Damage in optical materials for semiconductor lithography applications caused by exposure to 248 or 193 nm light is usually two-photon driven, hence it is a nonlinear function of incident intensity. Materials should be tested with flat- topped temporal and spatial laser beam profiles to facilitate interpretation of data, but in reality this is hard to achieve. Sandstrom provided a formula that approximates any given temporal pulse shape with a two- photon equivalent rectangular pulse (Second Symposium on 193 nm Lithography, Colorado Springs 1997). Known as the integral-square pulse duration, this definition has been embraced as an industry standard. Originally faced with the problem of comparing results obtained with pseudo-Gaussian spatial profiles to literature data, we found that a general solution for arbitrarily inhomogeneous spatial beam profiles exists which results in a definition much similar to Sandstrom's. In addition, we proved the validity of our approach in experiments with intentionally altered beam profiles.
Properties of single-layer and multilayer Al2O3/SiO2 coatings deposited by Plasma Ion Assisted Deposition (PIAD) and Low Loss Reactive Evaporation (LL-RE) have been studied with emphasis on their use in the UV and VUV spectral region. The influence of significant deposition parameters, mainly the bias voltage in the case of PIAD and the substrate temperature in the case of LL-RE, on the optical and structural properties as well as on the film stress is investigated by spectrophotometry, IR- spectroscopy, light scattering, atomic force microscopy, and laser beam deflection stress measurements. Laser photon interaction with single-layer films and multilayer coatings was studied for the different wavelengths of excimer lasers (ArF (193 nm), KrF (248 nm), XeCl (308 nm) and the 3rd harmonic (355 nm) of the Nd:YAG solid state laser. High laser damage resistance and environmentally stable optical characteristics have been accomplished for multilayer coatings, especially for KrF (248 nm) excimer laser. The influence of the surface roughness of the substrates on the surface topography and the related scatter losses of the coatings has been investigated by integrated light scattering and atomic force microscopy measurements.
Stable and efficient UV-laser systems require stable and efficient optics which can only be realized by applying high performance, low loss and highly damage resistant thin film interference coatings. The interaction between excimer laser photons and optical coatings is determined by combined effects of high repetition rates and high energy densities. Optical stability investigations on oxide and fluoride thin films have been performed to estimate ns-laser-induced- damage-thresholds. By using thermal mirage technique both the detection of the damage onset after illumination with KrF excimer laser (248 nm, 20 ns) and the exploitation of the increase of the signal per unit fluence for the interpretation of the origins of radiation damage were possible. Damage sensitive defects were identified to origin both from residual gas and evaporation sources. By optimizing electron beam evaporation technology the defect density was drastically reduced.
Some of the commercial available ion based technologies like APS (Leybold AG), Mark II (CSC), Ion-Plating (Balzers) and NTQ (Carl Zeiss) have been compared and investigated for supporting our coating production lines. A comparative study has been worked out including the ability of upgrading conventional plants, resulting production costs, batch time, temperature load and scatter and mechanical durability of the coatings.
The performance of UV photolithography lens-systems with usually several ten optical components is limited by both the quality of the substrates and by the quality of the optical coatings. The key problems are the quality of the reflectance over large and strongly curved surfaces, the absorption and scatter losses and the behavior during heavy UV irradiation in production lines.
For building gravitational wave detectors large interferometer mirrors with extreme low total losses, especially low absorption losses are required. With Ion Beam Sputter Technology total losses of a few ten ppm are standard on small substrates e.g. 1 inch diameter. For the optimization of thickness uniformity on large substrates, a special double masking technique has been introduced into the coating process. Recent results for the losses of large optics at 512 nm are presented. The thickness uniformity on a 240 mm diameter substrate including the mirror center is better than +/- 0.25%. The contribution of the HR coatings to the spherical aberration of the mirrors is of the order of (lambda) /100. The absorption losses are typically below 2 ppm. The total losses are typically below 25 ppm.
We have realized an in-situ multichannel spectrometer system for on line optical thin film controlling during deposition. The system is working as a thin film optical monitor as well as an universal process control system. The measurements are made on the original optics during rotation, to avoid tooling factor errors and to take advantage of error compensation of successive layers. The typical thickness accuracy is 0.25% of the design wavelength.
We have implemented a process control system, based on in-situ spectral photometry, for the manufacturing of advanced optical coatings. The coating process parameters as well as the theoretical design values, like refractive indices and thicknesses, are stored in a database for documentation. Based on the high accuracy of the spectral optical thickness control, several examples for advanced coatings with enhanced optical performances are given.