Study on Absolute Flatness Measurement of Silicon Single Crystalline Plane Mirrors Using Near-Infrared Interferometry

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Single-crystalline silicon plane mirrors are widely used for the optical elements of synchrotron radiation. The purpose of this study is to measure the absolute flatness of those mirrors at nanometric accuracy by performing a three flat method with a near-infrared laser and a detector. Silicon is opaque to visible light, which prevents us from performing interference measurements with visible light and silicon as a transparent reference. We have developed the near-infrared phase-shifting Fizeau interferometer in which a near-infrared (λ =1315 nm) semiconductor laser and a near-infrared CCD camera is a source of light and a light detector, respectively. The near-infrared light is transparent to silicon, which enables us to observe interference fringes with a silicon plane mirror as a transparent reference. However, silicon has high refractive index even in the near-infrared region. A high reflectance induces a multiple-beam interference. This distorts interference fringes, which leads t … More o the measurement error of the phase-shifting method. Hence silicon dioxides acting as an anti-reflection film have been formed on the mirror in order to achieve a two-beam interference. As a result, we have confirmed that the distortion of interference fringes is suppressed. In order to investigate the measurement accuracy of a three flat method, the straightness of the center lines of three optical flats has been measured by the three flat method with visible light. And the accuracy of absolute straightness of each optical flat is 5 nm (peak-to-valley height). As a next step, flatness measurements have been performed by the near-infrared phase-shifting interferometry in which interference fringes changing with the scan of the reference along the light axis are recorded. In order to suppress the multiple-beam interference at silicon plane mirrors, we have paid attention to the fact that the contrast of fringes lowers as the cavity length becomes long. By controlling the cavity length, we have achieved a nearly ideal two-beam interference. And the reproducibility of the interferometry is 6.38 nm (peak-to-valley height) that is lower than λ/200. Less