Techniques for formation of the surface profile of diffractive optical elements

We give a review of the findings of an investigation into a photolithographical method of fabricating diffractive optical elements (DOEs), based on the use of only one photomask with two transmittance gradations. The binarization of the continuous transmission function of the diffractive element is conducted using a technology of half-toning of continuous-tone images and the exposure of the light-sensitive medium is performed using an incoherent spatial filtration in combination with a conventional photolithographical process. We analyze how the DOE diffractive efficiency depends on photomask parameters, binarization methods and fabrication errors. Comparison of the characteristics of the method proposed with the well-known multilevel method is made. Peculiarities of fabricating DOEs with continuous phase profile resulting from the projection, contact, and X-ray lithography are discussed. The results of experimental research are given. A possibility of fabricating DOEs with an 80% diffractive efficiency using only one photomask is demonstrated. (C) 1998 Elsevier Science Ltd. All rights reserved.