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Mass spectrometry and absorption spectroscopy for oxidation of titanium target in rf magnetron sputtering
Vacuum 59 (2000) 600}605
Mass spectrometry and absorption spectroscopy for oxidation of titanium target in rf magnetron sputtering Kunio Okimura *, Tadashi Nakamura , Akira Shibata Department of Electronics, Faculty of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan Department of Electrical Engineering, Fukui National College of Technology, Sabae, Fukui 916-8507, Japan
Abstract Mass spectrometry and absorption spectroscopy were performed for a study of surface oxidation of titanium (Ti) target in rf magnetron sputtering. Line-averaged Ti atom density was measured with a resolution of 5;10 cm\ by means of an atomic absorption method. Hysteresis characteristic between Ti atom density and oxygen pressure was obtained for Ar pressure of 20 mTorr with rf power of 200 W. In a mass spectrometry for substrate-incident ions, TiO> and TiO> were detected in He}O discharge. Transient characteristics of substrate-incident ion currents were presented showing transient time scale from oxide to metallic mode. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In increasing number of applications of a reactive sputtering for a deposition of various oxide "lms, the oxidation of target surface is a key issue for optimum operations to get desirable "lms. It is known that the surface oxidation causes change in the sputtering yield, leading the hysteresis loop between deposition rate and the oxygen #ow rate [1]. Sputtering of Ti target in oxygen gas is widely used for deposition of TiO "lms which have attracted much attention recently in optical and photochemical applications. Several modeling studies in Ti}O system have been performed to describe the physical mechanism of the hysteresis characteristics in addition to the experimental observations [2,3]. It is pointed out that the surface adsorption and gettering e!ect decide the reactive gas #ow balance. Kusano developed time-dependent simulation modeling, showing
* Corresponding author. 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 3 2 2 - 5
K. Okimura et al. / Vacuum 59 (2000) 600}605
601
changes in several parameters such as target coverage, Ti #ux and oxygen pressure as a function of elapsed time, in addition to hysteresis curves for each parameter [4,5]. On the other hand, quantitative measurements of Ti atom, Ti oxide molecules and transition characteristics between metallic and oxide modes are insu$cient for the understanding of the surface oxidation on the basis of modeling studies. In this paper, line-averaged Ti atom density in Ar}O sputtering, spectra of substrate-incident ion (SII) current and transient characteristics of SII current after O stop in Ar}O and He}O sputtering, were presented for the study of oxidation of Ti target in rf magnetron sputtering.
2. Experimental A planar rf magnetron sputtering apparatus with a titanium target (99.99% purity) of 100 mm diameter was used in this study. The target}substrate (lower grounded electrode) spacing was 35 mm. For mass spectrometry of SII current, a small ori"ce of 0.2 mm diameter was placed at a center on the grounded electrode to lead ions to a mass spectrometer (ULVAC MSQ-400) mounted in the lower chamber which was di!erentially pumped below 1;10\ Torr. Ionization "lament in mass spectrometer was switched o! in this measurement. Atomic absorption spectroscopy for getting line-averaged Ti atom density at 15 mm away from the target surface was performed by using a hollow cathode lamp. In the conversion of the absorption intensity into the number density, we assumed a Gaussian line pro"le corresponding to a Ti atom temperature of 350 K for both the emission line of the Ti atom in the lamp and the absorption line of Ti in the sputtering [6,7] Wavelength of 363.5 nm was used as an absorption line. We can get an absorption intensity with a sensitivity of 5;10\, which corresponds to a resolution of 5;10 cm\ in density, by the use of auxiliary pipes and lens at an optimum condition. Details of the experimental setup were reported in a previous paper [8]. First, we measured Ti atom densities in the transition region between metallic mode and oxide mode by changing oxygen partial pressure. We introduced oxygen gas maintaining the valve opening which gave a certain (20 mTorr) pressure for Ar or He gas. The partial pressure of oxygen gas was measured by an ionization gauge and that of Ar or He gas was measured by the capacitance manometer. Next, we measured SII spectra for Ar}O and He}O discharges. Finally, transient characteristics of SII current were investigated. We closed the stopping valve for O #ow, and recorded the changes in SII current.
