Investigating fabrication errors for diffractive optical elements

As the use of DOEs has become ever more popular, there has been a concurrent increase in the development of the design algorithms used to optimise their phase profiles. The earliest design methods claimed efficiencies of around 75% and an image spot intensity variation of +/- 15%. Methods used today can give efficiency percentages in the high 90s and non-uniformities below 2%. As developments made in the design algorithms continue it increasingly becomes the case that the major factor contributing to losses in efficiency and increases in non-uniformity are not the ability of the algorithm to optimise the phase profile but the errors introduced by the fabrication process. In this paper we simulate the effects of misalignment and feature rounding on the quality of the output intensity of 7 different fan-out gratings. From these simulations we observe that the affect of misalignment on efficiency is generally greater for masks with deeper etches, although the extent of the drop in efficiency can be influenced by the direction of misalignment. Non-uniformity is less consistently affected, in some cases the pi level is dominant, in others it is the pi/2 level and there is often strong asymmetry between negative and positive misalignment. Study of feature rounding produces results which, as one might expect, indicate levels with deeper etches have a greater influence on the drop in efficiency and increased non-uniformity.