Abstract: A method of manufacturing an optoelectronic semiconductor device whereby a surface (1) of a semiconductor (2) built up from a number of layers of semiconductor material (4, 5, 6, 7) grown epitaxially on a semiconductor substrate (3), with a top layer (4) of GaAs adjoining the surface (1) and a subjacent layer (5) comprising InP, in particular made of (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P with 0.5<x<0.8 and 0.4<y<0.6, is provided with an etching mask (8), after which the top layer (4) and the subjacent layer (5) are locally etched in a plasma generated in a gas mixture comprising SiCl.sub.4 and Ar. According to the invention, CH.sub.4 is added to the gas mixture in which the plasma is generated. This measure leads to the creation of a smooth surface during etching of both layers, and in particular during etching of the layer comprising InP. The walls (10) of the ridge (9) formed in the layers are also smooth and steep.