The most frequently-used design method for diffractive optical elements (DOE) is scalar diffraction theory, but it is unsuitable for the design of sub-wavelength DOE with large diffraction angles. In this paper, we propose a hybrid iterative design method, which effectively utilizes the large-scale global optimization characteristics of scalar diffraction theory and the accuracy of the rigorous coupled wave analysis (RCWA) theory. A 5*7 beam splitter was designed to verify the proposed model. The design resolution is around 200nm with diffraction angle at 55.61°x61.82°. Several initial solutions were obtained by the non-paraxial scalar diffraction theory. Those solutions were later used as the inputs for continuous optimization through the Genetic Algorithm (GA). The RCWA model was used to analyze the diffraction efficiency and uniformity of the beam splitter DOE. All these structures were fabricated by lithography and duplicated by nano-imprinting process. The optical uniformity of 5*7 beam splitter pattern is improved to 46.30% from its initial values which is bigger than 70%. The testing data from RCWA optimized pattern matches with the design value from scatter plot analysis. This provides an effective method for the design of sub-wavelength DOE with large diffraction angle.
Diffractive optical elements developed based on divergent light illumination enable a much broader range of applications compared to those designed based on plane wave approximation model. This is due to their easier module process, lower mass production cost and higher integration reliability. Multi-level phase stages and small design resolution are essential to achieve good design properties, but on the contrary, they increase the complication of simulation and fabrication and hence the gap between them. In this paper, a design performance approaching method using lithography process parameter optimization is proposed and verified in both simulation and fabrication. This method is firstly reported and systematically analyzed in this paper to our knowledge. Basing on 22°*22° divergent light illumination model, a 9*9 spot array pattern with diagonal Field of View 72.34° was designed with eight level phase stages ranging from 0 to 7π/4. Master wafers were fabricated through repeatable stepper lithography and plasma dry etch. Nano-imprinting (NIL) was implemented to duplicate DOE samples. DOE uniformity has been improved from 110.6.% to 90.9.% through process parameters optimization, with better center zero order intensity observed. It’s most probably due to the optimization of phase stage shift from the pattern mis-order in fabrication process. This work provides a new metrology for DOE’s design and fabrication, and is helpful to reduce the overall research and development cost.