Since nanotechnology has progressed rapidly, new methods for length measurement applicable to the millimeter range with sub-nanometer resolution are required. Currently, the standard method for length measurement is laser interferometry. A heterodyne interferometer with a Zeeman laser or a homodyne interferometer using the bi-fringes counting method is widely used in the industry. However, it is difficult to determine arbitrary length with the sub-nanometer accuracy using the interferometers, because they have the non-linearity problem of the fringe interpolation. A phase modulation homodyne interferometer, that can determine the optical path difference of wavelength times integer with the accuracy of 10 picometer or less, is proposed. The lattice spacing of approximately 0.2 nanometer for some regular crystalline lattices is uniform and stable over a long range, when the crystals are stress free. These crystals can be used as reference scales with a sub-nanometer resolution instead of laser interferometry. X-ray interferometry (XRI) using silicon crystal has been developed to determine the lattice spacing of silicon at National Metrological Laboratories. Moreover, the combined optical and x-ray interferometer (COXI) has been developed for absolute length measurement with sub-nanometer accuracy at European metrological laboratories. However, XRI is very delicate for an adjustment to obtain x-ray fringe, and not commonly used in the industry. On the other hand, a scanning tunneling microscope (STM) and an atomic force microscope (AFM) are becoming a powerful and popular tool in surface engineering fields and can be used to obtain "images of atoms" on a regular crystalline surface. Therefore, such crystalline lattice can be used as a "crystalline lattice scale" with sub-nanometer resolution by combining them with STM/AFM. We have shown the feasibility of the crystalline lattice scale using a graphite crystal (highly oriented pyrolytic graphite: HOPG) as the reference scale and a dual-tunneling-unit scanning tunneling microscope as the detector. In this article, we propose a compact absolute length measuring machine (ALMM) with sub-nanometer accuracy and sub-millimeter travel by combining "the crystalline lattice scale" as a fine scale and the phase modulation homodyne laser interferometry (= optical fringe) as a coarse scale.
展开▼
