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p-Chloro-α-methylstyrene (PCMS) has the chemical structure as the chlorine atom is introduced in phenyl group of α- methylstyrene (MS). In the present study, we synthesized a ternary copolymer consisted with methyl-α-chloroacrylate (ACM), PCMS and MS having the composition ratio of 53:16:31 with the averaged molecular weight (Mw) of 370k, in order to improve sensitivity while maintaining the same resolution as conventional ACM-MS resist. ACM-MS resist having the composition ratio of 46:54 with Mw of 290k is also synthesized to compare exposure characteristics. Firstly, solubility of ACM-PCMS-MS resist without the electron beam exposure for ester solvents is investigated. The dissolving rate of ACM-PCMS-MS resist for amyl solvent is markedly lower compared with ACM-PCMS resist having the composition ratio of 49:51 with Mw=30k reported before, which also includes the effect of the larger molecular weight. Sensitivity curves are made and line and space (L/S) patterns down to 20/20 nm (design value) are formed by using an electron beam writing system with an acceleration voltage of 50 kV. The 20/20 nm L/S pattern is successfully formed in ACM-PCMS-MS resist as well as ACM-MS resist. No significant difference between ACM-PCMS-MS resist and ACMMS resist is observed in L/S pattern shapes. The exposure doses required for pattern fabrication are larger in all L/S patterns compared to ACM-MS resist, reflecting sensitivity curves of them. The dry etching resistance of ACM-PCMSMS resist is also presented.

The positive tone resist consisted of methyl-α-chloroacrylate (ACM) and α-methylstyrene (MS) has higher sensitivity and higher dry etching resistance than poly (methylmethacrylate) (PMMA) due to the presence of a chlorine atom and a phenyl group. Copolymers consisted of ACM and p-chloro-α-methylstyrene (PCMS), where the additional chlorine atom is introduced in phenyl group compared with ACM-MS resist are synthesized and their exposure characteristics are investigated. ACM-PCMS resist with the ACM:PCMS composition ratio of 49:51 indicates the high solubility for amyl acetate developer. As the ACM composition ratio increases, the solubility of ACM-PCMS resist is suppressed. In both ACM-PCMS and ACM-MS resists, the sensitivity decreases while the contrast increases with increasing ACM ratio. When the composition ratio of ACM:PCMS is 69:31, 100/100 nm line and space pattern having a good shape is obtained at 120 μC/cm2 which is comparable to the required exposure dose for conventional ACM-MS resist with ACM:MS=50:50. Dry etching resistance of ACM:PCMS resists for Ar gas is also presented.

In this study, we examine exposure characteristics of a positive tone electron beam resist consisting of methyl α- chloroacrylate and α-methylstyrene by changing the development process conditions. 25/25 nm and 30/30 nm line-andspace (L/S) patterns (design value) are developed in amyl and heptyl acetates. The resist patterns developed at 0oC for 120 s show the better shapes having the vertical sidewalls than those developed at 22 °C for 60 s. The dose margins of pattern formation for 0°C development become wider, although the sensitivities are lower. The effect of post exposure baking (PEB) on exposure characteristics is also investigated. Adding PEB process performed at 120°C for 2 min, the dose margin also becomes wider although the sensitivity is lower. 20/20 nm L/S patterns are fabricated by using PEB and/or 0°C development. Though the required exposure dose is larger, the resist pattern is improved by PEB and/or 0°C development. The formation of 35 nm pitch pattern is also presented.

The sensitivity and the resolution are affected by not only the nature of the resist such as a chemical structure and a molecular weight but also the developing process such as a developer molecular size. Exposure characteristics of positive-tone polymer resists having various molecular weights (Mw’s) ranging from 60 k to 500 k are investigated using different ester solvents as a developer. The line-and-space (L/S) patterns are exposed by the electron beam writing system with an acceleration voltage of 50 kV and the samples are developed by amyl acetate, hexyl acetate and heptyl acetate. The pattern shape becomes better and the surface of the resist also becomes smoother with increasing developer molecular size, though the exposure dose required for the formation of the L/S pattern increases. The dose margin of pattern formation is also wider in all the resists having the different molecular weights. The dissolution in the unexposed portions of the 60k-Mw resist for heptyl acetate is reduced significantly compared with those for amyl acetate and hexyl acetate. The improvement of the pattern shape and the increasing of dose margin are remarkable in the low molecular weight resist. The 3σ of line width roughness tends to be smaller in the higher molecular weight resist and with the larger molecular size developer. Exposure experiment of the 35 nm pitch pattern using the 500k-Mw resist developed at the room temperature is presented.

Higher resolution is eagerly requested to the electron beam resist for the next generation photomask production as well as higher sensitivity. The performance of a polymer resist is mainly characterized by its chemical structure and molecular weight. Positive tone polymer resists with various molecular weights ranging from 60 k to 500 k are synthesized and the molecular weight dependence on exposure characteristics is examined by fabricating line-and-space patterns. The molecular weight dependence of sensitivity for amyl acetate developer is small in the molecular weight range in this study. In a low molecular weight resist, the cross-section profile of the resist pattern becomes rounder and then the disconnections are observed in the 20-nm line-and-space pattern. Although the pattern width change by changing the exposure dose for each resist is quite similar, the exposure dose margin of pattern formation becomes wider with the higher molecular weight. The line width roughness is smaller in a high molecular weight resist than in a low molecular weight resist. The shift amount of the pattern width from the design value for various line-and-space patterns and the dry etching resistance to CF 4 plasma are also presented.