The invention relates to controllable Fabry-Perot interferometers which are produced with micromechanical (MEMS) technology. Producing prior art interferometers includes a risk of deterioration of mirrors during the etching of the sacrificial layer (123). According to the solution according to the invention at least one layer (103, 105, 114, 116) of the mirrors is made of silicon-rich silicon nitride. In the inventive Fabry-Perot interferometer it is possible to avoid or reduce using silicon oxide in the mirror layers whereby the risk of deterioration of the mirrors is reduced. It is also possible to use mirror surfaces with higher roughness, whereby the risk of the mirrors sticking to each other is reduced.