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Evaporation–glow discharge hybrid source for plasma immersion ion implantation
Surface & Coatings Technology 186 (2004) 165 – 169 www.elsevier.com/locate/surfcoat
Evaporation–glow discharge hybrid source for plasma immersion ion implantation L.H. Li
a,b,c
, Ricky K.Y. Fu b, R.W.Y. Poon b, S.C.H. Kwok b, P.K. Chu b,*, Y.Q. Wu c, Y.H. Zhang c, X. Cai a, Q.L. Chen a, M. Xu a
a School of Material Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China c Department of Materials Processing and Controlling (702), School of Mechanical Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China b
Available online 25 May 2004
Abstract The plasma ion sources play a very important role in the plasma immersion ion implantation (PIII) process. In this paper, we report on our newly designed evaporation – glow discharge hybrid ion source for PIII. The high negative substrate bias not only acts as the plasma producer but also provides the implantation voltage. The sulfur vapor gas glow discharge shows that the electrons in the plasma are focused to the orifice of the inlet tube, thereby helping the ionization of the fleeing vapor gases. The sulfur depth profile confirms that this evaporation – glow discharge hybrid source is effective for materials with a low melting point and high vapor pressure. D 2004 Elsevier B.V. All rights reserved. Keywords: Plasma immersion ion implantation; Sulfur; Glow discharge
1. Introduction Plasma immersion ion implantation (PIII) is a useful niche technology for the modification of surface properties of materials and industrial components that are large or have irregular shapes. In the PIII technique, the substrate is usually immersed in the plasma and then biased by the high negative-pulsed voltage. The electrons are immediately repelled and the sheath dynamically expanded during the pulse. Ions are accelerated through the sheath and implanted into the surface of the substrate. PIII eliminates the need of beam rastering because the entire target is immersed in the plasma [1,2]. Hence, the plasma source usually plays a very important role in PIII. Many kinds of plasma production methods, such as radio frequency (RF) microwave, hot filament and vacuum arc are used as ion sources in the PIII device [3– 9]. However, up to now, there is no single method that can satisfactorily produce ions of a wide spectrum of elements, let alone all the elements in the periodic table. For example, it is still * Corresponding author. Tel.: +852-2788-7724; fax: +852-2788-9549. E-mail address: [email protected] (P.K. Chu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.04.025
difficult to ionize solid materials with poor electrical conductivity, such as sulfur, phosphorus and boron; semiconductor materials such as silicon, germanium; and elements with low melt point and/or high chemical activity like the group IA and IIA elements. In this article, we report an evaporation – glow discharge hybrid source for plasma immersion ion implantation. It is designed for elements with a low melting point and high vapor pressure such as sulfur and phosphorus that are very important elements in the surface modification of biomaterials. In this method, the material is initially vaporized in a source chamber and then introduced via a 6-mm-diameter gas inlet tube into a small implantation chamber with an internal glass shield to reduce contamination. The source chamber has a large surface area relative to that of the orifice (inner diameter of the gas inlet tube) through which the evaporated species escapes. This provides quasi-equilibrium and yields a steady and easily controllable implantation process. The high negative pulses exerted on the substrate play dual roles on creating plasma and implantation. The plasma is composed of a neutral evaporated gas introduced into the glass-surrounded chamber. The neutral evaporated gas is ionized such that a constant source of
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L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169
plasma is provided to surround the sample during the implantation process.
the implantation chamber was kept at 0.032 –0.01 Pa and 0.014– 0.01 Pa.
