ABSTRACT
Photolithography is the engine that empowered microelectronics and semiconductor industry for more than 50 years. Photolithography allows very complex micro- and nanostructures to be built by copying a pattern from a photomask to a wafer. Photolithography is the key enabling technology (KET) behind the powerful concept of shrinkage, also referred to as die shrink, the ability to reduce the minimum feature size of transistors, electronic wires and other components of a microchip from some 50 microns (µm) in the early 1960s to some tens of nanometers today. Die shrink allows more chips on a wafer to be manufactured, thereby reducing manufacturing costs, minimizing power consumption, and improving the performance in terms of speed, storage capacity, and customer convenience. In a photolithographic process, a thin layer of photosensitive resist is coated on a wafer, the photomask is illuminated by light, and the light propagates from the mask to the wafer surface, transferring the pattern from the mask onto the resist. As shown schematically in Figure 5.1, there are two different ways to accomplish the pattern transfer by light propagation. Light could propagate in free space, which is referred to as shadow printing lithography; or through an imaging system, which is referred to as projection lithography.
