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Development of interference lithography for 22 nm node and below
Microelectronic Engineering 88 (2011) 1944–1947
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Development of interference lithography for 22 nm node and below Yasuyuki Fukushima ⇑, Yuya Yamaguchi, Takafumi Iguchi, Takuro Urayama, Tetsuo Harada, Takeo Watanabe, Hiroo Kinoshita University of Hyogo, Laboratory of Advanced Science and Technology for Industry, 1-1-2 Koto Kamigori Ako-gun, Hyogo Pref. 678-1205, Japan
a r t i c l e
i n f o
Article history: Available online 19 February 2011 Keywords: Extreme ultraviolet Interference Transmission grating Undulator Resist
a b s t r a c t An extreme ultraviolet (EUV) interference lithographic exposure tool was installed at the long undulator beamline in NewSUBARU to evaluate EUV resists for 25 nm node and below. The two-window transmission grating of 40 and 50 nm half pitch (hp) were fabricated with techniques of spattering, electron beam lithography, dry etching and wet etching. hp patterns (20 and 25 nm) of chemically amplified resist (CAR) and non-CAR were successfully replicated using the EUV interference lithographic exposure tool. Ó 2011 Published by Elsevier B.V.
1. Introduction Extreme ultraviolet lithography (EUVL) [1] is a leading candidate manufacturing technology to realize half-pitch (hp) 22 nm node and below for the semiconductor industry. According to the ITRS roadmap, the development of resist materials and processing equipment is one of the three top issues. Furthermore, the requirement of line width roughness (LWR) is 1.5 nm (3r) in 20 nm hp resolution [2]. Consequently, an EUV interference lithographic tool was developed to accelerate the EUV resist development because today there are not enough EUV exposure tools for the resist and material manufacturer to advance the EUV resist development. There are some advantages of EUV interference lithography (EUV-IL) [3–8]. They have an almost unlimited depth of focus and large exposure field which is an important factor for the resist evaluation. In addition, a very low flare and aberration can be achieved, because no mask and no imaging optics are needed. Furthermore, replicated resist pattern size is half of the diffraction grating pattern size. Therefore, LWR measurements at high resolution of 22 nm hp or less can be realized. The EUV interference lithographic exposure tool was installed at the BL-9 long undulator beamline in the NewSUBARU synchrotron radiation facility. The two-window transmission grating is a key component to produce the interference fringes to replicate the resist pattern on a wafer in EUV-IL. We describe the two-window transmission grating and the exposure results of chemically amplified resist (CAR) [9,10] compared to non-CAR in more detail in the following sections.
⇑ Corresponding author. Tel.: +81 791 58 2546; fax: +81 791 58 2504. E-mail address: [email protected] (Y. Fukushima). 0167-9317/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.mee.2011.02.076
2. Extreme ultraviolet interference lithography system in NewSUBARU The EUV interference lithographic exposure tool was constructed at the BL9 long undulator (LU) beamline in the NewSUBARU synchrotron radiation facility. A long undulator source spectrum with the peak at a specific wavelength can be obtained by tuning the gap between the magnets of the undulator [11]. Because the long undulator in NewSUBARU has a total length of 10.8 m and has 200 periods for high-brilliance radiation, the brilliance of EUV light with the undulator as a source is approximately 50,000 times higher than that with a bending magnet as a source. The EUV light was focused on a pinhole using an optical component and 1 mm spatial coherence length was produced using 10 lm pinhole sizes [12,13]. Fig. 1 shows the inside of the EUV interference lithographic exposure tool chamber. It is composed of the grating stage of two axes and the wafer stage of three axes. Furthermore, positioning of the two-window transmission grating and measurement of light intensity can be carried out using a photodiode on the wafer stage, and the exposure time and dose can be adjusted using a fast speed shutter. It is possible to expose more than 1000 shots per wafer by controlling the wafer stage with computer. The pressure in the exposure chamber during exposures is less than 10 4 Pa, and a wafer sample can be smoothly exchanged without lowering the vacuum pressure of EUV-IL chamber by using a load lock mechanism. 3. Specifications of two-window transmission grating Fig. 2 shows the principle of EUV-IL. A two-window transmission grating system was utilized in this EUV-IL. From each diffraction window, 0th order light, ±1st order light, ±2nd order light, and
Y. Fukushima et al. / Microelectronic Engineering 88 (2011) 1944–1947
Fig. 1. Photograph of the EUV interference lithographic exposure tool chamber.
