- Email: [email protected]
The effect of ultrasonic agitation on the stripping of photoresist using supercritical CO2 and co-solvent formulation
Microelectronic Engineering 86 (2009) 171–175
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
The effect of ultrasonic agitation on the stripping of photoresist using supercritical CO2 and co-solvent formulation Sung Ho Kim a, Haldorai Yuvaraj a, Yeon Tae Jeong a, Chan Park b, Sok Won Kim c, Kwon Taek Lim a,* a
Division of Image Science and Engineering, Pukyong National University, San 100 Yongdang-dong, Nam-gu, Busan 608-739, Republic of Korea Division of Material Engineering, Pukyong National University, Busan 608-739, Republic of Korea c Department of Physics, University of Ulsan, Ulsan 680-749, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 15 April 2008 Received in revised form 24 October 2008 Accepted 24 October 2008 Available online 5 November 2008 Keywords: Photoresist Stripping Ultrasonic agitation Supercritical carbon dioxide High dose ion-implanted
a b s t r a c t A novel technology for removing high-dose ion-implanted photoresist (HDI PR) from semiconductor wafers using supercritical carbon dioxide (scCO2) and several co-solvent formulations have been described. A combination of ultrasonic agitation with scCO2/co-solvent stripping was found to be an effective method for photoresist removal. Ultrasonic agitation was an efficient technique for achieving higher stripping rates. The effects of temperature, pressure, reaction time and the type of organic co-solvent on the stripping rate of HDI PR were investigated. The microstructures of sample wafers after stripping were characterized by scanning electron microscopy and energy dispersive X-ray spectrometer. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction It is necessary in a photolithographic process that the photoresist material, following pattern transfer, be evenly and completely removed. Even the partial remains of a resist in an area to be further patterned are undesirable. Also, resist residues between patterned lines can have deleterious effects on subsequent processes, such as metallization, or cause undesirable surface states and charges. Therefore, one of the critical challenges for future generation devices is the effective removal of photoresist, without silicon loss or damage to fine structures [1]. A common technique for photoresist removal involves placing the substrate in an asher and burning the resist and associated coatings using gaseous plasma [2,3]. While the high temperature (some ashes are removed even at room temperature) in the plasma process chamber oxidizes the photoresist and removes it, the plasma process leaves post-ash residues-undesirable by-products from the reaction of the plasma gases, reactant species and the photoresist. These by-products cannot be completely removed by the ash process. Thus, the substrate must be subsequently placed in a wet cleaning tool to remove by-products, and then rinsed and dried. Another currently used photoresist removal process includes exposing the substrate to a liquid photoresist stripper containing at least one polar solvent [3]. At times, however, the by-products of * Corresponding author. Tel.: +82 51 629 6409; fax: +82 51 629 6408. E-mail address: [email protected] (K.T. Lim). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.10.017
the stripping process and the stripping solution itself may be left behind in fine features formed in the substrate. Therefore, additional steps of rinsing out the stripper and stripper residues and drying the wafer must follow the wet stripping process. Despite a long history of wet stripping photoresists and residues [4–8], the semiconductor industry is still faced with a challenging problem in removing hardened photoresist and/or resist residues. Conventional stripping methods need drastic conditions which further damage the underlying materials. In addition, the formulations of these strippers contain toxic solvents and solvent combinations, which are environmentally hazardous. Besides the consideration of environmental safety and health impact, any alternative technologies must not just be greener but provide valid technical advantages that meet innovative component designs. Supercritical carbon dioxide (scCO2) provides an alternative attractive method for cleaning and stripping photoresist [9]. scCO2 has near zero surface tension like a gas, and thus can penetrate easily into deep trenches and vias. It is non-polar and has the ability to dissolve non-polar chemicals. In conjunction with its environmentally friendly merits and moderate critical temperature and pressure, scCO2 has become the impetus for many innovative applications [10–12]. Several methods of photoresist stripping and/or residue removing by scCO2 have been reported in the literature [13–18]. It was known that physical forces as well as the dissolution ability of solutions were important for the efficient stripping of photoresist. Recently, it has been shown that supercritical CO2 together with co-solvents and specific additives could strip the ion-implanted
172
S.H. Kim et al. / Microelectronic Engineering 86 (2009) 171–175
