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Magnetic null discharge sputtering with full target erosion
Surface & Coatings Technology 193 (2005) 123 – 128 www.elsevier.com/locate/surfcoat
Magnetic null discharge sputtering with full target erosion Youl-Moon Sung* Department of Electrical Electronic Engineering, University of Miyazaki, 1-1 Gakuenkibanadai-Nishi, Miyazaki 889-2192, Japan Available online 19 September 2004
Abstract The article reports on the operation of a novel-type sputtering system based on the magnetic null discharge concept for the thin film fabrication process. The unique feature of the system is the plasma production around the magnetic null field region on the target surface. The calculated electron motion around the magnetic null region on the target surface showed the complex meandering motion, and the measured electron and ion properties possessed peaks on the null region like in the original null plasma concept. Experimentally, it was also found that the shape of high-density and low-temperature plasma in the magnetic null field region was also similar to that of the inductive-type original null plasma. It is therefore expected that the dynamic plasma control over the target surface will be realized since rotating and arranging the magnets can actively control the plasma peaks. With sputtering application, it is possible to achieve almost full target erosion and thus significantly increase the usage lifetime of target. Also, it can be found from the result of a thin film deposition that the system is very useful for the performance of sputtering. D 2004 Elsevier B.V. All rights reserved. Keywords: Magnetic null plasma; Neutral loop discharge; Target erosion; Sputtering
1. Introduction Presently, magnetron sputtering [1] is a well-established technology, successfully used in a wade range of applications, many of which have been scaled up for industrial use. However, the target utilization of magnetrons currently produced is low of about 20–30%. It can be increased up to 50–60% in the case when the magnetron has an optimized magnetic field. Since the magnetron discharge forms a closed loop, there is always a certain part of the target inside the closed loop where no target sputtering occurs. In principle, there are two ways on how to achieve full erosion of the target. (1) To design a magnetron with full erosion target, i.e., a target without a center, which is not sputtered. (2) To control the sputtering region dynamically on the target surface during discharge using an appropriate magnetic method. The first method resulted in the realization of magnetrons, for example, interpole target magnetron [2], toroidal plasma-type mag-
* Tel./fax: +81 985 58 7350. E-mail address: [email protected]. 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.122
netron [3] and rectangular magnetron [4], but some of these methods are somewhat complicated structurally. The second method, if it can be realized, will be very convenient and effective for full target erosion; however, there has been no report on this. This study addresses the latter method. In recent years, a neutral loop discharge (NLD) [5–9] is a new plasma source utilizing the magnetic null discharge, which moves in a non-linear manner around the null region with effective electron heating at pressures of less than 1 mTorr and yet achieving high plasma density. It is also that kind of technology where the null region can easily be controlled by varying currents in the coils. By this processing over large surfaces areas with high uniformity can be realized and thus is attracting a lot of attention. In etching applications, adequate work has been done making the NLD technology industrially viable with good results [7]. We have been conducting process application in the NLD technology [9–13]. In addition, we seek to suggest a capacitive-coupled NLD plasma without the use of an antenna in order to realize to sputtering applications [14]. With the dynamic controllability of the NLD technology made applicable to sputter
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processes, there is the possibility of overcoming the erosion problem as well as achieving low pressure and high efficiency in operation. In this research, we present the design and construct of the new type null plasma sputtering system. We also seek to quantify through simulation and experiments, to clarify the plasma formation and structure. For the evaluation of the sputter efficiency, thin film growth experiments have been conducted.
2. System geometry and concept Fig. 1 shows the target of the equipment with the magnetic field formed by the permanent magnets. The chamber is of stainless steel of internal diameter 308 mm. The target is of doughnut shape of width 70 mm with the anode on the inner and outer sides of the target. Eight pairs of permanent magnet (neodymium, size: 10205 mm) are arranged on the outer and inner peripherals of the target inside the anode. The target hold is made of stainless steel and the disc width, 71.5 mm. Cooling water is circulated through the target to prevent overheating. The internal diameter of the cylinder housing the permanent magnet is 53 mm with its corresponding external diameter 204 mm grounded. The gap between the two rings is set less than the Paschen minimum (2 mm) to prevent unnecessary discharge. In order to measure the two-dimensional distributions of the electron density and electron temperature of the area over the doughnut target, the measuring probe and its supporting flange were both designed to allow two-stage rotation. The two-stage rotational probe consists of the double probe itself, inner flange and outer
Fig. 1. Schematic arrangement of the experimental apparatus: (a) front view; (b) side view.
