Flexible reflecting membranes micromachined in silicon

Flexible reflecting membranes with an electrostatically controlled surface shape have been fabricated in a silicon wafer by means of bulk micromachining. A typical membrane has a square aperture with a side length of 10 mm. The membrane is formed by a 500 nm thick tensile stressed silicon nitride diaphragm coated with a 200 nm thick reflecting aluminium layer to form a flexible mirror. The mirror has an initially plane surface with a mean square deviation out of plane approximately lambda /10 for lambda =633 nm. The high initial optical quality and the possibility of electrostatic control of the reflecting surface make the mirror useful for various on-chip electro-optical applications.