A vent hole precursor structure (26) in an intermediate product for a semi-conductor device has delicate structures (27, 28), and said intermediate product has a cavity (21) with a pressure therein differing from the pressure of the surroundings. The intermediate product comprises a first wafer (20) in which there is formed a depression (21). The first wafer is bonded to a second wafer (22) comprising a device layer (23) from which the structures (27, 28) are to be made by etching. A hole or groove (26) having a predefined depth extends downwards into the device layer, such that the cavity (21) during etching is opened up before the etching procedure breaks through the device layer (23) to form the structures (27, 28).