3. Results and discussion Fig. 1 shows hysteresis characteristic between Ti atom density and oxygen pressure for constant Ar #ow (100 sccm, 20 mTorr) with rf power of 200 W. On increasing oxygen pressure, metallic mode which had Ti atom density of 8}10;10 cm\ changed into oxide mode with Ti atom density of 1}2;10 cm\ at an oxygen pressure of 1.6;10\ Torr. On the other hand, oxide mode reversed to metallic mode at an oxygen pressure of 8;10\ Torr on decreasing oxygen partial pressure. We estimated the deposition rate of TiO "lm from obtained Ti atom density. In this
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K. Okimura et al. / Vacuum 59 (2000) 600}605
Fig. 1. Hysteresis characteristic between Ti atom density and oxygen partial pressure for constant Ar #ow (100 sccm, 20 mTorr) with rf power of 200 W. Ti atom densities are line-averaged values at 15 mm distance from the target surface.
estimation, we used natural di!usion #ux of Ti, assuming Ti atom velocity with the temperature of 350 K. As for the formation of TiO compound, we assumed that Ti atom combines with O molecule since #ux of O is larger than that of Ti atom. As a result of calculation, we obtained a value of 0.34 nm/min for Ti atom of density 2;10 cm\. However, this value of the deposition rate was one order smaller than that of TiO "lms (&3 nm/min) prepared by Ar}O sputtering with same total pressure and rf power. On the other hand, TiO> current was one order larger than Ti> current in Ar}O discharge as shown in Fig. 2. We consider that the deposition of TiO proceeds with incoming TiO in addition to Ti. Mass spectra of SII in the mass-to-charge ratio ranging from 1 to 90 for (a) pure Ar, (b) Ar}O and (c) He}O discharge are shown in Fig. 2. In pure Ar discharge, the SII current was mainly composed of Ar>(40) and Ti>(48). Ar dimor ion, Ar>(80), was also found as minor species. In the case of metallic mode of Ar}O discharge, the mass spectrum was almost the same as that of pure Ar discharge. On the other hand, in oxide mode of Ar}O discharge (see Fig. 2b), we recognized Ti>, ArO>(56), TiO>(64) and Ar>(80) as minor ions in addition to major species such as O>, O> and Ar>. Although Ar> overlaps with TiO> in mass number of 80, Ar> was thought to be dominant on the basis of the similarity with pure Ar discharge. TiO> current was one order larger than Ti> current, indicating the promotion of oxidation of Ti target. In the case of He}O discharge, we found both TiO> and TiO> (80) clearly in addition to major species such as O>, O> and He>. We can see that Ti> current was relatively larger than TiO> current in contrast to Ar}O discharge. Thus we should note that the target surface is kept more metallic at the He sputtering. Fig. 3 shows the transient characteristics of Ar>, Ti>, TiO>, ArO> and O> current after the stop of O introduction. After the stop of O introduction, the discharge was sustained at Ar of
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Fig. 2. Mass spectra of substrate-incident ion (SII) current in the mass-to-charge ratio ranging from 1 to 90 for (a) pure Ar (50 sccm), (b) Ar (50 sccm)}O (3 sccm) and (c) He (50 sccm)}O (3 sccm) discharge. Ar and He pressures were 20 mTorr and rf power was 200 W in all cases.
20 mTorr with rf power of 200 W. We found that O> current decreased rapidly and disappeared within 12 s, while TiO> current decreased gradually from 12 to 40 s. The Increase of Ti> current almost corresponded to the variation of TiO>, indicating that target surface serves the sources of both Ti and TiO. It took several tens of seconds to transit from oxide to metallic mode as shown in
604
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Fig. 3. Transient characteristics of Ar>, Ti>, TiO>, ArO> and O> current after the stop of O (3 sccm) introduction. After the stop of O , discharge was sustained by Ar of 20 mTorr with rf power of 200 W.
Fig. 4. Transient characteristics of TiO> and TiO> current after the stop of O (3 sccm) introduction. After the stop of O , discharge was sustained by He of 20 mTorr with the rf power of 200 W.
modeling studies [4]. ArO> current disappeared synchronously with O> and Ar> current increased for sustaining discharge without O>. Further, we show transient characteristics of TiO> and TiO> current in He}O discharge in Fig. 4. From Fig. 4, TiO> current decreased with short time constant coincident with the change of O>. It is suggested that the mechanism of TiO formation is di!erent from TiO which is formed as a surface compound layer. Reactions of TiO with dissociated oxygen atoms in vapor phase or on substrate surface are considered to contribute to the TiO formation.
