A Primer on Displacement Measuring Interferometers

A Displacement Measuring Interferometer (DMI) measures linear and angular displacements with very high accuracy and precision. DMI's are used in a variety of applications which can be broken into two broad categories: • high resolution real time position control systems, such as those used in semiconductor lithography, e-beam and laser reticle writers, CD measurement tools, process equipment, and memory repair tools, • characterization of high resolution, high frequency mechanical motions such as piezo transducers, linear and rotary scale calibration, AFM stage calibration. This primer outlines some of the practical issues which face the user when integrating a DMI as part of an instrument design or as a laboratory tool. Due to their inherent accuracy, DMI's have become an attractive tool for the most demanding displacement measuring applications. The fundamental accuracy of a DMI system is based on the precise knowledge of the wavelength of light. A DMI allows the user to minimize geometrical errors, such as Abbe offset error and opposite axis error, that are associated with mechanical displacement measuring techniques. Depending on the system configuration, a DMI can resolve displacements to 0.15 nanometers and track velocities up to 4.2 meters per second over a large displacement range. DMI's allow the user to measure displacements at the point of interest. With the simple detection scheme of a heterodyne system, multiple axes can be measured simultaneously (X, Y, q & more).