Fig. 2. Calculated magnetic field line distribution.
flange. By the simultaneous rotation of the probe and the inner flange, the measurement points over the target can be easily chosen. Measurements were performed in a 0–908 sector that included two null regions and the parameters of measurements were the electron temperature and electron density distributions. Expressing the null regions in coordinate form (r, h) they become (64.3 mm, 22.58) and (64.3 mm, 67.58). The measured data was acquired with the probe tips set at 20 mm from the target surface and in intervals of 58. The double probe technique (which minimizes errors from magnetic field) used was tungsten tips of diameter 0.5 mm and length 5 mm. The I–V curve was recorded on an X–Y plotter. The experiments were mainly conducted with Ar gas of 3.6 mTorr at RF power of 400 W. In the probe experiments, a stainless steel target (comparatively of low sputter rate) was used. For a sputter experiments, Cu and Mo targets were used. The calculation of the magnetic field on the target surface was done and the results were shown in Fig. 2. The permanent magnets for the production of the null plasma are arranged symmetrically as shown Fig. 1. As shown in Fig. 1a with that arrangement of permanent magnet (of 120 mT) is the resulting magnetic field in a sector of 908 (from 08 to 908). With the same magnet strength of any four adjacent groups of magnets arranged as shown in the figure, the magnetic null region is formed as a point at the center of the plane containing the magnets; and spatially a line in the zdirection. The null regions, as seen in Fig. 2, are formed at (64.3 mm, 22.58) and (64.3 mm, 67.58) from the target surface in the z-direction; the B=0 line can be seen formed. When the high frequency electric field is applied to the null region via the target by a 13.56 MHz RF generator, the structure and direction of the electric and magnetic fields are similar to that produced in the antenna-type NLD plasma
Y.-M. Sung / Surface & Coatings Technology 193 (2005) 123–128
[9–12]. Thus electrons move in a similar manner (meandering motion) in the electric and magnetic fields as in the inductive-type NLD. The position of the null region in any adjacent two pairs can be achieved by varying the magnetic strength and position. Since the erosion region of the target is concentrated around the null region, it is possible to control it by varying the magnet strength and position. Fig. 3 shows the calculated results for the variation of magnetic null field distribution with the rotation of outer magnets. The surface magnetic flux density of permanent magnets was 120 mT. The number ! in this figure (also as shown in Fig. 1) indicates the permanent magnet at 08 with respect to the Y-axis. Fig. 3 shows the results of magnetic field distributions when the location of the number ! magnet was rotated from 08 to 458 by rotating the outer magnets. As shown in the figures, with a rotation angle of 458, the initial magnetic null at (64.3 mm, 22.58) was rotated 22.58, with its new coordinates at (64.3 mm, 458)—there was no significant change in radius in method of rotation. This implies that the angular location of null region on the surface of target can be varied with rotating the outer magnets. Moreover, if magnets of different flux densities are used, varying the
125
radial position of the null region is possible. This calculation result of Fig. 3 indicates that it is possible to dynamically control the plasma on the target surface, and target erosion and its uniformity can be greatly enhanced.
3. Experimental results and discussion Fig. 4a and b shows examples of the electron density and electron temperature, respectively, measured using the double probe method over the target surface (z=20 mm). Experimental conditions were as follows: gas pressure=3.6 mTorr, RF power =400 W and magnetic strength (at the magnet surface)=120 mT. The width of the region with 10 G and below the mentioned conditions was 10 mm. For electron density of 4a, they were 1.351017 and 1.21017 m 3, respectively, for the same points of measurement. In the profiles, the peak values are about 1.3 and 1.65 times the minimum values for the ion current density and electron density, respectively. Also, the peaks of the ion current density profile at 22.58 and 67.58 were 1.12 and 1.05 mA/ cm2, respectively. The peaks show at these points (null regions) in the system that it is very similar to the inductive type [8,10,12]. Therefore, the same type of electron heating
Fig. 3. Variation of magnetic null field distribution with the rotation of outer magnets.