photoresist from wafer surface [19,20]. It was also reported that the scCO2 process that involves pressurization/depressurization debonded the resist from the substrate and carried it away using flow dynamics [21]. The processes, however, require unduly long processing time for complete photoresist removal, use a large amount of process fluids, and require toxic substances. It is, therefore, desirable to provide a process for stripping photoresists that is fast, efficient and environmentally friendly. In this study, we have demonstrated the effect of ultrasonic agitation on high-dose ion-implanted photoresist (HDI PR) stripping by using scCO2/co-solvent mixture. The stripping characteristics have been compared with those using magnetic stirring and no agitation. 2. Experimental section Analytical grade of acetone (Aldrich) and dimethylsulfoxide (Aldrich) were purchased and used as co-solvents. The sample wafers have been prepared to have the structure of 10 nm silicon dioxide and 970 nm thick patterned KrF type photoresist on 0.77 mm thick silicon substrate. The arsenic ion source was implanted on the surface with a dose level of 3 1015 ions/cm2 with the constant ion energy of 50 keV. The scCO2 stripping system consists of an ISCO automatic high pressure syringe pump (Model No. 260D), a temperature controller (water bath) and a stainless steel reactor equipped with an ultrasonic agitator (28 kHz) of power 600 W. The schematic representation of the experimental set-up is shown in Fig. 1. In a typical processing sequence, the wafer sample was loaded into the processing chamber. The high pressure syringe pump previously filled with high purity carbon dioxide (99.999%, Daeyoung Co., Korea) was used to introduce CO2 in the chamber. Small amount of cosolvent was injected into the chamber followed by the addition of liquid CO2 which was pressurized to a required pressure. The temperature controller kept the temperature of the chamber constant. While ultrasonically agitating the scCO2/co-solvent mixture, the processing chamber was heated to a particular temperature. After stripping was complete, the scCO2/co-solvent mixture was then vented into a high pressure trap to collect any residual chemical or photoresist residue during the phase separation. The final microstructure of the wafer after photoresist stripping was studied by using a Hitachi S-2400 scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDX). The extent of strip-
ping was calculated by dividing the stripped area by the original photoresist area. 3. Results and discussion The photoresist stripping efficiency is strongly dependant on the implant-ion concentration level to the photoresist. The complete removal of polymer photoresist exposed to high-dose ion implant is problematic with conventional stripping and cleaning methods. Ion implantation treatment results in the formation of a tough, carbonized crust that renders the removal of the bulk photoresist difficult. In this work, a rapid and efficient stripping method was investigated using a combination of ultrasonic agitation and scCO2/co-solvent formulations. Ion implanted photoresists on wafer surfaces were not removed by a pure scCO2 process. However, the addition of a small amount of co-solvent to scCO2 improved the removal of photoresist up to 25% at 40 °C and 27.6 MPa with the reaction time of 5 min. Interestingly it was found that the effect of ultrasonic agitation significantly influences the stripping rate of photoresist from the wafer surface. The addition of co-solvent is considered to promote the swelling and dissolution of the resist. The effect is much further improved by ultrasonic agitation which facilitates the delamination of photoresist layers (crust and PR). In order to find a suitable solvent for stripping, several organic solvents were tested for their solubility with the HDI PR. Amines (triisopropanolamine, N-tert-butyl diethanolamine, and triethylamine) hydrocarbons (hexane and heptane) and carbonates (propylene carbonate and ethylene carbonate) had no effect, while alcohols (methanol, ethanol, isopropyl alcohol and benzyl alcohol) had a little solubility. From observations, dimethylsufoxide (DMSO) and acetone showed the best solubilities and were selected as candidates for the stripping experiments. Fig. 2 shows the SEM image of HDI PR wafer consisting of 10 nm SiO2, 870 nm photoresist and 100 nm hardened crust. After processing the wafer for 120 s with ultrasonically agitating 10% w/w acetone/scCO2 formulations at 40 °C and 27.6 MPa, SEM analysis showed that approximately 50% HDI PR was stripped off. The photoresist was mostly dissolved laterally from the edges of the resist and only top-down in certain areas. This was expected due to the 100 nm hardened crust on the top of photoresist by the implanted process. Although the crust is somewhat permeable in the highly diffusive scCO2 solution, it is difficult to strip completely. The effect of pressure on the stripping rate of photoresist by ultrasonically
Fig. 1. Schematic representation of the experimental set-up for photoresist stripping.
173
S.H. Kim et al. / Microelectronic Engineering 86 (2009) 171–175
Table 2 The effect of temperature and density on the stripping rate of photoresist by ultrasonically agitating 10% w/w DMSO/scCO2 mixture for 120 s.
Fig. 2. SEM images of (a) as received HDI PR sample and (b) wafer processed by ultrasonically agitating 10% w/w acetone/scCO2 mixture at 40 °C and 27.6 MPa for 120 s.
