Holographic fabrication of two-dimensional scale gratings for surface encoder by using an orthogonal type two-axis Lloyd's mirror interference lithography

In this paper, an orthogonal type two-axis Lloyd’s mirror interference lithography technique was employed to fabricate two-dimensional planar scale gratings for surface encoder application. The two-axis Lloyd’s mirror interferometer is composed of a substrate and two reflective mirrors, which are placed edge by edge perpendicularly. An expanded and collimated beam was divided into three beams by this interferometer, a direct beam and two reflected beam. The unnecessary beam section was blocked by a home designed filter. These two reflected beams interfere with the direct beam respectively, generates perpendicularly cross patterns for forming two-dimensional scale gratings. The two reflected beams also interfere with each other and generate an undesired grating pattern along a 45 degree direction, which influence the pattern uniformity in a certain degree. The undesired grating pattern can be eliminated by polarization modulation, yet which will influence the grating area. Theoretical and experimental study was carefully carried out to evaluate the fabrication quality with and without polarization modulation. Two-dimensional scale gratings with a 1 μm period in X- and Y-directions were achieved by using the constructed experiment system with a 442 nm HeCd laser source. Atomic force microscope (AFM) images and diffraction performances verified that the two-axis Lloyd’s mirror interferometer with a small undesired interference between two reflected beams under an order of nominal value of 0.1 can provide a better fabrication result for scale gratings application.