A novel, very simple technique for the direct adjustment of the slope of facets in a reactive ion etching process is presented. The etching process is performed by screening the sample partially with an aluminum shadow mask, which is located inside the dark space of the plasma. The sidewall angle of etched lines, defined by a pattern mask on the sample, depends strongly on the distance between the shadow mask and the sample. The investigations are carried out with a methane-hydrogen plasma. The angle of inclination of the sidewalls can be varied in a controllable manner by more than 45 degrees. This technique is well suited to fabricate 90 degrees reflectors, which are necessary for three-dimensional optical interconnects with planar waveguide structures. To analyze the optical quality of the etched mirror plane, the beam transformation was investigated by reflectivity measurements using a single aluminum-evaporated mirror structure, which is illuminated by a focused laser beam at a wavelength of 633 nm.