Table 1 The effect of pressure on stripping rate of photoresist by ultrasonically agitating 10% w/w acetone/scCO2 and DMSO/scCO2 formulations at 40 °C for 120 s. Entry
Pressure (MPa)
scCO2 density (g/cc)
Extent of stripping (%) Acetone/scCO2
DMSO/scCO2
1 2 3 4
17 20.7 24 27.6
0.81 0.84 0.87 0.89
30 30 40 50
30 30 40 40
agitating acetone/scCO2 and DMSO/scCO2 formulations are summarized in Table 1. The stripping rate of photoresist was increased with operational pressure. It is suggested that the higher treatment pressure increases the amount of photoresist swollen for a given time and therefore the extent of stripping. At high pressures, the density of scCO2 increases and solvating ability of the fluid is enhanced. Moreover the pressure raises the reaction kinetics and interfacial attack of the fluid. Experimental results on the effect of temperature and scCO2 density on the stripping rate by ultrasonically agitating scCO2/DMSO (10% w/w to CO2) formulation is given in Table 2. It was observed that the increase in the temperature significantly raised the extent of photoresist stripping in spite of decrease in the scCO2 density and would, therefore, reduce the overall treatment time. This could be due to a greater effect of T on solubility than density. The maximum photoresist stripping was found to be 80% at 70 °C with the scCO2 density of 0.77 g/cc for a given reaction time of 120 s. At higher temperature solvent molecules penetrate better into the hardened photoresist and de-
Entry
Temperature (°C)
Pressure (MPa)
scCO2 density (g/cc)
Extent of stripping (%)
1 2 3 4 5 6
40 50 60 70 80 90
27.6 27.6 27.6 27.6 17.2 17.2
0.89 0.85 0.81 0.77 0.51 0.45
40 60 70 80 65 50
bond the resist polymer, resulting in an increase in the dissolution of the photoresist. For the purpose of decreasing the process pressure, we have tested lower pressures with higher temperatures, namely 17.2 MPa with 80 and 90 °C. At these conditions, however, the scCO2 density decreases too much (0.51 and 0.45 g/cc) and the extents of stripping were not comparable with that from 27.6 MPa and 70 °C (the density of 0.77 g/cc) (see Table 2). Thus, it is most likely that the density of CO2 should be above a certain value to give an effective stripping. From above results it is clear that the combined behavior of temperature and density are necessary, but even though combined behavior gives an efficient stripping, temperature plays a major role rather than density. It is obvious that the effect of stripping rate increased linearly with the increasing reaction time and the results are given in Table 3. The scCO2/DMSO process was then optimized using the design of experiments and the optimum conditions for complete (100%) stripping of photoresist were determined to be 70 °C and 27.6 MPa for 180 s under ultrasonic agitation. Fig. 3a shows the SEM image of the wafer from which HDI PR is completely stripped off, while leaving the underlying silicon substrate untouched by the scCO2/co-solvent formulation. We also compared the effect of stripping of photoresist using scCO2/DMSO (10% w/w to CO2) formulations under different agitation methods namely ultrasonic agitation, magnetic stirring and no agitation. The experimental results are illustrated in Table 4. Results show that the stripping rate is 100% for ultrasonically agitated sample whereas 60% and 50% in the case of magnetic stirring and no agitation samples, respectively. This is evident from the SEM images shown in Fig. 3b and c. From this it is clear that ultrasonic agitation is a key factor to achieve higher stripping rates compared to magnetic stirring and no agitation. EDX analysis also revealed that the combination of scCO2/co-solvent and ultrasonic agitation completely stripped the HDI PR while leaving the SiO2 layer (Fig. 4). We have investigated the impact of the ultrasonication on the patterns (120/1500 nm (line/space) poly Si pattern), but did not find any pattern damage in these conditions. The mechanism by which the co-solvent/scCO2 mixture and ultrasonication removes the photoresist completely has not been elucidated. However, it is suggested that polymeric material can be swollen by diffusion of CO2 and a reduction in the glass transition temperature can be produced. The swelling of the photoresist matrix reduces the polymer–polymer and polymer–substrate interaction, promoting the de-bonding of photoresist from the supporting substrate [14]. The solubility of the photoresist in scCO2 is relatively low, but this can be facilitated by tuning the operational parameters (temperature and pressure) Table 3 The effect of reaction time on stripping rate of photoresist by ultrasonically agitating 10% w/w DMSO/scCO2 mixture at 70 °C and 27.6 MPa. Entry
Time (s)
Extent of stripping (%)
1 2 3
120 150 180
80 90 100
174
S.H. Kim et al. / Microelectronic Engineering 86 (2009) 171–175
Fig. 3. SEM images of the wafers after photoresist stripping at 70 °C and 27.6 MPa for 180 s using 10% w/w DMSO/scCO2 mixture under (a) ultrasonic agitation, (b) magnetic stirring and (c) no agitation.
