Abstract
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 (X- and Y-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 beams, projected onto the substrate, X- and Y-mirrors, respectively. The unexpected beam sections having twice reflected off the mirrors were blocked by a filter. The remaining two reflected beams interfered with the direct beam on the substrate, generating perpendicularly cross patterns thus forming two-dimensional scale gratings. However, the two reflected beams undesirably interfere with each other and generate a grating pattern along 45-degree direction against the two orthogonal direction, which influence the pattern uniformity. Though an undesired grating pattern can be eliminated by polarization modulation with introduction of waveplates, spatial configuration of waveplates inevitably downsized the eventual grating, which is a key parameter for grating interferometry application. For solving this problem, 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 He-Cd laser source. Atomic force microscopy (AFM) images and the result of diffraction performances demonstrated that the orthogonal type two-axis Lloyd’s mirror interferometer can stand a small order undesired interference, that is, a degree of orthogonality between two reflected beams, denoted by γ, no larger than a nominal value of 0.1.
Highlights
Planar encoders play a key role in precision positioning of linear stages due to their high resolution, high robustness, and relatively low cost [1,2,3,4,5,6,7,8,9]
The exposure can be based on an amplitude-division type two-beam Laser interference lithography (LIL), called two beam two exposures (TBTE) or a wavefront-division type Lloyd’s mirror type LIL, called one beam two exposures (OBTE) [11]
According to about polarization modulation, we used halfhalf-wavelength plates to realize the optimal combination of initial polarization status
Summary
Planar encoders play a key role in precision positioning of linear stages due to their high resolution, high robustness, and relatively low cost [1,2,3,4,5,6,7,8,9]. Laser interference lithography (LIL), engine machining, and imprinting are three typical methods for fabrication of the scale grating. Sci. 2018, 8, 2283 convenient and cost-effective because only a coherent laser source is required and neither sophisticated precision positioning system nor expensive photolithography equipment and mask preparation are required. For fabrication of a 2D planar grating, according to the exposure times, the LIL process can be separated into two categories, double exposure and single exposure. A big challenge existing in the method of the two-exposure process is that the grating structures generated in the first exposure will be influenced by the background light in the second exposure. Diffraction efficiencies of both the negative and positive diffraction beams will not be consistent
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