The ability to reduce the feature dimensions of silicon-based transistors has enabled the semiconductor industry to enhance the performance, increase the speed, and reduce the price of integrated circuits for the past 55 years. In the late 1960s, Gordon Moore predicted that the transistor density for an integrated circuit would double every 18 months (Moore's law). So far, the semiconductor industry has been able to follow Moore's law by enhancing the resolution attributes of photolithography tools. A photolithography tool is an optical system that projects the image of a mask (representing a layer of an integrated circuit) onto a wafer that has been coated with a light-sensitive material (resist). After exposure, the resist is developed to reveal a pattern which is transferred to the wafer's surface through etching techniques. Semiconductor analysts have theorized that by 2010 photolithography tools with 157-nm light sources will approach their theoretical resolution limit (120 nm).