Optimization of test gratings and their measurement at manufacturing of diffractive optics and conformal correctors

Measurement of parameters of test blazed gratings made on a substrate together with the manufactured element makes possible to avoid measuring and identification of complicated structure in the working area of the multilevel diffractive optical elements or the energy characteristics of the wavefront transformed by the element. This approach allows one to make the inspection process less dependable on the element type. In this case, the profile shape, depth, and period of the test gratings can be optimized to specific task of technological inspection. Measurement of diffraction efficiency of the test blazed gratings in reflected light with visible wavelength makes possible to define parameters of multilevel diffractive optical elements designed for operation in transmission mode for DUV range. The wavelength of probe laser beam and incidence angle should provide a 2π phase jump at diffractive zones boundaries in the test grating structure at measurement in reflection mode. Elimination of disturbing laser radiation reflected from the back side of the substrate during the measurement is carried out by installing the test element on a layer of immersion liquid dropped on surface of a support substrate, which can be made as wedged or single-side polished with ground back side. The problem of the influence of the backward slopes of the blazed test gratings on the measurement of the characteristic curve of the technological process is proposed to be solved by using the sinusoidal form of the test grating profile. In this paper, a method for processing profilograms of such a profile is considered and experimental results obtained at fabrication of conformal correctors by means of proximity half-tone lithography are discussed.