Fabrication characteristics of a line-and-space pattern and a dot pattern on a roll mold by using electron-beam lithography

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1 Bulletin of the JSME Journal of Advanced Mechanical Design, Systems, and Manufacturing Vol.10, No.5, 2016 Fabrication characteristics of a line-and-space pattern and a dot pattern on a roll mold by using electron-beam lithography Kai OJIMA *, Masashi SAITO *, Noriyuki UNNO * and Jun TANIGUCHI * * Department of Applied Electronics, Tokyo University of Science Niijyuku, Katsushika, Tokyo , Japan junt@te.noda.tus.ac.jp Received 29 February 2016 Abstract Roll-to-roll nanoimprint lithography (RTR-NIL) has received considerable attention because it permits large-area nanopatterning with both high resolution and high throughput. However, the application of RTR-NIL is restricted by difficulties in fabricating nanoscale roll molds. Seamless roll molds are especially desirable, because the presence of seams reduces the yield of the imprinted product. We have previously developed a technique producing seamless molds by direct writing with an electron beam onto a rotating cylindrical substrate. We have now developed a method for fabricating fine patterns on roll molds for RTR-NIL by using electron-beam lithography (EBL) with a positive-type electron-beam resist and an aluminum roll as a substrate, which is rotated in a scanning electron microscope. The electron beam is focused at a single point on the surface of the roll mold and the dot pattern is produced by switching the beam on and off. Dot and line-and-space patterns are obtained by developing the exposed substrate. In this study, we investigate the fabrication characteristics of a line-and-space pattern and a dot pattern by using EBL with a rotating stage. As a result, we produced fine dot patterns with a diameter of less than 70 nm and a fine line-and-space pattern with a width of less than 70 nm. Key words : Roll-to-roll nanoimprint, Nanoimprint lithography, Roll mold, Electron-beam lithography, Line-and-space pattern, Dot pattern 1. Introduction Nanoscale fabrication techniques with a high throughput are needed for the low-cost fabrication of devices of the next generation. One such technique is ultraviolet (UV) nanoimprint lithography (UVNIL) (Haisma et al., 1996), which uses a mold with nanopatterns in conjunction with a UV-curable resin. The process of UVNIL is simple: first, the UV-curable resin is dropped onto a mold and pressed against a transfer substrate. Secondly, the resin is cured by exposure to UV light. Consequently, either the mold or the substrate must be transparent. The mold is then released from the substrate, leaving a duplicate of the nanopattern on the transfer substrate. By means of these simple steps, UVNIL can be used to fabricate nanoscale patterns. Roll-to-roll UVNIL (RTR-UVNIL) (Mikami et al., 1994; Ahn et al., 2006; Ahn and Guo, 2008), in which a roll mold is used, has the potential for even greater throughput, because in RTR-UVNIL, the nanopattern can be transferred continuously at high speeds; for example, Yoshikawa et al. (2013) achieved a speed of 18 m/min. Furthermore, the force necessary to release the mold from the substrate in the RTR-UVNIL process is smaller than that in a planar NIL process; furthermore, the roll mold is in linear contact with the substrate, whereas large-scale planar NIL processes require high pressures and heavy machinery. Patterns on large areas can therefore be produced more efficiently by RTR-UVNIL. However, difficulties exist in the fabrication of roll molds. A roll mold for RTR-UVNIL is typically prepared by attaching planar replica molds onto a roll substrate. This produces a roll mold with seams that reduce the yield of the product. A seamless roll mold can be fabricated by mechanical or laser cutting (Sekkat and Kawata, 2014), but it is difficult to achieve a resolution of less than 100 nm. Self-organized structures, such as those produced by anodic oxidation of aluminum (Masuda et al., 1997), have no seams, and fine pattern sizes can be fabricated by these techniques; however, it is difficult to produce patterns with a Paper No

