Fabrication of thin films with highly porous microstructures

TLDR

The fabrication uses multiple evaporation sources or substrate motion to deliver a highly oblique flux (≈80°) and a ballistic‑deposition simulator to model and optimize self‑shadowing growth. The evaporation method yields highly porous films (≈15 % bulk density) with vertical, columnar microstructures that can exhibit zigzag patterns and anisotropic conductivity up to a factor of two, and the anisotropy orientation changes when the incident flux exceeds ~80°, confirming self‑shadowing as the dominant growth mechanism.

Abstract

An evaporation process has been developed for depositing highly porous insulator or metal films with densities as low as 15% of bulk. The process utilizes either multiple evaporation sources or substrate movement to provide a symmetrical but very oblique (≳80%) flux incident on the substrate. Extreme self-shadowing produced a vertical columnar microstructure consisting of isolated and evenly spaced columns including a unique zigzag structure in a number of insulator films. Features of the film are often anisotropic, leading to conductivity differences of as much as a factor of two along perpendicular axes in the plane of the film surface. The direction of anisotropic growth was observed to switch orientation as the incident flux angle was increased to very oblique, beyond approximately 80°. A line segment simulator incorporating ballistic deposition and minimization of chemical potential has been used to aid in the understanding of the growth mechanisms of these films and to optimize the evaporation process. The simulator helped to confirm that self-shadowing was the dominant mechanism in this porous structure formation.