Table 4 The effect of stripping rate of photoresist using 10% w/w DMSO/scCO2 mixture under different mechanical methods; (a) ultrasonic agitation, (b) magnetic stirring and (c) no agitation. Experimental conditions: 70 °C and 27.6 MPa for 180 s. Entry
Type of mechanical agitation
Extent of stripping (%)
1 2 3
Ultrasonic agitation Magnetic stirring No agitation
100 60 50
removal in this system assumes to be based on both chemical and physical mechanisms. 4. Conclusions We have successfully shown the complete stripping of ion-implanted photoresist using scCO2/co-solvent formulations while maintaining the structural integrity of the underlying layer. The result indicates that ultrasonic agitation is a key factor to increase the stripping rate of photoresist compared to the mechanical stirring and no agitation. Besides ultrasonication, our investigations reveal that the effects of temperature, pressure, and type of cosolvent are the important factors for optimal stripping efficiency. Acknowledgements This work was supported by ‘System IC 2010’ Project of Korea Ministry of Knowledge Economy and by the Medium-term Strategic Technology Development Program funded by the Ministry of Commerce, Industry and Energy (MOICE, Korea). References
Fig. 4. EDX analysis data of the wafer surface after photoresist stripping with ultrasonication and DMSO/scCO2 formulation.
[1] [2] [3] [4] [5]
and the addition of co-solvent into the scCO2 further induces swelling and de-bonding. In addition, ultrasonic agitation can stimulate polymer film delamination and increases the stripping rate of photoresist. It is suggested that the interfacial attack is promoted by increased temperature and the agitation. Therefore, the crust layer
[6] [7] [8] [9] [10] [11]
P.W. Mertens, G. Vereecke, R. Vos, Semicond. Fabtech. 31 (2006) 86. M.A. Hartney, D.W. Hess, D.S. Soane, J. Vac. Sci. Technol. B 7 (1989) 1. W. Kern, Handbook of Semiconductor Wafer Cleaning Technology, 1993. M. Heyns, P.W. Martens, J. Ruzyllo, M.Y.M. Lee, Solid State Technol. 42 (1999) 37. H. Abe, H. Iwamoto, T. Toshima, T. Iino, G.W. Gale, IEEE Trans. Semicond. Manuf. 16 (2003) 401. P.G. Clark, K.K. Christenson, Adv. Semicond. Manuf. Conf. (2005) 1. J.W. Metselaar, V.I. Kuznetsov, A.G. Zhidkov, J. Appl. Phys. 75 (1994) 4910. K. Choi, S. Ghosh, J. Lim, C.M. Lee, Appl. Surf. Sci. 206 (2003) 355. K. Saga, T. Hattori, Solid State Phenom. 134 (2008) 97. A. O’Neil, J.J. Watkins, Green Chem. 6 (2004) 363. M.Y. Lee, K.M. Do, H.S. Ganapathy, Y.S. Lo, J.J. Kim, S.J. Choi, K.T. Lim, J. Supercrit. Fluids 42 (2007) 150.
S.H. Kim et al. / Microelectronic Engineering 86 (2009) 171–175 [12] J.Y. Kim, Y.H. Park, J.S. Kim, K.T. Lim, Y.T. Jeong, J. Ind. Eng. Chem. 13 (2007) 1023. [13] J.B. Rubin, L.B. Davenhall, C.M.V. Taylor, L.D. Sivils, T. Pierce, K. Tiefert, Carbon dioxide-based supercritical fluids as IC manufacturing solvents, in: IEEE International Symposium on Electronics and the Environment, Denver, MA, 1999. [14] J.W. King, L.L. Williams, Curr. Opin. Solid State Mater. Sci. 7 (2003) 413. [15] E.N. Hoggen, K. Wang, D. Flowers, J.M. DeSimone, R.G. Carbonell, IEEE Trans. Semicond. Manuf. 17 (2004) 510. [16] J.L. Chen, Y.S. Wang, H.I. Kuo, D.Y. Shu, Talanta 70 (2006) 414.
