Multi-adduct derivatives of C60 for electron beam nano-resists

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Abstract

Methanofullerenes, C60 derivatives, work as negative type electron beam resists, which have 10-nm scale resolution and high dry-etch durability. Their sensitivity (∼10−3 C/cm2) is one order of magnitude better than that of C60. We studied side-chain dependence of the sensitivities of methanofullerene resists, and found that the methano bridge and >CO near the C60 cage in the side chains play important roles in the sensitivity enhancement. We have improved the sensitivity further by synthesizing multi-adduct methanofullerene (four to six side chains), which has about two orders of magnitude higher sensitivity (3.8×10−4 C/cm2) than that of C60.

Introduction

We discovered that C60 films can be used as a high resolution negative electron beam (e-beam) resist with high dry-etch durability [1]. However C60 has some disadvantage as an e-beam resist: C60 films cannot be prepared by spin-coating because of the low viscosity of the C60 solutions, and the sensitivity is not very high (∼ 0.01 C/cm2), although it is better than that of inorganic resists [2], [3].

We adopted C60 derivatives, methanofullerenes, as e-beam resists to solve these problems [4], [5], [6], [7], [8]. Chemical modification of C60 enhanced the solubility in organic solvents and the viscosity of its solution, which allowed spin-coating preparation.

We found that the methanofullerenes actually functioned as a negative type e-beam resist with 10-nm resolution and high dry-etch durability. Their sensitivities were ∼10−3 C/cm2, about one order of magnitude higher than that of C60. They are, however, still one order of magnitude lower than that of PMMA (poly-methyl-methacrylate).

In this paper, we study what influences the sensitivity and develop methanofullerene resists with higher sensitivity.

Section snippets

Methanofullerene resists

Methanofullerene is a fullerene derivative where a carbon atom is added to the fullerene cage to form a methano bridge, three membered carbon ring, as shown in Fig. 1. Three kinds of methanofullerenes with different atoms at the end of side chains, sulfur, chlorine, and iodine (MTF-Ts, MTF-Cl, and MTF-I), were used to investigate the side-chain dependence (Fig. 1).

Because a heavier atom generally has a larger cross-section against e-beam, and iodine is the heaviest of the three, it is expected

Resist performance of the methanofullerene

Here we demonstrate the performance of methanofullerene resist. A nanometer scale grid pattern was defined in MTF13 film using the SEM equipped with a pattern generator (Crestec, CPG1000). Fig. 6(a) shows SEM pictures of the defined pattern. The line width was 14 nm. The minimum pattern size is not limited by the resist resolution, but by the performance of the exposure system. There seems to be minimal loss of resolution due to proximity effects although the intersection received a double dose.

Summary

The methanofullerene is negative type e-beam resist with nano-scale resolution and high dry-etch durability. It has higher sensitivity than that of C60, which is due to the presence of the methano bridge and >CO. We succeeded obtaining enhancement two orders of sensitivity higher than that of C60 by synthesizing methanofullerene with four to six addends.

Acknowledgments

This work is partly supported by NEDO. The authors would like to thank Professor J. Nishimura and Dr Y. Nakamura at Gunma University for the synthesis of the diels-alder adduct of C60.

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