The ongoing development in microlithography towards further miniaturization of structures creates a strong demand for
lens material with nearly ideal optical properties. Beside the highly demanding requirements on homogeneity and stress
induced birefringence (SIB), low absorption is a key factor. Even a small absorption is associated with a temperature
increase and results in thermally induced local variations of refractive index and SIB. This could affect the achievable
resolution of the lithographic process.
The total absorption of the material is composed of initial absorption and of absorption induced during irradiation. Thus,
the optimization of both improves the lifetime of the material.
In principal, it is possible to measure transmission and scattering with a suitable spectrometer assembly and calculate
absorption from them. However, owing to the influence of sample surfaces and errors of measurement, these methods
usually do not provide satisfactory results for highly light-transmissive fused silica. Therefore, it is most desirable to find
a technique that is capable of directly measuring absorption coefficients in the range of (1...10)•10<sup>-4</sup> cm<sup>-1</sup> (base 10)
We report our first results for fused silica achieved with the LID technique. Besides a fused silica grade designed for 193
nm applications, grades with higher absorption at 193 nm were measured to test the LID technique. A special focus was
set on the possibility of measuring initial absorption without the influence of degradation effects.