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Fig. 5. Thickness profile of a Cu film deposited by the null plasma sputter system.
min. On the other hand, the thickness uniformity was remarkably improved with the substrate rotation, and the best thickness uniformity is F0.26% within a 20-cm diameter. This wave-like thickness profile on the substrate implies, as with the Knudsen’s cosine law [15], that the
Fig. 4. Measured electron temperature and electron density: (a) electron density; (b) electron temperature.
in this new type NLD can be expected. From Fig. 4b, it is seen that the minimum occurs around the null regions with values 4.2 and 4.6 eV. This may be attributed to the fact that the magnetic field around the two null regions similar to a magnetic mirror field configuration. The electrons flow inwards the null region easily and thus bulk electrons are formed. The shape of high-density (low-temperature) plasma in the magnetic mirror field region is also similar to that of the inductive-type NLD [8,10,12]. In addition, it can be seen that the erosion profile has peaks around null region of the cathode surface because of the inhomogeneity of the plasma distribution as shown in Fig. 4a and b. Fig. 5 shows the thickness profile of a copper (Cu) film deposited by the null plasma sputter system. The Si samples of 1.51.5 cm2 size, which were located in A–AV line in Fig. 2, were 100 mm away from the target surface. The film thickness was evaluated by a SEM observation, and deposition time was 45 min. As shown in this figure, when d s=100 mm, the maximum thickness with value of about 1.55 Am was obtained at the position of 08, 458 and 908 between the null region. The minimum of the thickness profile at the null position of 22.58 and 67.58 was about 1.4 Am. The uniformity of film thickness was F5.1%. From this thickness profile, the average deposition rate is about 32 nm/
Fig. 6. SEM images of the Mo/Si and MoO3/Si samples prepared by the null plasma sputtering: (a) Mo (fractured cross-section); (b) MoO3 (fractured cross-section).
Y.-M. Sung / Surface & Coatings Technology 193 (2005) 123–128
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small amount of carbon (C1s) contamination was also detected, but it was a peculiar characteristic of XPS analysis [16]. The concentration ratio (%) for Mo and O was about 22.9:77.1 at a top surface, whereas it was 26.1:73.9 at a depth of 100 2. These results strongly suggested that a highquality MoO3 film could be obtained under the abovementioned conditions. As can be seen in the results of Figs. 6 and 7, good quality films were achievable with the settings.
4. Conclusion
Fig. 7. XPS spectra of the Mo3d and O1s: (a) Mo3d; (b) O1s.
erosion area in the target surface during the deposition is non-uniform, which is closely related to the spatial distribution of the bombarding ions on the target surface. Also, it can be seen that the thickness uniformity is well enhanced with the dynamical plasma control although further detail work is necessary to establish this. Reactive sputtering was performed for the Mo and MoO3 thin film preparation using the system. Fig. 6a and b shows the SEM images of the samples grown on Si wafer using the null plasma sputter equipment. The MoO3 film was prepared under the sputter conditions where the gas pressure was 3.6 mTorr, RF power was 400 W, the ratio of mixture gas was Ar–O2 (10%), substrate temperature was 400 8C, and deposition time was 60 min. For the Mo film, the substrate temperature and O2 gas ratio were room temperature and 0%, respectively. Other conditions were the same in the case of MoO3 fabrication. As shown in Fig. 6a and b, the Mo and MoO3 layer of about 0.9 and 1 Am thickness, respectively, are observed in the fractured cross-section of the sample. It can be found from the SEM images of Fig. 6a and b that the substrate temperature and reactivity have large effect on the structure and growth of the film, and the system is very useful for performing the reactive deposition. Fig. 7 shows the XPS spectra of the Mo3d and O1s for the sample of Fig. 6. The Mo3d photoelectron two peaks were detected at the binding energies of 228.9 and 232.4 eV, while the binding energy of the O1s peak was 532.6 eV. A
The article reports on the operation of a novel-type sputtering system based on the magnetic null discharge concept for the thin film fabrication process. The calculated electron motion around the magnetic null region on the target surface showed the complex meandering motion, and the measured electron and ion properties possessed peaks on the null region like in the original null plasma concept. Experimentally, it was also found that the shape of highdensity and low-temperature plasma in the magnetic null field region was also similar to that of the inductive-type original null plasma. Also, it was ascertained that the position of the electron heating area coincides with the null region. Thus with the variation of the magnets strength and their arrangement, effective electron heating around the null region or electron heating area can be controlled. Although further work is necessary to establish the plasma dynamic control, the target erosion and its uniformity can be greatly enhanced with sputter application. Finally, it can be found from the result of a thin film deposition that the system is very useful for the performance of sputtering.
