Interferometric measurement of profile deviations of large precision mirrors

In numerous interferometric applications of nanopositioning and nanomeasuring technology, plane mirrors are used as flatness or straightness standards for the movement of a positioning device. During this process, the shape deviations of the mirrors used lead to systematic errors of the position measurements, which can be corrected in later applications if known. Most often the effective shape deviations are described sufficiently well by profile deviations along a profile line which is fixed by the movement of the positioning or measuring device. The novel precision device - a so-called interferometric nanoprofilometer - for measuring the profile deviations of plane mirrors presented in this paper is based on the comparison of the profile line of the mirror to be tested with a straightness standard embodied by a mirror of very high flatness. A specially designed point-based measuring interferometer is moved along the profile line. As the measurement is directly referenced to the straightness standard, the influence of the guide errors was greatly reduced. The uniform movement of the interferometer is ensured through a linear measurement table, which is driven using speed control and pulse-width modulation. Apart from the mechanical and optical design of the interferometric nanoprofilometer, a hardware module was assembled which enables the control of the linear measurement table and the extraction of pulses for the synchronous acquisition of position and profile deviation values. In addition, a software tool was developed for configuring the measurement process and for data recording as well as a program to perform various analyses of the profile deviations. In measurements performed with the interferometric nanoprofilometer covering the maximum scanning length with lx=250 mm under laboratory conditions, the expanded uncertainty was U(l)=7.8 nm at a confidence level of p=95% (k=2).