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A new, room-temperature, high-rate plasma-based copper etch process
ARTICLE IN PRESS
Vacuum 74 (2004) 473–477
A new, room-temperature, high-rate plasma-based copper etch process Yue Kuo*, Sangheon Lee Thin Film Nano and Microelectronics Research Laboratory, Texas A&M University, College Station, TX 77843-3122. USA
Abstract A new plasma-based copper etch process that operated at room temperature using a conventional parallel-plate electrode design is discussed. In this paper, authors review the reaction mechanism and compare process parameters, such as gas type, pressure, and power, affecting the plasma–copper reaction rate and the edge profile of the etched pattern. Small geometry copper patterns, e.g., less than 0.8 mm, are successfully etched with this new method. This new process is applicable to many nano and microelectronic devices. r 2004 Elsevier Ltd. All rights reserved. Keywords: Copper; Plasma etch; Plasma etch of copper; Reactive ion etch
1. Introduction The combination of copper (Cu) interconnects and low k interlayer dielectric materials are critical to the success of future generation high-speed VLSIC [1]. Cu buslines are also necessary for large area, high-resolution flat panel displays. It has been desirable to develop a plasma-based Cu fine line etching process since the early stage of the IC industry. The conventional plasma etching process is based on the principle of continuously forming and removing the volatile plasma-material reaction product. The reactive gas used in the plasma etching process usually contains halogen elements. However, Cu halides have a very low volatility at room temperature. For example, Fig. 1 shows the vapor pressures of various Cu halides as a function of the *Corresponding author. Chemical Engineering Department, Texas A & M University, College Station, TX 77843-3122, USA. E-mail address: [email protected] (Y. Kuo).
temperature. Therefore, when Cu is exposed to the halide plasma, the substrate temperature has to be raised to a high temperature, such as >200 C, to remove the reaction product. Otherwise, an extra energy source, such as a high ion bombardment energy, an IR, UV, or laser beam, has to be added to the film surface to remove the products [2–5]. These methods are not practical for mass production. Recently, multi-level Cu lines for VLSICs have been successfully prepared with the chemical– mechanical polishing method. However, this method is sensitive to the pattern density and needs delicate process control and end-point monitoring. It will have problems satisfying many requirements in the sub 0.1 mm design. The wastewater treatment is another challenging issue.
2. New plasma-based Cu etch process Recently, Kuo and Lee [6–10] reported a new plasma-based Cu etch method that had a high Cu
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.01.072
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Y. Kuo, S. Lee / Vacuum 74 (2004) 473–477
Fig. 2. Flow chart of the plasma-based Cu etch process. Fig. 1. Vapor pressure of copper halides vs. temperature.
consumption rate at room temperature. The reaction was carried out with a conventional plasma etching or reactive ion etching (RIE) reactor at a wide range of temperatures, e.g., room temperature to 300 C, using halogen chemistry, such as HCl, Cl2, or HBr. The edge profile can be controlled by the process condition. Different wall profiles can be prepared. The process is simple as described in Fig. 2. Fig. 3 shows an example of (a) the photoresist patterned Cu layer after plasma reaction and (b) after the removal of the reaction product and stripping of the photoresist [7]. The original Cu layer thickness was 250 nm. The plasma exposure condition was Cl2 20 sccm, 20 mTorr, 600 W, 2 min, and 25 C substrate temperature. The Cu compound was removed by a dilute HC1 solution. The photoresist was stripped with acetone. The Cu layer was etched into fine patterns in a short period of time. Fig. 4 shows a 0.8 mm wide Cu line etched with this method using the HCl/Ar mixture. Cu lines narrower than 0.5 mm have been demonstrated. Thus far, the limitation of the line width is the lithography step since we only have access to the conventional optical aligner. Currently, we are testing the sub 0.2 mm Cu pattern using the e-beam lithography tool.
Fig. 3. (a) Photoresist patterned Cu layer after Cl2 plasma, exposure and (b) after removal of CuClx and photoresist [7].
3. Mechanism of plasma–Cu reaction Plasma is critical to the reaction process. For example, when Cu was exposed to HCl gas
without plasma, a very thin layer, e.g., o10 nm, of Cu compound was formed. The thickness did not increase with the exposure time.
