A method of etching a semiconductor substrate is described, the method comprising the steps of applying a paste containing an etchant to the substrate, and carrying out a thermal processing step to etch a part or a layer of the substrate where the paste has been applied. The etchant paste is preferably a caustic etching paste. The etchant paste may be applied selectively to a major surface of the substrate to form a pattern of applied paste. For example, the paste may be applied by a printing method, such as screen-printing. The method may be used to produce solar cells.