A new fabrication technique for silicon microcavities is described. The technique makes use of lithographic patterning and etching techniques, high-temperature bonding between hydrated surfaces and selective etching. Cavities with dimensions 100 × 250 × 12 μm and with a confining membrane consisting of a 1 μm-thick SiO2, film are demonstrated. The technique is potentially totally IC-compatible and can be used to realise a variety of sensor and actuator structures.