ABSTRACT
This chapter discusses the advancements in the area of optical photomasks. One sub-group requires etching into the substrate of a chrome-on-glass (COG) mask known as alternating phase shift mask (PSM), and the other sub-group uses different materials to induce phase shift in light and known as halftone PSM. The chapter describes the basic structure of photomask with an example of a generic COG mask and we showed how those masks have evolved into today's advanced masks like PSMs, and masks with OPCs and sub-resolution assist features. Considering the time factor, there are lot of dead times in the stage movement in the first case but in the second case the scale is in a continuous motion and the system gives considerable higher throughput. The electrons used for writing the pattern are extracted from their source mounted near the top of the column where the column is furnished with a set of electrostatic lenses and deflectors.
