SCANNING DIFFERENTIAL-HETERODYNE-INTERFEROMETER WITH ACOUSTOOPTIC DEFLECTORS

A heterodyne interferometer principle is applied for a noncontact opti cal profilometer that scans the probe by means of variation of the dri ver frequency of an acousto-optic deflector. The reference and measuri ng beams are focused on the same surface under test to reduce environm ental influences. The phase of the beat frequency of the interfering r eturn beams is a measure of the height difference between reference an d measuring beam. A height sensitivity of 5 nm was achieved with a bre adboard set-up and a 550 mu m line-scan length. Measurements on differ ent samples and their features are discussed,