Fig. 2. The principle of EUV-IL.
higher order light are generated. In this system, double periodic interference fringes are created at a cross position of two rays
1945
which consists of the +1st order ray diffracted from one diffraction window and 1st order ray diffracted from another window of the transmission grating. Since it is impossible to replicate the resist if the contrast of the interference fringes on the wafer is low, it is necessary to design a transmission grating to produce high contrast interference fringes. Therefore, the window size was set to 300 50 lm2 to avoid Fresnel diffraction causing low contrast interference fringes [14]. Furthermore, a 70 nm-thick tantalum nitride (TaN) was employed as grating pattern material to achieve high diffraction efficiency when taking in account dry etching requirements, the stress control and the weak adhesion on the wafer. Fig. 3 shows the fabrication process chart of the two-window transmission grating. A low-stress 100 nm-thick silicon nitride (Si3N4) film coated on both sides of a 4-in.-silicon wafer was prepared. A 70 nm-thick TaN was sputtered on a Si3N4 layer. Then a 20 nm-thick silicon dioxide (SiO2) was sputtered on a TaN layer. A 150 nm-thick non-CAR ZEP520A was spin-coated on a SiO2 layer. A diffraction grating pattern was replicated using an electron beam writing tool (ELS7500, Elionix). SiO2 was etched using the resist as mask utilizing an inductively coupled plasma (ICP) dry etching tool (TCP9400SE, LAM Research) using CF4 as a gas. TaN was etched by serving SiO2 as a mask utilizing ICP dry etching tool and Cl2 as a gas. Back side Si3N4 is etched utilizing reactive ion etch (RIE) dry etching tool and CF4 as a gas. The Si substrate was etched by using Si3N4 as mask in an aqueous solution of potassium hydroxide (KOH). Then the two-window transmission grating was fabricated. Fig. 4 shows the scanning electron microscope (SEM) image of TaN pattern of 40 nm hp. The interference fringes are created on a wafer to replicate resist pattern. However, the back ground light could transmit through the TaN and Si3N4 layers as shown. Thus, the back ground light has to be avoided by adding a filter film on the surface between the two diffraction grating windows the so called ‘‘centerstop’’. A 1 lm-thick ZEP520A resist which was employed as the center stop film was coated on the transmission grating. TaN (70 nm-thick) and a 1-lm-thick ZEP520A resist were used as additional absorbers for the outside of a TaN grating patterned area, which has a total absorbance of 99.998% including the TaN and the additional resist layer of ZEP520A. Then after electron beam exposure was carried out only on the region patterned with diffraction grating, ZEP520A resist was developed, and the residual ZEP520A resist was removed by dry-etching in an oxygen environment using RIE. As such, the resist strip was only carried out on the region patterned with diffraction grating, to keep the center stop between the two diffraction gratings.
Fig. 3. The fabrication process chart of the two-window transmission grating.
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Y. Fukushima et al. / Microelectronic Engineering 88 (2011) 1944–1947
Fig. 4. SEM image of TaN pattern of 40 nm hp.
