Kazumasa Okamoto

Proton Dynamics in Chemically Amplified Electron Beam Resists

The proton dynamics of poly(4-hydroxystyrene) (PHS) films were investigated using Coumarin 6 (C6). The acid density was 0.022 nm-3 at the exposure dose of 10 µC cm-2 (75 keV electron beam). The absorption intensity of C6 proton adducts was saturated at a certain concentration of C6, indicating an almost complete addition of protons at this C6 concentration. Protons can move in PHS films near C6 molecules even at room temperature. Also, the absorbed dose was estimated using 60Co γ-rays. The acid yield can be well explained by an acid generation model involving the ionization of a base polymer.

Potential cause of inhomogeneous acid distribution in chemically amplified resists for post optical lithography

In chemically amplified resists for ionizing radiation such as an electron beam and EUV, protons of acids are mainly generated by the deprotonation of base polymers. Therefore, the acid generation efficiency depends highly on polymer structure. In recent resist formulas, partially protected poly(4-hydroxystyrene) has often been used as a base polymer. In this work, the effects of protecting groups on acid generation were investigated. We found differences in acid generation efficiency caused by protecting groups. These differences are likely to affect acid distribution.

Protonation Sites in Chemically Amplified Resists for Electron-Beam Lithography

In chemically amplified resists that utilize acid-catalytic reactions for pattern formation, proton dynamics is important from the viewpoints of the insoluble layer formation due to acid loss, the resolution decrease due to acid diffusion, and the image quality improvement due to base-quencher effects. For electron-beam lithography, the protons and anions of the acid are initially generated at different places. Protons migrate in the resist matrix toward counter anions, attracted by the opposite electric charges. However, the details of proton migration are still unclear. In this study, we investigated proton quenching in poly(4-hydroxystyrene) films using base quenchers with different proton affinities. When the proton affinity of the base quencher was increased, the equimolecular proton adduct of the acid-sensitive dye was quenched without postexposure bake. Although the proton affinity is a gas-phase value, the quenching effect correlated well with the proton affinity.

Study of Acid-Base Equilibrium in Chemically Amplified Resist

Protons generated in chemically amplified resists drive pattern formation reactions such as acid catalytic deprotection reactions. Proton dynamics is controlled by the addition of base quenchers so that ultrafine patterns are obtained. However, the details of interaction between protons and base quenchers are still unclear. In this study, we investigated the reactions of protons with base quenchers in a model system of chemically amplified resists with a typical backbone polymer, poly(4-hydroxystyrene). We confirmed that an acid-base equilibrium was reached in the model system without elevating the temperature of the films. The proton affinities of resist ingredients give us a general estimation of protonation sites. When the proton affinities of resist ingredients are close to each other, we have to take into account acid-base equilibrium using pKa for the accurate prediction of protonation sites.

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.

Polymer screening method for chemically amplified electron beam and X-ray resists

In this paper, we discuss the dependency of acid generation on polymer structures and a polymer screening method for the development of chemically amplified resists. Convenient screening method is expected to reduce laborious tasks for the selection of base polymers.

Dependence of acid generation efficiency on the protection ratio of hydroxyl groups in chemically amplified electron beam, x-ray and EUV resists

In chemically amplified resists for ionizing radiation such as an electron beam, protons of acids come from not acid generators but base polymers. This means that the modification of base polymers has a great effect on the acid generation efficiency. The relation between relative acid yield and protecting groups of poly(4-hydroxystyrene) was examined. The selection of protecting groups is important for the acid generation efficiency and the acid distribution uniformity.

Feasibility Study on High-Sensitivity Chemically Amplified Resist by Polymer Absorption Enhancement in Extreme Ultraviolet Lithography

The strong photoabsorption of typical backbone polymers such as poly(4-hydroxystyrene) (PHS) has been a concern in extreme ultraviolet (EUV) lithography. The development of highly sensitive chemically amplified resists by polymer absorption enhancement seems an unacceptable strategy for overcoming this problem because the side wall angle is basically determined by the gradient of energy absorption, namely the absorption coefficient of the polymer. In this study, the feasibility of a high-absorption resist process was investigated by a simulation based on EUV sensitization mechanisms. Compared with PHS-based resists, the fourfold enhancement of polymer absorption is feasible without side wall degradation partly due to the long migration range of secondary electrons, although it is necessary to reduce the resist thickness to 20 nm.

Pulse Radiolysis Study on Proton and Charge Transfer Reactions in Solid Poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA) is a good matrix for a case study aimed at understanding reaction mechanisms of electron beam, X-ray and EUV resists. Radiation-induced reactions and proton dynamics of solid PMMA were studied under room temperature by pulse radiolysis. PMMA samples were doped with pyrene (Py), crystal violet lactone (CVL) or an anion scavenger to observe charge and proton transfer reactions in the solid matrix. The time-dependent behavior of ionic radicals was observed with the time resolution of 10 ns. The electron transfer from PMMA anion radicals to the anion scavenger or Py was observed in the time range of several hundred nanoseconds. The dynamics of protons generated in irradiated PMMA was traced using CVL. The growth of optical absorbance attributed to proton adducts of CVL was observed in the time range of several hundreds minutes. Protons derived from PMMA cation radicals have a long lifetime and migrated in the solid PMMA matrix.