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Development of CO2 gas cluster cleaning method and its characterization
Microelectronic Engineering 102 (2013) 87–90
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Development of CO2 gas cluster cleaning method and its characterization Hoomi Choi a, Hojoong Kim a, Deokjoo Yoon c, Jong W. Lee c, Bong-Kyun Kang d, Min-Su Kim d, Jin-Goo Park d, Soon-Bark Kwon e, Taesung Kim a,b,⇑ a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi, Republic of Korea c R&D Center, ZEUS Co. Ltd., 163-1 Busan-dong, Osan, Gyeonggi 447-050, Republic of Korea d Department of Materials Engineering, Hanyang University, Ansan 426-791, Republic of Korea e Railroad Environment Research Department, Korea Railroad Research Institute, Woram-dong, Uiwang-si, Gyeonggi-do 437-757, Republic of Korea b
a r t i c l e
i n f o
Article history: Available online 27 December 2011 Keywords: Gas cluster Cleaning PBMS (particle beam mass spectrometer)
a b s t r a c t As devices become smaller to the extent of nano-scale, dry damage-free cleaning process has become an important area of study. Gas cluster cleaning technology has a potential to remove contaminant particles from the surface of semiconductor wafers without damage because a gas cluster is smaller than 100 nm in diameter. In this paper, a new dry cleaning process using a CO2 gas cluster beam is developed and a characterization method for gas cluster generation was studied experimentally. First, a CO2 gas cluster cleaning system was built to evaluate the feasibility of cleaning by use of a gas cluster beam, which is generated by expansion of gas through a converging–diverging nozzle and nozzle chilling. Cleaning tests were performed several times. Twenty-five to three hundred nanometers silica particles were cleaned off from the surface of a bare Si wafer and of a wafer with 60 nm poly-silicon structure by using this cleaning system. Preliminary result showed that the contaminants were successfully removed without damage to the wafer surface. To characterize the gas cluster, we used the particle beam mass spectrometer (PBMS) for the measurement of the size distribution of the gas clusters. The size distribution of clusters is measured by varying flow rate and nozzle coolant temperature. From the measurement results, the size of a gas cluster was less than 40 nm, which is expected to cause no damage on a surface based on the measurement of pattern collapse force. By using the PBMS, CO2 gas cluster cleaning system can be optimized to provide the maximum cleaning efficiency for nano-scale contaminant particles without damage to substrate surfaces. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Generally, surface contaminants have been removed from substrate surfaces by wet cleaning processes. Although wet cleaning processes efficiently remove contaminants, they have several undesirable effects such as irregular supersonic, slow diffusion velocity, surface tension which bring pattern damage, reattachment, and leaning problem [1,2]. So, dry damage-free cleaning processes have been investigated extensively, and of these processes, cryogenic aerosol cleaning, has been used successfully to remove contaminant particles from the surface of semiconductor wafers. The cryogenic aerosol cleaning system consists of a number of aerosol jets, equal to the number of orifices in the nozzle. The size of a single orifice is on the order of 0.1 mm. The aerosol jets are formed by the rapid expansion of cryogenic fluid from high pressure to low pressure in the chamber. The fluid within the nozzle is either argon or nitro⇑ Corresponding author at: School of Mechanical Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi, Republic of Korea. Tel.: +82 31 290 7466. E-mail address: [email protected] (T. Kim). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.12.007
gen or a mixture of the two, in a gaseous or partially liquefied state and at a temperature of about 100 K. The aerosol is formed due to the super-saturation of the expanding vapor. The growth of the clusters is governed by the interaction of gas dynamics [3]. However, the damage issue has not been resolved fully because it is difficult to maintain the size of the cryogenic aerosol below submicrometer level. Direct size measurement of the cryogenic aerosol has not been done, but based on theoretical work is well substantiated [4]. To overcome these problems, the gas cluster cleaning method is investigated. Basically, both cryogenic cleaning and gas cluster cleaning use the phenomenon of phase change through a nozzle. Solid state clusters are generated directly from the gas that passes through the converging–diverging type nozzle during gas cluster cleaning. However, aerosols are formed from the critical state between gas and liquid as they pass through a converging type nozzle during cryogenic aerosol cleaning. This means that the cluster cleaning method has relatively shorter particle condensation and growth time. Therefore, smaller sized particles (nanosized particles) can be generated. Generating aerosol size that has been reported by FSI Inc. is mainly distributed in the 8–45 lm range with 30–60 m/s velocity, respectively. We can compare the momentum
