Tsutomu Shimokawa

Dependence of Absorption Coefficient and Acid Generation Efficiency on Acid Generator Concentration in Chemically Amplified Resist for Extreme Ultraviolet Lithography

The absorption coefficient and acid generation efficiency are elemental key factors for the design of chemically amplified resist because the acid distribution in resist films is primarily determined by these two factors. In this study, the number of acid molecules generated in a model system of chemically amplified extreme ultraviolet (EUV) resists [poly(4-hydroxystyrene) film dispersed with triphenylsulfonium-triflate (TPS-tf)] was evaluated using an acid sensitive dye. The absorption coefficient and acid generation efficiency were evaluated by changing film thickness. The acid generation efficiency was 1.7 (5 wt % TPS-tf), 2.5 (10 wt % TPS-tf), and 3.1 per photon (20 wt % TPS-tf), respectively. The absorption coefficient of the model film was 3.8±0.2 µm-1. The effect of acid generator concentration on the absorption coefficient of resist films was negligible within the concentration range of 0–20 wt %.

Analysis of acid yield generated in chemically amplified electron beam resist

Acid-related matters are a critical issue in a chemically amplified resist, in which photo- or radiation (ionizing radiation)-generated acids drive pattern formation reactions in exposed areas. The photosensitization of resist materials has been formulated by Dill et al. [IEEE Trans. Electron. Dev. 22, 445 (1975)]. The applicability of the formulation by Dill et al. to acid generation in chemically amplified photoresists has been proven by many researchers. The acid yields in photoresists are predicted well by the formulation of Dill et al. However, the formulation of Dill et al. cannot be applied to chemically amplified resists for ionizing radiation such as electron beams and extreme ultraviolet rays because polymer ionization significantly contributes to acid generation in these resists. In this study, the authors formulated acid generation in a chemically amplified resist for ionizing radiation. By the analysis of the dependence of acid yield on acid generator concentration, the details of acid genera...

Acid Generation Mechanism of Poly(4-hydroxystyrene)-Based Chemically Amplified Resists for Post-Optical Lithography: Acid Yield and Deprotonation Behavior of Poly(4-hydroxystyrene) and Poly(4-methoxystyrene)

With the shrinkage of patterns, the elucidation of reaction mechanisms at the molecular level has become essential in resist design. In particular, proton dynamics is one of the most important issues on the sensitivity and resolution of chemically amplified resists. In chemically amplified resists for post-optical lithographies, such as extreme ultraviolet and electron beam lithographies, it has been reported that protons mainly come from not acid generators but polymers. Determining proton sources is a key to understanding reaction mechanisms at the molecular level. In this article, we investigated the deprotonation mechanism of poly(4-hydroxystyrene) and poly(4-methoxystyrene) upon exposure to ionizing radiation. We found that the difference between the proton labilities of polymer radical cations (proton source for acid generation) leads to a difference in acid yield.

Dependence of Acid Generation Efficiency on Molecular Structures of Acid Generators upon Exposure to Extreme Ultraviolet Radiation

The trade-off between resolution, sensitivity, and line edge roughness (LER) is the most serious problem for the development of sub-30 nm resists based on chemical amplification. Because of this trade-off, the increase in acid generation efficiency is essentially required for high-resolution patterning with high sensitivity and low LER. In this study, we investigated the dependences of acid generation efficiency on the molecular structure and concentration of acid generators upon exposure to extreme ultraviolet (EUV) radiation. The acid generation efficiency (the number of acid molecules generated by a single EUV photon) was obtained within the acid generator concentration range of 0–30 wt % for five types of ionic and nonionic acid generators.

Reactivity of Acid Generators for Chemically Amplified Resists with Low-Energy Electrons

In chemically amplified resists for ionizing radiations such as electron beams and extreme ultraviolet (EUV), low-energy electrons play an important role in the pattern formation processes. The reactivity of acid generators with low-energy electrons was evaluated using solvated electrons in tetrahydrofuran, which were generated by a pulsed electron beam. The rate constants of acid generators with the solvated electrons ranged from 0.6 to 1.9 ×1011 M-1 s-1.

