Surface Roughness Study of Low-temperature PECVD a -Si:H.
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A5.14.1
Surface Roughness Study of Low-temperature PECVD a-Si:H. George T. Dalakos1, Joel L. Plawsky2, and Peter D. Persans3 1 General Electric Global Research Center, Niskayuna, NY 2 Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, NY 3 Department of Physics, Rensselaer Polytechnic Institute, Troy, NY
ABSTRACT Surface topography of a-Si:H thin films, deposited at 75°C by Plasma-enhanced Chemical Vapor Deposition (PECVD) has been examined using helium/silane feedstock mixtures under different substrate bias conditions. Notable differences in the surface roughness evolution are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Smooth and apparently featureless surfaces result from deposition on RF powered surfaces, upon which a self-bias induces high-energy ion bombardment. Rougher surfaces result from films deposited on electrically grounded surfaces. These anodic films show that after a transition period, surface roughness grows linearly with processing time, exhibiting mounded type growth as evidenced by 2-D power spectral density functions of surface height measurements. Linear growth in roughness has been predicted for shadow growth models assuming film precursor sticking coefficients of one and random angle approach of film precursor species. Growth of this nature has not been reported before in a-Si:H studies, which usually assume directional deposition conditions and sticking coefficients less than unity – occurring even at low processing temperatures.
INTRODUCTION Thin films with relatively smooth surfaces are highly desirable and even critical in some high-performance optical thin film applications. For example, applications in thin film waveguides and laser mirrors, which inherently involve many reflections or require very high reflectance. Surface roughness leads to diffuse scattering and power loss, which gets worse as the refractive index contrast at the optical interface increases. When silicon is considered, whose high refractive index is advantageous in optical applications, scattering problems become more important versus other materials [1]. One successful method for fabricating thin films with smooth surfaces is chemical vapor deposition, albeit processed at relatively high temperatures. These high temperatures provide smooth interfaces from very good film precursor diffusion on the surface. Highly mobile film precursors are able to move about the surface and able to fill in voids suppressing overhangs and columnar morphologies, both of which lead to surface roughness. Although high substrate temperatures are favorable processing conditions for forming smooth film interfaces, they are incompatible with temperature-sensitive materials such as some optical plastics. Despite this, smooth surfaces with a featureless microstructure are still obtainable under certain low-temperature deposition conditions. Indeed, by providing kinetic energy to the surface, ion bombardment can be u
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