Phase-Shifting Point Diffraction Interferometer Having Two Point Light Sources of Single-Mode Optical Fibers

Abstract

State-of-the-art research fields such as projection optics for Extreme Ultraviolet Lithography (EUVL) (Otaki et al., 2002a; Stulen & Sweeney, 1999; Goldberg et al., 2004), focusing optics for X-ray microscope (Yamamura & Takai, 2008; Handa et al., 2008; Yumoto et al., 2008; Matsuyama et al., 2008), and Michelson interferometers for detection of gravitational waves (Ando et al., 2001; Sato et al., 1999) require ultra precise mirrors with large surface areas and extremely low surface figure error (in the sub-nanometer scale). In EUVL, a light wave of 13.5 nm is used for photoresist exposure, and such an optical system accepts reflection optics instead of refractive optics owing to the short wavelength. In addition, EUV exposure of a dynamic random access memory (DRAM) pattern with a half pitch (hp) of 45 nm or less requires a numerical aperture (NA) of 0.2 or more. An EUV optical system is designed using Mo/Si multilayer mirrors. The surface figure accuracy of the mirror substrate determines the reflectance of the mirror and the long-range error must be less than 0.2 nm rms. Production of high-precision mirrors with large surface areas is usually supported by highly accurate surface figure measurement methods. For example, the slope integration method (Higashi et al., 2007), three-flat method (Schulz & Schwider, 1967), and point diffraction interferometry (PDI) (Smartt & Steel, 1975; Sommargren, 1996; Medecki et al., 1996; Millerd et al., 2004; Otaki et al., 2002b) have been studied and applied to the measurement of mirrors. The slope integration method determines the surface normal vector at each point on the test surface and the resultant angle of inclination is integrated to obtain the absolute surface figure. However, this method requires highly accurate locations of each measurement point. The three-flat method uses three mirrors including the test mirror and measures the relative surface figure between two of the mirrors at a time using conventional optical interferometry. The relative figure data are solved as a set of simultaneous equations to obtain each surface figure. However, this method returns a line profile of the mirrors rather than a whole surface figure. PDI is a novel absolute surface figure measurement system using an optical diffraction wave from a tiny aperture as the reference surface. A conventional Fizeau interferometer uses a substantial reference surface for the measurement and the machining accuracy of the reference results in measurement accuracy of approximately ┣/50. In contrast, the wavefront accuracy of the optical diffraction wave used in the PDI system has been estimated to be 10-5┣ or less at an NA of 0.2 (Sommargren, 1996).