**1. Introduction**

68 Recent Advances in Nanofabrication Techniques and Applications

[34] Yamauchi, Y.; Noro, T.; Kurahashi, M.; Suzuki, T. and Ju, X. (2005). Metastable-atom-

[35] Setoyama, H.; Kera, S.; Okudaira, K. K.; Hara, M.; Harada, Y. and Ueno, N. (2003).

[36] Thywissen, J. H.; Johnson, K. S.; Dekker, N. H.; Prentiss, M.; Wong, S. S.; Weiss, K. and

[38] Nowak, S.; Pfau, T. and Mlynek, J. (1996). Nanolithography with metastable helium.

[39] Johnson, K. S.; Thywissen, J. H.; Dekker, N. H.; Berggren, K. K.; Chu, A. P.; Younkin, R.

[41] Rehse, S. J.; Glueck, A. D.; Lee, S. A.; Goulakov, A. B.; Menoni, C. S.; Ralph, D. C.;

*Surf. Sci.* 241 141-145.

impacts. *Jpn. J. Appl. Phys.* 42 597-601.

lithography. *Appl. Phys.* B 70 651-655.

fabrication. *Appl. Phys. Lett.* 71 1427-1429.

*Appl. Phys.* B 63 203-205.

2775.

stimulated Desorption from Dodecanethiolate Self-assembled Monolayer. *Appl.* 

Outermost surface reactions of molecular thin films induced by metastable-atom

Grunze, M. (1998). Metastable-atom-activated growth of an ultrathin carbonaceous resist for reactive ion etching of SiO2 and Si3N4. *J. Vac. Sci. Technol*. B 16 1155-1160. [37] Close, J. D.; Baldwin, K. G. H.; Hoffmann, K. and Quaas, N. (2000). Fragmentation of

dodecanethiol molecules: application to self-assembled monolayer damage in atom

and Prentiss, M. (1998). Localization of metastable atom beams with optical standing waves: nanolithography at the heisenberg limit. *Science* 280 1583-1586. [40] Johnson, K. S.; Berggren, K. K.; Black, A. T.; Chu, A. P.; Dekker, N. H.; Ralph, D. C.;

Thywissen, J. H.; Younkin, R.; Thinkham, M.; Prentiss, M. and Whitesides, G. M. (1996). Using neutral metastable argon atoms and contamination lithography to form nanostructure in silicon,silicon dioxide,and gold. *Appl. Phys. Lett.* 69 2773-

Johnson, K. S. and Prentiss, M. (1997). Nanolithography with metastable neon atoms:Enhanced rate of contamination resist formation for nanostructure In the recent fabrication of semiconductor devices, quite various devices are produced while most of them result in small production volumes. A small production volume of ICs leads to a rise of the price of an IC because the expensive investment made in its photomask set must be redeemed by passing on the price. The price of photomasks increases rapidly as the transistor integration advances. The price of photomasks has a great impact on the price of semiconductor devices.

Electron beam direct writing (EBDW) is a solution to fabricating small-lot ICs at a cheap cost. The EBDW can draw patterns onto silicon wafers masklessly or quasi-masklessly (Inanami, 2000; Pfeiffer, 1979). The throughput of the conventional EBDW equipment which adopts the variable shaped beam (VSB) method (Pfeiffer, 1978) is, however, extremely low. In the VSB method, exposed patterns are divided into a large number of small rectangular and triangular shapes to draw them as shown in the left of Fig. 1. In this figure Letter "E" is divided into four rectangles and consequently needs four "EB shots" to be drawn. The conventional VSB equipment *shoots* rectangular and triangular shapes onto silicon wafers and results in a lot of EB shots, which deteriorate the throughput of the equipment.

Character projection (CP) lithography is a promising one in which a pattern more complex than a triangle or a rectangle, called a character, is projected onto a silicon wafer with an EB shot as shown in the right of Fig. 1 (Sakitani et al., 1992; Hattori et al., 1993; Hirumi et al., 2003; Inanami et al., 2000; Inanami et al., 2003; Nakamura et al., 2006; Nakasugi et al., 2003). The e-BEAM Corporation developed a low-energy electron beam direct writing (LEEBDW) system, which was named "EBIS" (Electron Beam Integrated System) (Inanami et al., 2000;

Character Projection Lithography for Application-Specific Integrated Circuits 71

throughput to produce ICs than sequential projection systems by parallelizing projection operations. Several ASIC design techniques were discussed (Sugihara, 2008, 2010). This

Standard cell methodology is a quite popular design method to design an ASIC. The standard cell methodology exploits a cell library which is a collection of low-level logic functions, called cells, such as NAND gates, NOR gates, flip-flops, latches and buffers. From a viewpoint of character projection lithography, it is important to project as many cells as

Cell library development methodologies were studied for character projection lithography (Sugihara et al., 2005, 2006a, 2006c, 2007b, 2008, 2010, Inanami et al., 2000). In this section, we focus on a cell library development methodology for a single-column-cell system (Sugihara

*Cells*, which are components for IC designs, are ordinarily utilized as the basis of characters. The characters are placed in an array on a CP aperture mask as shown in Fig. 4. It accommodates several hundred characters, which are several-m squares. The number of characters available on a single CP aperture mask is limited to a small one and not all cells in a cell library can be realized on it. For example, if there are four variations for every cell function as shown in Fig. 5, a CP aperture mask accommodates about 100 or less of cell functions at the 90 nm technology. Even if multiple CP aperture masks are utilized for placing all the cells on them, it takes forbiddingly long time to switch CP aperture masks for setting and adjusting. This chapter assumes that a single CP aperture mask is allowed to use for each layer and that the cells off the CP aperture mask are projected with the VSB lithography. It is quite important to select frequently-utilized cells to put on a CP aperture mask because a CP aperture mask is a precious resource to increase the throughput of the

chapter focuses on design techniques for single-column-cell projection equipment.

**2. Cell library development for character projection equipment** 

possible with character projection lithography.

et al., 2005, 2006a, 2006c, 2007b).

system.

Fig. 4. A CP mask

Fig. 5. Cell directions

EB

EB

EB

basic mirror-X mirror-Y mirror-XY

EB

Inanami et al., 2003; Nakamura et al., 2006; Nakasugi et al., 2003). The system can accommodate 400 characters on a CP aperture mask and any character can be chosen at every EB shot, so that the throughput of the system can be enhanced quite effectively with the CP lithography. The projection system can also project rectangular and triangular shapes with the VSB lithography. Their system is capable of projecting patterns with both the VSB and CP lithographies.

Fig. 2. Single-column-cell character projection equipment

Fig. 3. An Advantest multi-column-cell character projection equipment

Yasuda et al. proposed a multi-column-cell (MCC) system, which can project multiple characters in parallel by equipping it with multiple projection mechanisms called columncells (Yasuda et al., 2004). The motivation to develop the MCC system is to achieve higher throughput to produce ICs than sequential projection systems by parallelizing projection operations. Several ASIC design techniques were discussed (Sugihara, 2008, 2010). This chapter focuses on design techniques for single-column-cell projection equipment.
