Biomedical subjects
Publications and source records attributed to Angela Duparré.
Films with specific nanoroughness were designed by "virtual coating" to yield ultrahydrophobicity while the light scatter remained below an application-relevant threshold. Examples of coating experiments demonstrate the realization of predicted properties.
An instrumentation for total and angle-resolved scattering (ARS) at 193 and 157 nm has been developed at the Fraunhofer Institute in Jena to meet the severe requirements for scattering analysis of deep- and vacuum-ultraviolet optical components. Extremely low backscattering levels of 10(-6) for the total scattering measurements and more than 9 orders of magnitude dynamic range for ARS have been accomplished. Examples of application extend from the control of at-wavelength scattering losses of superpolished substrates with rms roughness as small as 0.1 nm to the detection of volume material scattering and the study into the scattering of multilayer coatings. In addition, software programs were developed to model the roughness-induced light scattering of substrates and thin-film coatings.
We introduce two parameters, large-scale and small-scale rms roughness, to take into account the interface properties of thin films and multilayers in the calculation of their specular reflectance and transmittance. A theoretical motivation for the introduction of these two parameters instead of a standard single rms roughness is provided. Experimental power spectral density functions of several samples are used to illustrate ways in which the parameters introduced can be evaluated.
Single layers of MgF2 and LaF3 were deposited upon superpolished fused-silica and CaF2 substrates by ion-beam sputtering (IBS) as well as by boat and electron beam (e-beam) evaporation and were characterized by a variety of complementary analytical techniques. Besides undergoing photometric and ellipsometric inspection, the samples were investigated at 193 and 633 nm by an optical scatter measurement facility. The structural properties were assessed with atomic-force microscopy, x-ray diffraction, TEM techniques that involved conventional thinning methods for the layers. For measurement of mechanical stress in the coatings, special silicon substrates were coated and analyzed. The dispersion behavior of both deposition materials, which was determined on the basis of various independent photometric measurements and data reduction techniques, is in good agreement with that published in the literature and with the bulk properties of the materials. The refractive indices of the MgF2 coatings ranged from 1.415 to 1.440 for the wavelength of the ArF excimer laser (193 nm) and from 1.435 to 1.465 for the wavelength of the F2 excimer laser (157 nm). For single layers of LaF3 the refractive indices extended from 1.67 to 1.70 at 193 nm to approximately 1.80 at 157 nm. The IBS process achieves the best homogeneity and the lowest surface roughness values (close to 1 nm(rms)) of the processes compared in the joint experiment. In contrast to MgF2 boat and e-beam evaporated coatings, which exhibit tensile mechanical stress ranging from 300 to 400 MPa, IBS coatings exhibit high compressive stress of as much as 910 MPa. A similar tendency was found for coating stress in LaF3 single layers. Experimental results are discussed with respect to the microstructural and compositional properties as well as to the surface topography of the coatings.
An instrument for total backscattering and forward-scattering measurements of optical coating components at 157 and 193 nm is described. The system is operated in both vacuum and nitrogen purge gas. An excimer laser as well as a deuterium lamp can be used as a radiation source. Suppression of the background signal level to 1 part in 10(6) permits measurements even of low-scatter samples such as superpolished substrates and antireflection coatings. Results of investigations of antireflective and highly reflective multilayers and CaF2 substrates reveal scattering from surface and interface roughness as well as from the volume of the substrate material. First steps to extend the instrument for angle-resolved scatter, transmittance, and reflectance measurements are described.
Optical coatings with enhanced roughness offer promising prospects for ultrahydrophobic transparent surfaces with controlled scatter losses. Our approach links roughness characteristics with both wetting behavior and scattering. Experiments with rough oxide layers yielded surfaces with a high water contact angle.
The determination of optical parameters of thin films from experimental data is a typical task in the field of optical-coating technology. The optical characterization of a single layer deposited on a substrate with known optical parameters is widely used for this purpose. Results of optical characterization are dependent on not only the choice of the thin-film model but also on the quality of experimental data. The theoretical results presented highlight the effect of systematic errors in measurement data on the determination of thin-film parameters. Application of these theoretical results is illustrated by the analysis of experimental data for magnesium fluoride thin films.
Surface topography and light scattering were measured on 15 samples ranging from those having smooth surfaces to others with ground surfaces. The measurement techniques included an atomic force microscope, mechanical and optical profilers, confocal laser scanning microscope, angle-resolved scattering, and total scattering. The samples included polished and ground fused silica, silicon carbide, sapphire, electroplated gold, and diamond-turned brass. The measurement instruments and techniques had different surface spatial wavelength band limits, so the measured roughnesses were not directly comparable. Two-dimensional power spectral density (PSD) functions were calculated from the digitized measurement data, and we obtained rms roughnesses by integrating areas under the PSD curves between fixed upper and lower band limits. In this way, roughnesses measured with different instruments and techniques could be directly compared. Although smaller differences between measurement techniques remained in the calculated roughnesses, these could be explained mostly by surface topographical features such as isolated particles that affected the instruments in different ways.