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Photoresist removal process by hydrogen radicals generated by W catalyst
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Thin Solid Films 516 (2008) 847 – 849 www.elsevier.com/locate/tsf
Photoresist removal process by hydrogen radicals generated by W catalyst M. Takata a,⁎, K. Ogushi a , Y. Yuba a , Y. Akasaka a , K. Tomioka b , E. Soda b , N. Kobayashi b a b
Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, 560-8531, Japan Semiconductor Leading Edge Technologies Inc., 16-1 Onogawa Tsukuba, Ibaraki, 305-8569, Japan Available online 17 September 2007
Abstract Hydrogen radical process for photoresist removal by use of hot W catalyst has been investigated for a possible application to advanced Cu/lowk dielectric interconnects in LSI. It is found that etching rates of resists depend critically on sample temperature (Ts) and are higher than 1 μm/min at the optimized condition. H radical irradiation effects on porous methylsilsesquioxane (p-MSQ) have been studied from measurements of k value and capacitance of the advanced interconnect test sample. No radical process is observed to induce the increase in k value of p-MSQ films. These results suggest that the hydrogen radical process for resist removal with W catalyst is promising for production of advanced interconnects. © 2007 Published by Elsevier B.V. Keywords: Hydrogen radical; Catalyst; Photoresist; Low-k
1. Introductions The device dimensions in LSI are being continually scaled down and the increase of RC delay in interconnect has become problematic. Thus interlayer materials with a low dielectric constant (low-k) and copper conductor have been intensively investigated. According to the ITRS2005, bulk effective dielectric constant (k) below 2.4 is required. Typically, low-k materials such as p-MSQ consist of silicon oxide frame and methyl (–CH3) bonds and have porosity which reduces the k value. However low-k materials have some issues such as their thermal instability and oxygen plasma damage. Annealing at high temperature and resist removal process by conventional oxygen plasma process replaces most of the Si–CH3 bonds with Si–OH bonds in low-k materials, and so leads to the increase in the k value [1]. Recently resist removal process by use of hydrogen and helium remote plasma has been investigated [2]. As a lower damage process without plasma, hydrogen radical process with W catalyst for resist removal has been proposed [3–6]. This method also has several advantages, that is; easy application to large size wafer and low equipment cost and so on. In the present study, we have investigated basic performance of resist removal process using hydrogen radicals generated by
⁎ Corresponding author. E-mail address: [email protected] (M. Takata). 0040-6090/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.tsf.2007.06.206
W catalyst, characterized the damage of p-MSQ films and measured W contamination density on sample surface. 2. Experimental details Hydrogen radical process was performed in an in-house apparatus composed of W catalyst wire, a temperature variable sample stage and a shower head of hydrogen gas in a vacuum chamber equipped with a pressure controller and a pumping system. High density hydrogen radicals were generated by a dissociative reaction on W surface. Catalyst temperatures (Tc) ranged between 1500 and 2000 °C and were monitored with a radiation thermometer. The process was performed at hydrogen pressure of 66.7 Pa and Ts from 150 to 300 °C. Sample temperature (Ts) was measured by the thermocouple which was set on the top surface and was controlled to keep it constant by the heater in the sample stage. Before heating up catalyst and samples, the chamber was evacuated to 1.33 × 10− 3 Pa and then hydrogen gas was fed into the chamber at pressure of 66.7 Pa. The flow rate of H2 gas is 80 sccm and the distance between catalyst and sample surface is 6 cm. In this study, photoresists for KrF and ArF excimer laser lithography were treated. Etching rates of photoresists were evaluated by Ellipsometer and sample surfaces and residues were observed by Scanning Electron Microscope (SEM). The effects of hydrogen radical irradiation on p-MSQ films with the k value of 2.3 were determined from Fourier Transform Infrared
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Fig. 1. Substrate temperature (Ts) dependence of etching rate of KrF photoresist as a function of catalyst temperature (Tc).
