Fabrication of scale gratings for surface encoders by using laser interference lithography with 405 nm laser diodes

This paper presents a cost-effective fabrication method of grating structures with a period of sub-μm for surface encoders. By employing an inexpensive and compact blue laser diode, which has a wavelength of 405 nm and is used in Blu-ray disc drives, a Lloyd's mirror interferometer has been developed to carry out laser interference lithography for the fabrication of grating structures. Experimental results showed that the developed fabrication system is capable of fabricating grating structures with a pitch of 570 nm. The relative pitch deviation is confirmed to be approximately 1%, which meets the requirement for the scale grating used in a surface encoder. The influence of the limited coherence length of the laser diode on the fabrication area of the grating structures has also been investigated. To expand the fabrication area of the grating structure, a laser source, which has a wavelength of 405 nm and a longer coherence length with a mode selection cavity, is employed in the fabrication system. It has been verified by experiments that the fabrication area of the sub-μm periodic grating structures can be expanded to 300 mm² with the laser source.