8 is introduced into the mask layer 3 for example by an implantation examples of other suitable materials for the additional substance 8 are p dopants such as aluminium gallium indium thallium and boron the layer 2 is then etched in accordance with the configuration illustrated in fig2 0029 fig2 shows the configuration illustrated in fig1 at a later time first of all etching has been carried out using a fluorohydrocarbon containing etching gas such that a trench 5 has been formed in the layer 2 and a facet 6 has been formed on the mask layer 3 the substance 8 first has the effect of reducing the rate at which material is removed from the mask layer 2 and in particular of reducing the rate at which material is removed at the facet angle being formed the facet angle is in this case measured between the substrate surface and the facet 6 a further exemplary embodiment in accordance with fig1 includes initially forming the mask layer 3 without the additional substance 8 then with reference to fig2 the additional substance 8 is admixed with the etching gas and during the etching operation leads to an increase in the etching selectivity since the levels of the additional substance 8 in and or on the mask layer 3 increase as a result the rate at which material is removed from the mask layer 3 is reduced during the etching.