nm- and μm-Scale Surface Roughness on Glass with Specific Optical Scattering Characteristics on Demand

Hindawi Publishing Corporation Advances in OptoElectronics Volume 2007, Article ID 27316, 7 pages doi:10.1155/2007/27316 Research Article nm- and μm-Scale Surface Roughness on Glass with Specific Optical Scattering Characteristics on Demand Henning Fouckhardt,1 Ingo Steingoetter,1 Matthias Brinkmann,2 Malte Hagemann,2 Helmut Zarschizky,3 and Lin Zschiedrich3 1 Integrated Optoelectronics and Microoptics Research Group, Physics Department, Kaiserslautern University of Technology, P.O. Box 3049, D-67653 Kaiserslautern, Germany 2 Faculty of Mathematics and Science, Darmstadt University of Applied Sciences, Haardtring 100, D-64295 Darmstadt, Germany 3 JCMwave GmbH, Haarer Straße 14a, D-85640 Putzbrunn, Germany Received 10 January 2007; Accepted 20 April 2007 Recommended by Stefan A. Maier During maskless ion etching of amorphous glass, self-organization can arise in certain etch parameter ranges, which leads to dense-lying dots/cones with typical diameters and heights in the 30–300 nm range. Another phenomenon, which results in cone sizes around 1 μm or more, is self-masking especially in the case of heterogeneous glasses like borosilicate glass as used in this contribution. Thus, a wide range of characteristic sizes and shapes of individual scatterers on the glass surface, jointly acting as a defined roughness, can be achieved resulting in specific optical scattering characteristics. This contribution gives results on borosilicate thin-glass dry etching. Certain surface morphologies are reported together with experimental results on their optical scattering characteristics. Copyright © 2007 Henning Fouckhardt et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION Optics and laser physics are enabling technologies for the 21st century. Light sources and optical elements have to be tailored very specifically depending on application. Glasses have become important materials for functional substrates of devices and modules. Transmission, reflection, scattering loss, and spatial scattering distribution or an inherent antireflection function of the front facet a la moth’s eye effect [1] have to be controlled and tailored. Thus, more and more optical substrates do not just have to provide for mechanical stability of the devices, but should incorporate optical functions—including certain scattering characteristics. For example, in organic light emitting diode (OLED) technology, care has to be taken such that as much of the electroluminescence as possible is not guided sideways out of the OLED by total internal reflection, but rather leaves the device perpendicularly to the emitting layer sequence (see, e.g., [2, 3]). Or substrates for thin film solar cells could redirect the light power portion, not absorbed during the first passage through the active layers, back into that layer sequence to give higher efficiency. The possible applications for certain scattering characteristics are manifold. One approach to achieve rough optical surfaces on purpose is pulsed-laser ablation and even pulsed-laser assisted growth [4–6]. On the other hand, as scanning techniques, these approaches cannot easily be upscaled to large substrates. A maskless nonscanning procedure is favorable. Far back between 1956 and 1962, Navez et al. [7] ionbeam-bombarded glass surfaces and observed some unexpected surface morphologies: wave-like structures for flat ion beam incidence and dots/cones for nearly perpendicular incidence. Typical wave periods and characteristic dot sizes were in the range of some 10 nm to some 100 nm. As described in a review article by Valbusa et al. [8], subsequently, many groups picked up these investigations—not only with (reactive) ion-beam machines, but also with (reactive) ion etching (RIE). Those investigations were usually not performed with amorphous glass, but rather for semiconductors or even metals [9–23]. The phenomenon observed and described in all of these publications is self-organization due to two compensating effects, which together stabilize the surface profile: first a tendency of surface structure shrinkage due to a preferred etch erosion at oblique flanks and secondly diffusion of the eroded particles into the etched depressions 2 Advances in OptoElectronics and adsorption. Theoretical description is based on the socalled damped Kuramoto-Sivashinsky equation for the rate of the height profile change [19, 24–26]. The etch-based dots lie close to each other in the surface plane. Dot shapes (cones, pyramids, . . .) depend on dry-etch parameters, like ion energy or ion-beam divergence. Another important phenomenon at least in glass etching is self-masking [27]. Especially for heterogeneous glasses like the inexpensive borosilicate glass, certain components can give new nonvolatile compounds during wet or dry etching, which function as a randomly distributed ensemble of usually undesired tiny etch masks and locally prohibit further etching. These effects give a roughness on the scale of 1 μm to many microns. In an early paper by Affatigato et al. [28], the influence of an initial surface roughness on wet etch rates was investigated, while using optical scattering behavior to characterize the roughness. In our current contribution, however, the scattering characteristics themselves and their dependence on the shape of the single scatterers are in the focus of the interest. Light scattering at rough microstructured or nanostructured surfaces or by volume scattering centers arranged three-dimensionally within a transparent host material (glass, plastic, etc.) is a classical topic in optics [29– 35], but still a field of very active current research. Moreover, modern nano-structuring technologies led to a renaissance of this topic and surface roughness can be employed for new optical functions. A problem is the lack of sufficiently realistic physical models predicting the scattered light distribution from a given specific surface profile or scattering center arrangement. Moreover, there are only very few attempts to tackle the corresponding inverse problem, that is, designing the surface structure from a desired angular light distribution. Thus thorough investigations have to be performed. 2. HARVEY MODEL The Harvey model [29, 30] has been developed to describe the optical scattering characteristics, for example, of polished glass surfaces in terms of macroscopic quantities, that is, the specular transmission, diffuse transmission, specular reflection, and diffuse reflection. In the model, the scattering spot on the surface, on one hand, is regarded as an illuminated surface, which experiences an irradiance E due to the laser light source. On the other hand, this very scattering spot can be seen as a secondary light source, from which a radiation density (radiance) L originates. Thus, the sca (...truncated)