175
[17] G. Levitin, S. Myneni, D.W. Hess, J. Electrochem. Soc. 151 (2004) G380. [18] H. Kiyose, K. Saito, I. Mizobata, T. Iwata, S. Hirae, K. Saga, H. Kuniyasu, T. Hattori, Solid State Phenom. 134 (2008) 341. [19] A. Sehgal, in US20040050406, US, 2004. [20] K. Saga, H. Kuniyasu, T. Hattori, M.B. Korzenski, P.M. Visintin, T.H. Baum, Ionimplanted photoresist stripping using supercritical carbon dioxide, in: 208th Meeting of the Electrochemical Society, Los Angeles, CA, 2005. [21] L.B. Rothman, R.J. Robey, M.K. Ali, D.J. Mount, Supercritical fluid process for photoresist stripping, Wafer Cleaning and Surface Preparation Workshop, 2002.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
The effect of ultrasonic agitation on the stripping of photoresist using supercritical CO2 and co-solvent formulation Sung Ho Kim a, Haldorai Yuvaraj a, Yeon Tae Jeong a, Chan Park b, Sok Won Kim c, Kwon Taek Lim a,* a
Division of Image Science and Engineering, Pukyong National University, San 100 Yongdang-dong, Nam-gu, Busan 608-739, Republic of Korea Division of Material Engineering, Pukyong National University, Busan 608-739, Republic of Korea c Department of Physics, University of Ulsan, Ulsan 680-749, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 15 April 2008 Received in revised form 24 October 2008 Accepted 24 October 2008 Available online 5 November 2008 Keywords: Photoresist Stripping Ultrasonic agitation Supercritical carbon dioxide High dose ion-implanted
a b s t r a c t A novel technology for removing high-dose ion-implanted photoresist (HDI PR) from semiconductor wafers using supercritical carbon dioxide (scCO2) and several co-solvent formulations have been described. A combination of ultrasonic agitation with scCO2/co-solvent stripping was found to be an effective method for photoresist removal. Ultrasonic agitation was an efficient technique for achieving higher stripping rates. The effects of temperature, pressure, reaction time and the type of organic co-solvent on the stripping rate of HDI PR were investigated. The microstructures of sample wafers after stripping were characterized by scanning electron microscopy and energy dispersive X-ray spectrometer. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction It is necessary in a photolithographic process that the photoresist material, following pattern transfer, be evenly and completely removed. Even the partial remains of a resist in an area to be further patterned are undesirable. Also, resist residues between patterned lines can have deleterious effects on subsequent processes, such as metallization, or cause undesirable surface states and charges. Therefore, one of the critical challenges for future generation devices is the effective removal of photoresist, without silicon loss or damage to fine structures [1]. A common technique for photoresist removal involves placing the substrate in an asher and burning the resist and associated coatings using gaseous plasma [2,3]. While the high temperature (some ashes are removed even at room temperature) in the plasma process chamber oxidizes the photoresist and removes it, the plasma process leaves post-ash residues-undesirable by-products from the reaction of the plasma gases, reactant species and the photoresist. These by-products cannot be completely removed by the ash process. Thus, the substrate must be subsequently placed in a wet cleaning tool to remove by-products, and then rinsed and dried. Another currently used photoresist removal process includes exposing the substrate to a liquid photoresist stripper containing at least one polar solvent [3]. At times, however, the by-products of * Corresponding author. Tel.: +82 51 629 6409; fax: +82 51 629 6408. E-mail address: [email protected] (K.T. Lim). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.10.017
the stripping process and the stripping solution itself may be left behind in fine features formed in the substrate. Therefore, additional steps of rinsing out the stripper and stripper residues and drying the wafer must follow the wet stripping process. Despite a long history of wet stripping photoresists and residues [4–8], the semiconductor industry is still faced with a challenging problem in removing hardened photoresist and/or resist residues. Conventional stripping methods need drastic conditions which further damage the underlying materials. In addition, the formulations of these strippers contain toxic solvents and solvent combinations, which are environmentally hazardous. Besides the consideration of environmental safety and health impact, any alternative technologies must not just be greener but provide valid technical advantages that meet innovative component designs. Supercritical carbon dioxide (scCO2) provides an alternative attractive method for cleaning and stripping photoresist [9]. scCO2 has near zero surface tension like a gas, and thus can penetrate easily into deep trenches and vias. It is non-polar and has the ability to dissolve non-polar chemicals. In conjunction with its environmentally friendly merits and moderate critical temperature and pressure, scCO2 has become the impetus for many innovative applications [10–12]. Several methods of photoresist stripping and/or residue removing by scCO2 have been reported in the literature [13–18]. It was known that physical forces as well as the dissolution ability of solutions were important for the efficient stripping of photoresist. Recently, it has been shown that supercritical CO2 together with co-solvents and specific additives could strip the ion-implanted