K. Okimura et al. / Vacuum 59 (2000) 600}605
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4. Conclusions Hysteresis characteristic in Ar}O sputtering was obtained in terms of Ti atom density against oxygen partial pressure. It was shown that Ti atom density at oxide mode was nearly two orders smaller than that at the metallic mode in Ar}O sputtering. In oxide mode, substrate-incident TiO> current was larger than Ti> current in Ar}O sputtering. TiO> current was detected in addition to TiO> in He}O sputtering. Transient characteristics of ion current showed that TiO is formed as a surface compound layer. Transient time constant of several tens of seconds from oxide to metallic mode coincident with modeling calculations was revealed.
Acknowledgements We acknowledge Professor A.Inoue and N.M. Horii of Fukui National College of Technology for their assistance in experiments. We are also indebted to K. Tsuchida of Fukui Industrial Technical Center for his continual guiding.
References [1] Hata T, Okada H, Nakano S, Sasaki K. Proceedings of the Fourth International Symposium Sputtering & Plasma Processes, (1997): 135. [2] Berg S, Blom H-O, Larsson T, Nender C. J Vac Sci Technol 1987;A5:202. [3] Touzeau M, Pagnon D, Bretagne J. Vacuum 1999;52:33. [4] Kusano E. J Appl Phys 1991;70:7089. [5] Kusano E. J Appl Phys 1993;73:8565. [6] Tachibana K, Tadokoro H, Harima H, Urano Y. J Phys D 1982;15:177. [7] Mitchell ACG, Zemansky MW. Resonance radiation and exited atoms. London: Cambridge University Press, 1971. [8] Okimura K, Shibata A. Jpn J Appl Phys 1997;36:2849.
Mass spectrometry and absorption spectroscopy for oxidation of titanium target in rf magnetron sputtering Kunio Okimura *, Tadashi Nakamura , Akira Shibata Department of Electronics, Faculty of Engineering, Tokai University, 1117 Kitakaname, Hiratsuka, Kanagawa 259-1292, Japan Department of Electrical Engineering, Fukui National College of Technology, Sabae, Fukui 916-8507, Japan
Abstract Mass spectrometry and absorption spectroscopy were performed for a study of surface oxidation of titanium (Ti) target in rf magnetron sputtering. Line-averaged Ti atom density was measured with a resolution of 5;10 cm\ by means of an atomic absorption method. Hysteresis characteristic between Ti atom density and oxygen pressure was obtained for Ar pressure of 20 mTorr with rf power of 200 W. In a mass spectrometry for substrate-incident ions, TiO> and TiO> were detected in He}O discharge. Transient characteristics of substrate-incident ion currents were presented showing transient time scale from oxide to metallic mode. 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction In increasing number of applications of a reactive sputtering for a deposition of various oxide "lms, the oxidation of target surface is a key issue for optimum operations to get desirable "lms. It is known that the surface oxidation causes change in the sputtering yield, leading the hysteresis loop between deposition rate and the oxygen #ow rate [1]. Sputtering of Ti target in oxygen gas is widely used for deposition of TiO "lms which have attracted much attention recently in optical and photochemical applications. Several modeling studies in Ti}O system have been performed to describe the physical mechanism of the hysteresis characteristics in addition to the experimental observations [2,3]. It is pointed out that the surface adsorption and gettering e!ect decide the reactive gas #ow balance. Kusano developed time-dependent simulation modeling, showing
* Corresponding author. 0042-207X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 0 ) 0 0 3 2 2 - 5
K. Okimura et al. / Vacuum 59 (2000) 600}605
601
changes in several parameters such as target coverage, Ti #ux and oxygen pressure as a function of elapsed time, in addition to hysteresis curves for each parameter [4,5]. On the other hand, quantitative measurements of Ti atom, Ti oxide molecules and transition characteristics between metallic and oxide modes are insu$cient for the understanding of the surface oxidation on the basis of modeling studies. In this paper, line-averaged Ti atom density in Ar}O sputtering, spectra of substrate-incident ion (SII) current and transient characteristics of SII current after O stop in Ar}O and He}O sputtering, were presented for the study of oxidation of Ti target in rf magnetron sputtering.
