The present invention provides a method for fabricating a shadow mask in a trench of a microelectronic or micromechanical structure, comprising the steps of: providing a trench in the microelectronic or micromechanical structure; providing a partial filling in the trench; providing a first liner mask layer on the partial filling; providing a sacrificial filling on the liner mask layer to completely fill the trench; shallow etching back of the sacrificial filling into the trench; forming a first mask on the top side of the sacrificial filling in the trench; removing a subregion of the sacrificial filling in the trench using the first mask; and optionally removing a subregion of the first liner mask layer below it on the partial filling, the remaining subregion of the sacrificial filling in the trench serving as a second mask.