Spectroscopy (FTIR) measurement of CH3 bonds and capacitance measurement of large coplanar and microstructure capacitors. The capacitance bridge used was Agilent 4284 Å operating at 1 MHz. The density of W contamination on Si wafer from the heated catalyst was obtained by Secondary Ion Mass Spectrometer (SIMS) analysis. 3. Results and discussion Fig. 1 shows Ts dependence of etching rate (ER) of KrF photoresists as a function of Tc from 1600 to 2000 °C. The ER increased significantly with Ts and showed the maximum of 1500 nm/min at Ts of 240 °C. Additionally its activation energy was 0.9 eV at high Ts side from Arrhenius plots of ER. The Tc was one of the key factors to control the radical yield [4] but the
ER depended little on Tc, suggesting the etching process was not limited by the amount of radical but by the reaction rate. In addition, it was found that the ER was independent of pressure from 13.3 to 66.7 Pa [6], so we think that sufficient hydrogen radicals were supplied to the surface of the resist. Fig. 2 shows SEM images of samples after hydrogen radical irradiation for 2 min. The samples had line and space patterns in low-k films fabricated by using ArF lithography and CFn dry etching, and thin films of ArF resists remained on the line part of the patterned low-k films. The residues of ArF resists on patterned low-k films were observed after the process at Ts of 150 °C and Tc of 1600 °C. But ArF photoresist was completely removed without residues in the process at either higher Tc or Ts and the side walls of the p-MSQ films were hardly etched by hydrogen radicals. We found that ER of CFn plasma treated ArF photoresist was lower than the ER of the sample without CFn plasma treatment. We expected that surfaces of the resists were modified by irradiation of charged particles during CFn plasmas etching. To characterize process induced damage, we measured the amount of Si-CH3 bonds in p-MSQ films, which decreases corresponding to the increase in damage. Fig. 3 shows amount of Si–CH3 bonds after 2 min of processing normalized with the asdeposited films. It was clear that the ratio of Si–CH3 bonds decreased with Tc at Ts of 250 and 300 °C and did not depend on Tc at Ts of 180 °C. These suggested that the damages increased gradually with Tc, higher damage was induced by high Ts processing. For the low damage process, Ts below 180 °C and Tc between 1600 °C and 1800 °C should be selected. We also measured the k value in blanket p-MSQ films before and after hydrogen radical irradiation for 2 min by using mercury probe. Fig. 4 shows the difference of k value in the processed samples from the value of the as-deposited p-MSQ film (k = 2.3). We found the irradiation to p-MSQ films caused no increase but rather a little decrease in k value, which might
Fig. 2. SEM images of ArF photoresist mask and low-k film with line and space patterns after H radical irradiation for 2 min.
M. Takata et al. / Thin Solid Films 516 (2008) 847–849
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Fig. 3. Amount of Si–CH3 bonds in p-MSQ after processing at various temperatures for 2 min. The value shown is normalized with that of the starting sample.
Fig. 5. Normalized parasitic capacitance change of micro capacitor after the processing for 2 min. The inset shows schematic of the micro capacitor with Cu and p-MSQ line width of 100 nm.