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H. Choi et al. / Microelectronic Engineering 102 (2013) 87–90
value between these two methods. Momentum of 1 lm sized aerosol particle with 50 m/s velocity is 104 times higher than 30 nm sized cluster with 500 m/s velocity. So, gas cluster cleaning is expected to remove contaminants effectively. A gas cluster is an aggregate of a few to several thousands of gaseous atoms or molecules, and it can be accelerated to a desired energy level after ionization. The kinetic energy of an atom in a cluster is equal to the total energy of the cluster divided by the cluster size [5]. This kinetic energy is converted to energy needed to remove contamination by collision of clusters on a substrate surface. Therefore, the sizes of the generated clusters need to be controlled to optimize this process. In this paper, the gas cluster cleaning method was investigated. The gas cluster cleaning method can be an alternative to commercial cleaning methods because the size of a gas cluster is below 100 nm [6–8]. The characterization method for gas cluster generation is studied experimentally. In the analysis of the gas cluster, the particle beam mass spectrometer (PBMS) is used to measure the size distribution of gas clusters. 2. Experimental 2.1. CO2 gas cluster cleaning apparatus Fig. 1 shows a schematic diagram of a CO2 gas cluster ion beam system. Clusters are formed by adiabatic expansion through a De
Laval type nozzle. This converging–diverging nozzle shape determines the thermodynamic conditions of the operating gas, such as temperature, pressure, and saturation ratio during gas flow. The resulting cluster size dominantly depends on these thermodynamic conditions. The source pressure was changed from 10 to 20 atm. A mechanical booster pump and turbo pump evacuated the cleaning chamber. Variables such as gas flow rate, cooling temperature, vacuum level, and inlet gas pressure that affect this process are related to the size distribution of the formed gas clusters. Operating gas expands through multi arranged nozzles and forms a 500 m/s cluster beam. This supersonic beam removes contaminants on a surface by collision. The CO2 cluster cleaning process is depicted on Fig. 2. 2.2. PBMS apparatus A particle beam mass spectrometer (PBMS) was developed by the McMurry group to measure concentrations and size-distributions of particles suspended in a low-pressure gas [9]. It consists of four modules: accelerating and focusing, charging, deflection, measurement. In this research, accelerating/focusing module was excluded from a PBMS as shown in Fig. 3 due to the several hundred m/s velocity and enough concentration at the nozzle exit. To measure the concentrations and size distributions of particles suspended in a low pressure condition. Particles with sufficient inertia pass through a skimmer into a high vacuum (<10 2 mtorr)
Fig. 1. Schematic diagram of the CO2 gas cluster cleaning apparatus.
Fig. 2. Process of the CO2 gas cluster cleaning method.
H. Choi et al. / Microelectronic Engineering 102 (2013) 87–90
89
Samples were prepared by using the following methods. To make contaminated wafer samples, 25–300 nm size silica particles were doped on a wafer surface by using colloid and dried using an airgun with a particle filter in the fume hood. After confirming the position where cluster beam would arrive, the prepared sample wafer was aligned. Then, the sample was exposed to the gas cluster beam for 5–10 s. For the 60 nm poly-silicon pattern damage test, the test process was the same. To characterize the size distribution of the generated clusters, we examined by the conditions of Table 1 using PBMS. The temperature profile in the nozzle is a critical condition of particle condensation and growth time. And the flow rate of operating gas controls the spray velocity. Fig. 3. Schematic diagram of PBMS system for cluster measurement.
