1.1 Optical Lithography
Complex circuitries of modern microelectronic devices are created by building and wiring millions of transistors together. At the heart of this technology is optical lithography. Optical lithography technology is similar in concept to printing, which was invented more than 3000 years ago . In optical lithography systems, a mask is used as the template, on which the target circuit patterns are carved. A light-sensitive polymer (photoresist) coated on the semiconductor wafer is used as the recording medium, on which the circuit patterns are projected. Light is used as the writing material, which is transmitted through the mask, thus optically projecting the circuit patterns from the mask to the wafer. The lithography steps are typically repeated 20–30 times to make up a circuit, where each underprinting pattern must be aligned to the previously formed patterns. After a lengthy lithography process, a complex integrated circuit (IC) structure is built from the interconnection of basic transistors. Moore's law, first addressed by Intel cofounder G. E. Moore in 1965, describes a long-term trend in the history of computing hardware. Moore's law predicted that the critical dimension (CD) of the IC would shrink by 30% every 2 years. This trend has continued for almost half a century and is not expected to stop for another decade at least. As the dimension of IC reduces following Moore's law, optical lithography has become a critical driving ...