A method of manufacturing a semiconductor device wherein a laminate structure comprising a sacrificial layer is sandwiched between two etch stop layers (8,11) and which separates a semiconductor membrane (9) from a bulk substrate (1) is used to provide an underetched structure. Access trenches (4) and support trenches (5) are formed in the layered structure through the thickness of the semiconductor layer (9) and through the upper etch stop layer (8). The support trenches extend deeper through the sacrificial layer (12) and the lower etch stop layer and are filled. The sacrificial layer is exposed and etched away selectively to the etch stop layers to form a cavity (30) and realise a semiconductor membrane which is attached to the bulk substrate via a vertical support structure comprising the filled support trenches.