Imaging coordinate measuring machines (CMMs) are widely used because of their ability to perform non-contact and high-precision coordinate measurement. Imaging CMMs measures not only the coordinate values but also forms of complex structures. However, the accuracy evaluation of the complex form measurement is not sufficient. ISO10360-7[1] defines the method for evaluating the accuracy of imaging CMMs. In this ISO, the length measurement errors are to be evaluated using a calibrated line scale, and the probing errors are to be evaluated using a calibrated roundness standard. The evaluation of probing errors is important for the accuracy evaluation of complex form measurement. Therefore, we started development of a calibration system for the photomask test circles, which serves as reference roundness standards for imaging CMMs. We constructed a rotary-table-based roundness measuring system. This system consists of an optical microscope and a high-precision rotary table, which equipped with a self-calibrating rotary encoder (SelfA) developed at NMIJ, instead of orthogonal linear stage, to eliminate the influence of geometric errors. To evaluate the validity of the developed method, the roundness of a circular mask with a nominal diameter of 0.22 mm was measured using a multistep method. As a result, a measurement result of 39 nm was obtained with respect to a reference value of 0.02 μm.
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