Preferably using a positive resist, a resist ridge (20) is formed in a photosensitive resist (16) applied on a semiconductor wafer (1) above a hard mask layer (12). The resist ridge (20) serves as a mask for a subsequent implantation step (46). This makes use of an effect whereby the material of the hard mask layer (12), in a part (122) shaded by the resist ridge (20), can be etched out selectively with respect to the implanted part (121). The consequently patterned hard mask layer is used as an etching mask with respect to an underlying layer or layer stack (102-104) that is actually to be patterned. From the resist ridge (10) that has been formed as a line in the photosensitive resist (16), in a type of tone reversal, an opening (24) has been formed in the hard mask layer and a trench (26) has been formed in the layer/layer stack (102-104). According to the invention, the width (51, 52) of the resist ridge (20) is reduced by exposing the resist ridge (20) to an oxygen plasma (42). As a result, it is possible to form a trench (26) in the hard mask layer (12) and in the layer/layer stack (102-104) the width (52) of which trench is smaller than the lithographic resolution limit during the lithographic patterning of the resist (16).