be a preferable change as a low-k material, although the Si–CH3 bonds decreased by hydrogen process shown in Fig. 4. Fig. 5 shows parasitic capacitance of micro-structured capacitor shown in the inset as a function of Ts and Tc after hydrogen radical process. It is clear that parasitic capacitance decreases by about 10%, and the ratio depends a little on Tc and Ts. From these results, we think that the decrease in k value and the improvement of a parasitic capacitance are caused by replacing Si–CH3 and Si–O bonds with Si–H bonds in p-MSQ films and reinforcing the surface damage by LSI process. Aerial densities of deposited W on Si wafer were obtained when the W wire was heated at 1800 or 2000 °C for 10 min. Contamination densities of W at Tc of 2000 °C in H2 atmosphere of 66.7 Pa and vacuum condition of 1.33 × 10− 4 Pa were 9.0 × 1010 and 2.7 × 1014 atm/cm2. W deposition depended strongly on vacuum condition and was suppressed considerably by introducing H2 gas, probably due to the collision of W atoms with H2 in the atmosphere. If a process time is limited to 2 min
or less, W density is around 1.2 × 1010 atm/cm2 which is allowable for device fabrication. 4. Conclusion In the photoresist removal process by hydrogen radicals generated by W catalyst, ER of KrF photoresist higher than 1 ìm/min. at high Ts processing was obtained. The decrease of Si–CH3 bonds in p-MSQ films was suppressed by use of hydrogen radical process at low Ts and Tc, compared with a conventional oxygen plasma process. No increase of k value in p-MAQ and capacitance of the advanced interconnect test sample was found after the processing in the present study. In addition, we verified W contamination is acceptable for trial manufacture. From these results, this process is of potential use for advanced low-k/Cu interconnect process. Acknowledgement The authors express their thanks to K. Kawasaki for the help with making our apparatus. References [1] T.K. Goh, T.K.S. Wong, Microelectron. Eng. 75 (2004) 330. [2] A. Matsushita, N. Ohhashi, K. Inukai, H.J. Shin, S. Sone, K. Suduo, K. Misawa, I. Matsumoto, N. Kabayashi, Proc IITC2003, 2003, p. 147. [3] T. Miki, A. Izumi, H. Matsumura, Solid State Phenom. 92 (2003) 231. [4] H. Umemoto, K. Ohara, D. Morita, Y. Nozaki, A. Masuda, H. Matsumura, J. Appl. Phys. 91 (2002) 1650. [5] A. Izumi, H. Matsumura, Jpn. J. Appl. Phys. 41 (2002) 4639. [6] K. Tomioka, E. Soda, N. Kobyashi, K. Mochidzuki, M. Takata, S. Uda, Conf. Proc. AMC, (AMC 2005) XXI (2006, MRS, Warrendale) (2005) 399.
Fig. 4. The change in k value of p-MSQ films after the processing for 2 min.
Thin Solid Films 516 (2008) 847 – 849 www.elsevier.com/locate/tsf
Photoresist removal process by hydrogen radicals generated by W catalyst M. Takata a,⁎, K. Ogushi a , Y. Yuba a , Y. Akasaka a , K. Tomioka b , E. Soda b , N. Kobayashi b a b
Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, 560-8531, Japan Semiconductor Leading Edge Technologies Inc., 16-1 Onogawa Tsukuba, Ibaraki, 305-8569, Japan Available online 17 September 2007
Abstract Hydrogen radical process for photoresist removal by use of hot W catalyst has been investigated for a possible application to advanced Cu/lowk dielectric interconnects in LSI. It is found that etching rates of resists depend critically on sample temperature (Ts) and are higher than 1 μm/min at the optimized condition. H radical irradiation effects on porous methylsilsesquioxane (p-MSQ) have been studied from measurements of k value and capacitance of the advanced interconnect test sample. No radical process is observed to induce the increase in k value of p-MSQ films. These results suggest that the hydrogen radical process for resist removal with W catalyst is promising for production of advanced interconnects. © 2007 Published by Elsevier B.V. Keywords: Hydrogen radical; Catalyst; Photoresist; Low-k
1. Introductions The device dimensions in LSI are being continually scaled down and the increase of RC delay in interconnect has become problematic. Thus interlayer materials with a low dielectric constant (low-k) and copper conductor have been intensively investigated. According to the ITRS2005, bulk effective dielectric constant (k) below 2.4 is required. Typically, low-k materials such as p-MSQ consist of silicon oxide frame and methyl (–CH3) bonds and have porosity which reduces the k value. However low-k materials have some issues such as their thermal instability and oxygen plasma damage. Annealing at high temperature and resist removal process by conventional oxygen plasma process replaces most of the Si–CH3 bonds with Si–OH bonds in low-k materials, and so leads to the increase in the k value [1]. Recently resist removal process by use of hydrogen and helium remote plasma has been investigated [2]. As a lower damage process without plasma, hydrogen radical process with W catalyst for resist removal has been proposed [3–6]. This method also has several advantages, that is; easy application to large size wafer and low equipment cost and so on. In the present study, we have investigated basic performance of resist removal process using hydrogen radicals generated by
