Optical lithography is a photographic process by which a light-sensitive polymer, called a photoresist, is exposed and developed to form 3D relief images on the substrate. Each pattern being printed on the wafer is aligned to the previously formed patterns and slowly the conductors, insulators, and selectively doped regions are built up to form the final device. To build the complex structures that make up a transistor and the many wires that connect the millions of transistors of a circuit, lithography and pattern transfer steps are repeated at least 10 times, but more typically are done 20 to 30 times to make one circuit.
In the case of semiconductor lithography, our stones are silicon wafers and our patterns are written with a light-sensitive polymer called photoresist. It means quite literally writing on stones. The word lithography comes from the Greek lithos, meaning stones, and graphia, meaning to write. Fundamental to all of these processes is lithography, i.e., the formation of three-dimensional (3D) relief images on the substrate for subsequent transfer of the pattern to the substrate. By creating structures of these various components, millions of transistors can be built and wired together to form the complex circuitry of a modern microelectronic device. Selective doping of various regions of silicon allows the conductivity of the silicon to be changed with the application of voltage. Films of both conductors (such as polysilicon, aluminum, and more recently copper) and insulators (various forms of silicon dioxide, silicon nitride, and others) are used to connect and isolate transistors and their components. In general, the various processes used to make an IC fall into three categories: film deposition, patterning, and semiconductor doping. The fabrication of an integrated circuit (IC) requires a variety of physical and chemical processes performed on a semiconductor (e.g., silicon) substrate.
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