Nanostructure Fabrication Using Electron Beam
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SEM [9]. Furthermore, carbon contamination patterns of 8 nm have been fabricated with SEM [10], and 8-nm PMMA patterns have been demonstrated by using Ga' FIB [11]. In this paper, nanofabrication using EB and its application to devices are described. ELECTRON BEAM NANOLITHOGRAPHY 10-NM LITHOGRAPHY USING ORGANIC RESIST Nanodevice fabrication requires not only high resolution but also high overlay accuracy. High-speed exposure very effectively meets the requirements because overlay accuracy is improved due to less beam drift on the nanometer scale. Moreover, it enables the use of a highly sensitive resist such as ZEP520 [12], which has sufficient resolution and high dry etching durability for nanolithography. A 10-nm-scale resist pattern was obtained using ZEP520 positive resist. The ZEP520 resist was spin-cast onto a Si wafer (layer thickness of 50 nm), and prebaked at 200 0C. After EB exposure, the ZEP520 was developed with hexyl acetate for 2 min and rinsed with 2-propanol. Figure 2 shows a ZEP520 resist pattern, in which the lines are 10 rum wide and have a pitch of 50 rum [3]. CALIXARENE has a cyclic structure, as shown in Fig. 3, and works as an ultrahigh293
Mat. Res. Soc. Symp. Proc. Vol. 584 ©2000 Materials Research Society
resolution negative EB resist [5, 6]. Such characteristics seem to be convenient for a nanodevice fabrication process. It is roughly a ring-shaped molecule with a diameter of about 1-nm. The basic component of CALIXARENE is a phenol derivative which seems to have high durability and stability, originating from the strong chemical coupling of the benzene ring. The threshold of sensitivity is about 800 U C/cm 2, which is almost 20 times higher than that of PMMA. CALIXARENE negative resist exposure was carried out. A thirty nm-thick resist was coated on a bare Si wafer. After prebaking at 170 OC for 30 min, EB exposure was carried out and then the resist was developed in xylene for 20 s and was rinsed in IPA for 1 min. The etching durability of CALIXARENE was tested using a DEM-451 (ANELVA Corp.) plasma dry-etching system with CF4 gas. The etching rate of CALIXARENE is almost comparable to that of Si, and the durability is about four times higher than that of PMMA. This durability seems to be sufficient to make a semiconductor or a metal nanostructure. Nanodot arrays are useful not only for quantum devices but also for studying exposure properties. In this experiment, the EB current was fixed to 100 pA at 50 kV accelerating voltage, for which the spot size is estimated to be about 5 nm. All the dot arrays were5 fabricated on Si substrates. The typical exposure dose (spot dose) was about lx10 electrons/dot. Figure 4 shows typical dot array patterns having 15-nm diameter with 35-nm pitch. Germanium pattern transfer is shown in Fig. 5. The 20-nm-thick Ge layer requires at least a 5-nm-thick CALIXARENE layer to be etched down, and the resist thickness was 30 nm. Figure 5(a) shows the line patterns of the resist on Ge film exposed at a line dose of 20 nC/cm. Delineation was done using the S-500
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