2. Experimental details
3. Results and discussion
The plasma immersion ion implanter is schematically illustrated in Fig. 1. In addition to the evaporation –glow discharge plasma source, the PIII equipment has some other plasma producing systems—the pulse arc plasma ion source, the RF plasma source, and the hot filament system, which are not shown here. The evaporation – glow discharge hybrid plasma ion source comprises a sulfur evaporation source container, argon feed tube and valve, gas inlet tube, and the implantation chamber. The substrate holder is placed in the implantation chamber with a diameter of 150 mm and height of 240 mm. The distance between the vapor outlet and substrate is 220 mm. A 100-mm silicon wafer was used as the substrate and placed on the substrate holder. Sulfur was vaporized in a quartz evaporation container and fed into the implantation chamber through a grounded gas inlet tube with an inner diameter of 6 mm. The sulfur vapor was introduced by two means: (1) Sulfur vapor was used as the source and fed into the implantation chamber directly. (2) The sulfur vapor was first mixed with argon, the carrier gas, and then fed into the implantation chamber. The sulfur vapor and sulfur + argon vapor were fed in separately, and high negative pulses of 10 kV for 1 h or 15 kV for 1 h, and 25 kV for 2 h were applied to the substrate. The pressure in
Fig. 2 is a picture depicting the glow discharge of the hybrid plasma source. Some of the typical characters of glow discharge are noticeable. The brightest part of the discharge is different from the normal parallel plane electrode glow discharge where the brightest part is usually the negative glow. The electrodes used here can be considered as a tube anode and a plate cathode. The brightest part in this glow discharge is the anode glow where the electrons are obviously focused. Under this field, the electrons highly interact with the outlet vapor gas atoms and assist in the vapor atom ionization. The tube anode can be considered as a hollow anode. The second brightest part is the negative glow, which is separated from the cathode by the cathode dark space. The negative glow and the positive column are separated by the Faraday dark space. A focused light at the cathode can also been observed. The diameter of the negative glow is obviously less than that of the Si substrate, and this is because the conductance of the Si substrate is much less than that of the copper substrate holder whose diameter is less than that of the Si substrate. It can be concluded that to achieve uniform implantation, the diameter of the substrate holder should not be less than the substrate. Because the electrons in the plasma are focused to
Fig. 1. Schematic of the plasma immersion ion implantation equipment with the evaporation – glow discharge hybrid source (the ball shape feature near the capacitive RF antenna is the exit of the third pulsed arc plasma source).
L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169
Fig. 2. Photograph showing the glow discharge of the sulfur vapor.
the tube outlet orifice where the evaporated atoms flee out, we believe that this will help the ionization of the gasses. This is being further studied by using computer simulation in our laboratory. Fig. 3a,b displays the implantation current and voltage waveforms at 25 and 15 kV, respectively. The current
167
waveforms exhibit two spikes at the beginning and at the end of the pulse. These spikes are the sums of the true implanted ion current, secondary electron current, as well as system capacitance. To subtract the system capacitance, Fig. 3c,d shows the spike current waveforms attributable to the system capacitance without plasma production at 25 and 15 kV, respectively. From Fig. 3a,b, it can be observed that the implantation current does not show a continuously decreasing trend, which is typical of the expansion of the plasma sheath at low pressure [10,11]. It is also different from high-pressure, high-voltage implantation in which the current waveform usually exhibits an obvious breakdown current increase [12]. The implantation current increases obviously only at the beginning. Once the glow discharge is established, the current increase slowly. This is true for at least in the given pulse duration and the current increase may not reach arc breakdown. As a result, this system is safe and steady with control of the sulfur evaporation rate. A further discussion can be found elsewhere [13]. The sulfur depth profiles acquired by sputtering X-ray photoelectron spectroscopy are shown in Fig. 4. The simulation results by TRIM are also given. The sulfur depth distribution for 10 kV implantation reveals a roughly Gaussian distribution. The peak position is at 15 nm and consistent with the TRIM simulation. The width of the S distribution (Gaussian peak) profile is less
Fig. 3. Glow discharge and spike current and voltage waveforms. (The high voltage was operated at a frequency of 100 Hz with a pulse duration of 250 As, corresponding to a duty factor of 2.5%.): (a) 25 kV bias, (b)15 kV, (c) spike for 25 kV, and (d) spike for 15 kV.
