Much work still remains, but there are some promising results.
As previously discussed, conventional chemically-amplified resists are struggling to balance the competing requirements of EUV lithography. Simultaneously meeting the industry’s targets for resolution, sensitivity, and line-edge roughness may require new resist concepts. Inpria’s resist technology, based on tin-oxide nano clusters, is one possibility. Recently published work at SUNY Albany suggests another.
The Albany group, funded by Sematech, examined a collection of molecular organometallic resists of the general type , where R is a hydrocarbon group (phenyl, butyl, or benzyl) and CR’ is a corresponding carboxylate group. The compounds were dissolved in toluene, spin-coated onto silicon wafers, and developed in hexanes after EUV exposure. Neither toluene nor hexanes are traditionally used in integrated circuit manufacturing, but these solvents gave, respectively, better quality spin-coated films and better dissolution contrast than the conventional alternatives.
In all cases, the compounds showed negative-tone behavior. Within each sub-group of materials (R held constant), cross-linking efficiency increased with increasing molecular weight, leading to increased photosensitivity. Between sub-groups, resist sensitivity was correlated with the stability of the free radical corresponding to R. Thus, it appears that the ultimate resist sensitivity depends on the strength of the alkyl-tin bond or, rather, how easily it is cleaved during exposure.
The group evaluated 20 different compounds. Of these, the most promising appear to be the dibenzyltin dicarboxylates, which achieved line edge roughness as low as 1.1 nm for line and space patterns with a 35 nm CD. Unfortunately, these materials also had relatively high exposure dose requirements. Dibenzyltin acrylates required much lower exposure doses, but at the cost of resolution and line edge roughness.
Collectively, the compounds studied here can achieve either the resolution, line edge roughness, and sensitivity required for EUV lithography. Much work remains to be done, however, as the group has yet to meet all three requirements with a single material.