Heterodyne Three-Degree-of-freedom Grating Interferometer for Ultra-Precision Positioning of Lithography Machine

In this paper, we proposed a new reflection type heterodyne three-degree-of-freedom (three-DOF) grating interferometer for ultra-precision positioning of the worktable of the lithography machine. The grating interferometer is based on a 780 nm wavelength dual-frequency laser that can generate a 40 kHz beat signal through two acoustic-optical modulators (AOM). The reflected lights from the grating and the mirror are used to obtain the heterodyne interference signal in the z-direction, and the four beams of diffracted light from the two-dimensional planar grating are used to obtain the heterodyne interference signal in the x- and y-directions. By comparing the phase of the reference signal, the phase changes in the x-, y-, and z- directions are calculated to derive the three-DOF displacement. Based on optical subdivision and electronic subdivision, the analysis result shows that the resolutions in the x- and y-directions are 0.069 nm, and in the z-direction is 0.108 nm, providing a nanometer resolution for all three axes. In the experiment, the z-direction measurement optical path was constructed to test the static stability of the zero-crossing point of the heterodyne interference signal and the straightness of the z-direction displacement. The preliminary experiment result verified that this interferometer can simultaneously provide the static stability of the zero-crossing point of ±0.1 nm and high straightness. Due to the common optical path design of x-, y- and z-directions, the heterodyne three-DOF grating interferometer structure can be compact and allow a high-performance uniformity of the three axes.