ARTICLE IN PRESS Y. Kuo, S. Lee / Vacuum 74 (2004) 473–477
475
Fig. 5. Plasma–Cu reaction process [6].
Fig. 4. A 0.8 mm Cu line etched by HCl/Ar plasma.
Fig. 5 shows a simplified diagram of the mechanism [6]. The plasma–Cu reaction includes two main steps: surface reaction and transport of reactants. When Cu is in contact with Cl or Br generated in the plasma, it is converted into CuClx or CuBrx. The plasma generated halogen particles can exist in various forms, charges (neutral radicals or charged ions), or energy levels. The reaction product is non-stoichiometric. For example, Cu(I) and Cu(II) compounds were detected in the reaction product. Therefore, the product is expressed as CuClx or CuBrx in the Cl or Br plasma, separately. The sputter deposited Cu is polycrystalline with a main XRD peak of (1 1 1) [9]. The Cl2 or HC1 plasma–Cu reaction product film is also in the polycrystalline form and contains several XRD CuCl peaks of (1 1 1), (2 2 0), and (3 1 1) [7,9]. The crystal orientation changes with the plasma exposure time. Since the extent of chlorination of the film increases with the exposure time, the film structure is related to the reaction process. The volume of the Cu layer expands several times after being converted into halides. For example, the film thickness increases by 7 or 5 times after being exposed to the Cl2 or HBr plasma, separately [7,10]. The volume expansion phenomenon is similar to that of the thermal oxidation of silicon [7]. The actual expansion amount is also related to the porosity of the product film, which distributes non-uniformly from the surface to the bulk of the film.
The reaction product surface is much rougher and more porous than that of the original Cu surface. In addition, the product layer contains the unreacted metallic Cu [7]. Therefore, the chlorination or bromination reaction proceeds along the grain boundary at a much higher rate than within the grain. In addition to the Cl or Br concentration in the plasma phase, the Cu consumption rate can be correlated to the cathode self-bias voltage ( Vdc), which is a reference of ion bombardment energy [7,10]. The transport of the Cl or Br reactant through the product layer appears to be related to the ion bombardment energy. However, it is unclear how the ion bombardment effect can propagate through a depth of several micrometers. The porous product structure probably plays an important role in the transport process.
4. Process influence on Cu conversion rate, undercut, edge shape, and final sidewall profile A plasma process is influenced by several parameters, such as gas composition, power, pressure, and substrate temperature. All of them can be correlated to two factors: the generation of reactants in the plasma phase and the ion bombardment energy. Main reactants are the fragments, e.g., free radicals and ions, dissociated from the feed gas. It is difficult to identify every reactant element in the plasma phase. However, some major components, such as Cl or Br, can be semi-quantitatively estimated by methods such as the optical emission actinometry [11]. The ion bombardment energy is also difficult to estimate because it is contributed by various ions. However,
ARTICLE IN PRESS 476
Y. Kuo, S. Lee / Vacuum 74 (2004) 473–477
a high cathode self-bias voltage ( Vdc) indicates a large ion bombardment energy. Under the same pressure and power condition, Vdc’s of different gases can be compared based on the bond strength in the molecule. The Cu consumption rate was calculated by the following steps: (1) after plasma exposure, removing the Cu compound layer by a dilute HCl solution and stripping the photoresist layer by acetone or a photoresist stripper solution, (2) measuring the Cu step height, and (3) dividing the Cu step height by the plasma exposure time. This rate depends on both the plasma phase chemistry and the ion bombardment energy. For example, the HBr plasma chemistry supplies a higher rate than the HCl chemistry although the former has a lower Vdc than the latter. However, since the high-power plasma always has a high Vdc, the rate increases with the power under the same gas and pressure condition. The change of pressure can affect both the concentration of the effective etchant, e.g., Cl or Br, and the Vdc. Therefore, the effect on the rate is dependent on which factor is more important. For example, in the HBr plasma case, the Br concentration increases with the increase of pressure, but the Vdc change follows the opposite trend. Since the Cu consumption rate increases with the increase of the HBr pressure, the Br etchant is more influential on the reaction than the ion bombardment energy [10]. In the HCl plasma case, since the Cu consumption rate decreases with the increase of pressure, the