We investigated the use of separation, or substrate recovery, layers (SRLs), to enable the reuse of optical substrates after the deposition of multilayer reflective coatings, in particular Mo/Si multilayers as used for EUV lithography. An organic material (polyimide), known from other work to reduce the roughness of the substrate, was applied to the optical substrate. It appeared to be possible to remove the multilayer coating, including the SRL, without any damage or roughening of the substrate surface. The SRL was spin-coated at 1500 to 6000 rpm on different substrate types (Si, quartz, Zerodur) with diameters up to 100 mm. For this range of parameters, the multilayer centroid wavelength value remained unchanged, and its reflectivity loss on applying the SRL was limited typically to 0.7%. The latter was shown to be caused by a minor increase of the SRL surface roughness in the high-spatial-frequency domain. The roughness, characterized with an atomic force microscope, remained constant at 0.2 nm during all stages of the substrate recovery process, independent of the initial substrate roughness.
Amorphous layers of metal oxides deposited by Plasma Ion Assisted Deposition (PIAD) are widely used in the field of optical coatings due to their salient properties which enable the deposition of complex multilayer stacks. However, their use in the Deep UV spectral range is restricted as the range of transparency is limited by the absorption due to the first electronic band transition. The only oxide suitable for applications at 193 nm seems to be Al2O3 for which a band gap energy of 8.7 eV (143 nm) is reported for the crystalline state. Yet for thin layers of Al2O3 no work reports the making of absorption free layers at 193 nm. In this study we investigate how the amorphous structure of PIAD-deposited Al2O3 thin films influences the electronic structure and as a consequence of that the absorption behaviour for wavelengths close to the absorption edge. The electronic structure is worked out by a theoretical approach where in a first step the geometric structure is simulated using a Monte Carlo approach. Using this geometric structure the electronic structure is calculated by the tight-binding method in a second step. With these data absorption spectra are calculated and compared to measurements on PIAD Al2O3 layers. The experimental data for the start of the absorption lie on the longer wavelength side of the limit set by the amorphous structure - a fact, that encourages further work on the optimization of the deposition parameters.
Ag-dielectric multilayers are widely used in the production of heat reflecting filters, induced transmission filters, beam splitters, etc. The performance of such coatings in the visible part of the spectrum is sometimes strongly influenced by a plasmon absorption in the Ag-layer or a surface plasmon absorption in the Ag-dielectric interfaces. The strength of the plasmon absorption is very sensitive to the layer structure, the light polarization and the angle of incidence. As a result, the target specifications for reflection and transmission are not reached easily. We investigate PVD-deposited TiO2-Ag-TiO2 multilayers by means of optical reflection and transmission and Grazing Incidence X-ray Reflectometry (GIXR). The GIXR-method yields the individual layers thicknesses and the interface roughness. Some of the coatings have a broad absorption peak between 500 and 400nm that cannot be modeled using the bulk dielectric function of Ag. The magnitude of the absorption peak is correlated with the measured roughness of the TiO2-Ag interfaces. The analysis of the results shows the critical parameters for the deposition process.
Thin film multilayer dielectric coatings are widely used for the fabrication of various optical components. The precise knowledg of the optical constants and the thickness of the individual layers is one of the most important factors for the successful design and production of optical interference coatings with optimal performance. As thin film materials within a multilayer stack often have different optical constants compared to single layers deposited at the same conditions a disagreement between measured and predicted optical response of a multilayer system is observed. A better agreement can be achieved if the optical constants of the layer materials are determined from measurements of multilayer stacks. In the present work such an approach is applied for the optical characterization of popular optical coating materials. The optical constants of TiO2, Ta2O5, Al2O3 and HfO2 are determined in the spectral region 200 nm up to 800 nm using the following measurement techniques: spectroscopic ellipsometry, intensity transmission and X-ray grazing incidence reflectometry. The measured samples are periodic stacks consisting of 12 layers made of one of these materials in combination with SiO2. The ellipsometric and intensity transmission data are fitted simultaneously using the Tauc-Lorentz parameterization for the optical constants of the layers. The results are compared with the thickness of the layers obtained from X-ray grazing incidence reflectometry. The comparison of the predicted and measured optical response of a 3 material multilayer stack demonstrates the accuracy of the extracted optical constants.
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