Acknowledgements This work was supported by Grant-in-Aid for Scientific Research of the Ministry of Education, Science, Sports and Culture, Japan. The author wishes to thank Profs. C. Honda, M. Otsubo and Mr. S. Atsuta of University of Miyazaki for their useful contribution.
References [1] [2] [3] [4] [5] [6] [7]
J.A. Thornton, Met. Finish. 77 (1997) 45. R. Kukla, J. Kieser, M. Mayer, IEEE Trans. Magn. 23 (1987) 137. T. Takahashi, M. Naoe, IEEE Trans. Magn. 25 (1987) 4174. J. Musil, J. Vac. Sci. Technol., A 17 (1999) 555. T. Uchida, Jpn. J. Appl. Phys. 33 (1994) L43. Z. Yoshida, et al., Phys. Rev. Lett. 81 (1998) 2458. W. Chen, T. Hayashi, M. Itoh, Y. Morikawa, Jpn. J. Appl. Phys. 38 (1999) 4296. [8] T. Sakoda, T. Miyao, K. Uchino, K. Muraoka, Jpn. J. Appl. Phys. 36 (1997) 6981. [9] Y.M. Sung, K. Uchino, K. Muraoka, T. Sakoda, J. Vac. Sci. Technol. A18 (2000) 2149.
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[10] Y. Okraku-Yirenkyi, Y.M. Sung, et al., J. Vac. Sci. Technol., A 19 (2001) 2590. [11] T. Sakoda, Y. Okraku-Yirenkyi, Y.M. Sung, et al., Jpn. Appl. Phys. 40 (2001) 6607. [12] Y.M. Sung, C. Honda, J. Vac. Sci. Technol., B 20 (2002) 1457.
[13] T. Sakoda, Y.M. Sung, J. Vac. Sci. Technol., A 20 (2002) 1964. [14] Y.M. Sung, et al., IEEE Trans. Plasma Sci. 30 (2002) 142. [15] B. Chapman, Glow Discharge Processes, John Wiley and Sons, New York, 1980. [16] M. Yahaya, et al., Proc. SPIE Int. Soc. Opt. Eng. 42 (1998) 3175.
Magnetic null discharge sputtering with full target erosion Youl-Moon Sung* Department of Electrical Electronic Engineering, University of Miyazaki, 1-1 Gakuenkibanadai-Nishi, Miyazaki 889-2192, Japan Available online 19 September 2004
Abstract The article reports on the operation of a novel-type sputtering system based on the magnetic null discharge concept for the thin film fabrication process. The unique feature of the system is the plasma production around the magnetic null field region on the target surface. The calculated electron motion around the magnetic null region on the target surface showed the complex meandering motion, and the measured electron and ion properties possessed peaks on the null region like in the original null plasma concept. Experimentally, it was also found that the shape of high-density and low-temperature plasma in the magnetic null field region was also similar to that of the inductive-type original null plasma. It is therefore expected that the dynamic plasma control over the target surface will be realized since rotating and arranging the magnets can actively control the plasma peaks. With sputtering application, it is possible to achieve almost full target erosion and thus significantly increase the usage lifetime of target. Also, it can be found from the result of a thin film deposition that the system is very useful for the performance of sputtering. D 2004 Elsevier B.V. All rights reserved. Keywords: Magnetic null plasma; Neutral loop discharge; Target erosion; Sputtering
1. Introduction Presently, magnetron sputtering [1] is a well-established technology, successfully used in a wade range of applications, many of which have been scaled up for industrial use. However, the target utilization of magnetrons currently produced is low of about 20–30%. It can be increased up to 50–60% in the case when the magnetron has an optimized magnetic field. Since the magnetron discharge forms a closed loop, there is always a certain part of the target inside the closed loop where no target sputtering occurs. In principle, there are two ways on how to achieve full erosion of the target. (1) To design a magnetron with full erosion target, i.e., a target without a center, which is not sputtered. (2) To control the sputtering region dynamically on the target surface during discharge using an appropriate magnetic method. The first method resulted in the realization of magnetrons, for example, interpole target magnetron [2], toroidal plasma-type mag-
* Tel./fax: +81 985 58 7350. E-mail address: [email protected]. 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.07.122
netron [3] and rectangular magnetron [4], but some of these methods are somewhat complicated structurally. The second method, if it can be realized, will be very convenient and effective for full target erosion; however, there has been no report on this. This study addresses the latter method. In recent years, a neutral loop discharge (NLD) [5–9] is a new plasma source utilizing the magnetic null discharge, which moves in a non-linear manner around the null region with effective electron heating at pressures of less than 1 mTorr and yet achieving high plasma density. It is also that kind of technology where the null region can easily be controlled by varying currents in the coils. By this processing over large surfaces areas with high uniformity can be realized and thus is attracting a lot of attention. In etching applications, adequate work has been done making the NLD technology industrially viable with good results [7]. We have been conducting process application in the NLD technology [9–13]. In addition, we seek to suggest a capacitive-coupled NLD plasma without the use of an antenna in order to realize to sputtering applications [14]. With the dynamic controllability of the NLD technology made applicable to sputter
124