A novel noria (water-wheel-like cyclic oligomer) derivative as a chemically amplified electron-beam resist material

A novel ladder-type cyclic oligomer (molecular water-wheel = noria) derivative containing t-butyl ester groups was synthesized. This derivative (noria-COOtBu) had good thermal stability, good solubility in common organic solvents, and good film-forming ability. The photo-induced deprotection (UV irradiation for 30 min followed by heating at 130 °C) of films of noria-COOtBu was examined in the presence of a photo-acid generator, and it was found that deprotection of the t-butyl groups proceeded smoothly to give the corresponding carboxylic acid derivative (noria-COOH). Furthermore, when noria-COOtBu(71) (ratio of t-butyl ester groups: 71%) was examined as an electron-beam resist material, a clear line and space pattern was obtained at a resolution of 70 nm.

Difference between Acid Generation Mechanisms in Poly(hydroxystyrene)- and Polyacrylate-Based Chemically Amplified Resists upon Exposure to Extreme Ultraviolet Radiation

Chemically amplified resists have been used in the mass production of semiconductors. Poly(hydroxystyrene)-based resists have been extensively developed for KrF lithography. After KrF lithography, polyacrylate-based resists have been developed for ArF lithography and used in sub-60 nm fabrication. Both types of resist are candidate platforms of extreme ultraviolet (EUV) resists. In this study, the acid generation mechanisms induced by EUV radiation in both types of resist were investigated from the viewpoint of the deprotonation of polymer radical cations using poly(4-hydroxystyrene), poly(methyl methacrylate), and poly(methyl methacrylate-co-4-hydroxystyrene) as model polymers. The dependence of the quantum efficiencies on the molecular structures and acid generator concentration indicated that the deprotonation mechanisms induced by EUV radiation is the same as those induced using an electron beam.

Study of the Reaction of Acid Generators with Epithermal and Thermalized Electrons

Ionizing radiation such as extreme ultraviolet (EUV) radiation and electron beams generates secondary electrons in resist materials. Acid generators are mainly decomposed by reaction with these electrons. The reaction of acid generators with solvated electrons has been well investigated. However, the reaction with electrons before solvation, in particular, during thermalization, has not been investigated because of measurement difficulties. Because the ejected electrons are not solvated in solid resist films, it is important to elucidate the reactions of acid generators with the electrons before solvation. The reactions before solvation are expected to give direct insight into the reactivity of acid generators with electrons in solid films. In this study, we investigated the reactivity of acid generators with electrons before solvation using picosecond pulse radiolysis. The dependence of reactivity on molecular structures was clarified in terms of C37 parameters.

Deprotonation mechanism of poly(4-hydroxystyrene) and its derivative

With the shrinkage of pattern sizes, the elucidation of reaction mechanisms at molecular level has become essential to resist design. Especially, proton dynamics is the most important issue for sensitivity and resolution of chemically amplified resists. Also, nanoscale topography of patterned resist surface such as line edge roughness may be explained by precise proton dynamics. In chemically amplified resists for post-optical lithographies such as EUV and electron beam lithography, it has been reported that protons come not from acid generators but from base polymers. Determining proton sources is a key to understanding reaction mechanisms at molecular level. In this article, we investigated deprotonation mechanism of poly(4-hydroxystyrene) and poly(4-methoxystyrene) on the exposure to ionizing radiation.

Relationship between Acid Generator Concentration and Acid Yield in Chemically Amplified Electron Beam Resist

Acids generated upon exposure play the most important role in the pattern formation of current standard resists, called chemically amplified resists. It is well known that acid yield depends on the acid generator concentration. However, the precise relationship between acid yield and acid generator concentration has not been investigated thus far. Understanding the details in acid generation is important for the development of resist materials and the process simulation. In this work, the dependence of acid yield on the acid generator concentration was studied using poly(4-hydroxystyrene) (PHS) and poly(methyl methacrylate) (PMMA) as a matrix. Both results showed a nonlinear dependence. With increase in acid generator concentration, acid yield showed a saturation tendency. The dependence differed between PHS and PMMA. This reflects the fact that protons of acids originate from the radical cation of polymers generated by ionization.