A semiconductor device includes a semiconductor layer (2) and a dielectric stack (3) on the semiconductor layer. A plurality of etchant openings (24-1,2 . . . ) are formed through the dielectric stack (3) for passage of etchant for etching a plurality of overlapping sub-cavities (4-1,2 . . . ), respectively. The etchant is introduced through the etchant openings to etch a composite cavity (4) in the semiconductor layer by simultaneously etching the plurality of overlapping sub-cavities into the semiconductor layer.