Y.-M. Sung / Surface & Coatings Technology 193 (2005) 123–128
processes, there is the possibility of overcoming the erosion problem as well as achieving low pressure and high efficiency in operation. In this research, we present the design and construct of the new type null plasma sputtering system. We also seek to quantify through simulation and experiments, to clarify the plasma formation and structure. For the evaluation of the sputter efficiency, thin film growth experiments have been conducted.
2. System geometry and concept Fig. 1 shows the target of the equipment with the magnetic field formed by the permanent magnets. The chamber is of stainless steel of internal diameter 308 mm. The target is of doughnut shape of width 70 mm with the anode on the inner and outer sides of the target. Eight pairs of permanent magnet (neodymium, size: 10205 mm) are arranged on the outer and inner peripherals of the target inside the anode. The target hold is made of stainless steel and the disc width, 71.5 mm. Cooling water is circulated through the target to prevent overheating. The internal diameter of the cylinder housing the permanent magnet is 53 mm with its corresponding external diameter 204 mm grounded. The gap between the two rings is set less than the Paschen minimum (2 mm) to prevent unnecessary discharge. In order to measure the two-dimensional distributions of the electron density and electron temperature of the area over the doughnut target, the measuring probe and its supporting flange were both designed to allow two-stage rotation. The two-stage rotational probe consists of the double probe itself, inner flange and outer
Fig. 1. Schematic arrangement of the experimental apparatus: (a) front view; (b) side view.
Fig. 2. Calculated magnetic field line distribution.
flange. By the simultaneous rotation of the probe and the inner flange, the measurement points over the target can be easily chosen. Measurements were performed in a 0–908 sector that included two null regions and the parameters of measurements were the electron temperature and electron density distributions. Expressing the null regions in coordinate form (r, h) they become (64.3 mm, 22.58) and (64.3 mm, 67.58). The measured data was acquired with the probe tips set at 20 mm from the target surface and in intervals of 58. The double probe technique (which minimizes errors from magnetic field) used was tungsten tips of diameter 0.5 mm and length 5 mm. The I–V curve was recorded on an X–Y plotter. The experiments were mainly conducted with Ar gas of 3.6 mTorr at RF power of 400 W. In the probe experiments, a stainless steel target (comparatively of low sputter rate) was used. For a sputter experiments, Cu and Mo targets were used. The calculation of the magnetic field on the target surface was done and the results were shown in Fig. 2. The permanent magnets for the production of the null plasma are arranged symmetrically as shown Fig. 1. As shown in Fig. 1a with that arrangement of permanent magnet (of 120 mT) is the resulting magnetic field in a sector of 908 (from 08 to 908). With the same magnet strength of any four adjacent groups of magnets arranged as shown in the figure, the magnetic null region is formed as a point at the center of the plane containing the magnets; and spatially a line in the zdirection. The null regions, as seen in Fig. 2, are formed at (64.3 mm, 22.58) and (64.3 mm, 67.58) from the target surface in the z-direction; the B=0 line can be seen formed. When the high frequency electric field is applied to the null region via the target by a 13.56 MHz RF generator, the structure and direction of the electric and magnetic fields are similar to that produced in the antenna-type NLD plasma
Y.-M. Sung / Surface & Coatings Technology 193 (2005) 123–128
[9–12]. Thus electrons move in a similar manner (meandering motion) in the electric and magnetic fields as in the inductive-type NLD. The position of the null region in any adjacent two pairs can be achieved by varying the magnetic strength and position. Since the erosion region of the target is concentrated around the null region, it is possible to control it by varying the magnet strength and position. Fig. 3 shows the calculated results for the variation of magnetic null field distribution with the rotation of outer magnets. The surface magnetic flux density of permanent magnets was 120 mT. The number ! in this figure (also as shown in Fig. 1) indicates the permanent magnet at 08 with respect to the Y-axis. Fig. 3 shows the results of magnetic field distributions when the location of the number ! magnet was rotated from 08 to 458 by rotating the outer magnets. As shown in the figures, with a rotation angle of 458, the initial magnetic null at (64.3 mm, 22.58) was rotated 22.58, with its new coordinates at (64.3 mm, 458)—there was no significant change in radius in method of rotation. This implies that the angular location of null region on the surface of target can be varied with rotating the outer magnets. Moreover, if magnets of different flux densities are used, varying the
125
radial position of the null region is possible. This calculation result of Fig. 3 indicates that it is possible to dynamically control the plasma on the target surface, and target erosion and its uniformity can be greatly enhanced.
