**5.1 Dry etching (RIE and ashing)**

20 Recent Advances in Nanofabrication Techniques and Applications

The EB writing with thin calixarene resist promises to open the way toward ultrahigh-

Fig. 4.12. SEM images (a)-(d) and the dot size variation (e) of very fine pitch resist dot arrays at center of the drawing area on Si substrate at a dosage of 16 mC/cm2 with a pitch of (a) 20

Fig. 4.13. SEM images (a), (b) and histograms (c)-(e) of 25 nm x 25 nm pitch resist dot arrays on Si substrate at a dosage of 16 mC/cm2, (a) at side of the written pattern area, (b) at corner,

(c) at center, (d) at side and (e) at corner.

nm x 20nm, (b) 25 nm x 25 nm, (c) 30 nm x 30 nm and (d) 40 nm x 40 nm.

density recording at >1 Tb/in2 and quantum devices.

We tried to apply the EB drawing to dry etching process. We checked if the pattern is available for reactive ion etching (RIE) process, and we carried out CF4-RIE using the resist patterns. We performed to do RIE of the Si substrate with the resist dot arrays pattern after post baking of the resist pattern, and to remove the remained resist by O2 ashing. The process flow is shown in Fig. 5. 1. The experiments conditions are represented in Table 5. 1.


Table 5.1. RIE and ashing conditions for Si dot formation and resist removal, respectively.

As the experimental results, we obtained very fine Si dot arrays with a pitch of 30 nm x 25 nm to 25 nm x 25 nm (Fig. 5. 2). The minimum diameter of the Si dot is < 10 nm, and the height is about 20 nm. Figure 5. 3 shows histograms of the EB drawn resist dot and the RIE Si dot sizes in a pitch of 25 nm x 25 nm. According RIE and ashing, the resist dot size is

Electron Beam Lithography for Fine Dot Arrays with Nanometer-Sized Dot and Pitch 23

Fig. 5.4. Etching rates of Si, calixarene resist and Si dot diameter using RIE.

By using the RIE Si dot arrays pattern as a mold, we tried to do nano-imprinting (NIP) to transfer the etched Si dot arrays pattern into UV photo polymer. Figure 5. 5 shows the nano-imprinting process flow with UV photo polymer method. After coating the polymer on the dot arrays, putting the polycarbonate substrate on the polymer and illuminating the UV light into the polymer, we obtained ultrahigh packed pit arrays in the polymer as shown in Fig. 5. 6. We could demonstrate to fabricate the 25 nm x 25 nm pitch pit arrays using EB drawing, RIE and NIP, although the polymer surfaces are in slight curvature because of deformation in the SEM observation or localized stretching in separating the polymer from the mold. From SEM images of the Si mold with 25 nm x 25 nm pitch Si dot arrays before and after NIP, we have confirmed that it is clear that the mold surface has no damage after NIP. From the SEM images of Figs. 5. 3 and 5. 6, we observed that the pit diameters are about 2 nm larger than those in the Si dots. We can consider that the polymer have a shrink property with a rate of about 8 % in the fixing process. In a detail, however, the rate of 8 % does not agree with the increase of about 2 nm against mean dot diameter of about 9 nm. We should consider the shrinkage in the future. We clarified that the very fine pitch resist dot arrays pattern written by EB drawing with calixarene are available for dry etching and nano-imprinting process. We demonstrated to form the Si nano-dot and pit arrays with a pitch of 25 nm x 25 nm using RIE and nano-imprinting

**5.2 Nano-imprinting using UV photo polymer** 

with the resist pattern as a mask.

transferred to Si substrate. Mean dot size decreases from about 14.6 nm to about 9 nm. This may be caused by in-plane etching in RIE. On the other hand, the standard deviation increases from about 1.3 nm to about 1.5 nm. Although the cause is not clear, it may be due to the EB resist toughness or the resist pattern edge sharpness. From the etching experiments, we measured the etching rates of 7 nm/min and 10 nm/min in calixarene and Si(100) in normal direction, respectively, as shown in Fig. 5. 4. The etching rate in plane component is about 2 nm/min. The data supported the dot size decrease.

Fig. 5.2. Dot arrays patterns before and after RIE and ashing (SEM images; A: 25 nm x 25 nm in pitch and B: 25 nm x 30 nm.

Fig. 5.3. Histograms of the resist and Si dot diameters before and after RIE and ashing, respectively, with a pitch of 25 nm x 25 nm.

transferred to Si substrate. Mean dot size decreases from about 14.6 nm to about 9 nm. This may be caused by in-plane etching in RIE. On the other hand, the standard deviation increases from about 1.3 nm to about 1.5 nm. Although the cause is not clear, it may be due to the EB resist toughness or the resist pattern edge sharpness. From the etching experiments, we measured the etching rates of 7 nm/min and 10 nm/min in calixarene and Si(100) in normal direction, respectively, as shown in Fig. 5. 4. The etching rate in plane

Fig. 5.2. Dot arrays patterns before and after RIE and ashing (SEM images; A: 25 nm x 25 nm

Fig. 5.3. Histograms of the resist and Si dot diameters before and after RIE and ashing,

in pitch and B: 25 nm x 30 nm.

respectively, with a pitch of 25 nm x 25 nm.

component is about 2 nm/min. The data supported the dot size decrease.

Fig. 5.4. Etching rates of Si, calixarene resist and Si dot diameter using RIE.