ion bombardment energy is a more influential factor than the Cl etchant concentration [9]. The difference in HCl plasma- and HBr plasma–Cu reaction rate can be explained by the reactivity difference between Cl and Br. The Br radical is more reactive than the Cl radical because the former has a larger size with the outmost electron shell farther from the nucleus than the latter. This is reflected on the bond strengths of HCl and HBr. The substrate temperature is an important parameter in comparing the effect of the etchant concentration and the ion bombardment energy. For example, the HCl pressure effect discussed in the previous section is for the unheated substrate, e.g., 25 C. When the substrate is raised to 250 C,
the opposite effect is observed, i.e., the etch rate increases with the increase of the Cl concentration rather than the Vdc. The substrate temperature affects the kinetics of the reaction rather than the plasma phase chemistry or the ion bombardment energy. This indicates that not all Cl radicals generated in the plasma are reactive in the Cu consumption process. Some of these radicals need extra energy to activate the reaction. The HBr plasma–Cu reaction has a larger undercut of the photoresist mask than the HC1 plasma–Cu reaction. Since the reaction underneath the photoresist mask is independent of the ion bombardment phenomenon, the reaction rate is affected by the reactivity of the etchant, e.g., Cl or Br. The undercut result is consistent with the reactivity of the radical generated in the plasma. The HBr plasma etched Cu pattern is much rougher than that of the HCl plasma etched pattern. Since the Br radical is much reactive than the Cl radical, the former attacks the grain boundary at a higher rate than the latter, which shows up as the roughness of the etched pattern. The grain boundary effect is not obvious when the ion bombardment factor dominates the reaction process. However, when the etchant–Cu reaction controls the Cu consumption process, e.g., at a high substrate temperature or a highly reactive etchant, the grain boundary effect shows up and the pattern becomes rough. The angle of the etched Cu sidewall is influenced by the pressure of the HC1 plasma [9]. The lower the pressure is, the more close to vertical the sidewall is. Ion bombardment is perpendicular to the surface to the substrate and the surface reaction is anisotropic. The contribution of ion bombardment to the wall angle is more pronounced than the etchant concentration. Since the plasma–Cu reaction product layer can be several micrometers thick, ions have to penetrate through the whole layer to show up the ion bombardment effect. Although the CuClx product layer is porous, it is difficult for these ions to reach the underneath fresh Cu without being scattered or consumed. Therefore, the ion transport mechanism needs further studies.
ARTICLE IN PRESS Y. Kuo, S. Lee / Vacuum 74 (2004) 473–477
477
5. Summary
Acknowledgements
Cu fine patterns could be effectively prepared with a new plasma-based etch process. The etch rate and the edge profile of the Cu layer can be controlled by the plasma process. Author reviewed the basic mechanism of the plasma–Cu reaction that is the key to the success of the new process. In addition to demonstrate the sub-micrometer pattern definition capability, we discussed influences of various process parameters on the reaction. Plasma phase chemistry and ion bombardment energy are the two critical factors that correlate the process parameter to the final Cu consumption rate and pattern control. The influence of ion bombardment to the process is well documented. However, the exact mechanism, e.g., the transport of energized ions through the thick reaction product, is not understood. This new process is applicable to many nano and microelectronic devices and circuits in VLSICs, displays, and other products.
Authors acknowledge the partial financial support of this work was through the NSF ECS0103022 Grant. Mr. Jiang Lu prepared the sample in Fig. 4. References [1] National Technology Roadmap for Semiconductors (NTRS), Semiconductor Industry Association (SIA), 1997. [2] Lee JW, Park YD, Childress JR, Pearton SJ, Sharifi F, Ren F. J Electrochem Soc 1998;145(7):2585. [3] Ohshita Y, Hosoi N. Thin Solid Films 1995;262:67. [4] Schwartz GC, Schaible PM. J Electrochem Soc 1983;130:1777. [5] Howard BJ, Steinbruchel CH. Appl Phys Lett 1991;59:914. [6] Kuo Y, Lee S. Jpn J Appl Phys 2000;39(3AB):L188. [7] Kuo Y, Lee S. Appl Phys Lett 2001;78(7):1002. [8] Lee S, Kuo Y. J Electrochem Soc 2001;148(9):G524. [9] Lee S, Kuo Y. Jpn J Appl Phys 2002;41(12):7345. [10] Lee S, Kuo Y. Thin Solid Films 2003, in press. [11] Richards AD, Thompson BE, Allen KD, Sawin HH. J Appl Phys 1987;62:792.
