Shuhei Yoshizawa

VUV transmittance of fused silica glass influenced by thermal disorder

The technological development of projection photolithography at the 157 nm wavelength of the F2 laser followed by the 193 nm of the ArF excimer laser has been progressing rapidly. Fused silica glass is most promising candidate for 157 nm photomasks, while its absorption edge is extremely close to 157 nm. In this paper the vacuum,-UV transmittance curve at temperatures of 298 to 498 K was examined. The curve gradually shift to longer wavelengths due to thermal disorder and the transmittance at 157nm decreased with increasing temperature. The rise in temperature induced by F2 laser energy density and the irradiation time. For examples, in the case of 0.9 internal transmittance and a fluence of 0.1 mJ/cm(superscript 2/pulse with 1 kHz frequency, the temperature raised by about 8K. The change of transmittance at 157 nm accompanying the increment of 8K is negligibly small. The data suggests that the transmission loss caused by thermal disorder during F2 laser irradiation can be neglected as long as the silica glass with high internal transmittance is used in 157 nm photomasks.