Surface Roughness Evolution of PECVD Cathodic and Anodic a -Si:H.
- PDF / 1,422,362 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 48 Downloads / 197 Views
Surface Roughness Evolution of PECVD Cathodic and Anodic a-Si:H. George T. Dalakos1, Joel L. Plawsky2, and Peter D. Persans3 General Electric Corporate Research and Development, Niskayuna, NY 2 Department of Chemical Engineering, Rensselaer Polytechnic Institute, Troy, NY 3 Department of Physics, Rensselaer Polytechnic Institute, Troy, NY 1
ABSTRACT Surface or interface roughness can impact optical, electronic, and MEMS applications of thin a-Si:H films. Deposition at lower temperatures can be advantageous for some applications of aSi:H, but lower temperature deposition generally leads to rougher films. We have found that the evolution of surface roughness growth can be modified substantially by ion bombardment due to the self-bias of the plasma during Plasma-Enhanced Chemical Vapor Deposition (PECVD). Notable differences in the surface roughness evolution and deposition rate are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Suppression of surface roughness growth of a-Si:H can be achieved under conditions of relatively high ion bombardment even at deposition temperatures as low as 75 C. Atomic force microscopy (AFM) was used to measure the relative surface roughness profile as a function of deposition time. Analysis of the power spectral density of the roughness yielded important statistical surface parameter information. Based on these observations, insight is given into growth mechanisms under the two deposition conditions.
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 in 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. Film precursors with high surface mobilities are able to move about the surface and able to fill in voids suppressing overhangs and columnar morphologies, which lead to surface roughness. Although high substrate temperatures are favorable processing conditions towards forming smooth film interfaces, they are incompatible with temperature-sensitive materials such as some optical plastics. Despite this, smooth surfaces with featureless microstructure are still obtainable under certain low-temperature deposition conditions. Indeed, by providing kinetic energy to the surface, ion bombardment can be used to make up for the lack in thermal energy to
Data Loading...
