Masafumi Takimoto

Super-smooth polishing on aspherical surfaces

This paper discusses super-smooth polishing technology researched and developed for excimer lasers, soft X-rays, and other short-wavelength light applications. Short-wavelength light requires surface quality and contour accuracy superior to traditional specifications, as well as free-form contour elements. For this reason, the final target for free-form contours of OE500 mm or more was set to 0.08 mu m PV for contour accuracy, and 0.2 nm RMS for surface roughness. To improve surface quality we employed local pitch polishing, utilizing a flexible tool laminated with an elastic layer, which adapts well to various contours. For greater contour accuracy, we developed the CSSP (Canon Super-Smooth Polisher), which polishes OE500 mm optical elements, and has a unique structure incorporating an on-machine contour measurement device built onto a single base plate. The CSSP polishing process achieves a contour accuracy of 0.078 mu m PV and a surface roughness of 0.13 nm RMS on a OE500 mm fused silica toroidal mirror. We also achieved the targets in the case of both CaF2 and CVD-SiC, materials widely used in short-wavelength light applications.

Super-smooth surface polishing on aspherical optics

The study of microscopes for shortwave light such as X rays has made much progress in recent years. Accordingly, the demand for a super-smooth aspherical optical element is increasing. The purpose of this research is to achieve a 0.08 p m or less surface figure error (relative accuracy 1O),and a 0.2 nm rms or less surface roughness on freeform surface optical elements of 500 mm in diameter or more. In order to improve the degree of surface roughness, we used a local pitch polishing method that had a profile generation possibility. A surface roughness of less than O.O6nm rms was achieved in the end of 1990 using the fused silica. In order to improve figure accuracy and ripple, we developed the Canon Super Smooth Polisher (CSSP)(Fig.1) which corrects the figure error by an on-machine measurement and developed a profile generating algorithm by early 1991. In this paper, we describe the results of studies on super-smooth polishing methods and the evaluation results of the CSSP system.

A high-precision coordinate measuring system for super-smooth polishing

For the purpose of fabricating free-form optical elements to an accuracy of 80 nm PV, the CSSP (Canon Super-Smooth Polisher) has been developed. This device finishes workpieces by alternately repeating contour measuring and corrective polishing. In such a system, contour measuring 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 contours with maximum tilt angles of 35 degrees . A unique probe structure is proposed, by which both inclination and motion errors of the probe are simultaneously compensated. The flaw problem is discussed from an experimental point of view; it was found that the major cause of flaws is dirt on the probe or the workpiece. Dirt also causes unstable irregular figure errors. Thus, the influence of dirt can be estimated from the results of contour measurement. A model of dirt is proposed, and the distribution on a width-height chart of irregular figure error predicted by the model agrees well with the experiment. A cleaning procedure was developed that is effective in reducing the problem of dirt. Using a ceramic air slide and a linear motor, the contact force was controlled to a constant of 2 mN, which is much smaller than the force of 260 mN that causes a yield stress on the CaF2 workpiece. The probe`s scanning speed of 4 mm s-1 was achieved by speeding up this force controller. The margin of force error during contour measurement was under 0.2 mN. A coordinate measuring method using fourteen-axis interferometers was also proposed for compensation of the major mechanical motion errors of the probe and the tables. Some methods of compensation for system errors are discussed. A new method was proposed by which three kinds of angle error, on the x, y, and z axes of measurement, are simultaneously compensated with a repeatability of 0.08 mu rad RMS. The measurement results showed good repeatabilities of 3 nm RMS for a 540 mm line measurement, and 9 nm RMS for a OE500 mm aspherical surface measurement.

Super-smooth polishing on aspherical surfaces (I): high-precision coordinate measuring and polishing systems

For the purpose of fabricating free-form optical elements with the accuracy of 80 nm PV, the Canon Super-Smooth Polisher has been developed. It has a high resolution on-machine coordinate measuring system that utilizes a contact probe. A unique probe structure is proposed, by which both inclination and motion errors of the probe are simultaneously compensated. The flaw problem is discussed from an experimental point of view, and it is found that the major cause of flaws is dirt on the probe or the workpiece. Fourteen axes interferometers are used to compensate for mechanical motion errors. The scanning speed of 4 mm/s is achieved by speeding up the force controller. The measurement results show good repeatabilities of 3 nm RMS for a 540 mm line measurement, and 9 nm RMS for a (phi) 500 mm aspherical surface measurement.

Super-smooth polishing on aspherical surfaces(II) Achievement of a super-smooth polishing

This paper discusses super-smooth polishing technology for excimer lasers, soft X-rays, and other short-wavelength light applications. Short-wavelength light elements require surface quality and contour accuracy superior to traditional specifications, as well as free-form contours. For this reason, our target for free-form contours of a 500 mm diameter was set to 0.08 mm PV for contour accuracy, and 0.2 nm RMS for surface roughness. To improve surface quality we employed local pitch polishing, utilizing a flexible tool laminated with an elastic sheet, which adapts well to various contours. For greater contour accuracy, we developed the CSSP (Canon Super-Smooth Polisher), which polishes 500 mm diameter optical elements. The CSSP polishing process achieves a contour accuracy of 0.078 mm PV and a surface roughness of 0.13 nm RMS on a 500 mm diameter fused silica toroidal mirror. We also fabricated both CaF2 and CVD-SiC, materials widely used in short-wavelength light elements.