The present publication discloses a method for forming cavities in prefabricated silicon wafers comprising a first silicon layer (1), a second monocrystalline silicon layer, or a so-called structural layer (3), oriented substantially parallel with said first silicon layer (1) and an insulating layer (2) situated between said first and second layers (1, 3). According to the method, in at least one of the conducting silicon layers (1, 3) are fabricated windows (4) extending through the thickness of the layer, and cavities are etched in the insulating layer (2) by means of etchants passed to the layer via said fabricated windows (4). According to the invention, subsequent to the fabrication step of the windows (4) and prior to the etching step, a thin porous layer (5) is formed on the surface to be processed such that the etchants can be passed through said porous layer into said cavities (6) being etched and, after the cavities (6) are etched ready, at least one supplementary layer (7) is deposited in order to render to the material of said porous layer impermeable to gases.