4. Resist patterning by extreme ultraviolet interference lithography Silicon wafers coated with ZEP520A and CA molecular resist coated at 40 nm film thickness were prepared. EUV exposure was carried out at optimal exposure time, and ZEP520A was developed in o-xylene for 15 s. The CA molecular resist was baked on the hot
plate and developed with tetra-methyl ammonium hydroxide (TMAH) 2.38% during 60 s. Fig. 5 shows SEM images of 25 and 20 nm hp resist patterns in ZEP520A. Fig. 6 shows 25 and 20 nm hp resist patterns of the molecule resist. The replicated pattern was observed in the whole range of the interference fringes because there was neither the proximity effect nor backscattering unlike EB lithography. Furthermore, the resist patterns do not easily collapse compared to the EB exposed resist patterns due to the lack of undercut in EUV-IL. However, causes of the pattern collapse of CA molecular resist of 20 nm hp were through to be aspect ratio and problems during the development. Therefore, in order to improve the pattern collapse, it is important to improve the adhesion between the Si substrate and the resist material. 5. Conclusion An extreme ultraviolet interference lithographic exposure tool was installed at the long undulator beamline in NewSUBARU for evaluation of resists for the 25 nm node and below. A two-window transmission grating was designed. The window size was optimized to be 300 50 mm2, and a 70 nm-thick TaN film was
Fig. 5. SEM image of (a) 25 nm hp and (b) 20 nm hp resist pattern of ZEP520A.
Fig. 6. SEM image of (a) 25 nm hp and (b) 20 nm hp resist pattern of CA molecule resist.
Y. Fukushima et al. / Microelectronic Engineering 88 (2011) 1944–1947
employed as grating pattern material to obtain high diffraction efficiency. Furthermore, it was fabricated with techniques of spattering, EB lithography, dry etching, and wet etching. CA molecular resist and non-CAR ZEP520A were evaluated with EUV-IL, and it was possible to replicate 20 nm hp resist patterns. In future studies, we will approach replicating 18 nm hp resist patterns and smaller. Acknowledgement We would like to thank the Japan Society for the Promotion Science for the Grant-in-Aid for supporting the scientific research. References [1] H. Kinoshita, K. Kurihara, Y. Ishii, Y. Torii, J. Vac. Sci. Technol. B 7 (1987) 1648. [2] ITRS Roadmap [
1947
[3] H.H. Solak, D. He, W. Li, S. Singh-Gasson, F. Cerrinaa, B.H. Sohn, X.M. Yang, P. Nealey, Appl. Phys. Lett. 75 (1999) 2328. [4] H.H. Solak, C. David, J. Gobrecht, V. Golovkina, F. Cerrina, S.O. Kim, P.F. Nealey, Microelectron. Eng. 67 (2003) 56. [5] P.P. Naulleau, C.N. Anderson, S.F. Horne, Proc. SPIE 6517 (2007) 65172T. [6] H.H. Solak, D. He, W. Li, F. Cerrina, J. Vac. Sci. Technol. B 17 (1999) 3052. [7] S. Suzuki, Y. Fukushima, R. Ohnishi, T. Watanabe, H. Kinoshita, J. Photopolym. Sci. Technol. 21 (2008) 435. [8] H. Shiotani, S. Suzuki, D. Gun Lee, P. Naulleau, Y. Fukushima, R. Ohnishi, T. Watanabe, H. Kinoshita, Jpn. J. Appl. Phys. 47 (2008) 4881. [9] H. Ito, J. Polym. Sci. Part A, Polym. Chem. 41 (2003) 3863. [10] H. Ito, Proc. SPIE (1999) 3678. [11] S. Hashimoto, A. Ando, M. Nibe, S. Miyamoto, Y. Shoji, Y. Fukuda, T. Tanaka, Y. Gomei, Nucl. Instrum. Methods Phys. Res. Sect. A 467 (2001) 141. [12] Y. Nagata, K. Furusawa, Y. Nabekawa, K. Midorikawa, Opt. Lett. 32 (2007) 722. [13] Yasuyuki Fukushima, Naoki Sakagami, Teruhiko Kimura, Yoshito Kamaji, Takafumi Iguchi, Yuya Yamaguchi, Masaki Tada, Tetsuo Harada, Takeo Watanabe, Hiroo Kinoshita, Jpn. J. Appl. Phys. 49 (2010) 06GD06. [14] Yasuyuki Fukushima, Yuya Yamaguchi, Teruhiko Kimura, Takafumi Iguchi, Tetsuo Harada, Takeo Watanabe, Hiroo Kinoshita, J. Photopolym. Sci. Technol. 23 (2010) 673.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Development of interference lithography for 22 nm node and below Yasuyuki Fukushima ⇑, Yuya Yamaguchi, Takafumi Iguchi, Takuro Urayama, Tetsuo Harada, Takeo Watanabe, Hiroo Kinoshita University of Hyogo, Laboratory of Advanced Science and Technology for Industry, 1-1-2 Koto Kamigori Ako-gun, Hyogo Pref. 678-1205, Japan
a r t i c l e
i n f o
Article history: Available online 19 February 2011 Keywords: Extreme ultraviolet Interference Transmission grating Undulator Resist
a b s t r a c t An extreme ultraviolet (EUV) interference lithographic exposure tool was installed at the long undulator beamline in NewSUBARU to evaluate EUV resists for 25 nm node and below. The two-window transmission grating of 40 and 50 nm half pitch (hp) were fabricated with techniques of spattering, electron beam lithography, dry etching and wet etching. hp patterns (20 and 25 nm) of chemically amplified resist (CAR) and non-CAR were successfully replicated using the EUV interference lithographic exposure tool. Ó 2011 Published by Elsevier B.V.
