Anodization of Al in microwave-discharged oxygen plasma

This paper describes the plasma anodization of aluminum thin films in microwave-discharge oxygen plasma with electron-cyclotron-resonance conditions from the following two points of view. First, the change in the current efficiency with the anodizing conditions and the factors affecting the oxide growth are examined. The current efficiency depends on the microwave input power, the magnetic field, the distance between the specimen and the location where the plasma is generated, the pressure of oxygen gas, and the presence of ozone. Second, some considerations are given on the generation of the atomic oxygen that closely relates to the growth of the oxide film. The generation process was calculated with a simple model and an approximation. The result suggests that atomic oxygen is generated when electrons and positive ions are extinguished by dissociative recombination in the course of ambipolar diffusion in the plasma. In addition, on the supposition that only the rate of atomic oxygen supplied to the surface of the specimen limits the growth rate of the oxide film, the minimum density of atomic oxygen necessary for the oxide growth was estimated. It was found to be maximal under the appropriate conditions of oxygen gas pressure and position of the specimen. However, the atomic oxygen density is also a complicated function of other conditions such as the electron density, the molecular oxygen density, and the electron energy.