Vacuum 74 (2004) 473–477
A new, room-temperature, high-rate plasma-based copper etch process Yue Kuo*, Sangheon Lee Thin Film Nano and Microelectronics Research Laboratory, Texas A&M University, College Station, TX 77843-3122. USA
Abstract A new plasma-based copper etch process that operated at room temperature using a conventional parallel-plate electrode design is discussed. In this paper, authors review the reaction mechanism and compare process parameters, such as gas type, pressure, and power, affecting the plasma–copper reaction rate and the edge profile of the etched pattern. Small geometry copper patterns, e.g., less than 0.8 mm, are successfully etched with this new method. This new process is applicable to many nano and microelectronic devices. r 2004 Elsevier Ltd. All rights reserved. Keywords: Copper; Plasma etch; Plasma etch of copper; Reactive ion etch
1. Introduction The combination of copper (Cu) interconnects and low k interlayer dielectric materials are critical to the success of future generation high-speed VLSIC [1]. Cu buslines are also necessary for large area, high-resolution flat panel displays. It has been desirable to develop a plasma-based Cu fine line etching process since the early stage of the IC industry. The conventional plasma etching process is based on the principle of continuously forming and removing the volatile plasma-material reaction product. The reactive gas used in the plasma etching process usually contains halogen elements. However, Cu halides have a very low volatility at room temperature. For example, Fig. 1 shows the vapor pressures of various Cu halides as a function of the *Corresponding author. Chemical Engineering Department, Texas A & M University, College Station, TX 77843-3122, USA. E-mail address: [email protected] (Y. Kuo).
temperature. Therefore, when Cu is exposed to the halide plasma, the substrate temperature has to be raised to a high temperature, such as >200 C, to remove the reaction product. Otherwise, an extra energy source, such as a high ion bombardment energy, an IR, UV, or laser beam, has to be added to the film surface to remove the products [2–5]. These methods are not practical for mass production. Recently, multi-level Cu lines for VLSICs have been successfully prepared with the chemical– mechanical polishing method. However, this method is sensitive to the pattern density and needs delicate process control and end-point monitoring. It will have problems satisfying many requirements in the sub 0.1 mm design. The wastewater treatment is another challenging issue.
2. New plasma-based Cu etch process Recently, Kuo and Lee [6–10] reported a new plasma-based Cu etch method that had a high Cu
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.01.072
ARTICLE IN PRESS 474
Y. Kuo, S. Lee / Vacuum 74 (2004) 473–477
Fig. 2. Flow chart of the plasma-based Cu etch process. Fig. 1. Vapor pressure of copper halides vs. temperature.
consumption rate at room temperature. The reaction was carried out with a conventional plasma etching or reactive ion etching (RIE) reactor at a wide range of temperatures, e.g., room temperature to 300 C, using halogen chemistry, such as HCl, Cl2, or HBr. The edge profile can be controlled by the process condition. Different wall profiles can be prepared. The process is simple as described in Fig. 2. Fig. 3 shows an example of (a) the photoresist patterned Cu layer after plasma reaction and (b) after the removal of the reaction product and stripping of the photoresist [7]. The original Cu layer thickness was 250 nm. The plasma exposure condition was Cl2 20 sccm, 20 mTorr, 600 W, 2 min, and 25 C substrate temperature. The Cu compound was removed by a dilute HC1 solution. The photoresist was stripped with acetone. The Cu layer was etched into fine patterns in a short period of time. Fig. 4 shows a 0.8 mm wide Cu line etched with this method using the HCl/Ar mixture. Cu lines narrower than 0.5 mm have been demonstrated. Thus far, the limitation of the line width is the lithography step since we only have access to the conventional optical aligner. Currently, we are testing the sub 0.2 mm Cu pattern using the e-beam lithography tool.
Fig. 3. (a) Photoresist patterned Cu layer after Cl2 plasma, exposure and (b) after removal of CuClx and photoresist [7].
3. Mechanism of plasma–Cu reaction Plasma is critical to the reaction process. For example, when Cu was exposed to HCl gas
without plasma, a very thin layer, e.g., o10 nm, of Cu compound was formed. The thickness did not increase with the exposure time.
ARTICLE IN PRESS Y. Kuo, S. Lee / Vacuum 74 (2004) 473–477
475
Fig. 5. Plasma–Cu reaction process [6].
