Micro projection lithography inside deep trenches using microlens on a mask

This paper presents a micro projection lithography process which enables the patterning of photoresist at the bottom surface of deep cavities, using microlenses formed directly on the mask. Arrays of cylindrical plano-convex microlenses on conventional photomasks have been fabricated by a photoresist melting and reflow technique, thereby augmenting the proximity lithography performance of these masks. Patterning results inside deep trenches have been analyzed. A maximum pattern width reduction of 64% for a 60 mum rectangular pattern at the bottom of a 216 mum-deep trench has been achieved with a microlens with a focal length of 267 mum. The effect of the focal length variation on the pattern size variation has been analyzed.