Eric Eva

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.

Compaction and Rarefaction of Fused Silica with 193-nm Excimer Laser Exposure

Extensive testing of the laser damage behavior of fused silica has been performed over the past few years by several researchers. The results have shown that compaction and rarefaction / expansion of the material can occur. The actually observed process depends on the used energy density and laser pulse number at constant pulse length. In order to check the influence of the different laser parameters in more detail, an experimental set up has been constructed that allows us to investigate not only the influence of the energy density and laser pulse number but also the effect of the integrated square pulse width on the laser damage behavior. An optical delay line is used to create a longer integrated pulse width than the natural laser pulse width. To make these tests relevant to the microlithography community, the integrated energy densities chosen for these tests span the range typically found in the projection optics of a 193-nm excimer laser-based microlithography tool. The samples are exposed to several billions of pulses with wavefront measurements made periodically.

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.

Characterization of absorption and scatter losses on optical components for ArF excimer lasers

The performance of DUV optical components is assessed by measuring both absorption and scatter losses during ArF excimer laser irradiation. Absolute absorptance is determined by employing a high-resolution calorimetric technique which provides greatly enhanced sensitivity compared to transmissive measurements. Thus, the determination of single and two-photon absorption coefficients at energy densities of several 10 mJ/cm2 is accomplished. As a result of its sensitivity, UV laser calorimetry can be also employed for fast monitoring of laser induced aging phenomena like color center formation in fused silica or CaF2. For monitoring of the scatter losses in UV optics, a total scattering setup was recently installed, using an ArF excimer laser as pulsed 193nm light source and a Coblentz hemisphere as integrating element. Results of quantitative absorption and scatter measurements at 193 and 248 nm are presented for coated and uncoated optics, and the contribution of the various loss channels is discussed.

Nonlinear absorption phenomena in optical materials for the UV-spectral range

A laser calorimetric technique is employed to measure absorptance and laser-induced changes therein on optical components at 248 nm. In BaF2 a linear increase of absorptance with intensity is observed, but a pure two- photon absorption mechanism can be ruled out because the slope lessens towards shorter pulse lengths. Fused silica, at moderate energy densities, exhibits a reversible, linear absorption increase with energy density; an AR-coated sample mirrors the behavior of the uncoated substrate but discloses an added offset due to coating imperfections. In a series of measurements at constant irradiation parameters, uncoated substrates of Suprasil and CaF2 reveal a slight exponential decrease in absorptance which might be attributed to surface cleaning or conditioning effects, and absolute absorptance is virtually independent of the sample thickness.