172
S.H. Kim et al. / Microelectronic Engineering 86 (2009) 171–175
photoresist from wafer surface [19,20]. It was also reported that the scCO2 process that involves pressurization/depressurization debonded the resist from the substrate and carried it away using flow dynamics [21]. The processes, however, require unduly long processing time for complete photoresist removal, use a large amount of process fluids, and require toxic substances. It is, therefore, desirable to provide a process for stripping photoresists that is fast, efficient and environmentally friendly. In this study, we have demonstrated the effect of ultrasonic agitation on high-dose ion-implanted photoresist (HDI PR) stripping by using scCO2/co-solvent mixture. The stripping characteristics have been compared with those using magnetic stirring and no agitation. 2. Experimental section Analytical grade of acetone (Aldrich) and dimethylsulfoxide (Aldrich) were purchased and used as co-solvents. The sample wafers have been prepared to have the structure of 10 nm silicon dioxide and 970 nm thick patterned KrF type photoresist on 0.77 mm thick silicon substrate. The arsenic ion source was implanted on the surface with a dose level of 3 1015 ions/cm2 with the constant ion energy of 50 keV. The scCO2 stripping system consists of an ISCO automatic high pressure syringe pump (Model No. 260D), a temperature controller (water bath) and a stainless steel reactor equipped with an ultrasonic agitator (28 kHz) of power 600 W. The schematic representation of the experimental set-up is shown in Fig. 1. In a typical processing sequence, the wafer sample was loaded into the processing chamber. The high pressure syringe pump previously filled with high purity carbon dioxide (99.999%, Daeyoung Co., Korea) was used to introduce CO2 in the chamber. Small amount of cosolvent was injected into the chamber followed by the addition of liquid CO2 which was pressurized to a required pressure. The temperature controller kept the temperature of the chamber constant. While ultrasonically agitating the scCO2/co-solvent mixture, the processing chamber was heated to a particular temperature. After stripping was complete, the scCO2/co-solvent mixture was then vented into a high pressure trap to collect any residual chemical or photoresist residue during the phase separation. The final microstructure of the wafer after photoresist stripping was studied by using a Hitachi S-2400 scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDX). The extent of strip-
ping was calculated by dividing the stripped area by the original photoresist area. 3. Results and discussion The photoresist stripping efficiency is strongly dependant on the implant-ion concentration level to the photoresist. The complete removal of polymer photoresist exposed to high-dose ion implant is problematic with conventional stripping and cleaning methods. Ion implantation treatment results in the formation of a tough, carbonized crust that renders the removal of the bulk photoresist difficult. In this work, a rapid and efficient stripping method was investigated using a combination of ultrasonic agitation and scCO2/co-solvent formulations. Ion implanted photoresists on wafer surfaces were not removed by a pure scCO2 process. However, the addition of a small amount of co-solvent to scCO2 improved the removal of photoresist up to 25% at 40 °C and 27.6 MPa with the reaction time of 5 min. Interestingly it was found that the effect of ultrasonic agitation significantly influences the stripping rate of photoresist from the wafer surface. The addition of co-solvent is considered to promote the swelling and dissolution of the resist. The effect is much further improved by ultrasonic agitation which facilitates the delamination of photoresist layers (crust and PR). In order to find a suitable solvent for stripping, several organic solvents were tested for their solubility with the HDI PR. Amines (triisopropanolamine, N-tert-butyl diethanolamine, and triethylamine) hydrocarbons (hexane and heptane) and carbonates (propylene carbonate and ethylene carbonate) had no effect, while alcohols (methanol, ethanol, isopropyl alcohol and benzyl alcohol) had a little solubility. From observations, dimethylsufoxide (DMSO) and acetone showed the best solubilities and were selected as candidates for the stripping experiments. Fig. 2 shows the SEM image of HDI PR wafer consisting of 10 nm SiO2, 870 nm photoresist and 100 nm hardened crust. After processing the wafer for 120 s with ultrasonically agitating 10% w/w acetone/scCO2 formulations at 40 °C and 27.6 MPa, SEM analysis showed that approximately 50% HDI PR was stripped off. The photoresist was mostly dissolved laterally from the edges of the resist and only top-down in certain areas. This was expected due to the 100 nm hardened crust on the top of photoresist by the implanted process. Although the crust is somewhat permeable in the highly diffusive scCO2 solution, it is difficult to strip completely. The effect of pressure on the stripping rate of photoresist by ultrasonically
Fig. 1. Schematic representation of the experimental set-up for photoresist stripping.
173
S.H. Kim et al. / Microelectronic Engineering 86 (2009) 171–175
Table 2 The effect of temperature and density on the stripping rate of photoresist by ultrasonically agitating 10% w/w DMSO/scCO2 mixture for 120 s.
Fig. 2. SEM images of (a) as received HDI PR sample and (b) wafer processed by ultrasonically agitating 10% w/w acetone/scCO2 mixture at 40 °C and 27.6 MPa for 120 s.
