As next generation immersion lithography, combined with double patterning, continues to shrink feature sizes, the
industry is contemplating a move to non-chemically amplified resists to reduce line edge roughness. Since these resists
inherently have lower sensitivities, the transition would require an increase in laser exposure doses, and thus, an increase
in incident laser fluence to keep the high system throughput.
Over the past several months, we have undertaken a study at MIT Lincoln Laboratory to characterize performance
of bulk materials (SiO2 and CaF2) and thin film coatings from major lithographic material suppliers under continuous
193-nm laser irradiation at elevated fluences. The exposures are performed in a nitrogen-purged chamber where samples
are irradiated at 4000 Hz at fluences between 30 and 50 mJ/cm2/pulse. For both coatings and bulk materials, in-situ laser
transmission combined with in-situ laser-induced fluorescence is used to characterize material performance. Potential
color center formation is monitored by ex-situ spectrophotometry. For bulk materials, we additionally measure spatial
birefringence maps before and after irradiation. For thin film coatings, spectroscopic ellipsometry is used to obtain
spatial maps of the irradiated surfaces to elucidate the structural changes in the coating.
Results obtained in this study can be used to identify potential areas of concern in the lens material performance if
the incident fluence is raised for the introduction of non-chemically amplified resists. The results can also help to
improve illuminator performance where such high fluences already occur.