A resist strip is the final operation in the lithographic process, after the resist pattern has been transferred into the underlying layer. The general sequence of processing steps for a typical photolithography process is as follows: substrate preparation, photoresist spin coat, prebake, exposure, post-exposure bake, development, and postbake. This binary pattern is needed for pattern transfer since the parts of the substrate covered with resist will be protected from etching, ion implantation, or other pattern transfer mechanism. Thus, the final resist pattern is binary: parts of the substrate are covered with resist while other parts are completely uncovered. In general, the ideal photoresist image has the exact shape of the designed or intended pattern in the plane of the substrate, with vertical walls through the thickness of the resist. Optical lithography is basically a photographic process by which a light sensitive polymer, called a photoresist, is exposed and developed to form three-dimensional relief images on the substrate. Although lithography is certainly not the only technically important and challenging process in the IC manufacturing flow, historically, advances in lithography have gated advances in IC cost and performance. Obviously, one must carefully understand the trade-offs between cost and capability when developing a lithography process. Second, lithography tends to be the technical limiter for further advances in feature size reduction and thus transistor speed and silicon area. First, due to the large number of lithography steps needed in IC manufacturing, lithography typically accounts for about 30 percent of the cost of manufacturing. The importance of lithography can be appreciated in two ways. 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 etch 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 (also called photolithography) our stones are silicon wafers and our patterns are written with a light sensitive polymer called a 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 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 allow 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. Semiconductor Lithography (Photolithography) - The Basic Process
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