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Fig. 4. Sulfur depth profiles acquired from (a) 15 + 25 kV with sulfur + argon vapor and (b) 10 kV with sulfur vapor and the simulation results using the TRIM.
than that of the simulated one. This may be due to the measurement error by XPS and other factors. However, by considering that the width of the 15 + 25 kV implantation sample is also less than that of the simulation one, we believe that the sputtering depth per minute is underestimated. The outline shape of the sulfur depth distribution implanted for 15 + 25 kV biases seems also to be like the TRIM calculated one without considering the peak position. The peak position of the XPS result is nearer to the surface than that of the calculated one. The reason is that during the TRIM simulation, Ar is neglected. During the 15 + 25 kV process, Ar was used as the carrying gas. Because Ar has a higher sputtering yield, surface etching of sample B should be more serious, thereby resulting in an apparently shallower distribution. Anyway, from Fig. 4, it can be concluded that the evaporation – glow discharge hybrid source is suitable for plasma immersion ion implantation.
atoms implanted by this method exhibit a typical Gaussian distribution. The XPS depth profile results show that the evaporation –glow discharge hybrid source is an effective method for elements possessing a low melting point and high vapor pressure. If Ar is mixed in the feeding vapor gases, it can be ionized at the same time and will etch the surface of the substrate seriously.
4. Conclusion
References
The high negative voltage exerted on the substrate plays two roles in PIII as the plasma producer as well as implantation. The grounded gas inlet tube is the anode for the glow discharge and the electrons in the plasma are focused to the end of the inlet tube where the vapor gas escapes. This helps the ionization of the vapor gases and the anode glow becomes the brightest discharge area. The current also reveals spikes at the beginning and end of the negative pulse. The slight increase in the implantation current shows that arc breakdown can be avoided. The S
Acknowledgements This work was jointly supported by the National Natural Science Foundation of China, No. 50271004, Hong Kong Research Grants Council (RGC) Competitive Earmarked Research Grant (CERG) #CityU1137/03E, Germany/Hong Kong RGC Joint Research Scheme #G_HK001/02 (CityU designation 9050165) and RGC/NSFC Joint Scheme N_CityU101/03.
[1] J.R. Conrad, J.I. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. [2] P.K. Chu, S. Qin, C. Chan, N.W. Cheung, L.A. Larson, Mater. Sci. Eng., Rep. 17 (1996) 207. [3] X.B. Tian, T. Zhang, R.K.Y. Fu, P.K. Chu, Surf. Coat. Technol. 161 (2 – 3) (2002) 232. [4] H. Bender, J. Halder, F. Hilschert, H. Klein, J. Scha¨fer, B. Seiler, R. Thomae, Surf. Coat. Technol. 93 (2 – 3) (1997) 213. [5] B. Seiler, H. Bender, H. Klein, J. Halder, J. Scha¨fer, R. Thomae, J. Brutscher, R. Gu¨nzel, W. Ensinger, B. Rauschenbach, Nucl. Instrum. Methods Phys. Res., B 113 (1 – 4) (1996) 266. [6] I.G. Brown, Surf. Coat. Technol. 136 (1 – 3) (2001) 16.
L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169 [7] L.H. Li, T. Zhang, P.K. Chu, I.G. Brown, Rev. Sci. Instrum. 73 (8) (2002) 2971. [8] S. Schoser, J. Forget, K. Kohlhof, Surf. Coat. Technol. 93 (2 – 3) (1997) 339. [9] C. Ruset, E. Grigore, Surf. Coat. Technol. 156 (1 – 3) (2002) 159. [10] P.K. Chu, S. Qin, C. Chan, N.W. Cheung, P.K. Ko, IEEE Trans. Plasma Sci. 26 (1) (1998) 79.
169
[11] J. Pelletier, F. Le Coeur, Y. Arnal, A. Lacoste, A. Straboni, Surf. Coat. Technol. 136 (2001) 7. [12] J.N. Matossian, R.H. Wei, Surf. Coat. Technol. 85 (1996) 92. [13] L.H. Li, R.W.Y. Poon, S.C.H. Kwok, P.K. Chu, R.Q. Wu, Y.H. Zhang, Rev. Sci. Instrum. 74 (10) (2003) 4301.