3. Results and discussion Table 1 Experimental conditions for PBMS measurement.
3.1. Cleaning result
Case 1 Coolant temp. Inlet flow rate
50 °C 2 lpm
2.5 lpm
PBMS pressure 3 lpm
5 10 4 torr 3.5 lpm 4 lpm
PBMS pressure 50 °C
5 10 52 °C
Case 2 Inlet flow rate Coolant temp.
2 lpm 48 °C
4
torr
compartment. At this point, particles are charged to saturation by electron impact and are classified by a 90° electrostatic deflector, which deflects particles with a kinetic energy-to-charge ratio below a critical value. The beam current of deflected particles is measured by a Faraday detector and an electrometer. Particle size distribution is obtained from the relation that particle current is a function of deflection voltage [10]. 2.3. Experimental conditions and sample preparation To check the feasibility of the gas cluster cleaning technology, 25 nm silica particles and 80 nm poly-silicon patterns were used.
Results of each case are depicted on Fig. 4. As shown in the figures, CO2 cluster cleaning effectively removes 25 nm silica particles with 400 sccm at 49.7 °C cooling temperature condition. In addition, the pattern is not damaged under the same condition. Overall, experimental results show that higher cleaning efficiency can be achieved at higher flow rates and lower pre-cooling temperatures. However, to reduce pattern damage, flow rate and pre-cooling temperature should be optimized. 3.2. Cluster size distribution Size distributions by flow rate and coolant temperature are depicted in Fig. 5. These graphs used average value of 3 times realtime measurement results. According to uncertainty analysis by Nijhawan et al. [11], for such small particles, considerable uncertainty exists in determining particle number concentration due to transport losses, which increase sharply with decreasing size below 0.02 lm. However the overall uncertainty of PBMS particle size measurements is typically less than 10%. Primary cluster size
Fig. 4. Experimental results of CO2 gas cluster cleaning: (a) 25 nm silica particle removal experiment and (b) 60 nm poly-silica pattern damage experiment.
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Fig. 5. CO2 cluster size distribution by PBMS measurement: (a) by flow rate and (b) by nozzle coolant temperature.
ranges from about 22 to 30 nm, independent of experimental conditions. This agrees with the design expectation of cleaning without pattern damage by small clusters of size below 40 nm. The measured cluster size increases in proportion to the CO2 flow rate (Fig. 5a) and in inverse proportion to the nozzle coolant temperature (Fig. 5b). These results can be explained by the higher concentration of clusters at higher flow rates and by the relatively longer growth time at higher nozzle coolant temperatures. The mode diameter of generating gas cluster is about 30 nm. Gas clusters remove contaminant particles by momentum transfer. The limitation value of momentum was calculated from the collapse force measurement [12]. The collapse force of 60 nm poly-silicon line was 2 lN and increased by the square of the line width. The cluster size which does not cause the pattern damage is about 270 nm. This size was calculated from the impulse (I = F t = m v) with 500 m/s velocity, 1562 kg/m3 density. So, 30 nm size is suitable to remove particles without 60 nm pattern damage theoretically. 4. Conclusion In this paper, a dry cleaning process using CO2 gas clusters was developed and the gas cluster generation in this process was characterized by experiments. First, the CO2 gas cluster cleaning system was built to evaluate its feasibility. Cleaning tests were performed for cleaning of 25–300 nm silica particles on a bare Si wafer, and on a wafer with 60 nm poly-silicon structures. Overall, experimental results showed that higher cleaning efficiency can be achieved at higher flow rates and lower pre-cooling temperatures. To characterize the gas cluster generation, we used the particle beam mass spectrometer (PBMS) for the measurement of the size distribution of the gas clusters. Generating clusters were measured by flow rate and nozzle coolant temperature. From the measurement results,