Table 1 The effect of pressure on stripping rate of photoresist by ultrasonically agitating 10% w/w acetone/scCO2 and DMSO/scCO2 formulations at 40 °C for 120 s. Entry
Pressure (MPa)
scCO2 density (g/cc)
Extent of stripping (%) Acetone/scCO2
DMSO/scCO2
1 2 3 4
17 20.7 24 27.6
0.81 0.84 0.87 0.89
30 30 40 50
30 30 40 40
agitating acetone/scCO2 and DMSO/scCO2 formulations are summarized in Table 1. The stripping rate of photoresist was increased with operational pressure. It is suggested that the higher treatment pressure increases the amount of photoresist swollen for a given time and therefore the extent of stripping. At high pressures, the density of scCO2 increases and solvating ability of the fluid is enhanced. Moreover the pressure raises the reaction kinetics and interfacial attack of the fluid. Experimental results on the effect of temperature and scCO2 density on the stripping rate by ultrasonically agitating scCO2/DMSO (10% w/w to CO2) formulation is given in Table 2. It was observed that the increase in the temperature significantly raised the extent of photoresist stripping in spite of decrease in the scCO2 density and would, therefore, reduce the overall treatment time. This could be due to a greater effect of T on solubility than density. The maximum photoresist stripping was found to be 80% at 70 °C with the scCO2 density of 0.77 g/cc for a given reaction time of 120 s. At higher temperature solvent molecules penetrate better into the hardened photoresist and de-
Entry
Temperature (°C)
Pressure (MPa)
scCO2 density (g/cc)
Extent of stripping (%)
1 2 3 4 5 6
40 50 60 70 80 90
27.6 27.6 27.6 27.6 17.2 17.2
0.89 0.85 0.81 0.77 0.51 0.45
40 60 70 80 65 50
bond the resist polymer, resulting in an increase in the dissolution of the photoresist. For the purpose of decreasing the process pressure, we have tested lower pressures with higher temperatures, namely 17.2 MPa with 80 and 90 °C. At these conditions, however, the scCO2 density decreases too much (0.51 and 0.45 g/cc) and the extents of stripping were not comparable with that from 27.6 MPa and 70 °C (the density of 0.77 g/cc) (see Table 2). Thus, it is most likely that the density of CO2 should be above a certain value to give an effective stripping. From above results it is clear that the combined behavior of temperature and density are necessary, but even though combined behavior gives an efficient stripping, temperature plays a major role rather than density. It is obvious that the effect of stripping rate increased linearly with the increasing reaction time and the results are given in Table 3. The scCO2/DMSO process was then optimized using the design of experiments and the optimum conditions for complete (100%) stripping of photoresist were determined to be 70 °C and 27.6 MPa for 180 s under ultrasonic agitation. Fig. 3a shows the SEM image of the wafer from which HDI PR is completely stripped off, while leaving the underlying silicon substrate untouched by the scCO2/co-solvent formulation. We also compared the effect of stripping of photoresist using scCO2/DMSO (10% w/w to CO2) formulations under different agitation methods namely ultrasonic agitation, magnetic stirring and no agitation. The experimental results are illustrated in Table 4. Results show that the stripping rate is 100% for ultrasonically agitated sample whereas 60% and 50% in the case of magnetic stirring and no agitation samples, respectively. This is evident from the SEM images shown in Fig. 3b and c. From this it is clear that ultrasonic agitation is a key factor to achieve higher stripping rates compared to magnetic stirring and no agitation. EDX analysis also revealed that the combination of scCO2/co-solvent and ultrasonic agitation completely stripped the HDI PR while leaving the SiO2 layer (Fig. 4). We have investigated the impact of the ultrasonication on the patterns (120/1500 nm (line/space) poly Si pattern), but did not find any pattern damage in these conditions. The mechanism by which the co-solvent/scCO2 mixture and ultrasonication removes the photoresist completely has not been elucidated. However, it is suggested that polymeric material can be swollen by diffusion of CO2 and a reduction in the glass transition temperature can be produced. The swelling of the photoresist matrix reduces the polymer–polymer and polymer–substrate interaction, promoting the de-bonding of photoresist from the supporting substrate [14]. The solubility of the photoresist in scCO2 is relatively low, but this can be facilitated by tuning the operational parameters (temperature and pressure) Table 3 The effect of reaction time on stripping rate of photoresist by ultrasonically agitating 10% w/w DMSO/scCO2 mixture at 70 °C and 27.6 MPa. Entry
Time (s)
Extent of stripping (%)
1 2 3
120 150 180
80 90 100
174
S.H. Kim et al. / Microelectronic Engineering 86 (2009) 171–175
Fig. 3. SEM images of the wafers after photoresist stripping at 70 °C and 27.6 MPa for 180 s using 10% w/w DMSO/scCO2 mixture under (a) ultrasonic agitation, (b) magnetic stirring and (c) no agitation.
