The performance of high quality optics relies heavily on their geometrical properties of surface finish and figure. The surface finish consists of the short spatial-wavelength departures from perfect smoothness and leads to scattering of the incident light with the resulting loss of intensity and contrast in the image. The figure represents the long spatial-wavelength departures of the surface topography from its designed form and leads to aberrations in the optical image. For soft x-ray (also termed extreme ultraviolet) optical elements operating at 13.6 nm the required figure accuracy for diffraction limited performance is 1 nm rms or better. The required surface finish is approximately 0.1 nm rms. These functional requirements mandate high accuracy in the measuring instruments. In this paper, we emphasize the calibration procedures for measuring instruments. Our work in surface finish primarily involves techniques to calibrate scanned probe microscopes. Towards that end we have developed a calibrated atomic force microscope (C-AFM). This instrument is calibrated in the x- and y?directions by laser interferometers and in the z-direction by a capacitance gauge, which in turn is calibrated periodically in situ by a laser interferometer. Presently, the instrument is being used to calibrate single atom steps on Si(111) fabricated by Williams et al. Our results differ from those measured by others in an interlaboratory comparison. For figure measurement, we have been developing techniques for improving the accuracy of commercial Fizeau, phase measuring interferometers for measuring flats, spheres, and aspheres. This has principally been an effort to compensate two types of errors: (1) error in the reference surface itself; and (2) the additional error introduced when the interferometer is operated off null, that is, when the wavefront reflected from the reference surface is not common path with respect to the wavefront returning from the measured surface. As a test of our correction procedure, we have applied it to a situation where a transmission sphere serves as the reference for measurement of a spherical ball, which is displaced axially from best focus, thus producing aberration in the wavefront reflected from the ball and leading to an erroneous result in the measured topography. Use of the correction procedure leads to a considerable improvement in the accuracy of the results.
Proceedings Title: Proceedings of Japan Society for Precision Engineering: U.S.-Japan Workshop on Soft X-ray Optics: Technical Challenges, T. Namioka, H. Kinoshita, and K. Ito, Editors
Conference Location: Tokyo, JA
Pub Type: Conferences
atomic force microscope, extreme ultraviolet, figure, finish, phase measuring interferometer, power spectral density, roughness, scanning tunneling microscope, soft x-ray, surface