Klaus R. Mann

Laser conditioning of LaF(3)/MgF(2) dielectric coatings at 248 nm.

Highly reflective LaF(3)/MgF(2) systems for a wavelength of 248 nm on MgF(2) and crystalline quartz substrates were investigated. The influence of laser conditioning on damage threshold and absorptance was remarkable in those coatings that had a high initial absorptance. Monitoring with a laser calorimeter revealed the conditioning effect to be a function of the irradiation dose rather than of energy density or pulse rate. Furthermore, x-ray photoelectron spectroscopy and transmission electron microscopy investigations showed that conditioning induces stoichiometric and structural changes in the multilayers, especially in near-surface sublayers, whereas scanning electron microscopy and atomic force microscopy investigations indicated that the surface remains unchanged.

Nonlinear absorption of thin Al 2 O 3 films at 193 nm

Absorption of thin Al2O3 films was measured at 193 nm with an ArF-laser calorimeter. In addition to the expected high linear absorption coefficient, we found, for the first time to our knowledge, that two-photon absorption and transient color-center formation are nonnegligible loss channels in thin films at 193 nm. The nonlinear absorption coefficient is of the order of several times 10(-4) cm/W.

Coated optics for DUV excimer laser application

Recent developments of DUV-excimer laser applications have gained in demands for radiation resistant coated components at interesting wavelengths. To meet the requirements of long term reliability and high pulse number throughput superior performance of the optical components with lowest absorption and scattering losses are necessary. In the framework of the German Joint Research Project OPUS II efforts are made to investigate the optical properties, the radiation resistance and long term stability of single layers and layer system of interest in the DUV. The evaluation of optical coatings and coating system on different substrate materials was carried out by scattering experiments, atomic force microscopy, IR spectroscopy, calorimetric absorption measurements, and determination of laser induced damage threshold. Additionally, from the spectralphotometric measurements the optical behavior of the films was examined.

Antireflection coatings for UV radiation obtained by molecular-beam deposition

We have developed fluoride antireflection (AR) coatings on MgF(2) substrates for a wavelength of 248 nm by molecular-beam deposition. Transmission and laser-induced damage threshold of the samples were measured and atomic force microscope (AFM) investigations were carried out. We compare a 14-layer design for AR coatings with sublayer thicknesses of 12 nm with a conventional two-layer design with quarter-wavelength thicknesses. The laser-induced damage threshold of the 14-layer coating is slightly higher than that of the two-layer coating. The AFM surface images show that the 14-layer coating has a smoother surface than the two-layer coating.

Surface finish and optical quality of CaF2 for UV lithography applications

CaF2 has received increasing attention as a promising substrate for coatings in the VUV range. Optimization of the optical properties of these optical components requires the study of basic characteristics of the coated and uncoated CaF2 substrates such as surface roughness, optical performance, absorption and scatter losses, and laser induced damage threshold. The investigations reveal the influence of different substrate polishing grades on the quality of the coated components.

Very high damage threshold Al2O3/SiO2 dielectric coatings for excimer lasers

In the EUREKA EU205 project the target products are industrial excimer lasers in the average power range of one kilowatt or more. The high power optical components and dielectric coatings have to be developed in close adaption to cavity design (optics), beam relay optics, mask imaging optics, and masks. Therefore, we used ultra low loss conventional e-beam evaporation for Al2O3/SiO2 dielectric multilayers. Based on a fundamental coating technique, both multilayer mean background absorption and absorption at localized spikes have been reduced drastically. The resulting KrF laser damage threshold of HR coatings is 16 J/cm2 (1-on-1, 30 ns, EMG-202-MSC). Measurements have been performed with an automated damage testing facility, being part of the EUREKA program. Multilayers have been characterized by Atomic Force Microscopy, Photothermal Microscopy, absorption measurements, and Spectroscopy of Sputtered Neutrals.

Characterizing the absorption and aging behavior of DUV optical material by high-resolution excimer laser calorimetry

Absorption loss in DUV optics during 193 nm irradiation is investigated by employing a high-resolution calorimetric technique which allows determining both single and two photon absorption coefficients at energy densities of several 10 mJ/cm2, avoiding a significant thermal load on the samples. UV calorimetry is also employed to investigate laser induced aging phenomena, e.g. color center formation in fused silica or CaF2. A separation of transient and cumulative effects as a function of intensity can be achieved, giving insight into various loss mechanisms. Moreover, the influence of dielectric coatings on the absorption characteristics is discussed.

High-resolution calorimetric absorption measurements on optical components for excimer lasers

In the past, we presented several works showing that laser calorimetry can be applied in the UV-spectral range with greatly enhanced sensitivity compared to transmission measurements. We have now finished developing an improved laser calorimeter which is two orders of magnitude more temperature sensitive than our previous setup. This added resolution can be employed to measure absorptance as a function of irradiation parameters such as energy density or pulse repetition rate over an even broader range. In particular, the irradiation energy dose per measurement can be kept low enough to allow cumulative laser-induced absorption increase to be monitored with very high resolution. We used the new setup to cast some light on the scaling laws of absorptance in fused silica as a function of the ample thickness. It was possible to distinguish between surface and bulk processes.

Laser conditioning of LaF3/MgF2 dielectric coatings for excimer lasers

LaF3/MgF2-dieletric thin film combinations can be applied in optics for wavelengths down to 150 nm. Several such HR systems for a wavelength of 248 nm were investigated. In these coatings, the influence of laser conditioning on damage threshold and absorptivity was found to be remarkable. XPS- and TEM-investigations showed that the conditioning effect is related to structural and stoichiometric changes in the multilayers, especially in the near-surface-sublayers.

Photothermal measurement of absorptance losses, temperature-induced wavefront deformation, and compaction in DUV optics

A measurement system for quantitative registration of transient and irreversible lens effects in DUV optics induced by absorbed UV laser radiation was developed at the Laser-Laboratorium Göttingen. It is based upon a strongly improved Hartmann-Shack wavefront sensor with an extreme sensitivity of ~λ/10000 RMS @ 193nm, accomplishing precise online monitoring of wavefront deformations of a collimated test laser beam transmitted through the laser-irradiated site of a sample. Caused by the temperature dependence of the refractive index as well as thermal expansion and compaction, the initially plane wavefront of the test laser is distorted into a convex or concave lens, dependent on sign and magnitude of index change and expansion. The observed wavefront distortion yields a quantitative measure of the absorption losses in the sample. Some results for fused silica and CaF2 are presented.