the size of the gas clusters was found to be less than 40 nm, which would not damage the surface. The CO2 gas cluster cleaning method can be developed into a commercial cleaning method through with optimization. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20100010760) and a grant (11 Urban Railroad A-01) from Urban Railroad Technology Development Program funded by Ministry of Land, Transport and Maritime Affairs of Korean government. References [1] Y. Ahn, D. Yoo, J. Yang, J. Min, J. Kim, H. Lee, A. Kullkarni, T. Kim, Electrochem. Solid-State Lett. 13 (2010) H222–H226. [2] Y. Lu, Y. Aoyagi, M. Takai, S. Namba, Jpn. J. Appl. Phys. 33 (1994) 7138–7143. [3] N. Narayanswami, J. Electrochem. Soc. 146 (1999) 767–774. [4] N. Narayanswami, J. Heitzinger, J. Patrin, Adhes. Removal (1999) 251–266. [5] N. Toyoda, IEEE Trans. Plasma Sci. 36 (2008) 1471–1488. [6] I. Yamada, Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 257 (2007) 632–638. [7] N. Toyoda, S. Houzumi, I. Yamada, Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 241 (2005) 609–613. [8] J. Song, S. Kwon, D. Choi, W. Choi, Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 179 (2001) 568–574. [9] J. Na, T. Kim, J. Choi, J. Yun, Y. Shin, S. Kang, Appl. Phys. Express 2 (2009) 035501. [10] J. Na, T. Kim, J. Choi, Y. Kim, Y. Shin, J. Yun, S. Kang, Electrochem. Solid-State Lett. 13 (2010) H248–H252. [11] S. Nijhawan, P. McMurry, M. Swihart, S. Suh, S. Girshick, S. Campbell, J. Brokmann, J. Aerosol Sci. 34 (2003) 691–711. [12] T. Kim, K. Wostyn, P. Mertens, A. Busnaina, J. Park, Nanotechnology 21 (2010) 015708.
Contents lists available at SciVerse ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Development of CO2 gas cluster cleaning method and its characterization Hoomi Choi a, Hojoong Kim a, Deokjoo Yoon c, Jong W. Lee c, Bong-Kyun Kang d, Min-Su Kim d, Jin-Goo Park d, Soon-Bark Kwon e, Taesung Kim a,b,⇑ a
SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi, Republic of Korea School of Mechanical Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi, Republic of Korea c R&D Center, ZEUS Co. Ltd., 163-1 Busan-dong, Osan, Gyeonggi 447-050, Republic of Korea d Department of Materials Engineering, Hanyang University, Ansan 426-791, Republic of Korea e Railroad Environment Research Department, Korea Railroad Research Institute, Woram-dong, Uiwang-si, Gyeonggi-do 437-757, Republic of Korea b
a r t i c l e
i n f o
Article history: Available online 27 December 2011 Keywords: Gas cluster Cleaning PBMS (particle beam mass spectrometer)
a b s t r a c t As devices become smaller to the extent of nano-scale, dry damage-free cleaning process has become an important area of study. Gas cluster cleaning technology has a potential to remove contaminant particles from the surface of semiconductor wafers without damage because a gas cluster is smaller than 100 nm in diameter. In this paper, a new dry cleaning process using a CO2 gas cluster beam is developed and a characterization method for gas cluster generation was studied experimentally. First, a CO2 gas cluster cleaning system was built to evaluate the feasibility of cleaning by use of a gas cluster beam, which is generated by expansion of gas through a converging–diverging nozzle and nozzle chilling. Cleaning tests were performed several times. Twenty-five to three hundred nanometers silica particles were cleaned off from the surface of a bare Si wafer and of a wafer with 60 nm poly-silicon structure by using this cleaning system. Preliminary result showed that the contaminants were successfully removed without damage to the wafer surface. To characterize the gas cluster, we used the particle beam mass spectrometer (PBMS) for the measurement of the size distribution of the gas clusters. The size distribution of clusters is measured by varying flow rate and nozzle coolant temperature. From the measurement results, the size of a gas cluster was less than 40 nm, which is expected to cause no damage on a surface based on the measurement of pattern collapse force. By using the PBMS, CO2 gas cluster cleaning system can be optimized to provide the maximum cleaning efficiency for nano-scale contaminant particles without damage to substrate surfaces. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Generally, surface contaminants have been removed from substrate surfaces by wet cleaning processes. Although wet cleaning processes efficiently remove contaminants, they have several undesirable effects such as irregular supersonic, slow diffusion velocity, surface tension which bring pattern damage, reattachment, and leaning problem [1,2]. So, dry damage-free cleaning processes have been investigated extensively, and of these processes, cryogenic aerosol cleaning, has been used successfully to remove contaminant particles from the surface of semiconductor wafers. The cryogenic aerosol cleaning system consists of a number of aerosol jets, equal to the number of orifices in the nozzle. The size of a single orifice is on the order of 0.1 mm. The aerosol jets are formed by the rapid expansion of cryogenic fluid from high pressure to low pressure in the chamber. The fluid within the nozzle is either argon or nitro⇑ Corresponding author at: School of Mechanical Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi, Republic of Korea. Tel.: +82 31 290 7466. E-mail address: [email protected] (T. Kim). 0167-9317/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.12.007
