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 %.

Dependence of acid generation efficiency on the protection ratio of hydroxyl groups in chemically amplified electron beam, x-ray and EUV resists Acid-related matters are a critical issue in a chemically amplified resist, in which photo-or radiation ͑ionizing radiation͒-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 ͑1975͔͒. 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 generation paths were clarified.

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.

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.

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