Frank Coriand

Bulk absorption measurements of highly transparent DUV/VUV optical materials

The laser induced deflection technique (LID) is introduced for measuring small absorption coefficients of highly transparent DUV/VUV optical materials with high sensitivity and accuracy. The measuring principle, the calibration and the developed experimental realization are explained. At 193 nm in situ absorption and fluorescence measurements of fused silica give evidence that a commonly observed absorption decrease at the onset of laser irradiation is a bulk effect and due to a diminution of oxygen deficient centers ODC II. This decline is caused by a single photon absorption process and terminates after a dose of 4-5 kJ/cm2. Fluence dependent bulk absorption measurements of fused silica are presented which indicate the presence of a nonlinear dependence between the absorption coefficient α and the fluence H. For calcium fluoride a very good agreement between direct absorption and conventional transmission measurements is obtained. At 157 nm, a modified compact experimental setup is introduced which exhibits a significantly higher sensitivity than that applied for 193 nm experiments. First measurements of high quality calcium fluoride show that the obtained absorption is independent on the laser repetition rate. The investigation of equivalent CaF2 samples of different thickness (10 mm and 20 mm) indicates that the measured absorption coefficient is virtually free of contributions from the irradiated surfaces. Finally, a very good agreement is obtained by comparing LID data with transmission measurements of 100 mm long samples.

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

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 H2 content fulfill the requirement for optical lithography which is determined by an α0 of less than (formula available in paper). Prolonged direct absorption measurements at relatively high fluences of 10 and 20 mJ/cm2 by the LID technique are compared to state of the art marathon durability tests for H2 poor fused silica at a H = 1.3 mJ/cm2. 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 α values of (formula available in paper) for calcium fluoride and fused silica, respectively.

Evaluation of fused silica for DUV laser application by short-time diagnostics

Excimer laser pulses ((lambda) equals 193 nm, (lambda) equals 248 nm) induce transient and permanent defects in highly UV transparent optical glass for microlithography. Usually laser damage of fused silica is evaluated by time consuming and expensive marathon tests characterized by about 109 pulses at repetition rates of 400-1000 Hz and fluences of 0.5-10 mJ/cm2. Alternatively, short time tests using high laser energy densities have been developed to quickly evaluate influences of changes in the production technology. The following evaluation methods are used: Laser induced absorption at 193 nm measured by laser induced deflection (LID), Laser induced fluorescence at 650 nm (LIF) excited by 193 nm or 248 nm laser irradiation, H2 content measurement by means of a pulsed Raman spectroscopy at 248 nm laser excitation. Both, the LIF signal and the H2 concentration are measured locally resolved in a non-destructive way. The applied energy densities of the above methods vary from 1 mJ/cm2 to 600 mJ/cm2. The front face technique for investigating large diameter samples, e.g. mask blanks (6 inches and 9 inches), have been established.