1. Introduction Extreme ultraviolet lithography (EUVL) [1] is a leading candidate manufacturing technology to realize half-pitch (hp) 22 nm node and below for the semiconductor industry. According to the ITRS roadmap, the development of resist materials and processing equipment is one of the three top issues. Furthermore, the requirement of line width roughness (LWR) is 1.5 nm (3r) in 20 nm hp resolution [2]. Consequently, an EUV interference lithographic tool was developed to accelerate the EUV resist development because today there are not enough EUV exposure tools for the resist and material manufacturer to advance the EUV resist development. There are some advantages of EUV interference lithography (EUV-IL) [3–8]. They have an almost unlimited depth of focus and large exposure field which is an important factor for the resist evaluation. In addition, a very low flare and aberration can be achieved, because no mask and no imaging optics are needed. Furthermore, replicated resist pattern size is half of the diffraction grating pattern size. Therefore, LWR measurements at high resolution of 22 nm hp or less can be realized. The EUV interference lithographic exposure tool was installed at the BL-9 long undulator beamline in the NewSUBARU synchrotron radiation facility. The two-window transmission grating is a key component to produce the interference fringes to replicate the resist pattern on a wafer in EUV-IL. We describe the two-window transmission grating and the exposure results of chemically amplified resist (CAR) [9,10] compared to non-CAR in more detail in the following sections.
⇑ Corresponding author. Tel.: +81 791 58 2546; fax: +81 791 58 2504. E-mail address: [email protected] (Y. Fukushima). 0167-9317/$ - see front matter Ó 2011 Published by Elsevier B.V. doi:10.1016/j.mee.2011.02.076
2. Extreme ultraviolet interference lithography system in NewSUBARU The EUV interference lithographic exposure tool was constructed at the BL9 long undulator (LU) beamline in the NewSUBARU synchrotron radiation facility. A long undulator source spectrum with the peak at a specific wavelength can be obtained by tuning the gap between the magnets of the undulator [11]. Because the long undulator in NewSUBARU has a total length of 10.8 m and has 200 periods for high-brilliance radiation, the brilliance of EUV light with the undulator as a source is approximately 50,000 times higher than that with a bending magnet as a source. The EUV light was focused on a pinhole using an optical component and 1 mm spatial coherence length was produced using 10 lm pinhole sizes [12,13]. Fig. 1 shows the inside of the EUV interference lithographic exposure tool chamber. It is composed of the grating stage of two axes and the wafer stage of three axes. Furthermore, positioning of the two-window transmission grating and measurement of light intensity can be carried out using a photodiode on the wafer stage, and the exposure time and dose can be adjusted using a fast speed shutter. It is possible to expose more than 1000 shots per wafer by controlling the wafer stage with computer. The pressure in the exposure chamber during exposures is less than 10 4 Pa, and a wafer sample can be smoothly exchanged without lowering the vacuum pressure of EUV-IL chamber by using a load lock mechanism. 3. Specifications of two-window transmission grating Fig. 2 shows the principle of EUV-IL. A two-window transmission grating system was utilized in this EUV-IL. From each diffraction window, 0th order light, ±1st order light, ±2nd order light, and
Y. Fukushima et al. / Microelectronic Engineering 88 (2011) 1944–1947
Fig. 1. Photograph of the EUV interference lithographic exposure tool chamber.
