Dual-mode on-machine metrology for SPDT tool alignment

Abstract

To address the critical demand for on-machine metrology (OMM) in precision optics fabrication, a unique dual-mode OMM system has been developed. By integrating polarization-based snapshot motionless phase shifting, this OMM system enables the measurement of both surface form and roughness under laser interferometry mode and LED interference microscopy mode. Its compact and dual-mode design makes it ideal for on-machine tool alignment within single-point diamond turning (SPDT) machines, without the need to remove parts. To enhance its OMM capabilities, a defocus-model-based least square (L2) regression fitting algorithm and a convex-hull-based L2 regression are proposed to achieve precise retrieval of deviations in X and Y axes with robustness. Additionally, a high-precision calibration method for testing system misalignment, based on the Zernike high-order approximation model, is applied to relax the OMM system alignment requirements. As a result, rapid tool alignment is achieved without stringent alignment needs. The proposed OMM eliminates the necessity for offline metrology feedback during the tool alignment process and increases process efficiency by at least 50 %. Furthermore, it eliminates errors caused by removing, repositioning, and rebalancing the part, offering a novel alternative solution to address the critical demand for SPDT tool alignment and surface characterization.