Interferometric length and roughness measurements with nanometer accuracy level

Abstract

Gauge blocks are known to be the most important and commonly used material standards in maintaining traceability in dimensional metrology. They are regularly used in international comparisons of interferometric measurements to assess the accuracy of the length SI-unit, reproduced in different countries, as with the help of interferometric comparators the length of the block can be directly determined in terms of vacuiini wavelengths of standard radiations [1]. In many countries the primary level comparators are similar to the big KOsters interferometer [2-3], where a plane light wave of big aperture is propagating in vacuum and after reflection from a plane mirror forms a standing wave, with a precisely known separation between the points of equal intensity (fringes). This standing wave is then used to find the length of a gauge block in terms of the vacuum wavelength, which is calculated as the ratio of the speed of light (explicitly fixed in the present definition of the Metre) and the laser frequency, known from the absolute frequency measurements performed against the primary Cs standard. The procedure of length measurement in this instrument requires a flat reference plate to be wrung to one of the faces of the gauge block. For short gauge blocks, of a few mm nominal lengths, the largest uncertainties in the accuracy budget of the interferometric measurement [4] arises due to the phase change correction [2] (±12 nm at 95% confidence level [4]) and due to the variability of the wringing film thickness (±12 nm at 95% confidence level [4]). In some papers [3,5], for selected blocks with smaller deviations in flatness, the variability of the wringing film thickness is reported to be about nm (at 95% confidence level). In this paper, we describe a new approach in the interferometric length measurements that is based on the technique of the reproducible wringing [6-9]. It gives the opportunity to measure the physical length of the material artifact without the excessive wringing film thickness and results in the accuracy of the phase change measurements at the optical reflection from surface of the material standard of better than I nm.