Fabrication of arrays of two-dimensional micropatterns using microspheres as lenses for projection photolithography

This letter demonstrates the use of an array of transparent microspheres in forming repetitive, micrometer-scale patterns in photoresist, starting from masks with centimeter-scale patterns. A transparent microsphere with diameter d1.5 μm acts as a lens and reduces centimeter-scale images into micrometer-scale images on its image plane. A planar array of microspheres projects the image of an illuminated mask onto a corresponding array of micropatterns on their common image plane. We have prepared arrays of polystyrene microspheres (d=1.5–10 μm) embedded in a transparent membrane to generate repetitive patterns in photoresist, and have transferred the resulting patterns into metal films by liftoff. The optical system of this technique is related to that used in conventional projection photolithography, but differs in that the lens that accomplishes size reduction is positioned within 10 μm of the photoresist. The microspheres generate uniform patterns over an area of ∼2 cm2, using a mask with area ∼25×25 cm2 illuminated with a white light source. This method can generate submicron features either within a micropattern or between neighboring patterns.