Displacement measurement of planar stage by diffraction planar encoder in nanometer resolution

This paper presents a novel planar diffraction grating interferometer (PDGI), which is based on the principle of diffractive interferometry. The PDGI is composed of two linear diffraction grating interferometers (LDGIs) arranged in orthogonal configuration for simultaneously measuring the two-dimensional displacement of the planar grid, which is mounted on the bottom surface of an XY stage. It adopts a special design in optical path that can increase the alignment tolerance between the optical head and the grid. The signal process circuit and software are also developed, including the pulse count and phase subdivision. The fabricated planar grid has 55 mm×55 mm size and 1740 line/mm. The resolution can reach to 1 nm. Experimental results on an air bearing XY stage showed that even in the normal laboratory environment the standard deviation of measured values can be controlled to within 15 nm for a long stroke up to 25 mm in both axes.