The major etching processes of a large-aperture multilayer diffraction grating, including the uniformity of the ion beam current along the major axis and the on-line measurement of the diffraction intensity distribution are described. A largeaperture ion beam etcher with radio frequency linear source has been developed to fabricate large-aperture diffractive optical elements. The length with ±5.1% uniformity of the ion beam current distribution along the major axis is 30 cm. A series of multilayer diffraction gratings were etched successfully by using this etcher with CHF 3 chemistry. Multilayer diffraction gratings on a 80 mm×150 mm BK7 substrate etched for laser systems are shown. The grating exhibits an averaged diffraction efficiency about 96% at TE polarization of 1 064 nm light viewed at Littrow angle.
DOE is often produced by lithography and ion beam etching. The etching depth error directly affects the diffraction performance of DOE. The uniformity of ion beam etching depth is particularly important for large-sized DOEs in that errors by ion beam etching uniformity would result in an obvious aberrant spot of intensity in the focal area of DOE, which consumedly reduces the uniformity of target field in uniform illumination. On the basis of the KZ-400 ion beam etching equipment the method of improving DOE ion beam etching uniformity is investigated. The step-by-step method is used to improve the uniformity of ion beam etching, in which the etching time and location are adjusted. Experimental result shows that in the range of 190mm along the major axis of ion beam source the etching uniformity of DOE increases from ±5% to ±1.3%.
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