Evaporation–glow discharge hybrid source for plasma immersion ion implantation L.H. Li
a,b,c
, Ricky K.Y. Fu b, R.W.Y. Poon b, S.C.H. Kwok b, P.K. Chu b,*, Y.Q. Wu c, Y.H. Zhang c, X. Cai a, Q.L. Chen a, M. Xu a
a School of Material Science and Engineering, Shanghai Jiao Tong University, Shanghai 200030, China Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, China c Department of Materials Processing and Controlling (702), School of Mechanical Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China b
Available online 25 May 2004
Abstract The plasma ion sources play a very important role in the plasma immersion ion implantation (PIII) process. In this paper, we report on our newly designed evaporation – glow discharge hybrid ion source for PIII. The high negative substrate bias not only acts as the plasma producer but also provides the implantation voltage. The sulfur vapor gas glow discharge shows that the electrons in the plasma are focused to the orifice of the inlet tube, thereby helping the ionization of the fleeing vapor gases. The sulfur depth profile confirms that this evaporation – glow discharge hybrid source is effective for materials with a low melting point and high vapor pressure. D 2004 Elsevier B.V. All rights reserved. Keywords: Plasma immersion ion implantation; Sulfur; Glow discharge
1. Introduction Plasma immersion ion implantation (PIII) is a useful niche technology for the modification of surface properties of materials and industrial components that are large or have irregular shapes. In the PIII technique, the substrate is usually immersed in the plasma and then biased by the high negative-pulsed voltage. The electrons are immediately repelled and the sheath dynamically expanded during the pulse. Ions are accelerated through the sheath and implanted into the surface of the substrate. PIII eliminates the need of beam rastering because the entire target is immersed in the plasma [1,2]. Hence, the plasma source usually plays a very important role in PIII. Many kinds of plasma production methods, such as radio frequency (RF) microwave, hot filament and vacuum arc are used as ion sources in the PIII device [3– 9]. However, up to now, there is no single method that can satisfactorily produce ions of a wide spectrum of elements, let alone all the elements in the periodic table. For example, it is still * Corresponding author. Tel.: +852-2788-7724; fax: +852-2788-9549. E-mail address: [email protected] (P.K. Chu). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.04.025
difficult to ionize solid materials with poor electrical conductivity, such as sulfur, phosphorus and boron; semiconductor materials such as silicon, germanium; and elements with low melt point and/or high chemical activity like the group IA and IIA elements. In this article, we report an evaporation – glow discharge hybrid source for plasma immersion ion implantation. It is designed for elements with a low melting point and high vapor pressure such as sulfur and phosphorus that are very important elements in the surface modification of biomaterials. In this method, the material is initially vaporized in a source chamber and then introduced via a 6-mm-diameter gas inlet tube into a small implantation chamber with an internal glass shield to reduce contamination. The source chamber has a large surface area relative to that of the orifice (inner diameter of the gas inlet tube) through which the evaporated species escapes. This provides quasi-equilibrium and yields a steady and easily controllable implantation process. The high negative pulses exerted on the substrate play dual roles on creating plasma and implantation. The plasma is composed of a neutral evaporated gas introduced into the glass-surrounded chamber. The neutral evaporated gas is ionized such that a constant source of
166
L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169
plasma is provided to surround the sample during the implantation process.
the implantation chamber was kept at 0.032 –0.01 Pa and 0.014– 0.01 Pa.