3. Experimental results and discussion Fig. 4a and b shows examples of the electron density and electron temperature, respectively, measured using the double probe method over the target surface (z=20 mm). Experimental conditions were as follows: gas pressure=3.6 mTorr, RF power =400 W and magnetic strength (at the magnet surface)=120 mT. The width of the region with 10 G and below the mentioned conditions was 10 mm. For electron density of 4a, they were 1.351017 and 1.21017 m 3, respectively, for the same points of measurement. In the profiles, the peak values are about 1.3 and 1.65 times the minimum values for the ion current density and electron density, respectively. Also, the peaks of the ion current density profile at 22.58 and 67.58 were 1.12 and 1.05 mA/ cm2, respectively. The peaks show at these points (null regions) in the system that it is very similar to the inductive type [8,10,12]. Therefore, the same type of electron heating
Fig. 3. Variation of magnetic null field distribution with the rotation of outer magnets.
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Y.-M. Sung / Surface & Coatings Technology 193 (2005) 123–128
Fig. 5. Thickness profile of a Cu film deposited by the null plasma sputter system.
min. On the other hand, the thickness uniformity was remarkably improved with the substrate rotation, and the best thickness uniformity is F0.26% within a 20-cm diameter. This wave-like thickness profile on the substrate implies, as with the Knudsen’s cosine law [15], that the
Fig. 4. Measured electron temperature and electron density: (a) electron density; (b) electron temperature.
in this new type NLD can be expected. From Fig. 4b, it is seen that the minimum occurs around the null regions with values 4.2 and 4.6 eV. This may be attributed to the fact that the magnetic field around the two null regions similar to a magnetic mirror field configuration. The electrons flow inwards the null region easily and thus bulk electrons are formed. The shape of high-density (low-temperature) plasma in the magnetic mirror field region is also similar to that of the inductive-type NLD [8,10,12]. In addition, it can be seen that the erosion profile has peaks around null region of the cathode surface because of the inhomogeneity of the plasma distribution as shown in Fig. 4a and b. Fig. 5 shows the thickness profile of a copper (Cu) film deposited by the null plasma sputter system. The Si samples of 1.51.5 cm2 size, which were located in A–AV line in Fig. 2, were 100 mm away from the target surface. The film thickness was evaluated by a SEM observation, and deposition time was 45 min. As shown in this figure, when d s=100 mm, the maximum thickness with value of about 1.55 Am was obtained at the position of 08, 458 and 908 between the null region. The minimum of the thickness profile at the null position of 22.58 and 67.58 was about 1.4 Am. The uniformity of film thickness was F5.1%. From this thickness profile, the average deposition rate is about 32 nm/
Fig. 6. SEM images of the Mo/Si and MoO3/Si samples prepared by the null plasma sputtering: (a) Mo (fractured cross-section); (b) MoO3 (fractured cross-section).
