Grayscale lithography by a polymer photomask doped with laser dye

Abstract

This article reports on the development of a novel grayscale photomask which operates on the basis of differential absorption of ultraviolet (UV) light in photoabsorbing material. The developed photomask is made of a patterned, moldable polymer (polydimethylsiloxane) doped with a laser dye (Coumarin 314). The doped polymer functions as photoabsorbing material. Due to the moldable nature of polydimethylsiloxane, micro- and nanostructures can be created on its surface from a complementary mold relief. By adjusting the thickness of patterns formed on the photomask, concentration of the dye in the polymer, and UV exposure dose, a multitude of unique multidimensional microstructures can be fabricated with desired geometries and dimensions. Using the developed polymer photomask with a standard UV source, three-dimensional microstructures with different heights have been formed in positive photoresist by a single UV exposure step. This method is inexpensive compared to other grayscale lithography techniques and relatively easy to implement due to its process simplicity.