All dielectric high-reflectance (HR) mirror coatings consisting of AlF3/LaF3/oxide layers were deposited on deep-ultraviolet-grade fused silica and CaF2. A novel technique was employed to measure the absorption of these mirrors during irradiation by a 193-nm ArF excimer laser source. The method involves the application of a photothermal measurement technique. The setup uses a Shack-Hartmann wavefront sensor to measure wavefront deformation caused by the heating of the coating by the ArF beam. Laser calorimetric measurements of absorption were used to calibrate the wavefront sensor. The new test setup was used to investigate HR mirror coatings both before and after exposure to high average power ArF laser beams. HR mirror samples were irradiated by a 193-nm kilohertz laser source for either 500 million or 18.6 billion pulses. The differences between wavefront distortion measured inside the beam footprint compared to measured outside the beam footprint can be explained by compaction of the coating in the area heated by the ArF laser. Interesting wavefront-distortion results from testing mirrors with either fused silica or CaF2 substrates can be explained by considering the figure of merit of these materials for excimer-laser mirror substrates.
High-reflectance mirrors, fabricated by use of fluoride coating materials, were irradiated for extended periods by a 193-nm kilohertz repetitive laser source. This irradiation promoted a spectral shift in the reflectance band towards shorter wavelengths. In efforts to determine the mechanism for the observed spectral shifts, various models were investigated by employing such techniques as spectrophotometry, surface profile interferometry, coating design simulation, and x-ray diffraction. The result of the investigation indicates that layers near the top surface of the coating structure underwent densification, which resulted in the observed spectral shift.
All dielectric HR mirror coatings consisting of AlF3/LaF3/Oxide layers were deposited on DUV grade fused silica and
CaF2. A novel technique was employed to measure the absorption of these mirrors during irradiation by a 193nm ArF
excimer laser source. The method involves the application of a photothermal measurement technique. The setup uses a
Shack-Hartmann wavefront sensor to measure wavefront deformation caused by the heating of the coating by the ArF
beam. Laser calorimetric measurements of absorption were used to calibrate the wavefront sensor. Gage R&R
(Repeatability & Reproducibility) measurements were done to show that this is a practical test technique for use in
production.
The new test setup was used to investigate HR mirror coatings both before and after exposure to high average power
ArF laser beams. HR mirror samples were irradiated by a 193 nm kilohertz laser source for either 500 million or 18.6
billion pulses. The spatial resolution is sufficient to make wavefront distortion measurements both inside and outside of
the laser beam footprint. The differences between wavefront distortion measured inside the beam footprint compared to
measured outside the beam footprint can be explained by compaction of the coating in the area heated by the ArF laser.
Interesting wavefront distortion results from testing mirrors with either fused silica or CaF2 substrates can be explained
by considering the figure of merit of these materials for excimer laser mirror substrates.
The laser induced damage thresholds (LIDT), N-on-1 test at 266 nm HfO2/SiO2 AR coatings, were measured for a 2
layer and 4 layer antireflection (AR) coating designs on fused silica. LIDT values for the 2 layer AR coating exhibited
a constant threshold level over a wide range of increasing number of laser pulses. LIDT values for the 4 layer AR
coating decreased relatively rapidly with increasing pulses in comparison. The projected lifetime of the 2 layer coating
design was thus determined to be much longer than that of the 4 layer design.
To explain the observed LIDT performance differences, this study effort employed the following metrological
techniques: 1) Measurement of the surface roughness with a surface profile interferometer, 2) Analysis of material
crystal structure with X-ray diffractometry, 3) Examination of surface damage morphology, 4) Spectrophotometric
analysis of the reflectance of AR coatings, 5) Investigation of the electric filed distribution utilizing optical coating
design software, and 6) Calculation of the maximum temperature rise.
High reflectance mirrors, using fluoride coating materials, have been irradiated for extended time periods by a 193 nm
kilohertz repetitive laser source. This irradiation promoted a spectral shift in the reflectance band towards shorter
wavelength. In efforts to determine the mechanism for the observed spectral shifts, various models were investigated by
employing such techniques as: spectrophotometry, surface profile interferometry, coating design simulation and x-ray
diffraction. The result of the investigation indicates that layers near the top surface of the coating structure underwent
densification which resulted in the observed spectral shift.
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