Absorption measurement of DUV optical materials at 193 nm and 157 nm by laser induced deflection

摘要

Under 193 nm excimer laser irradiation the laser induced deflection technique (LID) is applied to investigate directly the bulk absorption α of high quality fused silica and calcium fluoride. Fused silica samples are characterized by their fluence H dependent absorption α(H). Their small signal absorption coefficients α_0 are extrapolated by an appropriate fitting model. All investigated standard samples with high H_2 content fulfill the requirement for optical lithograohy which is determined by an α_0 of less than 5.10~(-3)cm~(-1). Prolonged direct absorption measurements at relatively high fluences of 10 and 20 mJ/cm~2 by the LID technique are compared to state of the art marathon durability tests for H_2 poor fused silica at a H = 1.3 mJ/cm~2. The very good agreement of the results demonstrates that the measurement time for durability tests of fused silica can be reduced considerably by increasing the applied fluencs H. Calcium fluoride is investigated by both, direct bulk absorption (LID) and conventional transmission measurements. A very good agreement is found by comparing the results of both experiments. For investigations at 157 nm laser irradiation a new compact LID measurement device is introduced. Calibration measurements show that the sensitivity is significantly increased compared to the previous setup. The detection limit of the new setup is estimated to a values of 1·10~(-3) cm~(-1) and 1·10~(-4) cm~(-1) for calcium fluoride and fused silica, respectively.