gen or a mixture of the two, in a gaseous or partially liquefied state and at a temperature of about 100 K. The aerosol is formed due to the super-saturation of the expanding vapor. The growth of the clusters is governed by the interaction of gas dynamics [3]. However, the damage issue has not been resolved fully because it is difficult to maintain the size of the cryogenic aerosol below submicrometer level. Direct size measurement of the cryogenic aerosol has not been done, but based on theoretical work is well substantiated [4]. To overcome these problems, the gas cluster cleaning method is investigated. Basically, both cryogenic cleaning and gas cluster cleaning use the phenomenon of phase change through a nozzle. Solid state clusters are generated directly from the gas that passes through the converging–diverging type nozzle during gas cluster cleaning. However, aerosols are formed from the critical state between gas and liquid as they pass through a converging type nozzle during cryogenic aerosol cleaning. This means that the cluster cleaning method has relatively shorter particle condensation and growth time. Therefore, smaller sized particles (nanosized particles) can be generated. Generating aerosol size that has been reported by FSI Inc. is mainly distributed in the 8–45 lm range with 30–60 m/s velocity, respectively. We can compare the momentum
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H. Choi et al. / Microelectronic Engineering 102 (2013) 87–90
value between these two methods. Momentum of 1 lm sized aerosol particle with 50 m/s velocity is 104 times higher than 30 nm sized cluster with 500 m/s velocity. So, gas cluster cleaning is expected to remove contaminants effectively. A gas cluster is an aggregate of a few to several thousands of gaseous atoms or molecules, and it can be accelerated to a desired energy level after ionization. The kinetic energy of an atom in a cluster is equal to the total energy of the cluster divided by the cluster size [5]. This kinetic energy is converted to energy needed to remove contamination by collision of clusters on a substrate surface. Therefore, the sizes of the generated clusters need to be controlled to optimize this process. In this paper, the gas cluster cleaning method was investigated. The gas cluster cleaning method can be an alternative to commercial cleaning methods because the size of a gas cluster is below 100 nm [6–8]. The characterization method for gas cluster generation is studied experimentally. In the analysis of the gas cluster, the particle beam mass spectrometer (PBMS) is used to measure the size distribution of gas clusters. 2. Experimental 2.1. CO2 gas cluster cleaning apparatus Fig. 1 shows a schematic diagram of a CO2 gas cluster ion beam system. Clusters are formed by adiabatic expansion through a De
Laval type nozzle. This converging–diverging nozzle shape determines the thermodynamic conditions of the operating gas, such as temperature, pressure, and saturation ratio during gas flow. The resulting cluster size dominantly depends on these thermodynamic conditions. The source pressure was changed from 10 to 20 atm. A mechanical booster pump and turbo pump evacuated the cleaning chamber. Variables such as gas flow rate, cooling temperature, vacuum level, and inlet gas pressure that affect this process are related to the size distribution of the formed gas clusters. Operating gas expands through multi arranged nozzles and forms a 500 m/s cluster beam. This supersonic beam removes contaminants on a surface by collision. The CO2 cluster cleaning process is depicted on Fig. 2. 2.2. PBMS apparatus A particle beam mass spectrometer (PBMS) was developed by the McMurry group to measure concentrations and size-distributions of particles suspended in a low-pressure gas [9]. It consists of four modules: accelerating and focusing, charging, deflection, measurement. In this research, accelerating/focusing module was excluded from a PBMS as shown in Fig. 3 due to the several hundred m/s velocity and enough concentration at the nozzle exit. To measure the concentrations and size distributions of particles suspended in a low pressure condition. Particles with sufficient inertia pass through a skimmer into a high vacuum (<10 2 mtorr)
Fig. 1. Schematic diagram of the CO2 gas cluster cleaning apparatus.
