Lisa Anne Moore

Fused silica for 157 nm transmittance

The use of silica glass for the photomask material in 157-nm lithography tools is proposed. While fused silica enjoys widespread application for 248 and 193-nm optics, its use for 157-nm applications has been largely discounted, in part because of low transmittance at this wavelength. It is demonstrated here that silica glass can be made to have high transmittance at 157-nm. This is accomplished by minimizing the OH content of the glass. It is also noted that the thermal and mechanical properties of so-called dry silica are very close to higher OH silicas that are commonly used for lithography applications.

Properties of fused silica for 157-nm photomasks

Modified fused silica is the primary candidate material for 157-nm photomask substrates. Standard UV excimer grade silica, such as Corning HPFSR, does not transmit below about 175- nm because of its high OH content. In contrast, we have prepared fused silicas with low OH contents and low levels of fluorine with measured transmissions up to 73.8%/6.4 mm and internal transmittances up to 87.9%/cm at 157-nm. Refractive index measurements at 157-nm are presented from which we calculate a theoretical limit for the measured transmission (reflection losses only) of about 88.5% at 157-nm. Modified fused silica is shown to have high resistance to laser-induced color center formation. The thermal and mechanical properties of modified fused silica are shown to be similar to those of standard excimer grade fused silica but not identical. For example, the thermal expansion and Youngs Modulus of modified fused silica are slightly lower than that of Corning HPFSR, while thermal conductivity is the same. Modified fused silica substrates have been shown to behave similarly to standard fused silica substrates in mask-making processes such as polishing and Cr film deposition. In summary, our property and process results support the selection of modified fused silica for the 157-nm photomask application.