Since the planar processing for integrated circuits (IC) manufacturing started nearly half century ago, optical lithography, or often called photolithography, has become the convenient choice of making planar microstructures. In optical lithography, a mask or photomask, also called reticle, is imaged onto a flat substrate surface coated with a thin layer of polymer material called photoresist. The photon energy is focused into the photoresist, causing polymer chain scission or cross-linking. The mask pattern is then delineated into the photoresist after development.