Studying electron-PAG interactions using electron-induced fluorescence

Abstract

In extreme ultraviolet (EUV) lithography, 92 eV photons are used to expose photoresists. Typical EUV resists are organic-based and chemically amplified using photoacid generators (PAGs). Upon exposure, PAGs produce acids which catalyze reactions that result in changes in solubility. In EUV lithography, photo-and secondary electrons (energies of 10-80 eV) play a large role in PAG acid-production. Several mechanisms for electron-PAG interactions (e.g. electron trapping, and hole-initiated chemistry) have been proposed. The aim of this study is to explore another mechanism -internal excitation -in which a bound PAG electron can be excited by receiving energy from another energetic electron, causing a reaction that produces acid. This paper explores the mechanism of internal excitation through the analogous process of electron-induced fluorescence, in which an electron loses energy by transferring that energy to a molecule and that molecule emits a photon rather than decomposing. We will show and quantify electron-induced fluorescence of several fluorophores in polymer films to mimic resist materials, and use this information to refine our proposed mechanism. Relationships between the molecular structure of fluorophores and fluorescent quantum yield may aid in the development of novel PAGs for EUV lithography.

What We Don’t Know About EUV Exposure Mechanisms

“…Researchers [2,3,6,8,10,[14][15][16][17][18][19] have proposed that the dominant chemical mechanisms involved in EUV chemically amplified resists include: (1) electron trapping (or dissociative electron attachment), (2) hole-initiated chemistry, or (3) internal excitation (or dissociative electron excitation), as described below ( Figure 6). In electron trapping, a low energy electron (perhaps 0-5 eV [17]) may be trapped by a PAG molecule, occupying an antibonding orbital in the PAG.…”

“…In electron trapping, a low energy electron (perhaps 0-5 eV [17]) may be trapped by a PAG molecule, occupying an antibonding orbital in the PAG. This leads to a change in the electronic structure of the PAG, causing the molecule to fall apart [2,10,15]. (2) Holes left in ionized atomic species within the resist may also contribute to resist chemistry [2,14].…”

Acid Generation Efficiency of EUV PAGs via Low Energy Electron Exposure

“…It is unclear why the limit exists and why some PAGs have higher efficiency limits, but it may be related to the mechanism in which a PAG generates acid. There are three proposed mechanisms for an acid generation event when interacting with an EUV photoelectron: electron ionization (hole initiated chemistry), electron excitation (internal excitation) and electron attachment (electron trapping) [15,16]. Each mechanism will be affected to some extent by a few key factors such as the number and energy of electrons generated through an electron's energy loss cascade, the energy available to be deposited, and any polymer mediated effects.…”

Molecular Modeling of EUV Photoresist Revealing the Effect of Chain Conformation on Line-Edge Roughness Formation

“…During elastic and inelastic scattering in the matrix, these electrons generate a cascade of secondary electrons, which interact with PAG molecules [16]. This results in the generation of acids (protons) where the material is exposed to light [17,18]. After exposure, a post-exposure bake (PEB) is applied.…”