Abstract: A high accuracy linear displacement interferometer capable of measuring changes in displacement of two plane mirror surfaces (60, 61) comprises a source (10) of an input beam (12) with two linear orthogonally polarized components which may or may not be of the same optical frequency, a birefringent optical element (40) and a quarter-wave phase retardation plate (42) for converting the input beam (12) into two separated, parallel, oppositely handed circularly polarized beams (16, 17); a first plane mirror (60) comprising one of the two (60, 61) plane mirror surfaces; a second plane mirror (61) nominally parallel to and rigidly connected to the first plane mirror (60) surface comprising the other of the two plane mirror surfaces; the birefringent optical element (40), the quarter-wave phase retardation plate (42), a right angle prism (48) with reflective, orthogonal faces, and a pair of retroreflectors (44, 45) causing each of the separated, parallel, oppositely handed circularly polarized output beams (32, 33)

Abstract: The deformable wafter chuck system includes a base with a recess having a diameter slightly smaller than the diameter of the wafer to be held. The base may have one or more orifices or channels running therethrough for distributing a vacuum to secure the wafer to the chuck, or it may have a plurality of clips attached at the rim of the chuck for holding the wafer. Attached to the chuck within the recess is a plurality of distortive actuators, such as piezoelectric crystals, which cause the wafer to be selectively deformed to assume arbitrary shapes, cancelling the warpage of the wafer to permit reduced distortion of the projected pattern. An interferometer system is included to combine light reflected from the wafer surface with a portion of incoming light modulated by a mask or reticle, thereby forming an interference pattern.