Absorption coefficient of metal-containing photoresists in the extreme ultraviolet

Abstract

The amount of absorbed light in thin photoresist films is a key parameter in photolithographic processing, but its experimental measurement is not straightforward. The optical absorption of metal oxide-based thin photoresist films for extreme ultraviolet (EUV) lithography was measured using an established methodology based on synchrotron light. Three types of materials were investigated: tin cage molecules, zirconium oxoclusters, and hafnium oxoclusters. The tin-containing compound was demonstrated to have optical absorption up to three times higher than conventional organic-based photoresists have. The absorptivity of the zirconium oxocluster was comparable to that of organic polymer-based photoresists, owing to the low absorption cross section of zirconium at EUV. The hafnium-containing resist shows about twice as high absorptivity as an organic photoresist, owing to the significantly higher absorbance of hafnium. From the chemical composition and crystal structure, the density of the spin-coated films was determined. Using the density of the films and the tabulated data for atomic cross section at EUV, the expected absorptivity of these resists was calculated and discussed in comparison to the experimental results. The agreement between measured and expected absorption was fairly good with some substantial discrepancies due to differences in the actual film density or to thickness inhomogeneity due to the spin coating. The developed method here enables the accurate measurement of the EUV absorption of the photoresists and can contribute to the further development of EUV resists and more accurate lithographic modeling.

Hybrid EUV Resists with Mixed Organic Shells: A Simple Preparation Method

“…The main consequence is that absorption of light in this energy regime (soft X‐ray) leads to photoionization instead of resonant electronic transitions and, therefore, the chemical reactions are mainly performed by the secondary electrons generated upon irradiation. In addition, the probability of absorption of EUV photons is mainly determined by the elemental composition of the photoresist,, rather than the selection rules of classical photochemistry, where the characteristics of the frontier molecular orbitals can be used to predict the absorption probability.…”

“…The results validate this new synthetic approach for the preparation of custom-made EUV resists towards better lithographic performance. sorption of EUV photons is mainly determined by the elemental composition of the photoresist, [4,5] rather than the selection rules of classical photochemistry, where the characteristics of the frontier molecular orbitals can be used to predict the absorption probability.…”

“…[12] In a recent work, we showed that Zr-based metal oxoclusters featuring methacrylate ligands are promising EUV photoresists (Scheme 1). [7] This material is comprised of an hexametalic oxocore and twelve ligands in bridging and chelating coordination modes [13] (Zr 6 O 4 (OH) 4 Mc 12 noted as Zr 6 Mc 12 in this paper). The change in solubility that this material undergoes upon EUV irradiation mainly originates from the cross-linking of the methacrylate ligands.…”

The role of the organic shell in hybrid molecular materials for EUV lithography

“…The linear absorption coefficient α of each doped material was the calculated according to the equation α= µ*δ/MW, where µ is the sum of atomic absorption cross-sections, δ is the density of the thin film and MW is the molecular weight. 16 ( Table 1). The choice of the doping ligands followed different criteria.…”

Current challenges and opportunities for EUV lithography

“…Increasing the optical absorption of EUV resists can help reduce the yield impact of photon statistics without a direct increase in exposure doses. Chemical elements that are highly absorbing at EUV wavelengths are typically not used to make resists, 12 so new chemistry needs to be explored. Many of these elements are considered chemical contaminates in semiconductor processing, so contamination issues will need to be addressed if such elements are incorporated into EUV resists.…”