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Micro projection lithography using microlens on a mask

A micro projection lithography process has been implemented using a photomask having arrays of cylindrical or spherical plano-convex microlenses formed by thermal reflow in proximity exposure mode. This approach provides a practical means to generate patterns inside deep cavities without proximity-based pattern size increases. The generated pattern size can be controlled by controlling the focal depth of the microlens. A maximum pattern size reduction of 62% has been achieved at the bottom of a 216 µm deep trench with a cylindrical microlens having a focal length of 254 µm for a 60 µm square pattern. The effect of the microlens on the lithography process, including the relationship between the focal depth, exposure dose and pattern size, has been analyzed.