Relative variation of stress-optic coefficient with wavelength in fused silica and calcium fluoride

Birefringence in refractive components such as lenses has become an increasingly serious problem in semiconductor lithography as exposure wavelength decreases. Most measurements of birefringence are made with visible light but the light used for photolithography is in the UV and deep UV spectral regions. Measurements of the relative variation of stress-optic constants have been made for fused silica and calcium fluoride, the two primary transmissive optical materials used by this industry. The method uses photoelastic modulators with optical elements made fi om fused silicaand calcium fluoride. Measurement of the PEM oscillation amplitude required to produce half-wave retardation amplitude as a function of wavelength yields the required relationship. For fused silica at 193nm, the stress-optic coefficient is about 1.32 times the value at 633 nanometers. For calcium fluoride, the increase is about a factor of 1.19.