Hiroki Jinbo

Dry and F-doped fused silica for photomask substrate in 157-nm lithography

In this paper, Dry & F doped fused silica (modified fused silica) expected as material of photo-mask substrate in the 157 nm (F2 laser) lithography has been evaluated with respect to the optical properties of transmittance, internal loss, F2 laser durability, index homogeneity and stress birefringence, etc. Obtained internal loss coefficient at 157.6 nm was approximately 0.015/cm (base 10), which was measured by several samples with different thickness. From this coefficient, internal loss was calculated as approximately 3% per 1 cm, 2% per quarter-inch. The uniformity in transmittance inside the sample diameter (120 mm) was nearly less than plus or minus 0.5% per 1 cm, plus or minus 0.3% per quarter-inch. Laser durability test was made by MIT/LL. The samples (3 mm, 20 mm and 40 mm thickness) were exposed to F2 laser light up to 80 - 520 million pulses with energy density of 0.1 to 1.4 mJ/cm2/pulse. No significant transmittance change was observed (change was less than 1.0% per quarter-inch at 0.1 to 1.4 mJ/cm2/pulse, 520 million pulses). We measured the index homogeneity by using interferometer and the stress birefringence by using phase modulation method. Inside the sample diameter (120 mm), index homogeneity was 150 ppm at 632.8 nm, the distribution configuration of relative refractive index has a central symmetry property. The stress birefringence was less than 5 nm per quarter-inch at 632.8 nm. As the result, we concluded that this new material had enough capability for the mask substrate of 157 nm lithography.