Abstract: A method of reactive ion etching a substrate 46 to form at least a first and a second etched feature (42, 44) is disclosed. The first etched feature (42) has a greater aspect ratio (depth:width) than the second etched feature (44). In a first etching stage the substrate (46) is etched so as to etch only said first feature (42) to a predetermined depth. Thereafter in a second etching stage, the substrate (46) is etched so as to etch both said first and said second features (42, 44) to a respective depth. A mask (40) may be applied to define apertures corresponding in shape to the features (42, 44). The region of the substrate (46) in which the second etched feature (44) is to be produced is selectively masked with a second maskant (50) during the first etching stage, The second maskant (50) is then removed prior to the second etching stage.