Fig. 2. Process of the CO2 gas cluster cleaning method.
H. Choi et al. / Microelectronic Engineering 102 (2013) 87–90
89
Samples were prepared by using the following methods. To make contaminated wafer samples, 25–300 nm size silica particles were doped on a wafer surface by using colloid and dried using an airgun with a particle filter in the fume hood. After confirming the position where cluster beam would arrive, the prepared sample wafer was aligned. Then, the sample was exposed to the gas cluster beam for 5–10 s. For the 60 nm poly-silicon pattern damage test, the test process was the same. To characterize the size distribution of the generated clusters, we examined by the conditions of Table 1 using PBMS. The temperature profile in the nozzle is a critical condition of particle condensation and growth time. And the flow rate of operating gas controls the spray velocity. Fig. 3. Schematic diagram of PBMS system for cluster measurement.
3. Results and discussion Table 1 Experimental conditions for PBMS measurement.
3.1. Cleaning result
Case 1 Coolant temp. Inlet flow rate
50 °C 2 lpm
2.5 lpm
PBMS pressure 3 lpm
5 10 4 torr 3.5 lpm 4 lpm
PBMS pressure 50 °C
5 10 52 °C
Case 2 Inlet flow rate Coolant temp.
2 lpm 48 °C
4
torr
compartment. At this point, particles are charged to saturation by electron impact and are classified by a 90° electrostatic deflector, which deflects particles with a kinetic energy-to-charge ratio below a critical value. The beam current of deflected particles is measured by a Faraday detector and an electrometer. Particle size distribution is obtained from the relation that particle current is a function of deflection voltage [10]. 2.3. Experimental conditions and sample preparation To check the feasibility of the gas cluster cleaning technology, 25 nm silica particles and 80 nm poly-silicon patterns were used.
Results of each case are depicted on Fig. 4. As shown in the figures, CO2 cluster cleaning effectively removes 25 nm silica particles with 400 sccm at 49.7 °C cooling temperature condition. In addition, the pattern is not damaged under the same condition. Overall, experimental results show that higher cleaning efficiency can be achieved at higher flow rates and lower pre-cooling temperatures. However, to reduce pattern damage, flow rate and pre-cooling temperature should be optimized. 3.2. Cluster size distribution Size distributions by flow rate and coolant temperature are depicted in Fig. 5. These graphs used average value of 3 times realtime measurement results. According to uncertainty analysis by Nijhawan et al. [11], for such small particles, considerable uncertainty exists in determining particle number concentration due to transport losses, which increase sharply with decreasing size below 0.02 lm. However the overall uncertainty of PBMS particle size measurements is typically less than 10%. Primary cluster size
Fig. 4. Experimental results of CO2 gas cluster cleaning: (a) 25 nm silica particle removal experiment and (b) 60 nm poly-silica pattern damage experiment.