2 designed shape. Therefore, no existing method is capable fabricating a seamless roll mold with a sub-100 nm resolution and intricate designs. We have developed a method for fabricating nanopatterned roll molds by electron-beam lithography (EBL) with a single point beam that permits the production of a seamless roll mold with high resolution (Taniguchi and Aratani, 2009); similar methods have also been investigated by other researchers (Tseng et al., 2010). In particular, a stencil mask has been used to achieve a high fabrication speed (Abe et al., 2011), but this gave a fabricated pattern with the size of the order of a few hundreds of nanometers with a roll substrate, because the stencil mask caused considerable blurring of the electron beam. We also produced a pattern with a width over 100 nm in a previous study (Saito and Taniguchi, 2014). In contrast, electron optics systems with a single point beam have the potential to fabricate sub-10 nm pattern on a planar substrate (Yamazaki and Namatsu, 2004). We therefore needed to determine the key factors in obtaining sub-100 nm patterns with a roll substrate by using the single point electron beam. In this study, we investigated the fabrication characteristics of a line-and-space pattern and a dot pattern by EBL with a single point beam to obtain a finer roll mold. A typical electron-beam resist and an aluminum roll substrate were used, and we performed the EBL process while the roll substrate was rotated in a vacuum. As a result, we succeed in fabricating both a line-and-space pattern and a dot pattern with sub-100 nm resolutions on a roll mold. 2. Experimental setup Figure 1 shows the process for direct writing with an EB on a rotating roll mold. We used a cylindrical aluminum substrate of diameter 27.9 mm or 30.0 mm. ZEP520A (Zeon Corp., Tokyo) was used as electron-beam resist; this was developed by using ZED-N50 (pentyl acetate). First, ZEP520A was diluted to 50% with ZEP-A (methoxybenzene) to decrease its viscosity. The roll substrate was then inserted into the diluted ZEP520A resist at a speed of 3 mm/s, kept in the solution for 5 s, and then pulled out at a speed of 0.2 mm/s. The sample was subsequently baked in air at 180 C for 20 min to give a ZEP layer about 120 nm thick. The sample was then mounted in a scanning electron microscope (ERA-8800FE; Elionix Inc., Tokyo), customized to permit direct EB writing. In this EB writing system, the beam can be switched into an on or off state by means of a blanking signal (chopping signal) as the roll mold is rotated, resulting in the fabrication of a dot pattern. The chopping signal and the deflection signal are generated by using a personal computer. Fig. 1 Process for the fabrication of roll molds The size of each dot was controlled by changing the number of chopping signals input over one lap of the roll. In this study, the number of the chopping signals was varied from 520,000 to 2,080,000 per rotation. (The minimum length of a dot was therefore 27.9 mm π/2,080,000 = 42.1 nm.) To obtain the same EB dose with a different number of signals, we adjusted the EB blanking signal frequency and rotation speed. The space width between the dot arrays was set to 2.0 μm, and the gap width was controlled by varying the off time of the EB. 2

3 After EB writing, the roll substrate was developed with ZED-N50 and rinsed with 2-propanol (IPA). Because ZEP520A is a positive-type EB resist, the EB-exposed area was removed after development. We also exposed a line-and-space pattern by adopting a similar approach. In the latter case, the EB dose was controlled by changing the speed of rotation while the EB was on. The EB dose can be calculated by using the following expression: EB dose = (EB current unit drawing time) / (beam diameter unit drawing length) (1). The EB parameters were as follows: acceleration voltage, 10 or 30 kv; EB dose μc/cm 2, dot design size, 168.4, 84.2, or 42.1 nm; EB current, 200, 100, 60, or 20 pa. The beam diameter was approximately 10 nm. The vacuum pressure during EB writing was less than Pa. 3. Results and discussion 3.1 Fabrication of dot patterns Effects of the electron-beam current on the drawing of nanodot patterns Figure 2 shows the relationship between the EB current and the diameter of the resulting nanodots, and Fig. 3 shows the patterns obtained at EB currents of 200, 100, 60, and 20 pa. The acceleration voltage for the EB was 10 kv. The error bars correspond to one standard deviation (1SD). It is clear that the diameter of the dots decreased with decreasing EB current. This is because large currents tended produce large coulomb forces, resulting in greater blurring. On the other hand, the error bars for the EB current of 20 pa were larger than those observed at higher currents. This is probably because the slower speed required produce more vibrations in the rotating motor gears. In this study, the EB current was set to 20 pa in subsequent experiments; lower currents could not be used to fabricate patterns because of the restriction imposed by the minimum rotation speed (0.1 rpm). For a current of less than 20 pa, the rotation speed would have to be slower than 0.1 rpm to provide a sufficient EB dose for the fabrication of the nanopattern; this would require the use of speed-reduction equipment designed for low vibration. Fig. 2 The relationship between the EB current and the diameter of the resulting nanodots Fig. 3 The patterns obtained at EB currents of (a) 200, (b) 100, (c) 60, and (d) 20 pa (The EB dose was a constant 1371 μc/cm 2 ) 23

