embodiment wherein only the trench 4 that forms the sealed cavity is formed at this juncture the trench that is open to ambient conditions is created at the end of the process the advantage of this latter option is that if both trenches are open it is difficult to remove material from the open trench during subsequent processing there is however a disadvantage in following this option in that the opening of the second trench at the end of the process introduces added steps to the process and may result in variations in the diaphragm thickness due to alignment variations the embodiment of the option of fig5 will be assumed for the remaining process steps a nitride protection layer 12 is then applied prior to the filling of the trench 4 with a sacrificial oxide layer 13 as shown in fig6 and step 130 after the trench 4 is filled the sacrificial oxide layer 13 is patterned and etched a partial etchback of the oxide 13 may be used to thin the oxide 13 prior to patterning and etching it as shown in fig7 and step 140 in step 150 a layer 5 is deposited patterned and etched to create a u201c cover u201d over the trenches this modification to the device of fig7 is shown in fig8 the sacrificial layer 13.