Fabrication Of Diffraction Gratings By Lithography Process

As one of the important optical elements, diffraction gratings have wide application fields in spectral measurement, optical calculation, optical-information processing and so on. To meet the demands of these extensive applications, the performance of the gratings is being gradually improved by increasing the area of gratings, which is also the effective approach to obtain a higher diffractive efficiency. This thesis is to fabricate large-size diffraction gratings by adjusting interference parameters based on laser interference technology, and their diffractive efficiency is tested and analyzed. The results show that the groove depth is one of the main factors which influence on their diffraction efficiencies. So this paper is mainly surrounding on the topic: How to increase the groove depth of gratings. In order to fabricate gratings with a large groove depth, thick Organic-inorganic Hybrid Material (OIHM) photosensitive films were prepared as recording material for lithography process. On the other hand, large groove depths were obtained by chemical etching method on the basis of preparation the inorganic film gratings by double-beam laser interferometric method as well as heat treatment process.At the beginning, optical path which could realize a interference pattern with a period of 833nm and a exposure-field diameter of 30.6mm by adjusting parameters of two-beam laser Interference optical path, such as incident angleθof interferometric beams, beam size R₀ of laser beams, critical divergence angle d of concave lens, focal length f of parallel mirror. Then such gratings were fabricated by using of the above optical path based on successful preparation of ZrO₂ photosensitive gel films. After gold coated, the diffractive efficiency of these gratings was up to 60%. Groove depth of the ZrO₂ inorganic film grating was sharply dropped when the gel film grating was subsequently heat treated at 600℃, which led to the diffractive efficiency to be below 2%. So a new recording material, PMMA/TiO₂ OIHM was explored and synthesized from photosensitive TiO₂ sol and Methyl Methacrylate (MMA) as the starting materials, and Dibenzoyl Peroxide as initiator. The UV and IR spectra indicated that the gel films showed the excellent UV photosensitivity. Then using the UV irradiation method, gratings with a pitch of 250μm and a groove depth over 5.58μm were successfully fabricated.As to high-density gratings fabricated by laser interference, the groove depth could be improved using chemical etching process. ZrO₂ gratings on (100)Si substrate with a pitch of 250μm were etched in KOH and KOH+IPA solutions in the experiment respectively, which led to their groove depth increasing from 0.08μm to over 5μm, and the corrosion results with KOH+IPA solution were better than that of KOH solution. Meanwhile, High-density ZrO₂ gratings on (100)Si substrate with a groove depth less than 50nm were etched in KOH+IPA solution which enabled the groove depth to increase to about 200nm.