4 3.1.2 Effects of the design value of the dot size on the nanodot pattern Figure 4 is a plot of the relationship between the designed value of the dot size and the actual diameter of the resulting nanodots, and Fig. 5 shows the dot patterns obtained for each designed value. The acceleration voltage of the EB was 10 kv. The error bar corresponds to 1SD. The diameter of the dot pattern produced tended to decrease as the design value decreased. However, it was difficult to fabricate dots with the same diameter as the designed value because of a proximity effect (Unno, and Taniguchi, 2008). As a result, the design value was set to 42.1 nm (the minimum value for the system) in subsequent experiments. The use of a signal generator with higher blanking speeds might permit the fabrication of finer patterns. Fig. 4 Relationship between the design value of a dot and the diameter of the resulting nanodots Fig. 5 The dot patterns obtained for design values of (a) nm, (b) 84.2 nm, (c) 42.1 nm Effect of the electron-beam dose per dot on the drawing of nanodot patterns Figure 6 shows the relationship between the EB dose and the diameter of the nanodots produced with a 10 kv EB. The diameter of the fabricated dot pattern decreased with decreasing EB dose. In the case of EB doses of 806 and 685 μc/cm 2, however, the dot pattern did not appear across the entire circumference of the roll substrate because the EB dose was too low. Fig. 6 Relationship between the EB dose and the diameter of the resulting nanodots 24

5 Figure 7 shows an SEM image of the dot pattern at 914 μc/cm 2. The design value for the gap width was nm. It became clear that the variation in the gap width in the radial direction of our EB writing system with a rotating stage was about 50 nm. This was probably caused by a combination of errors resulting from variations in the blanking signal or in the rotation speed. Since the initial thickness of the ZEP layer was about 120 nm and the diameter of the obtained dot pattern was less than 100 nm, it was difficult to measure the depth of the dot pattern by using a confocal laser microscope. Moreover, it was also difficult to measure the height of the roll substrate by using an AFM in this study. The aspect ratio of the obtained pattern will be investigated in the future. Fig. 7 SEM image of dot pattern at 914 μc/cm 2 We measured the diameter of the resulting dots at 30 intervals; Figs. 8(a) and 8(b) show the distribution of the diameters of the dot patterns. By optimizing the EB dose, we succeeded in fabricating a dot pattern with a diameter of approximately 93 nm with a standard deviation of less than ±10 nm. Figure 9 shows the characteristics of the gap width between dot patterns obtained by using the optimized EB dose of 914 μc/cm 2. In this case, we changed the gap width from nm to nm by varying the EB off time. As shown in Fig. 9, the dot patterns merged at a gap width of nm, because the variation in the gap width in the radial direction was about 50 nm, as noted above. Therefore, the minimum gap width attainable by the technique used in this study is nm. Fig. 8 (a) The measurement points for dot diameters at 30 intervals and (b) the result at 914 μc/cm 2 25