#### **5.2 Nano-imprinting using UV photo polymer**

By using the RIE Si dot arrays pattern as a mold, we tried to do nano-imprinting (NIP) to transfer the etched Si dot arrays pattern into UV photo polymer. Figure 5. 5 shows the nano-imprinting process flow with UV photo polymer method. After coating the polymer on the dot arrays, putting the polycarbonate substrate on the polymer and illuminating the UV light into the polymer, we obtained ultrahigh packed pit arrays in the polymer as shown in Fig. 5. 6. We could demonstrate to fabricate the 25 nm x 25 nm pitch pit arrays using EB drawing, RIE and NIP, although the polymer surfaces are in slight curvature because of deformation in the SEM observation or localized stretching in separating the polymer from the mold. From SEM images of the Si mold with 25 nm x 25 nm pitch Si dot arrays before and after NIP, we have confirmed that it is clear that the mold surface has no damage after NIP. From the SEM images of Figs. 5. 3 and 5. 6, we observed that the pit diameters are about 2 nm larger than those in the Si dots. We can consider that the polymer have a shrink property with a rate of about 8 % in the fixing process. In a detail, however, the rate of 8 % does not agree with the increase of about 2 nm against mean dot diameter of about 9 nm. We should consider the shrinkage in the future. We clarified that the very fine pitch resist dot arrays pattern written by EB drawing with calixarene are available for dry etching and nano-imprinting process. We demonstrated to form the Si nano-dot and pit arrays with a pitch of 25 nm x 25 nm using RIE and nano-imprinting with the resist pattern as a mask.

Electron Beam Lithography for Fine Dot Arrays with Nanometer-Sized Dot and Pitch 25

2. It is demonstrated that the calixarene negative resist is very suitable to form an

3. As the experimental results, the minimum pitch of about 18 nm x 18 nm has been demonstrated using 30-keV EB drawing with calixarene resist, while about 40 nm x 50

5. It is demonstrated that nanometer-sized Si dot arrays were fabricated by CF4-RI etching

6. It is also demonstrated that nanometer-sized polymer pit arrays were fabricated by nano-imprinting with 2P method using the nanometer-sized Si dot arrays as a mother

Furthermore, we try to form 15 nm x 15 nm pitched resist dot arrays using HSQ negative EB resist. We have gotten a prospect to form them by improving a developer for the resist. There are some papers to improve them using HSQ negative resist. X. Yang et al. have reported 12 nm x 12 nm or 15 nm x 15 nm pitch fine resist dot arrays using 100 keV EB drawing with hot developer of TMAH at 40 oC [6]. In addition, J. K. Yang et al. have reported 12 nm or 14 nm pitch resist line and space pattern EB-drawn using 100 keV EB

On the other hand, it is crucial to improve the resolution of SEM to check whether the EBdrawn pattern is complete or not. We should get high resolution observation method for

I would like to thank Profs. H. Sone, Y. Yin, K. Itoh, Z. Mohamad and T. Tamura, and students of Ms. H. Zhang, Mr. H. Sano, Mr. M. Shirai in Hosaka labo of Gunma University for the experiments, simulation and fruitful discussion, and Mr. K. Noguchi in Dept. of Electronic Eng., Gunma University for technical supports in EB writing and resist process. This research was performed in Kiryu Ohta Toshi Area project supported by the Ministry of

[3] G. Varnell, D. Spiecer, J. Hebley, R. Robbins, C. Carpenter and M. Malone, J. Vac. Sci.

[4] S. Hosaka, M. Ichihashi, H. Hayakawa, S. Nishi and M. Migitaka, Jpn. J. Appl. Phys. 21,

[5] S. Hosaka, Y. Tanaka, M. Shirai, Z. Mohamad and Y. Yin, Jpn. J. Appl. Phys. 49, 046503

[8] H. Zhang, T. Tamura, Y. Yin, and S. Hosaka, Key Engineering Material, to be published.

[10] W. Zhang, A. Potts, D.M. Bagnall, B.R. Davidson, Thin Solid Film 515, 3714 (2007).

[7] S. Hosaka, H. Sano, K. Itoh, and H. Sone, Microelectronic Eng. 83, 792 (2006).

ultrahigh packed bit arrays pattern, comparing with ZEP520 positive resist.

nm pitch has been demonstrated with ZEP520.

using calixarene resist dot arrays as a mask.

drawing with salty development [27].

**7. Acknowledgements** 

Monbukagaku-sho of Japan.

543 (1982).

(2010).

[1] J. A. Dohery, Solid State Technol. 22 83 (1979).

Technol. 16, 1787 (1979).

**8. References** 

pattern.

4. EB drawing with calixarene has extremely small proximity effect.

very fine pitch dot arrays with a pitch of less than 15 nm x 15 nm.

[2] E. V. Weber and R. D. Moor, J, Vac. Sci. Technol. 16, 1780 (1979).

[6] X. Yang, S. Xiao, et al., J. Vac. Sci. Technol. 25, 2202 (2007).

[11] K. Murata and T. Matsukawa, Jpn. J. Appl. Phys. 10, 678(1971).

[9] T. H. P. Chang, J. Vac. Sci. Technol. 12, 1271 (1975).

Fig. 5.5. Process flow of 2P (photo polymer) method for nano-imprinting.

Fig. 5.6. SEM images of very fine pitch pit arrays in photo-polymer using nano-imprinting with the Si mold made by RI etched Si dot arrays pattern with a pitch of 25 nm x 25 nm drawn by 30 keV electrons with exposure dosages, (a) 28 mC/cm2, (b) 36 mC/cm2 .
