Laser-interferometric nanometre comparator for length gauge calibration in advanced manufacturing

Abstract

We report on an instrument for calibration of the 8-mm length gauges, currently being developed and finalised at ISI. The design and construction were focused on elimination and compensation of the typical error sources associated with laser interferometric length measurement. The state-of-art four beam double-path differential plane interferometer with the common-path arrangement (with co-planar beams and co-axial arms) has its z-axis coincident with that of the calibrated gauge so that the system preserves Abbé principle. The differential arrangement efficiently reduces the metrological loop as the reference mirror of the interferometer is mounted on the preloaded grip holder of the tested sensor and the object (measurement) mirror simultaneously loading the measuring tip of the calibrated gauge. The latter is carried with a two-stage positioning comprising a precise linear ball bearing guide actuated with a DC motor (75 mm range, 2,6 mm/s velocity), and a triplet of piezoelectric elements that allow for a micro-positioning (0,015 mm range) and tilting of the object mirror. The displacement readout, compensated for the refractive index of air fluctuations using temperature, pressure and humidity sensors, provides feedback for the stabilisation of the object mirror position in the z-axis (to a nanometre). The interferometer also features a facility for detecting the lateral displacement of the beam in the measurement arm, providing feedback for the closed-loop stabilisation of the object mirror tilts that reduces the guidance-induced cosine error. With the series of test-run calibration of an optical ruler, we achieved the precision below 12 nm (k = 2) and accuracy below 34 nm (k = 2) over 25 mm range. A zero-drift test revealed the coincidence at a static position below 2,3 nm (k = 2) over six hours. Finally, the instrument is designed for automated operation with telemetry data collection compatible with the advanced manufacturing and Industry 4.0 demands.