Oliver Apel

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.

Absolute measurements of nonlinear absorption near LIDT at 193 nm

Previous investigations indicate that oxide coatings exhibit non-linear absorption phenomena below 200 nm. Hereby, absorption data of Al2O3 thin film coatings has been determined absolutely by laser calorimetry (LCA) at 193 nm in the low fluence regime. As an alternative, on the basis of the pulsed surface thermal lens technique (STL), photothermal measurements allow to determine the absorption relatively at fluence levels both in the subdamage fluence range far from the damage onset and close to the LIDT. By combining the two measurement techniques, the absolute determination of linear as well as multiphoton absorption can be achieved also in the vicinity of the laser damage fluences. This is of crucial interest because the initiation of damage onset can be observed immediately. Absolute absorption data of Al2O3 coatings at different laser fluences stating of some mJoule/cm2 will be presented for the wavelength 193 nm. Thus, the correlation between the increase of absorption and the onset of breakdown can be illustrated impressively. The evaluation and discussion of the experimental results are focused on the degree of non-linearity of the investigated absorption behavior of oxide single layers initiating the optical breakdown of UV oxide coatings.

Interfacial absorption of DUV coatings

Complicated dielectric coatings consist of a large number of layers and thus have many interfaces, that may contribute to the total absorption of the coatings. We examined this contribution of the interfaces using two different approaches. For the determination of the absorption of the first interface between the substrate and the coatings we varied the thickness of dielectric single layers. For the examination of the influence of the interfaces within a dielectric stack, coatings consisting of (lambda) /2-layers were produced and their absorption was measured.

Nonlinear absorption phenomena in oxide coatings for 193 nm

New absorption measurements for aluminum oxide optical coatings at 193nm are presented. Apart from the strong linear absorption at this wavelength the data indicate a nonlinear absorption within the thin dielectric layer. By varying the laser thickness, the intrinsic contribution of the layer material to the overall absorption was separated from the contribution of the substrate and the interface. In addition, the conditioning behavior of the coatings was examined. A strong long term conditioning in the linear absorption was found for Al2O3 containing systems. Comparing the absorption and conditioning behavior of the single layers and a high-reflective system, we can show that the absorption properties of the HR-system are determined by its Al2O3 layers.

Scatter and absortion losses from DUV optics: a comparative study

DUV optical components are examined with respect to their optical losses at 193nm. Scattering and calorimetric absorption measurements were performed. The used calorimetric measurement setup allows absolute absorptance measurement with ppm resolution. By varying the energy density on the sample, linear as well as nonlinear absorptance can be determined. The total-scattering measurement setup allows the determination of both forward and backward scattering independently. We examined bare fused silica substrates, single layer coatings and thin film stacks. The obtained data give a detailed overview over the strength and the origins of the losses due to the different mechanisms for these optical components.

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.

Testing of optical components for microlithography at 193-nm and 157-nm

Absorption loss in UV optics during 193nm and 157nm irradiation is investigated by employing a high-resolution calorimetric technique which allows determining both single and two-photon absorptance at low energy densities. UV calorimetry is also employed to investigate laser induced aging phenomena, e.g. color center formation in fused silica. A separation of transient and cumulative effects can be achieved, giving insight into the loss mechanisms. A strong wavelength dependence of the DUV and VUV absorption characteristics in CaF2 substrates is observed. In addition, Hartmann-Shack wavefront measurements are presented, which allow on-line monitoring of laser-induced compaction in fused silica.

Absorption limited performance of SiO 2 /Al 2 O 3 multi-layer coatings at 193nm – a systematic study

The optical performance and radiation stability of HR193nm - SiO2/Al2O3 – coatings deposited by different technologies (RE, PIAD, IBS) were systematically evaluated and causes for the low LIDT at 193nm compared to 248nm were investigated.

Quadratic increase of nonlinear absorption in thin Al2O3 films at 193 nm

Linear and nonlinear absorptance in Al2O3 films of different optical thicknesses are investigated using an ArF laser calorimeter. While the linear absorptance at 193 nm shows the linear increase expected for homogenous layers coated with identical process parameters, nonlinear absorptance increases nonlinearly with increasing film thickness. Thus, it cannot be described by a constant nonlinear absorption coefficient. The experimental findings are explained by a simple phenomenological approach using excited states with a finite interaction length longer than the actual film thickness. Due to the observed quadratical increase a new material constant is introduced which describes the nonlinear absorptance behavior correctly.

CHARISMA: a new way for angular-resolved scattering measurements

In this paper, we present our new Coblentz Hemisphere based Angular Resolved and Integral Scattering Measurement Apparatus (CHARISMA). It is based on the combination of the well known Coblentz-Hemisphere used for integral scattering measurements and highly sensitive camera system developed at the Laser-Laboratorium Goettingen. Using CHARISMA, single- shot determination of the bidirectional reflectance distribution (BRDF) of optical samples is possible. Due to the spherical aberrations in the high-precision machined Coblentz-Hemisphere, the light scattered from a sample is not imaged onto a single spot but rather onto a stratified area. From this light distribution monitored by the camera system the angular-resolved scattering distribution can be evaluated unambiguously using a sophisticated complicated mathematical transformation.