Mechanism and dissolution rates of anodic oxide films on silicon

Abstract

The electropolishing of p-type silicon has been investigated over a wide range of wafer conductivities (p- to p++) and HF concentrations (0.01-15 wt%) by potentiodynamic polarization. The rate of oxide dissolution was determined from the plateau current density observed in the electropolishing region of the IV curve; i.e. where the growth and dissolution rates of the anodic oxide film are equal. The IV curves of silicon are reminiscent of a corrosion process control by the dissolution of a salt film in which the rate of reaction is controlled by the removal of dissolved products away from the surface rather than reactants to the surface as previously proposed. For the silicon anodic oxide films this can be by either mass transport or further chemical reaction with HF species in solution. It is shown that this means the dissolution rate should be described by: Dissolution rate = DKsp[HF]2/δ + k/A Ksp[HF]6 This relationship is shown to hold for the whole concentration range investigated. Because hydrofluoric acid is a weak acid this rate equation converts to a cubic equation when written in terms of the total HF concentration as opposed to only the HF species, but in this form the connection with the dissolution mechanism is lost. © 2013 Elsevier Ltd. All rights reserved.