Abstract
Laser interference lithography is an attractive method for the fabrication of a large-area two-dimensional planar scale grating, which can be employed as a scale for multi-axis optical encoders or a diffractive optical element in many types of optical sensors. Especially, optical configurations such as Lloyd’s mirror interferometer based on the division of wavefront method can generate interference fringe fields for the patterning of grating pattern structures at a single exposure in a stable manner. For the fabrication of a two-dimensional scale grating to be used in a planar/surface encoder, an orthogonal two-axis Lloyd’s mirror interferometer, which has been realized through innovation to Lloyd’s mirror interferometer, has been developed. In addition, the concept of the patterning of the two-dimensional orthogonal pattern structure at a single exposure has been extended to the non-orthogonal two-axis Lloyd’s mirror interferometer. Furthermore, the optical setup for the non-orthogonal two-axis Lloyd’s mirror interferometer has been optimized for the fabrication of a large-area scale grating. In this review article, principles of generating interference fringe fields for the fabrication of a scale grating based on the interference lithography are reviewed, while focusing on the fabrication of a two-dimensional scale grating for planar/surface encoders. Verification of the pitch of the fabricated pattern structures, whose accuracy strongly affects the performance of planar/surface encoders, is also an important task to be addressed. In this paper, major methods for the evaluation of a grating pitch are also reviewed.
Highlights
A diffraction grating is one of the most important optical components employed in many academic and industrial fields
Some techniques for the fabrication of a diffraction grating based on the laser interference lithography have been reviewed, while focusing on the fabrication of orthogonal two-dimensional gratings to be employed as a scale for multi-axis planar/surface encoder
Lloyd’s mirror interferometers, which are based on the division of wavefront system, have superior stability against the external disturbances, and are suitable for the fabrication of a large-area two-dimensional scale grating that requires long pattern exposure time
Summary
A diffraction grating is one of the most important optical components employed in many academic and industrial fields. It is necessary to establish a method to fabricate two-dimensional grating pattern structures over a wide area since the measurement range of a planar/surface encoder is determined by the size of a scale grating. A method employing a multi-beam optical head and a mosaic-scale grating composed of small two-dimensional diffraction scale gratings aligned in a matrix has been proposed [23–25] This method can expand the in-plane measuring area of the planar/ surface encoder while reducing the influences of the gravitational deformation of the 2D scale grating and its manufacturing cost. The fabrication of grating pattern structures with complex profiles is difficult, the features of mask-less and low-cost grating fabrication of the laser interference lithography are suitable for the fabrication of a large two-dimensional scale grating for multi-axis planar/ surface encoders and other optical sensors [41]. Where I1 and I2 are the intensities of two light beams, and φ is the phase difference between the two light beams at a particular X-position on the substrate that can be expressed by the following equation [62]:
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