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H. Choi et al. / Microelectronic Engineering 102 (2013) 87–90
Fig. 5. CO2 cluster size distribution by PBMS measurement: (a) by flow rate and (b) by nozzle coolant temperature.
ranges from about 22 to 30 nm, independent of experimental conditions. This agrees with the design expectation of cleaning without pattern damage by small clusters of size below 40 nm. The measured cluster size increases in proportion to the CO2 flow rate (Fig. 5a) and in inverse proportion to the nozzle coolant temperature (Fig. 5b). These results can be explained by the higher concentration of clusters at higher flow rates and by the relatively longer growth time at higher nozzle coolant temperatures. The mode diameter of generating gas cluster is about 30 nm. Gas clusters remove contaminant particles by momentum transfer. The limitation value of momentum was calculated from the collapse force measurement [12]. The collapse force of 60 nm poly-silicon line was 2 lN and increased by the square of the line width. The cluster size which does not cause the pattern damage is about 270 nm. This size was calculated from the impulse (I = F t = m v) with 500 m/s velocity, 1562 kg/m3 density. So, 30 nm size is suitable to remove particles without 60 nm pattern damage theoretically. 4. Conclusion In this paper, a dry cleaning process using CO2 gas clusters was developed and the gas cluster generation in this process was characterized by experiments. First, the CO2 gas cluster cleaning system was built to evaluate its feasibility. Cleaning tests were performed for cleaning of 25–300 nm silica particles on a bare Si wafer, and on a wafer with 60 nm poly-silicon structures. Overall, experimental results showed that higher cleaning efficiency can be achieved at higher flow rates and lower pre-cooling temperatures. To characterize the gas cluster generation, we used the particle beam mass spectrometer (PBMS) for the measurement of the size distribution of the gas clusters. Generating clusters were measured by flow rate and nozzle coolant temperature. From the measurement results,
the size of the gas clusters was found to be less than 40 nm, which would not damage the surface. The CO2 gas cluster cleaning method can be developed into a commercial cleaning method through with optimization. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20100010760) and a grant (11 Urban Railroad A-01) from Urban Railroad Technology Development Program funded by Ministry of Land, Transport and Maritime Affairs of Korean government. References [1] Y. Ahn, D. Yoo, J. Yang, J. Min, J. Kim, H. Lee, A. Kullkarni, T. Kim, Electrochem. Solid-State Lett. 13 (2010) H222–H226. [2] Y. Lu, Y. Aoyagi, M. Takai, S. Namba, Jpn. J. Appl. Phys. 33 (1994) 7138–7143. [3] N. Narayanswami, J. Electrochem. Soc. 146 (1999) 767–774. [4] N. Narayanswami, J. Heitzinger, J. Patrin, Adhes. Removal (1999) 251–266. [5] N. Toyoda, IEEE Trans. Plasma Sci. 36 (2008) 1471–1488. [6] I. Yamada, Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 257 (2007) 632–638. [7] N. Toyoda, S. Houzumi, I. Yamada, Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 241 (2005) 609–613. [8] J. Song, S. Kwon, D. Choi, W. Choi, Nucl. Instrum. Methods Phys. Res. Sect. B: Beam Interact. Mater. Atoms 179 (2001) 568–574. [9] J. Na, T. Kim, J. Choi, J. Yun, Y. Shin, S. Kang, Appl. Phys. Express 2 (2009) 035501. [10] J. Na, T. Kim, J. Choi, Y. Kim, Y. Shin, J. Yun, S. Kang, Electrochem. Solid-State Lett. 13 (2010) H248–H252. [11] S. Nijhawan, P. McMurry, M. Swihart, S. Suh, S. Girshick, S. Campbell, J. Brokmann, J. Aerosol Sci. 34 (2003) 691–711. [12] T. Kim, K. Wostyn, P. Mertens, A. Busnaina, J. Park, Nanotechnology 21 (2010) 015708.