance (ECR) absorption of the microwave power. When the microwave is introduced into the space between the facing
targets through a tapered waveguide, ECR microwave plasma is produced around the target surface. A three-stub tuner was installed for impedance matching between the plasma and microwave circuit. The two targets are aluminum with a diameter of 50 mm and a thickness of 2 mm. Electrostatic probe measurements were made to obtain spatial distributions of the plasma parameters. The deposition rate was measured with thickness monitor located at a distance of 4 cm from the targets. Optical emission spectra of the sputtered particle ions and neutral species were measured using a monochrometer with fiber optics for light collection. The emission from the plasma was observed through the side-view port of the chamber. Ar gas was used as the discharge gas.
3. Experimental results and discussion The two-dimensional profile of the ion saturation current between the two targets is shown in Fig. 3, which is proportional to the plasma density. The gas pressure was 0.1 mTorr, microwave power was 220 W and target voltage was 200 V. It was found that the plasma was confined by mirror magnetic field (Fig. 2). Fig. 4 shows the radial electron density and electron temperature profiles at Z ¼ 0 cm for gas pressure 0.5 mTorr and
ARTICLE IN PRESS A. Yonesu et al. / Vacuum 74 (2004) 521–524
Fig. 3. Contour plots of ion saturation current between two targets. Microwave power is 220 W, and gas pressure is 0.1mTorr, and target voltage is 200 V.
Fig. 4. Radial profiles of electron density and electron temperature, measured at Z=0 cm. Microwave power is 220 W, and gas pressure is 0.5 mTorr, and target voltage is 200 V.
microwave power 220 W. The target voltage was 200 V. It is noted that the plasma density is high even at low gas pressure of 0.1 mTorr. Moreover, high electron temperature, which is a characteristic of ECR microwave plasma, was obtained in our apparatus. Fig. 5 shows the deposition rate and target current as a function of gas pressures. The microwave power, PIN, was 600 W and target voltage was 300 V. It appears that the operation
523
Fig. 5. Deposition rate and target current as a function of gas pressure. Microwave power is 600 W and target voltage is 300 V.
of the sputtering deposition has been achieved at pressure of the order of 105 Torr with the assistance of the ECR microwave plasma. Furthermore, it is found that the deposition rate increase with decreasing gas pressure, while the target current is almost constant over this gas pressure range. The sputtered atom may collide with background gas atoms on its way to the substrate at a rate which will decrease with decreasing the gas pressure. The result of the collision is to deflect the sputtered atom from the substrate. Therefore, the decrease in the gas pressure leads to the increase in the deposition rate. Aluminum films were deposited in this apparatus at rates 30 nm/ min at the low gas pressure of 0.06 mTorr. The lower operating pressure in sputtering deposition leads to a significant increase in the average kinetic energy and directionality of sputtered atoms. That is useful for obtaining high quality thin films at low substrate temperature. In order to investigate the influence of the microwave plasma on the sputtered atoms (aluminum), optical emission spectra were measured. Fig. 6 shows the optical emission intensity and intensity ratio as a function of microwave input power. Measured lines were Al ion line (Al II) at 390 nm and Al neutral line (Al I) at 394 nm. Here, the gas pressure was 0.4 mTorr and the target voltage was 300 V. This emission intensity ratio
ARTICLE IN PRESS 524
A. Yonesu et al. / Vacuum 74 (2004) 521–524
Fig. 7. Optical emission intensity ratio of AlII (390 nm) to AlI (394 nm) as a function of gas pressure. Microwave power is 720 W and target voltage is 300 V.
Fig. 6. Optical emission intensity of Al atoms line (AlI) at 390 nm and Al ions line (AlII) at 394 nm, and intensity ratio of AlII to AlI as a function of microwave power. Gas pressure is 0.4 mTorr and target voltage is 300 V.
the decrease in gas pressure results in the increase in the ionization rate of sputtered atoms.
4. Conclusion is considered to be qualitatively proportional to the ionization rate of sputtered atoms. It is found that the ionization rate of the sputtered Al atoms seems to increase with increasing microwave input power. Fig. 7 shows the emission intensity ratio of Al II to Al I as a function of gas pressure. Here, the microwave power was 720 W and the target voltage was 300 V. It can be seen that the ionization rate increases with decreasing the gas pressure. The sputtered atoms are ionized by the collision with electrons and excited gas atoms (Ar) which have higher energy than ionization potential of sputtered atoms. In an ECR microwave plasma, the electron temperature, i.e. average electron energy, increases drastically with decreasing the gas pressure, whereas the electron and the gas atom density decrease with the gas pressure. Thus,
A new facing-target sputtering system assisted by microwave plasma for enhanced ionization of sputtered atoms has been developed. The results of optical emission spectroscopy indicated that the ionization rate of the sputtered atoms was enhanced by the microwave plasma.
References [1] Naoe M, Yamanaka S, Hoshi Y. IEEE Trans Magn 1980;16:646. [2] Shi JR, Wang JP. Thin Solid Films 2002;420:172. [3] Hayden DB, Juliano DR, Neumann MN, Allain MC, Ruzic DN. Surf Coat Technol 1999;53:21. [4] Kusano Y, Christou C, Barber ZH, Evetts JE, Hutchings IM. Thin Solid Films 1999;355:117. [5] Chiu KF, Barder ZH. Thin Solid Films 2000;358:264.