A HIGH-PRECISION COORDINATE MEASURING SYSTEM FOR SUPER-SMOOTH POLISHING

For the purpose of fabricating free-form optical elements to an accura cy of 80 nm PV, the CSSP (Canon Super-Smooth Polisher) has been develo ped. This device finishes workpieces by alternately repeating contour measuring and corrective polishing. In such a system, contour measurin g is important because it limits the final accuracy of the workpiece. This paper focuses on the CSSP's on-machine contour measuring method. A contact probe is employed to ensure adaptability to free-form contou rs with maximum tilt angles of 35 degrees. A unique probe structure is proposed, by which both inclination and motion errors of the probe ar e simultaneously compensated. Th flaw problem is discussed from an exp erimental point of view; it was found that the major cause of flaws is dirt on the probe or the workpiece. Dirt also causes unstable irregul ar figure errors. Thus, the influence of dirt can be estimated from th e results of contour measurement. A model of dirt is proposed, and the distribution on a width-height chart of irregular figure error predic ted by the model agrees well with the experiment. A cleaning procedure was developed that is effective in reducing the problem of dirt. Usin g a ceramic air slide and a linear motor, the contact force was contro lled to a constant of 2 mN, which is much smaller than the force of 26 0 mN that causes a yield stress on the CaF2 workpiece. The probe's sca nning speed of 4 mm s(-1) was achieved by speeding up this force contr oller. The margin of force error during contour measurement was under 0.2 mN. A coordinate measuring method using fourteen-axis interferomet ers was also proposed for compensation of the major mechanical motion errors of the probe and the tables. Some methods of compensation for s ystem errors are discussed. A new method was proposed by which three k inds of angle error, on the x, y, and z axes of measurement, are simul taneously compensated with a repeatability of 0.08 mu rad RMS. The mea surement results showed good repeatabilities of 3 nm RMS for a 540 mm line measurement, and 9 nm RMS for a <empty set> 500 mm aspherical sur face measurement.