Paper SE+TF+NC-ThA7
Quantification of Porosity and Deposition Rate of Nano-Porous Films Grown by Oblique Angle Deposition

Thursday, October 23, 2008, 4:00 pm, Room 204

We propose analytic formulas accurately predicting the refractive index and film thickness of obliquely deposited thin films for a given incident angle. Recently, it was shown that accurate control of the refractive index of physical-vapor deposited thin film materials can be achieved through the use of oblique-angle deposition. Refractive index tunability and low refractive index (low-n) films are highly desirable for a variety of optical applications. For example, broadband antireflection coatings, omni-directional reflectors, distributed Bragg reflectors, optical micro-resonators, light-emitting diodes, photovoltaic solar cells, and optical interconnects. While the qualitative tenets of oblique-angle deposition were demonstrated over a century ago, no quantitative formulas for the porosity and deposition rate have been described in the literature. In this work, we propose a model relating the porosity and deposition rate of a material to its vapor flux incidence angle for oblique-angle deposition. Our model is based upon geometrical arguments, employs a single fitting parameter, and takes into account surface diffusion. We have measured the refractive index and thickness for SiO₂ and indium tin oxide (ITO) nano-porous films deposited over a wide range of deposition angles (0° < θ < 90°). The porosity of a material is determined from the measured refractive index. Comparison of experimental SiO₂ and ITO porosity values and deposition rates with theory reveals excellent agreement. The theoretical model allows for the predictive control of refractive index, porosity, and deposition rate for all deposition angles, potentially a very useful tool in the development of high quality low-n optical coatings. Furthermore, given the set of basic assumptions used, we expect these formulas to be valid for a wide range of materials.