A trench is formed in a semiconductor substrate between mesas of a first conductivity type. The trench extends from a process surface down to a bottom plane. A semiconductor layer of a second, complementary conductivity type is formed on sidewalls of the trench. At least in the mesas a vertical impurity concentration profile vertical to the process surface is non-constant between the process surface and the bottom plane. A portion of the semiconductor layer in the trench is removed by electrochemical etching. Thereafter, the thickness of the recessed semiconductor layer images the vertical impurity concentration profile in the mesa.