SPIE Advanced Lithography + Patterning
Immersion interference lithography for realizing a large slant angle beyond the limit
1Ushio Inc. (Japan)
In holographic systems such as AR and VR devices, the holographic grating is known to increase the efficiency by providing a slant angle. The Interference lithography is essential for the high-quality production of slant gratings. We have previously reported the development of an interference lithography system, equipped with two rotating optical arms and high-precision wafer stage for producing gratings with arbitrary slant angles.
However, there is a restriction on the angle of the slant structure fabricated by conventional dry interference lithography. The angle of inclined beams of exposure is decreased in photoresist due to the refraction from air with a refractive index of 1. Interference lithography with a pitch of 300 nm cannot achieve a slant angle of more than 22 degrees due to the geometrical limit of rays. We report on a novel system which combines a prism and immersion lithography to achieve a larger slant angle.
The apparatus consists of three components. The first is exposure optics used to generate interference fringes. The incident angle of beams can be controlled from 75 to -60 degrees. The second is, a wafer stage to move a substrate of 200 mm in diameter precisely. The wafer stage is controlled with an accuracy of 1 nm to achieve patterning of the entire substrate. The third is an immersion prism over a wafer. The prism is placed on top of the wafer, and the space between the prism and the wafer is filled with liquid. The beam incident on the prism reaches the inside of the resist without being significantly refracted by the prism-liquid interface and the top surface of the photoresist. Large inclination of rays inside the resist can be realized due to the small refraction angle. Therefore, a slant angle of 40 degrees can be realized with interference lithography at a pitch of 300 nm. We will present its mechanism and processing examples.
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