Acid generation efficiency: EUV photons versus photoelectrons

Abstract

EUV photoacid generation efficiency has been described primarily in terms of the EUV photon absorption by the PAG or the resist matrix and the production of low energy photoelectrons, which are reported as being ultimately responsible for the high quantum efficiencies reported in EUV resists (>1). Such observation led to a number of recent studies on PAGs with variable electron affinity (EA) and reduction potential (Ered) presumably conducive to a differential EUV photoelectron harvesting efficiency. However, such studies either did not disclose the PAG chemical structures, replaced the EUV source with an e-beam source, or lacked a fundamental discussion of the underlying physical mechanisms behind EUV PAG decomposition. In this work, we report the EUV photospeed of a methacrylatebased resist formulated with a battery of openly disclosed isostructural sulfonium PAGs covering a wide range of EA's and Ered's, to unveil any preferential photoelectron scavenging effect. In parallel, several iodonium PAGs are also tested in order to compare the direct EUV photon absorption route to the photoelectron-based decomposition path. Contrarily to what has been widely reported, we have found no direct correlation whatsoever between photospeed and the calculated EA's or experimental Ered's for the isostructural sulfonium PAGs studied. Instead, we found that iodonium PAGs make more efficient use of the available EUV power due to their higher photoabsorption cross-section. Additionally, we determined a cation size effect for both PAG groups, which is able to further modulate the acid generation efficiency. Finally, we present a formal explanation for the unselective response towards photoelectron harvesting based on the stabilization of the PAG cation by bulky substituent groups, the spatial and temporal range of the transient photoelectron and the differences in electron transfer processes for the different systems studied.

What We Don’t Know About EUV Exposure Mechanisms

“…This high energetic barrier effectively "knocks out" the holeinduced chemistry pathway for the PMS resist, and yet acid is still produced, presumably entirely from the PAG. In addition, Thackeray et al [14], Goldfarb et al [18], and Tarutani et al [19] have shown that PAG reduction potential correlates well with EUV clearing dose, E 0 , and acid yield, indicating that PAG electron affinity directly relates to acid production in EUV exposure. Bulk electrolysis of PAGs at their reduction potentials has shown a 1:1 correlation between electrons injected and acid produced, demonstrating acid production through direct electron-PAG interactions at the low energies involved in electrolysis [9].…”

“…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.…”

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.…”

“…then an increased likelihood for an electron-PAG interaction could explain the differing acid generation efficiencies. The reduction potential correlates to the electron affinity of a PAG, and other groups have shown that the tendency of a PAG to be reduced is related to the EUV photospeed such that a lower reduction potential usually results in a higher EUV photospeed [16,17]. Electrochemical PAG reduction experiments were conducted using a BASi Epsilon potentiostat with a Pt working electrode, Pt wire auxiliary electrode, and a Ag/AgCl reference electrode.…”

Studying electron-PAG interactions using electron-induced fluorescence

“…Experiments using electron energy loss spectroscopy (EELS) show evidence of internal excitation, 7,8 also known as electrical excitation 9 or dissociative electron excitation. 10 e-+ Previous work by Brainard 4,11 and Kozawa 6 ( Figure 2) indicates that chemically amplified resists can show maximum quantum yields of 5-6 acids produced per absorbed photon at very high PAG concentrations. Resists with higher quantum yield have lower Z-parameters, 12 so improving quantum yields should improve resist performance.…”