Fig. 4. A 0.8 mm Cu line etched by HCl/Ar plasma.
Fig. 5 shows a simplified diagram of the mechanism [6]. The plasma–Cu reaction includes two main steps: surface reaction and transport of reactants. When Cu is in contact with Cl or Br generated in the plasma, it is converted into CuClx or CuBrx. The plasma generated halogen particles can exist in various forms, charges (neutral radicals or charged ions), or energy levels. The reaction product is non-stoichiometric. For example, Cu(I) and Cu(II) compounds were detected in the reaction product. Therefore, the product is expressed as CuClx or CuBrx in the Cl or Br plasma, separately. The sputter deposited Cu is polycrystalline with a main XRD peak of (1 1 1) [9]. The Cl2 or HC1 plasma–Cu reaction product film is also in the polycrystalline form and contains several XRD CuCl peaks of (1 1 1), (2 2 0), and (3 1 1) [7,9]. The crystal orientation changes with the plasma exposure time. Since the extent of chlorination of the film increases with the exposure time, the film structure is related to the reaction process. The volume of the Cu layer expands several times after being converted into halides. For example, the film thickness increases by 7 or 5 times after being exposed to the Cl2 or HBr plasma, separately [7,10]. The volume expansion phenomenon is similar to that of the thermal oxidation of silicon [7]. The actual expansion amount is also related to the porosity of the product film, which distributes non-uniformly from the surface to the bulk of the film.
The reaction product surface is much rougher and more porous than that of the original Cu surface. In addition, the product layer contains the unreacted metallic Cu [7]. Therefore, the chlorination or bromination reaction proceeds along the grain boundary at a much higher rate than within the grain. In addition to the Cl or Br concentration in the plasma phase, the Cu consumption rate can be correlated to the cathode self-bias voltage ( Vdc), which is a reference of ion bombardment energy [7,10]. The transport of the Cl or Br reactant through the product layer appears to be related to the ion bombardment energy. However, it is unclear how the ion bombardment effect can propagate through a depth of several micrometers. The porous product structure probably plays an important role in the transport process.
4. Process influence on Cu conversion rate, undercut, edge shape, and final sidewall profile A plasma process is influenced by several parameters, such as gas composition, power, pressure, and substrate temperature. All of them can be correlated to two factors: the generation of reactants in the plasma phase and the ion bombardment energy. Main reactants are the fragments, e.g., free radicals and ions, dissociated from the feed gas. It is difficult to identify every reactant element in the plasma phase. However, some major components, such as Cl or Br, can be semi-quantitatively estimated by methods such as the optical emission actinometry [11]. The ion bombardment energy is also difficult to estimate because it is contributed by various ions. However,
ARTICLE IN PRESS 476
Y. Kuo, S. Lee / Vacuum 74 (2004) 473–477
a high cathode self-bias voltage ( Vdc) indicates a large ion bombardment energy. Under the same pressure and power condition, Vdc’s of different gases can be compared based on the bond strength in the molecule. The Cu consumption rate was calculated by the following steps: (1) after plasma exposure, removing the Cu compound layer by a dilute HCl solution and stripping the photoresist layer by acetone or a photoresist stripper solution, (2) measuring the Cu step height, and (3) dividing the Cu step height by the plasma exposure time. This rate depends on both the plasma phase chemistry and the ion bombardment energy. For example, the HBr plasma chemistry supplies a higher rate than the HCl chemistry although the former has a lower Vdc than the latter. However, since the high-power plasma always has a high Vdc, the rate increases with the power under the same gas and pressure condition. The change of pressure can affect both the concentration of the effective etchant, e.g., Cl or Br, and the Vdc. Therefore, the effect on the rate is dependent on which factor is more important. For example, in the HBr plasma case, the Br concentration increases with the increase of pressure, but the Vdc change follows the opposite trend. Since the Cu consumption rate increases with the increase of the HBr pressure, the Br etchant is more influential on the reaction than the ion bombardment energy [10]. In the HCl plasma case, since the Cu consumption rate decreases with the increase of pressure, the ion bombardment energy is a more influential factor than the Cl etchant concentration [9]. The difference in HCl plasma- and HBr plasma–Cu reaction rate can be explained by the reactivity difference between Cl and Br. The Br radical is more reactive than the Cl radical because the former has a larger size with the outmost electron shell farther from the nucleus than the latter. This is reflected on the bond strengths of HCl and HBr. The substrate temperature is an important parameter in comparing the effect of the etchant concentration and the ion bombardment energy. For example, the HCl pressure effect discussed in the previous section is for the unheated substrate, e.g., 25 C. When the substrate is raised to 250 C,
