Polymer ionization and reductive sensitization of PAG play an important role for acid generation in EUV lithography.
We have systematically investigated effects of PAG structure, polymer structure and their loadings on sensitivity of EUV
resists. With an increase in PAG loading, both sensitivity and acid generation yield were successfully improved, however,
these were saturated at higher PAG loadings. Least-square fitting of sensitivity as a function of PAG loading, polymer
loading and quencher loading indicates that both PAG and polymer have a positive effect on sensitivity improvement,
and contribution ratio of polymer to PAG on sensitivity is estimated as 1 to 2. This indicates that decrease of polymer
loading in place of increasing PAG loading reduce ionization frequency of polymer. To further improve sensitivity, we
have synthesized a series of PAGs to clarify how large the electron affinity of PAG affects acid generation yield. A
linear relationship between the reduction potential of PAG and EB sensitivity clearly revealed that the strong electron
affinity of PAG causes both high acid yield and sensitivity. To further increase acid generation yield, we have
synthesized a series of polymers to clarify how polymer structure affects sensitivity. Actually, acid generation yield and
sensitivity were both improved by using a newly developed polymer in EUV lithography.
EUV lithography performances of resist materials with different molecular weight of polymer were investigated. EUV
exposure experiment using a SFET at Selete clearly showed that line-width roughness (LWR) and 1:1 half-pitch (hp)
resolution were each improved using the polymers with middle and low molecular weights. These polymers showed high
dissolution contrast relative to polymer with high molecular weight. Mask linearity data also showed that the polymer
with low molecular weight gave a linear dependence on critical dimension (CD) against mask size down to hp 26 nm.
Thermal analysis of resist film revealed that thermal glass transition temperature (Tg) was dramatically decreased from
190 °C to 110 °C with decreasing molecular weight from high to low. In contrast with Tg which directly reflects
mobility of polymer, exposure latitude (EL) was increased from 12.3% to 14.5% at hp 32 nm by decreasing molecular
weight of polymer. Similarly, iso-dense bias was also improved by utilizing the low molecular weight polymer.
Combination of PAG-B with the low molecular weight polymer caused further improvement in mask linearity, EL, and
iso-dense bias at hp 32 nm, although LWR was rather increased.