Y.-M. Sung / Surface & Coatings Technology 193 (2005) 123–128
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small amount of carbon (C1s) contamination was also detected, but it was a peculiar characteristic of XPS analysis [16]. The concentration ratio (%) for Mo and O was about 22.9:77.1 at a top surface, whereas it was 26.1:73.9 at a depth of 100 2. These results strongly suggested that a highquality MoO3 film could be obtained under the abovementioned conditions. As can be seen in the results of Figs. 6 and 7, good quality films were achievable with the settings.
4. Conclusion
Fig. 7. XPS spectra of the Mo3d and O1s: (a) Mo3d; (b) O1s.
erosion area in the target surface during the deposition is non-uniform, which is closely related to the spatial distribution of the bombarding ions on the target surface. Also, it can be seen that the thickness uniformity is well enhanced with the dynamical plasma control although further detail work is necessary to establish this. Reactive sputtering was performed for the Mo and MoO3 thin film preparation using the system. Fig. 6a and b shows the SEM images of the samples grown on Si wafer using the null plasma sputter equipment. The MoO3 film was prepared under the sputter conditions where the gas pressure was 3.6 mTorr, RF power was 400 W, the ratio of mixture gas was Ar–O2 (10%), substrate temperature was 400 8C, and deposition time was 60 min. For the Mo film, the substrate temperature and O2 gas ratio were room temperature and 0%, respectively. Other conditions were the same in the case of MoO3 fabrication. As shown in Fig. 6a and b, the Mo and MoO3 layer of about 0.9 and 1 Am thickness, respectively, are observed in the fractured cross-section of the sample. It can be found from the SEM images of Fig. 6a and b that the substrate temperature and reactivity have large effect on the structure and growth of the film, and the system is very useful for performing the reactive deposition. Fig. 7 shows the XPS spectra of the Mo3d and O1s for the sample of Fig. 6. The Mo3d photoelectron two peaks were detected at the binding energies of 228.9 and 232.4 eV, while the binding energy of the O1s peak was 532.6 eV. A
The article reports on the operation of a novel-type sputtering system based on the magnetic null discharge concept for the thin film fabrication process. The calculated electron motion around the magnetic null region on the target surface showed the complex meandering motion, and the measured electron and ion properties possessed peaks on the null region like in the original null plasma concept. Experimentally, it was also found that the shape of highdensity and low-temperature plasma in the magnetic null field region was also similar to that of the inductive-type original null plasma. Also, it was ascertained that the position of the electron heating area coincides with the null region. Thus with the variation of the magnets strength and their arrangement, effective electron heating around the null region or electron heating area can be controlled. Although further work is necessary to establish the plasma dynamic control, the target erosion and its uniformity can be greatly enhanced with sputter application. Finally, it can be found from the result of a thin film deposition that the system is very useful for the performance of sputtering.
Acknowledgements This work was supported by Grant-in-Aid for Scientific Research of the Ministry of Education, Science, Sports and Culture, Japan. The author wishes to thank Profs. C. Honda, M. Otsubo and Mr. S. Atsuta of University of Miyazaki for their useful contribution.
References [1] [2] [3] [4] [5] [6] [7]
J.A. Thornton, Met. Finish. 77 (1997) 45. R. Kukla, J. Kieser, M. Mayer, IEEE Trans. Magn. 23 (1987) 137. T. Takahashi, M. Naoe, IEEE Trans. Magn. 25 (1987) 4174. J. Musil, J. Vac. Sci. Technol., A 17 (1999) 555. T. Uchida, Jpn. J. Appl. Phys. 33 (1994) L43. Z. Yoshida, et al., Phys. Rev. Lett. 81 (1998) 2458. W. Chen, T. Hayashi, M. Itoh, Y. Morikawa, Jpn. J. Appl. Phys. 38 (1999) 4296. [8] T. Sakoda, T. Miyao, K. Uchino, K. Muraoka, Jpn. J. Appl. Phys. 36 (1997) 6981. [9] Y.M. Sung, K. Uchino, K. Muraoka, T. Sakoda, J. Vac. Sci. Technol. A18 (2000) 2149.
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[10] Y. Okraku-Yirenkyi, Y.M. Sung, et al., J. Vac. Sci. Technol., A 19 (2001) 2590. [11] T. Sakoda, Y. Okraku-Yirenkyi, Y.M. Sung, et al., Jpn. Appl. Phys. 40 (2001) 6607. [12] Y.M. Sung, C. Honda, J. Vac. Sci. Technol., B 20 (2002) 1457.
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