Dual monopole exposure strategy to improve extreme ultraviolet imaging

Abstract

Background: In extreme ultraviolet lithography, the printable feature density is limited by stochastic defectivity, which can be reduced by increasing the optical contrast. The photomask induces pole-specific aerial image offsets. Consequently, illumination settings with multiple poles lead to contrast loss and focus offsets between different features.Aim: We aim to mitigate the contrast loss and best focus offsets between different features.Approach: Illumination was decomposed into monopoles. Each monopole was exposed separately using a fraction of the total dose. Each exposure was shifted by its pole-specific image offset to mitigate 3D mask effects.Results: Single monopoles mitigate contrast loss and best focus shifts, but in defocus, they suffer from aerial image shifts and distortions. Multiple aligned monopole exposures conserve these advantages but mitigate the problems in defocus. Because each monopole is exposed with only a fraction of the dose, the throughput penalty is limited to the scanner overhead.Conclusions: A multiple monopole exposure scheme can increase contrast, align the best foci, and mitigate single monopole exposure constraints. Additionally, it offers an improved pattern placement control through dose control knobs.