the opposite effect is observed, i.e., the etch rate increases with the increase of the Cl concentration rather than the Vdc. The substrate temperature affects the kinetics of the reaction rather than the plasma phase chemistry or the ion bombardment energy. This indicates that not all Cl radicals generated in the plasma are reactive in the Cu consumption process. Some of these radicals need extra energy to activate the reaction. The HBr plasma–Cu reaction has a larger undercut of the photoresist mask than the HC1 plasma–Cu reaction. Since the reaction underneath the photoresist mask is independent of the ion bombardment phenomenon, the reaction rate is affected by the reactivity of the etchant, e.g., Cl or Br. The undercut result is consistent with the reactivity of the radical generated in the plasma. The HBr plasma etched Cu pattern is much rougher than that of the HCl plasma etched pattern. Since the Br radical is much reactive than the Cl radical, the former attacks the grain boundary at a higher rate than the latter, which shows up as the roughness of the etched pattern. The grain boundary effect is not obvious when the ion bombardment factor dominates the reaction process. However, when the etchant–Cu reaction controls the Cu consumption process, e.g., at a high substrate temperature or a highly reactive etchant, the grain boundary effect shows up and the pattern becomes rough. The angle of the etched Cu sidewall is influenced by the pressure of the HC1 plasma [9]. The lower the pressure is, the more close to vertical the sidewall is. Ion bombardment is perpendicular to the surface to the substrate and the surface reaction is anisotropic. The contribution of ion bombardment to the wall angle is more pronounced than the etchant concentration. Since the plasma–Cu reaction product layer can be several micrometers thick, ions have to penetrate through the whole layer to show up the ion bombardment effect. Although the CuClx product layer is porous, it is difficult for these ions to reach the underneath fresh Cu without being scattered or consumed. Therefore, the ion transport mechanism needs further studies.
ARTICLE IN PRESS Y. Kuo, S. Lee / Vacuum 74 (2004) 473–477
477
5. Summary
Acknowledgements
Cu fine patterns could be effectively prepared with a new plasma-based etch process. The etch rate and the edge profile of the Cu layer can be controlled by the plasma process. Author reviewed the basic mechanism of the plasma–Cu reaction that is the key to the success of the new process. In addition to demonstrate the sub-micrometer pattern definition capability, we discussed influences of various process parameters on the reaction. Plasma phase chemistry and ion bombardment energy are the two critical factors that correlate the process parameter to the final Cu consumption rate and pattern control. The influence of ion bombardment to the process is well documented. However, the exact mechanism, e.g., the transport of energized ions through the thick reaction product, is not understood. This new process is applicable to many nano and microelectronic devices and circuits in VLSICs, displays, and other products.
Authors acknowledge the partial financial support of this work was through the NSF ECS0103022 Grant. Mr. Jiang Lu prepared the sample in Fig. 4. References [1] National Technology Roadmap for Semiconductors (NTRS), Semiconductor Industry Association (SIA), 1997. [2] Lee JW, Park YD, Childress JR, Pearton SJ, Sharifi F, Ren F. J Electrochem Soc 1998;145(7):2585. [3] Ohshita Y, Hosoi N. Thin Solid Films 1995;262:67. [4] Schwartz GC, Schaible PM. J Electrochem Soc 1983;130:1777. [5] Howard BJ, Steinbruchel CH. Appl Phys Lett 1991;59:914. [6] Kuo Y, Lee S. Jpn J Appl Phys 2000;39(3AB):L188. [7] Kuo Y, Lee S. Appl Phys Lett 2001;78(7):1002. [8] Lee S, Kuo Y. J Electrochem Soc 2001;148(9):G524. [9] Lee S, Kuo Y. Jpn J Appl Phys 2002;41(12):7345. [10] Lee S, Kuo Y. Thin Solid Films 2003, in press. [11] Richards AD, Thompson BE, Allen KD, Sawin HH. J Appl Phys 1987;62:792.