A sub-nanometric three-axis surface encoder with short-period planar gratings for stage motion measurement

A three-axis surface encoder was developed for stage motion measurement with sub-nanometric resolutions. The surface encoder was composed of a scale XY planar grating with X- and Y-directional periodic grating structures and an optical sensor head for reading the grating structures. A reference XY planar grating, which had the same periodic structures as those of the scale XY grating, was employed in the optical reading head. Four sets of interference signals, which were generated by superimposition of the X and Y-directional ±1 order diffracted beams from the two gratings, were employed for evaluation of the X-, Y- and Z-directional displacements of the optical sensor head with respect to the scale grating. The X- and Y-directional periodic structures of the gratings directly acted as the scale graduations for the X- and Y-directional displacement measurements, while the wavelength of the laser beam acted as the graduation period for the Z-directional displacement measurement. X- and Y-directional rectangular structures with a short period of 1 μm were designed and fabricated by the light lithography based on two beams interference for improvement of the resolutions of the surface encoder for X- and Y-directional position measurement. An optical sensor head was designed and constructed for reading the short period gratings. Experiments were carried out to confirm the basic performances of the three-axis surface encoder.