| Abstract: |
We have measured an intrinsic birefringence in calcium fluoride in the ultraviolet for wavelengths from 365 nm down to 156 nm, and compared these results with theory and calculations. The measured effect is largest for propagation in the [110] direction, increasing strongly (faster than 1/lambda^2) for shorter wavelengths, and is consistent with zero as predicted by theory for propagation in the [100] and [111] directions. For propagation in the [110] direction, n[001]-n[-110]=(6.5+/-0.4)x10^-7 for lambda=157.10 nm and n[001]-n[-110]=(3.6+/0.2)x10^-7 for lambda=193.09 nm. These values are considerably larger than the present semiconductor industry birefringence specificaitons for 193 nm and 157 nm lithography optics, and may force design reconsiderations. Calculations of the effect of finite photon q on the index of calcium fluoride agrees qualitatively with the measurements, but predicts larger values. |