Fig. 2. The principle of EUV-IL.
higher order light are generated. In this system, double periodic interference fringes are created at a cross position of two rays
1945
which consists of the +1st order ray diffracted from one diffraction window and 1st order ray diffracted from another window of the transmission grating. Since it is impossible to replicate the resist if the contrast of the interference fringes on the wafer is low, it is necessary to design a transmission grating to produce high contrast interference fringes. Therefore, the window size was set to 300 50 lm2 to avoid Fresnel diffraction causing low contrast interference fringes [14]. Furthermore, a 70 nm-thick tantalum nitride (TaN) was employed as grating pattern material to achieve high diffraction efficiency when taking in account dry etching requirements, the stress control and the weak adhesion on the wafer. Fig. 3 shows the fabrication process chart of the two-window transmission grating. A low-stress 100 nm-thick silicon nitride (Si3N4) film coated on both sides of a 4-in.-silicon wafer was prepared. A 70 nm-thick TaN was sputtered on a Si3N4 layer. Then a 20 nm-thick silicon dioxide (SiO2) was sputtered on a TaN layer. A 150 nm-thick non-CAR ZEP520A was spin-coated on a SiO2 layer. A diffraction grating pattern was replicated using an electron beam writing tool (ELS7500, Elionix). SiO2 was etched using the resist as mask utilizing an inductively coupled plasma (ICP) dry etching tool (TCP9400SE, LAM Research) using CF4 as a gas. TaN was etched by serving SiO2 as a mask utilizing ICP dry etching tool and Cl2 as a gas. Back side Si3N4 is etched utilizing reactive ion etch (RIE) dry etching tool and CF4 as a gas. The Si substrate was etched by using Si3N4 as mask in an aqueous solution of potassium hydroxide (KOH). Then the two-window transmission grating was fabricated. Fig. 4 shows the scanning electron microscope (SEM) image of TaN pattern of 40 nm hp. The interference fringes are created on a wafer to replicate resist pattern. However, the back ground light could transmit through the TaN and Si3N4 layers as shown. Thus, the back ground light has to be avoided by adding a filter film on the surface between the two diffraction grating windows the so called ‘‘centerstop’’. A 1 lm-thick ZEP520A resist which was employed as the center stop film was coated on the transmission grating. TaN (70 nm-thick) and a 1-lm-thick ZEP520A resist were used as additional absorbers for the outside of a TaN grating patterned area, which has a total absorbance of 99.998% including the TaN and the additional resist layer of ZEP520A. Then after electron beam exposure was carried out only on the region patterned with diffraction grating, ZEP520A resist was developed, and the residual ZEP520A resist was removed by dry-etching in an oxygen environment using RIE. As such, the resist strip was only carried out on the region patterned with diffraction grating, to keep the center stop between the two diffraction gratings.
Fig. 3. The fabrication process chart of the two-window transmission grating.
1946
Y. Fukushima et al. / Microelectronic Engineering 88 (2011) 1944–1947
Fig. 4. SEM image of TaN pattern of 40 nm hp.
