Etching and chemomechanical polishing of compound semiconductors using halogen-based reagents
The reactions of dichlorine and dibromine on compound semiconductors follow a pattern of halogenation of the substrate surface, followed by solvation of the halogenated products and removal of the secondary products in the etchant solutions. The reactions of hypochlorite and hydrogen peroxide follow a similar pattern, beginning with oxidation of the substrate surface, followed by subsequent hydration in the etchant solution and final product removal. The solubility of the products and the action of the polishing pad and abrasive are all critical to the passivation of the surface and the production of a polished substrate. The pH dependence of sodium hypochlorite is explained in terms of its composition. At low pH (<8) the main component of the etchant solution is dichlorine. At high pH (>8) the main component of the etchant solution is hypochlorite anion. Thus hypochlorite solution may be considered as a parallel to dibromine at low pH, but is more consistent with the behaviour of hydrogen peroxide at high pH. Thus, aqueous sodium hypochlorite solution may be formulated as part of the series O-O, O-X, X-X, where X represents a halogen. The General Model for Chemomechanical Polishing is formulated on the basis of the three stages of reaction observed for all the reagents examined: Oxidation of the substrate; solvation/hydration of the oxidation products; removal of the products from the substrate surface. Novel bromine-based organic reagents have been designed, synthesised and used to etch and polish gallium arsenide and cadmium telluride substrates. These compounds have shown that properties such as stock removal rate, pH dependence and selectivity may be predicted and even designed into etchants to produce the optimum conditions for polishing a given substrate.