⁎ Corresponding author. E-mail address: [email protected] (M. Takata). 0040-6090/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.tsf.2007.06.206
W catalyst, characterized the damage of p-MSQ films and measured W contamination density on sample surface. 2. Experimental details Hydrogen radical process was performed in an in-house apparatus composed of W catalyst wire, a temperature variable sample stage and a shower head of hydrogen gas in a vacuum chamber equipped with a pressure controller and a pumping system. High density hydrogen radicals were generated by a dissociative reaction on W surface. Catalyst temperatures (Tc) ranged between 1500 and 2000 °C and were monitored with a radiation thermometer. The process was performed at hydrogen pressure of 66.7 Pa and Ts from 150 to 300 °C. Sample temperature (Ts) was measured by the thermocouple which was set on the top surface and was controlled to keep it constant by the heater in the sample stage. Before heating up catalyst and samples, the chamber was evacuated to 1.33 × 10− 3 Pa and then hydrogen gas was fed into the chamber at pressure of 66.7 Pa. The flow rate of H2 gas is 80 sccm and the distance between catalyst and sample surface is 6 cm. In this study, photoresists for KrF and ArF excimer laser lithography were treated. Etching rates of photoresists were evaluated by Ellipsometer and sample surfaces and residues were observed by Scanning Electron Microscope (SEM). The effects of hydrogen radical irradiation on p-MSQ films with the k value of 2.3 were determined from Fourier Transform Infrared
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M. Takata et al. / Thin Solid Films 516 (2008) 847–849
Fig. 1. Substrate temperature (Ts) dependence of etching rate of KrF photoresist as a function of catalyst temperature (Tc).
Spectroscopy (FTIR) measurement of CH3 bonds and capacitance measurement of large coplanar and microstructure capacitors. The capacitance bridge used was Agilent 4284 Å operating at 1 MHz. The density of W contamination on Si wafer from the heated catalyst was obtained by Secondary Ion Mass Spectrometer (SIMS) analysis. 3. Results and discussion Fig. 1 shows Ts dependence of etching rate (ER) of KrF photoresists as a function of Tc from 1600 to 2000 °C. The ER increased significantly with Ts and showed the maximum of 1500 nm/min at Ts of 240 °C. Additionally its activation energy was 0.9 eV at high Ts side from Arrhenius plots of ER. The Tc was one of the key factors to control the radical yield [4] but the
ER depended little on Tc, suggesting the etching process was not limited by the amount of radical but by the reaction rate. In addition, it was found that the ER was independent of pressure from 13.3 to 66.7 Pa [6], so we think that sufficient hydrogen radicals were supplied to the surface of the resist. Fig. 2 shows SEM images of samples after hydrogen radical irradiation for 2 min. The samples had line and space patterns in low-k films fabricated by using ArF lithography and CFn dry etching, and thin films of ArF resists remained on the line part of the patterned low-k films. The residues of ArF resists on patterned low-k films were observed after the process at Ts of 150 °C and Tc of 1600 °C. But ArF photoresist was completely removed without residues in the process at either higher Tc or Ts and the side walls of the p-MSQ films were hardly etched by hydrogen radicals. We found that ER of CFn plasma treated ArF photoresist was lower than the ER of the sample without CFn plasma treatment. We expected that surfaces of the resists were modified by irradiation of charged particles during CFn plasmas etching. To characterize process induced damage, we measured the amount of Si-CH3 bonds in p-MSQ films, which decreases corresponding to the increase in damage. Fig. 3 shows amount of Si–CH3 bonds after 2 min of processing normalized with the asdeposited films. It was clear that the ratio of Si–CH3 bonds decreased with Tc at Ts of 250 and 300 °C and did not depend on Tc at Ts of 180 °C. These suggested that the damages increased gradually with Tc, higher damage was induced by high Ts processing. For the low damage process, Ts below 180 °C and Tc between 1600 °C and 1800 °C should be selected. We also measured the k value in blanket p-MSQ films before and after hydrogen radical irradiation for 2 min by using mercury probe. Fig. 4 shows the difference of k value in the processed samples from the value of the as-deposited p-MSQ film (k = 2.3). We found the irradiation to p-MSQ films caused no increase but rather a little decrease in k value, which might
Fig. 2. SEM images of ArF photoresist mask and low-k film with line and space patterns after H radical irradiation for 2 min.