4. Resist patterning by extreme ultraviolet interference lithography Silicon wafers coated with ZEP520A and CA molecular resist coated at 40 nm film thickness were prepared. EUV exposure was carried out at optimal exposure time, and ZEP520A was developed in o-xylene for 15 s. The CA molecular resist was baked on the hot
plate and developed with tetra-methyl ammonium hydroxide (TMAH) 2.38% during 60 s. Fig. 5 shows SEM images of 25 and 20 nm hp resist patterns in ZEP520A. Fig. 6 shows 25 and 20 nm hp resist patterns of the molecule resist. The replicated pattern was observed in the whole range of the interference fringes because there was neither the proximity effect nor backscattering unlike EB lithography. Furthermore, the resist patterns do not easily collapse compared to the EB exposed resist patterns due to the lack of undercut in EUV-IL. However, causes of the pattern collapse of CA molecular resist of 20 nm hp were through to be aspect ratio and problems during the development. Therefore, in order to improve the pattern collapse, it is important to improve the adhesion between the Si substrate and the resist material. 5. Conclusion An extreme ultraviolet interference lithographic exposure tool was installed at the long undulator beamline in NewSUBARU for evaluation of resists for the 25 nm node and below. A two-window transmission grating was designed. The window size was optimized to be 300 50 mm2, and a 70 nm-thick TaN film was
Fig. 5. SEM image of (a) 25 nm hp and (b) 20 nm hp resist pattern of ZEP520A.
Fig. 6. SEM image of (a) 25 nm hp and (b) 20 nm hp resist pattern of CA molecule resist.
Y. Fukushima et al. / Microelectronic Engineering 88 (2011) 1944–1947
employed as grating pattern material to obtain high diffraction efficiency. Furthermore, it was fabricated with techniques of spattering, EB lithography, dry etching, and wet etching. CA molecular resist and non-CAR ZEP520A were evaluated with EUV-IL, and it was possible to replicate 20 nm hp resist patterns. In future studies, we will approach replicating 18 nm hp resist patterns and smaller. Acknowledgement We would like to thank the Japan Society for the Promotion Science for the Grant-in-Aid for supporting the scientific research. References [1] H. Kinoshita, K. Kurihara, Y. Ishii, Y. Torii, J. Vac. Sci. Technol. B 7 (1987) 1648. [2] ITRS Roadmap [
1947
[3] H.H. Solak, D. He, W. Li, S. Singh-Gasson, F. Cerrinaa, B.H. Sohn, X.M. Yang, P. Nealey, Appl. Phys. Lett. 75 (1999) 2328. [4] H.H. Solak, C. David, J. Gobrecht, V. Golovkina, F. Cerrina, S.O. Kim, P.F. Nealey, Microelectron. Eng. 67 (2003) 56. [5] P.P. Naulleau, C.N. Anderson, S.F. Horne, Proc. SPIE 6517 (2007) 65172T. [6] H.H. Solak, D. He, W. Li, F. Cerrina, J. Vac. Sci. Technol. B 17 (1999) 3052. [7] S. Suzuki, Y. Fukushima, R. Ohnishi, T. Watanabe, H. Kinoshita, J. Photopolym. Sci. Technol. 21 (2008) 435. [8] H. Shiotani, S. Suzuki, D. Gun Lee, P. Naulleau, Y. Fukushima, R. Ohnishi, T. Watanabe, H. Kinoshita, Jpn. J. Appl. Phys. 47 (2008) 4881. [9] H. Ito, J. Polym. Sci. Part A, Polym. Chem. 41 (2003) 3863. [10] H. Ito, Proc. SPIE (1999) 3678. [11] S. Hashimoto, A. Ando, M. Nibe, S. Miyamoto, Y. Shoji, Y. Fukuda, T. Tanaka, Y. Gomei, Nucl. Instrum. Methods Phys. Res. Sect. A 467 (2001) 141. [12] Y. Nagata, K. Furusawa, Y. Nabekawa, K. Midorikawa, Opt. Lett. 32 (2007) 722. [13] Yasuyuki Fukushima, Naoki Sakagami, Teruhiko Kimura, Yoshito Kamaji, Takafumi Iguchi, Yuya Yamaguchi, Masaki Tada, Tetsuo Harada, Takeo Watanabe, Hiroo Kinoshita, Jpn. J. Appl. Phys. 49 (2010) 06GD06. [14] Yasuyuki Fukushima, Yuya Yamaguchi, Teruhiko Kimura, Takafumi Iguchi, Tetsuo Harada, Takeo Watanabe, Hiroo Kinoshita, J. Photopolym. Sci. Technol. 23 (2010) 673.