2. Experimental A planar rf magnetron sputtering apparatus with a titanium target (99.99% purity) of 100 mm diameter was used in this study. The target}substrate (lower grounded electrode) spacing was 35 mm. For mass spectrometry of SII current, a small ori"ce of 0.2 mm diameter was placed at a center on the grounded electrode to lead ions to a mass spectrometer (ULVAC MSQ-400) mounted in the lower chamber which was di!erentially pumped below 1;10\ Torr. Ionization "lament in mass spectrometer was switched o! in this measurement. Atomic absorption spectroscopy for getting line-averaged Ti atom density at 15 mm away from the target surface was performed by using a hollow cathode lamp. In the conversion of the absorption intensity into the number density, we assumed a Gaussian line pro"le corresponding to a Ti atom temperature of 350 K for both the emission line of the Ti atom in the lamp and the absorption line of Ti in the sputtering [6,7] Wavelength of 363.5 nm was used as an absorption line. We can get an absorption intensity with a sensitivity of 5;10\, which corresponds to a resolution of 5;10 cm\ in density, by the use of auxiliary pipes and lens at an optimum condition. Details of the experimental setup were reported in a previous paper [8]. First, we measured Ti atom densities in the transition region between metallic mode and oxide mode by changing oxygen partial pressure. We introduced oxygen gas maintaining the valve opening which gave a certain (20 mTorr) pressure for Ar or He gas. The partial pressure of oxygen gas was measured by an ionization gauge and that of Ar or He gas was measured by the capacitance manometer. Next, we measured SII spectra for Ar}O and He}O discharges. Finally, transient characteristics of SII current were investigated. We closed the stopping valve for O #ow, and recorded the changes in SII current.
3. Results and discussion Fig. 1 shows hysteresis characteristic between Ti atom density and oxygen pressure for constant Ar #ow (100 sccm, 20 mTorr) with rf power of 200 W. On increasing oxygen pressure, metallic mode which had Ti atom density of 8}10;10 cm\ changed into oxide mode with Ti atom density of 1}2;10 cm\ at an oxygen pressure of 1.6;10\ Torr. On the other hand, oxide mode reversed to metallic mode at an oxygen pressure of 8;10\ Torr on decreasing oxygen partial pressure. We estimated the deposition rate of TiO "lm from obtained Ti atom density. In this
602
K. Okimura et al. / Vacuum 59 (2000) 600}605
Fig. 1. Hysteresis characteristic between Ti atom density and oxygen partial pressure for constant Ar #ow (100 sccm, 20 mTorr) with rf power of 200 W. Ti atom densities are line-averaged values at 15 mm distance from the target surface.
estimation, we used natural di!usion #ux of Ti, assuming Ti atom velocity with the temperature of 350 K. As for the formation of TiO compound, we assumed that Ti atom combines with O molecule since #ux of O is larger than that of Ti atom. As a result of calculation, we obtained a value of 0.34 nm/min for Ti atom of density 2;10 cm\. However, this value of the deposition rate was one order smaller than that of TiO "lms (&3 nm/min) prepared by Ar}O sputtering with same total pressure and rf power. On the other hand, TiO> current was one order larger than Ti> current in Ar}O discharge as shown in Fig. 2. We consider that the deposition of TiO proceeds with incoming TiO in addition to Ti. Mass spectra of SII in the mass-to-charge ratio ranging from 1 to 90 for (a) pure Ar, (b) Ar}O and (c) He}O discharge are shown in Fig. 2. In pure Ar discharge, the SII current was mainly composed of Ar>(40) and Ti>(48). Ar dimor ion, Ar>(80), was also found as minor species. In the case of metallic mode of Ar}O discharge, the mass spectrum was almost the same as that of pure Ar discharge. On the other hand, in oxide mode of Ar}O discharge (see Fig. 2b), we recognized Ti>, ArO>(56), TiO>(64) and Ar>(80) as minor ions in addition to major species such as O>, O> and Ar>. Although Ar> overlaps with TiO> in mass number of 80, Ar> was thought to be dominant on the basis of the similarity with pure Ar discharge. TiO> current was one order larger than Ti> current, indicating the promotion of oxidation of Ti target. In the case of He}O discharge, we found both TiO> and TiO> (80) clearly in addition to major species such as O>, O> and He>. We can see that Ti> current was relatively larger than TiO> current in contrast to Ar}O discharge. Thus we should note that the target surface is kept more metallic at the He sputtering. Fig. 3 shows the transient characteristics of Ar>, Ti>, TiO>, ArO> and O> current after the stop of O introduction. After the stop of O introduction, the discharge was sustained at Ar of
K. Okimura et al. / Vacuum 59 (2000) 600}605
603
Fig. 2. Mass spectra of substrate-incident ion (SII) current in the mass-to-charge ratio ranging from 1 to 90 for (a) pure Ar (50 sccm), (b) Ar (50 sccm)}O (3 sccm) and (c) He (50 sccm)}O (3 sccm) discharge. Ar and He pressures were 20 mTorr and rf power was 200 W in all cases.