M. Takata et al. / Thin Solid Films 516 (2008) 847–849
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Fig. 3. Amount of Si–CH3 bonds in p-MSQ after processing at various temperatures for 2 min. The value shown is normalized with that of the starting sample.
Fig. 5. Normalized parasitic capacitance change of micro capacitor after the processing for 2 min. The inset shows schematic of the micro capacitor with Cu and p-MSQ line width of 100 nm.
be a preferable change as a low-k material, although the Si–CH3 bonds decreased by hydrogen process shown in Fig. 4. Fig. 5 shows parasitic capacitance of micro-structured capacitor shown in the inset as a function of Ts and Tc after hydrogen radical process. It is clear that parasitic capacitance decreases by about 10%, and the ratio depends a little on Tc and Ts. From these results, we think that the decrease in k value and the improvement of a parasitic capacitance are caused by replacing Si–CH3 and Si–O bonds with Si–H bonds in p-MSQ films and reinforcing the surface damage by LSI process. Aerial densities of deposited W on Si wafer were obtained when the W wire was heated at 1800 or 2000 °C for 10 min. Contamination densities of W at Tc of 2000 °C in H2 atmosphere of 66.7 Pa and vacuum condition of 1.33 × 10− 4 Pa were 9.0 × 1010 and 2.7 × 1014 atm/cm2. W deposition depended strongly on vacuum condition and was suppressed considerably by introducing H2 gas, probably due to the collision of W atoms with H2 in the atmosphere. If a process time is limited to 2 min
or less, W density is around 1.2 × 1010 atm/cm2 which is allowable for device fabrication. 4. Conclusion In the photoresist removal process by hydrogen radicals generated by W catalyst, ER of KrF photoresist higher than 1 ìm/min. at high Ts processing was obtained. The decrease of Si–CH3 bonds in p-MSQ films was suppressed by use of hydrogen radical process at low Ts and Tc, compared with a conventional oxygen plasma process. No increase of k value in p-MAQ and capacitance of the advanced interconnect test sample was found after the processing in the present study. In addition, we verified W contamination is acceptable for trial manufacture. From these results, this process is of potential use for advanced low-k/Cu interconnect process. Acknowledgement The authors express their thanks to K. Kawasaki for the help with making our apparatus. References [1] T.K. Goh, T.K.S. Wong, Microelectron. Eng. 75 (2004) 330. [2] A. Matsushita, N. Ohhashi, K. Inukai, H.J. Shin, S. Sone, K. Suduo, K. Misawa, I. Matsumoto, N. Kabayashi, Proc IITC2003, 2003, p. 147. [3] T. Miki, A. Izumi, H. Matsumura, Solid State Phenom. 92 (2003) 231. [4] H. Umemoto, K. Ohara, D. Morita, Y. Nozaki, A. Masuda, H. Matsumura, J. Appl. Phys. 91 (2002) 1650. [5] A. Izumi, H. Matsumura, Jpn. J. Appl. Phys. 41 (2002) 4639. [6] K. Tomioka, E. Soda, N. Kobyashi, K. Mochidzuki, M. Takata, S. Uda, Conf. Proc. AMC, (AMC 2005) XXI (2006, MRS, Warrendale) (2005) 399.
Fig. 4. The change in k value of p-MSQ films after the processing for 2 min.