Table 4 The effect of stripping rate of photoresist using 10% w/w DMSO/scCO2 mixture under different mechanical methods; (a) ultrasonic agitation, (b) magnetic stirring and (c) no agitation. Experimental conditions: 70 °C and 27.6 MPa for 180 s. Entry
Type of mechanical agitation
Extent of stripping (%)
1 2 3
Ultrasonic agitation Magnetic stirring No agitation
100 60 50
removal in this system assumes to be based on both chemical and physical mechanisms. 4. Conclusions We have successfully shown the complete stripping of ion-implanted photoresist using scCO2/co-solvent formulations while maintaining the structural integrity of the underlying layer. The result indicates that ultrasonic agitation is a key factor to increase the stripping rate of photoresist compared to the mechanical stirring and no agitation. Besides ultrasonication, our investigations reveal that the effects of temperature, pressure, and type of cosolvent are the important factors for optimal stripping efficiency. Acknowledgements This work was supported by ‘System IC 2010’ Project of Korea Ministry of Knowledge Economy and by the Medium-term Strategic Technology Development Program funded by the Ministry of Commerce, Industry and Energy (MOICE, Korea). References
Fig. 4. EDX analysis data of the wafer surface after photoresist stripping with ultrasonication and DMSO/scCO2 formulation.
[1] [2] [3] [4] [5]
and the addition of co-solvent into the scCO2 further induces swelling and de-bonding. In addition, ultrasonic agitation can stimulate polymer film delamination and increases the stripping rate of photoresist. It is suggested that the interfacial attack is promoted by increased temperature and the agitation. Therefore, the crust layer
[6] [7] [8] [9] [10] [11]
P.W. Mertens, G. Vereecke, R. Vos, Semicond. Fabtech. 31 (2006) 86. M.A. Hartney, D.W. Hess, D.S. Soane, J. Vac. Sci. Technol. B 7 (1989) 1. W. Kern, Handbook of Semiconductor Wafer Cleaning Technology, 1993. M. Heyns, P.W. Martens, J. Ruzyllo, M.Y.M. Lee, Solid State Technol. 42 (1999) 37. H. Abe, H. Iwamoto, T. Toshima, T. Iino, G.W. Gale, IEEE Trans. Semicond. Manuf. 16 (2003) 401. P.G. Clark, K.K. Christenson, Adv. Semicond. Manuf. Conf. (2005) 1. J.W. Metselaar, V.I. Kuznetsov, A.G. Zhidkov, J. Appl. Phys. 75 (1994) 4910. K. Choi, S. Ghosh, J. Lim, C.M. Lee, Appl. Surf. Sci. 206 (2003) 355. K. Saga, T. Hattori, Solid State Phenom. 134 (2008) 97. A. O’Neil, J.J. Watkins, Green Chem. 6 (2004) 363. M.Y. Lee, K.M. Do, H.S. Ganapathy, Y.S. Lo, J.J. Kim, S.J. Choi, K.T. Lim, J. Supercrit. Fluids 42 (2007) 150.
S.H. Kim et al. / Microelectronic Engineering 86 (2009) 171–175 [12] J.Y. Kim, Y.H. Park, J.S. Kim, K.T. Lim, Y.T. Jeong, J. Ind. Eng. Chem. 13 (2007) 1023. [13] J.B. Rubin, L.B. Davenhall, C.M.V. Taylor, L.D. Sivils, T. Pierce, K. Tiefert, Carbon dioxide-based supercritical fluids as IC manufacturing solvents, in: IEEE International Symposium on Electronics and the Environment, Denver, MA, 1999. [14] J.W. King, L.L. Williams, Curr. Opin. Solid State Mater. Sci. 7 (2003) 413. [15] E.N. Hoggen, K. Wang, D. Flowers, J.M. DeSimone, R.G. Carbonell, IEEE Trans. Semicond. Manuf. 17 (2004) 510. [16] J.L. Chen, Y.S. Wang, H.I. Kuo, D.Y. Shu, Talanta 70 (2006) 414.
175
[17] G. Levitin, S. Myneni, D.W. Hess, J. Electrochem. Soc. 151 (2004) G380. [18] H. Kiyose, K. Saito, I. Mizobata, T. Iwata, S. Hirae, K. Saga, H. Kuniyasu, T. Hattori, Solid State Phenom. 134 (2008) 341. [19] A. Sehgal, in US20040050406, US, 2004. [20] K. Saga, H. Kuniyasu, T. Hattori, M.B. Korzenski, P.M. Visintin, T.H. Baum, Ionimplanted photoresist stripping using supercritical carbon dioxide, in: 208th Meeting of the Electrochemical Society, Los Angeles, CA, 2005. [21] L.B. Rothman, R.J. Robey, M.K. Ali, D.J. Mount, Supercritical fluid process for photoresist stripping, Wafer Cleaning and Surface Preparation Workshop, 2002.