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.
The complete evaluation in the selection of coating designs for production of high performance mirrors must include the scattering losses that are associated with the thin film materials combination. This is especially true for deep ultraviolet [DUV] microlithography applications. Scattering loss data are presented at 193 nm for various coating material designs for operation in argon fluoride laser systems. For overall optimum performance tradeoffs of the spectral reflectance, environmental stability and pulsed laser irradiation lifetime survivability is also discussed.
The selection of coating materials for the design and production of optical components has to satisfy at least two criteria: (1) the component’s spectral performance requirements and (2) the survivability in the system’s operational environment. In many instances tradeoffs are required to satisfy both conditions. This paper offers tradeoffs in the material selection and deposition process conditions that provide high performance, stable coating structures for optical components in the deep ultraviolet (DUV) for excimer laser microlithography and micromachining applications. A critical necessity of these coatings is long lifetime survivability under the continuous operation of high fluence, pulsed laser irradiation; the continuous operation can number in the tens of billions pulses. Spectral performance data and lifetime survivability data will be presented.
Traditionally, high-energy lasers have used optics which have been actively cooled, primarily with temperature-conditioned, deionized water. The Ballistic Missile Defense Organization (BMDO) has successfully developed Very Low Absorptance (VLA) coatings for optics for Hydrogen-Fluoride (HF) lasers. These coatings have produced the next generation in high energy optics: uncooled optics. This paper addresses the successful transfer of this technology to Deuterium-Fluoride (DF) lasers. Both analytical predictions and experimental results are presented.
The High Altitude Observatory (HALO) is an instrumented gulfstream IIb aircraft sponsored by the US Army Space and Strategic Defense Command as an optical data collection platform. Capable of operation at altitudes above 50,000 feet, the HALO's highly diverse and flexible sensor suite has provided infrared, visible and ultraviolet data on numerous BMDO, DOD, and NASA test programs. This paper describes the design, fabrication, and testing of two large zinc selenide infrared windows employed by the aircraft's primary sensor system, the Infrared Instrumentation System. The paper describes the design, fabrication, and testing of the windows and coatings, the window proof testing for flight qualification, and summarizes the current operational experience with the windows.
Optical coatings with very low absorption losses have been produced on silicon and fused silica in the 1 to 3 micron spectral region. Absorption values below 30 ppm have been achieved at oxygen-iodine and hydrogen fluoride laser wavelengths with fluence levels as high as 40 kW/cm2. The optical performance of these coatings in producing ultra high reflectance mirrors (R equals 99.97%) and predicted performance, extrapolated from this data, for the deuterium fluoride laser wavelength region are presented. These results are critical in the production of optical components for application in high power, high fluence mid-infrared cw laser systems.
Gain enhancement from IR detectors could be realized if they were antireflection (AR) coated. The feasibility of AR coating silicon surfaces over the 6 to 11 micron and 11 to 17 micron bands has been demonstrated using single and double-layer coatings. The band averaged reflectance values were reduced from about 30% to about 4% with double-layer coatings. The actual spectral reflectance curves agreed very closely to the computer-modelled performances. Temperature and material-pair parameters were established that resulted in thermally durable combinations. The coated silicon was successfully cycled between room temperature and 77 degrees Kelvin. Application of this coating to Si:As Impurity Band Conduction detectors was successfully demonstrated in the 6 to 11 micron spectral band.
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