6 Fig. 9 Dense nanodot patterns fabricated by changing the gap width between dot patterns (a) nm, (b) nm, (c) 210.5nm and (d) nm Fabrication of the finest nanodot patterns Next, we changed the acceleration voltage to 30 kv and we examined the minimum diameter of the dot pattern. We increased the acceleration voltage to 30 kv because the beam diameter at 30 kv was less than that at 10 kv. The optimized EB current and dose were 20 pa and 1371 μc/cm 2, respectively. The diameter of the finest dot pattern obtained was 64 nm, as shown in Fig. 10. However, this dot pattern did not appear across the entire circumference of the roll substrate, probably because inaccuracies in centering of the rotation stage or the roll caused the EB to become defocused. This demonstrated that an additional autofocus system would be required to obtain finer patterns on a roll mold. Fig. 10 SEM image of the finest dot pattern (64 nm diameter) obtained at 30 kv 3.2 Fabrication of line-and-space patterns Finally, we fabricated a fine line-and-space pattern. In this case, we used a cylindrical aluminum substrate of diameter 30 mm, an acceleration voltage of 30 kv, an EB current of 10 pa, a dose of 120 μc/cm 2, and a rotation speed of 1.2 rpm. The diameter of the EB was about 45 nm. Fig. 11 shows the resulting fine line-and-space pattern, which had a line width of 60 nm. We succeeded in fabricating the 60 nm line-and-space pattern across the entire circumference of the roll mold. In this study, it was difficult to measure the heights of the developed patterns because no atomic-force microscope was available for use with a roll substrate, and the resolution of a laser confocal microscope was insufficient. 26

7 μm 2 μm μm 2 μm Fig. 11 Scanning electron micrographs at 90 intervals of the line-and-space pattern formed at 120 μc/cm 2 4. Conclusions We investigated the characteristics of the fabrication of dot patterns and a line-and-space pattern on roll-shaped substrates by using a single point EB. A typical EB resist was coated onto an aluminum roll and used in a similar manner to the normal EBL process on a planar substrate. As a result, we produced dot patterns with a diameter of less than 70 nm and line-and-space patterns with a width of less than 70 nm. To fabricate finer patterns across the whole circumference of the roll substrate, additional techniques such as autofocusing will be necessary. Acknowledgement This study was supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities, References Abe, M., Kitada, T., Ito, N., Tanaka, T., Ataka, M., Kishiro, T., and Matsui, S., Fabrication of Large Area Seamless Roller Mold using Fast EB Lithography (rebl) for R2R Process, Proceedings of the 37th International Conference on Micro and Nano Engineering, (2011), p.28. Ahn, S. H. and Guo, L. J., High-speed roll-to-roll nanoimprint lithography on flexible plastic substrates, Advanced Materials, Vol.20, No.11 (2008), pp Ahn, S., Cha, J., Myung, H., Kim, S.-m., and Kang, S., Continuous ultraviolet roll nanoimprinting process for replicating large-scale nano- and micropatterns, Applied Physics Letters, Vol.89, No. 21 (2006), Haisma, J., Verheijen, M., van den Heuvel, K., and van den Berg, J., Mold-assisted nanolithography: A process for reliable pattern replication, Journal of Vacuum Science and Technology B, Vol.14, No.6 (1996), pp Masuda, H., Yamada, H., Satoh, M., Asoh, H., Nakao, M., and Tamamura, T., Highly ordered nanochannel-array architecture in anodic alumina, Applied Physics Letters, Vol.71, No. 19 (1997), Mikami, Y., Nagae, Y., Mori, Y., Kuwabara, K., Saito, T., Hayama, H., Asada, H., Akimoto, Y., Kobayashi, M., Okazaki, S., Asaka, K., Matsui, H., Nakamura, K., and Kaneko, E., A new patterning process concept for large-area transistor circuit fabrication without using an optical mask aligner, IEEE Transactions on Electron Devices, Vol.41, No.3 (1994), pp

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