20 mTorr with rf power of 200 W. We found that O> current decreased rapidly and disappeared within 12 s, while TiO> current decreased gradually from 12 to 40 s. The Increase of Ti> current almost corresponded to the variation of TiO>, indicating that target surface serves the sources of both Ti and TiO. It took several tens of seconds to transit from oxide to metallic mode as shown in
604
K. Okimura et al. / Vacuum 59 (2000) 600}605
Fig. 3. Transient characteristics of Ar>, Ti>, TiO>, ArO> and O> current after the stop of O (3 sccm) introduction. After the stop of O , discharge was sustained by Ar of 20 mTorr with rf power of 200 W.
Fig. 4. Transient characteristics of TiO> and TiO> current after the stop of O (3 sccm) introduction. After the stop of O , discharge was sustained by He of 20 mTorr with the rf power of 200 W.
modeling studies [4]. ArO> current disappeared synchronously with O> and Ar> current increased for sustaining discharge without O>. Further, we show transient characteristics of TiO> and TiO> current in He}O discharge in Fig. 4. From Fig. 4, TiO> current decreased with short time constant coincident with the change of O>. It is suggested that the mechanism of TiO formation is di!erent from TiO which is formed as a surface compound layer. Reactions of TiO with dissociated oxygen atoms in vapor phase or on substrate surface are considered to contribute to the TiO formation.
K. Okimura et al. / Vacuum 59 (2000) 600}605
605
4. Conclusions Hysteresis characteristic in Ar}O sputtering was obtained in terms of Ti atom density against oxygen partial pressure. It was shown that Ti atom density at oxide mode was nearly two orders smaller than that at the metallic mode in Ar}O sputtering. In oxide mode, substrate-incident TiO> current was larger than Ti> current in Ar}O sputtering. TiO> current was detected in addition to TiO> in He}O sputtering. Transient characteristics of ion current showed that TiO is formed as a surface compound layer. Transient time constant of several tens of seconds from oxide to metallic mode coincident with modeling calculations was revealed.
Acknowledgements We acknowledge Professor A.Inoue and N.M. Horii of Fukui National College of Technology for their assistance in experiments. We are also indebted to K. Tsuchida of Fukui Industrial Technical Center for his continual guiding.
References [1] Hata T, Okada H, Nakano S, Sasaki K. Proceedings of the Fourth International Symposium Sputtering & Plasma Processes, (1997): 135. [2] Berg S, Blom H-O, Larsson T, Nender C. J Vac Sci Technol 1987;A5:202. [3] Touzeau M, Pagnon D, Bretagne J. Vacuum 1999;52:33. [4] Kusano E. J Appl Phys 1991;70:7089. [5] Kusano E. J Appl Phys 1993;73:8565. [6] Tachibana K, Tadokoro H, Harima H, Urano Y. J Phys D 1982;15:177. [7] Mitchell ACG, Zemansky MW. Resonance radiation and exited atoms. London: Cambridge University Press, 1971. [8] Okimura K, Shibata A. Jpn J Appl Phys 1997;36:2849.