2. Experimental details
3. Results and discussion
The plasma immersion ion implanter is schematically illustrated in Fig. 1. In addition to the evaporation –glow discharge plasma source, the PIII equipment has some other plasma producing systems—the pulse arc plasma ion source, the RF plasma source, and the hot filament system, which are not shown here. The evaporation – glow discharge hybrid plasma ion source comprises a sulfur evaporation source container, argon feed tube and valve, gas inlet tube, and the implantation chamber. The substrate holder is placed in the implantation chamber with a diameter of 150 mm and height of 240 mm. The distance between the vapor outlet and substrate is 220 mm. A 100-mm silicon wafer was used as the substrate and placed on the substrate holder. Sulfur was vaporized in a quartz evaporation container and fed into the implantation chamber through a grounded gas inlet tube with an inner diameter of 6 mm. The sulfur vapor was introduced by two means: (1) Sulfur vapor was used as the source and fed into the implantation chamber directly. (2) The sulfur vapor was first mixed with argon, the carrier gas, and then fed into the implantation chamber. The sulfur vapor and sulfur + argon vapor were fed in separately, and high negative pulses of 10 kV for 1 h or 15 kV for 1 h, and 25 kV for 2 h were applied to the substrate. The pressure in
Fig. 2 is a picture depicting the glow discharge of the hybrid plasma source. Some of the typical characters of glow discharge are noticeable. The brightest part of the discharge is different from the normal parallel plane electrode glow discharge where the brightest part is usually the negative glow. The electrodes used here can be considered as a tube anode and a plate cathode. The brightest part in this glow discharge is the anode glow where the electrons are obviously focused. Under this field, the electrons highly interact with the outlet vapor gas atoms and assist in the vapor atom ionization. The tube anode can be considered as a hollow anode. The second brightest part is the negative glow, which is separated from the cathode by the cathode dark space. The negative glow and the positive column are separated by the Faraday dark space. A focused light at the cathode can also been observed. The diameter of the negative glow is obviously less than that of the Si substrate, and this is because the conductance of the Si substrate is much less than that of the copper substrate holder whose diameter is less than that of the Si substrate. It can be concluded that to achieve uniform implantation, the diameter of the substrate holder should not be less than the substrate. Because the electrons in the plasma are focused to
Fig. 1. Schematic of the plasma immersion ion implantation equipment with the evaporation – glow discharge hybrid source (the ball shape feature near the capacitive RF antenna is the exit of the third pulsed arc plasma source).
L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169
Fig. 2. Photograph showing the glow discharge of the sulfur vapor.
the tube outlet orifice where the evaporated atoms flee out, we believe that this will help the ionization of the gasses. This is being further studied by using computer simulation in our laboratory. Fig. 3a,b displays the implantation current and voltage waveforms at 25 and 15 kV, respectively. The current
167
waveforms exhibit two spikes at the beginning and at the end of the pulse. These spikes are the sums of the true implanted ion current, secondary electron current, as well as system capacitance. To subtract the system capacitance, Fig. 3c,d shows the spike current waveforms attributable to the system capacitance without plasma production at 25 and 15 kV, respectively. From Fig. 3a,b, it can be observed that the implantation current does not show a continuously decreasing trend, which is typical of the expansion of the plasma sheath at low pressure [10,11]. It is also different from high-pressure, high-voltage implantation in which the current waveform usually exhibits an obvious breakdown current increase [12]. The implantation current increases obviously only at the beginning. Once the glow discharge is established, the current increase slowly. This is true for at least in the given pulse duration and the current increase may not reach arc breakdown. As a result, this system is safe and steady with control of the sulfur evaporation rate. A further discussion can be found elsewhere [13]. The sulfur depth profiles acquired by sputtering X-ray photoelectron spectroscopy are shown in Fig. 4. The simulation results by TRIM are also given. The sulfur depth distribution for 10 kV implantation reveals a roughly Gaussian distribution. The peak position is at 15 nm and consistent with the TRIM simulation. The width of the S distribution (Gaussian peak) profile is less
Fig. 3. Glow discharge and spike current and voltage waveforms. (The high voltage was operated at a frequency of 100 Hz with a pulse duration of 250 As, corresponding to a duty factor of 2.5%.): (a) 25 kV bias, (b)15 kV, (c) spike for 25 kV, and (d) spike for 15 kV.
