3-D surface profile measurements of large x-ray synchrotron radiation mirrors using stitching interferometry

摘要

Stitching interferometry, using small-aperture, high-resolution, phase-measuring interferometery, has been proposed for quite some time now as a metrology technique to obtain 3-dimensional profiles of surfaces of oversized optical components and substrates. The aim of this work is to apply this method to the specific case of long grazing-incidence x-ray mirrors, such as those used in beamlines at synchrotron radiation facilities around the world. Both fabrication and characterization of these mirrors would greatly benefit from this technique because it offers the potential for providing measurements with accuracy and resolution better than those obtained using existing noncontact laser profilers, such as the long trace profiler (LTP). Measurement data can be used as feedback for computer-controlled fabrication processes to correct for possible topography errors. The data can also be used for simulating and predicting mirror performance under realistic conditions. A semiautomated stitching system was built and tested at the X-ray Optics Metrology Laboratory of the Advanced Photon Source at Argonne National Laboratory. The initial objective was to achieve a measurement sensitivity on the order of 1 μrad rms. Preliminary tests on a 1m-long x-ray mirror showed system repeatability of less than 0.6 μrad rms. This value is comparable to that of a conventional LTP. The measurement accuracy was mostly affected by environmental perturbations and system calibration effects. With a full automated and improved system (to be built in the near future), we expect to achieve measurement sensitivity on the order of 0.01 μrad rms or better. In this paper, after a brief review of basic principles and general technical difficulties and challenges of the stitching technique, a detailed description of the measurement setup is given and preliminary results obtained with it are analyzed and discussed.