Abstract
Modifications made to the Long Trace Profiler (LTP II) system at the Advanced Photon Source at Argonne National Laboratory have significantly improved the accuracy and repeatability of the instrument. The use of a Dove prism in the reference beam path corrects for phasing problems between mechanical errors and thermally-induced system errors. A single reference correction now completely removes both error signals from the measured surface profile. The addition of a precision air conditioner keeps the temperature in the metrology enclosure constant within + or - 0.1 degrees C over a 24-hour period and has significantly improved the stability and repeatability of the system. We illustrate the performance improvements with several sets of measurements. We are able to put an error bar of 0.3 microradian on a 500mm long surface. The improved environmental control has reduced thermal drift error to less than 1 microradian on a 500 mm long surface. The improved environmental control has reduced thermal drift error to less than 1 microradian RMS over a 7.5 hour time period. Measurements made in the forward scan direction and the reverse scan direction differ by less than 0.5 microradian RMS over a 500mm trace length. We are now able to put a 0.2 microradian error bar on an average of 10 measurements over a 200mm trace length. The corresponding height error bar for this measurement is 3.5 nm. We are now able to see systematic errors on the order of 1 microradian in magnitude that limit the ultimate accuracy of the instrument.
© 1996 Optical Society of America
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