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L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169
Fig. 4. Sulfur depth profiles acquired from (a) 15 + 25 kV with sulfur + argon vapor and (b) 10 kV with sulfur vapor and the simulation results using the TRIM.
than that of the simulated one. This may be due to the measurement error by XPS and other factors. However, by considering that the width of the 15 + 25 kV implantation sample is also less than that of the simulation one, we believe that the sputtering depth per minute is underestimated. The outline shape of the sulfur depth distribution implanted for 15 + 25 kV biases seems also to be like the TRIM calculated one without considering the peak position. The peak position of the XPS result is nearer to the surface than that of the calculated one. The reason is that during the TRIM simulation, Ar is neglected. During the 15 + 25 kV process, Ar was used as the carrying gas. Because Ar has a higher sputtering yield, surface etching of sample B should be more serious, thereby resulting in an apparently shallower distribution. Anyway, from Fig. 4, it can be concluded that the evaporation – glow discharge hybrid source is suitable for plasma immersion ion implantation.
atoms implanted by this method exhibit a typical Gaussian distribution. The XPS depth profile results show that the evaporation –glow discharge hybrid source is an effective method for elements possessing a low melting point and high vapor pressure. If Ar is mixed in the feeding vapor gases, it can be ionized at the same time and will etch the surface of the substrate seriously.
4. Conclusion
References
The high negative voltage exerted on the substrate plays two roles in PIII as the plasma producer as well as implantation. The grounded gas inlet tube is the anode for the glow discharge and the electrons in the plasma are focused to the end of the inlet tube where the vapor gas escapes. This helps the ionization of the vapor gases and the anode glow becomes the brightest discharge area. The current also reveals spikes at the beginning and end of the negative pulse. The slight increase in the implantation current shows that arc breakdown can be avoided. The S
Acknowledgements This work was jointly supported by the National Natural Science Foundation of China, No. 50271004, Hong Kong Research Grants Council (RGC) Competitive Earmarked Research Grant (CERG) #CityU1137/03E, Germany/Hong Kong RGC Joint Research Scheme #G_HK001/02 (CityU designation 9050165) and RGC/NSFC Joint Scheme N_CityU101/03.
[1] J.R. Conrad, J.I. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran, J. Appl. Phys. 62 (1987) 4591. [2] P.K. Chu, S. Qin, C. Chan, N.W. Cheung, L.A. Larson, Mater. Sci. Eng., Rep. 17 (1996) 207. [3] X.B. Tian, T. Zhang, R.K.Y. Fu, P.K. Chu, Surf. Coat. Technol. 161 (2 – 3) (2002) 232. [4] H. Bender, J. Halder, F. Hilschert, H. Klein, J. Scha¨fer, B. Seiler, R. Thomae, Surf. Coat. Technol. 93 (2 – 3) (1997) 213. [5] B. Seiler, H. Bender, H. Klein, J. Halder, J. Scha¨fer, R. Thomae, J. Brutscher, R. Gu¨nzel, W. Ensinger, B. Rauschenbach, Nucl. Instrum. Methods Phys. Res., B 113 (1 – 4) (1996) 266. [6] I.G. Brown, Surf. Coat. Technol. 136 (1 – 3) (2001) 16.
L.H. Li et al. / Surface & Coatings Technology 186 (2004) 165–169 [7] L.H. Li, T. Zhang, P.K. Chu, I.G. Brown, Rev. Sci. Instrum. 73 (8) (2002) 2971. [8] S. Schoser, J. Forget, K. Kohlhof, Surf. Coat. Technol. 93 (2 – 3) (1997) 339. [9] C. Ruset, E. Grigore, Surf. Coat. Technol. 156 (1 – 3) (2002) 159. [10] P.K. Chu, S. Qin, C. Chan, N.W. Cheung, P.K. Ko, IEEE Trans. Plasma Sci. 26 (1) (1998) 79.
169
[11] J. Pelletier, F. Le Coeur, Y. Arnal, A. Lacoste, A. Straboni, Surf. Coat. Technol. 136 (2001) 7. [12] J.N. Matossian, R.H. Wei, Surf. Coat. Technol. 85 (1996) 92. [13] L.H. Li, R.W.Y. Poon, S.C.H. Kwok, P.K. Chu, R.Q. Wu, Y.H. Zhang, Rev. Sci. Instrum. 74 (10) (2003) 4301.