Nanometer positioning control by a multiple-pass

High accuracy positioning systems in the nanometer range are needed for IC fabrication, research in microelectro-mechanical systems (MEMS) and nanotechnology. A linear encoder with sub-nanometer resolution is needed for such applications as large field x-ray lithography, large field scanning microscopes, x-ray microscopes, x-ray micromachining, etc. For a conventional laser interferometer the resolution is about λ/64 or 10 nm. Higher fringe interpolation, may be limited by the optical non-linearity, electronic noises, detector shot noises, and environmental conditions. Furthermore, for nanometer positioning measurement by a laser interferometer in a laboratory environment, the accuracy or resolution is limited by the speed of light changes caused by air circulation or turbulence. To solve these problems, a multiple-pass optical arrangement can be used to increase the resolution and to minimize the effect of air turbulence. Reported here is a laser Doppler displacement meter with a multiple-pass optical arrangement developed by Optodyne to achieve sub-nanometer resolution with a measuring range of 100 mm and a maximum velocity of 600 mm/sec. The optical setup is compact and easy to align. The performance of the multiple-pass optical arrangement, a comparison with a single-pass laser interferometer, various sources of errors and uncertainties, the test setup and results, will be described.