A Reflective-Type Heterodyne Grating Interferometer for Three-Degree-of-Freedom Subnanometer Measurement

A heterodyne grating interferometer (HTGI) able to simultaneously measure the three-axis translational displacements of ultraprecision motion stages with a subnanometer resolution is demonstrated. The HTGI is composed of two main parts: a reflective planar grating with a uniform micron-scale period in two orthogonal directions and a reading head. Thanks to the reflective grating, the reading head design manages to make main optical paths common and located on the same side of this compact structure, potentially resulting in low Abbe errors and environmental variations. Relying on a laboratory-made dual-frequency laser source at 780 nm, two heterodyne interferometric measurement modules for X- / Y- direction motions and Z- direction motion were realized based on first-order and zeroth-order diffractions, respectively. Experiments show that the proposed HTGI, in all three axes, can distinguish a 0.5 nm step or even better, perform an up to $2.5\times10$ -5 linearity in a range of $80 \mu \text{m}$ , and hold a 5 nm stability within 300 s. These performances confirm that the proposed HTGI can greatly benefit nanoscience and technology, where multiaxis ultraprecision positioning is required, such as wafer stage in next-generation lithography machine, and so forth.