general, such as microelectromechanical systems. However, photolithography cannot be used to produce masks on surfaces that are not perfectly flat; and, like all chip manufacturing processes, it requires extremely clean operating conditions. History The root words photo, litho, and graphy all have Greek origins, with the meanings 'light', 'stone' and 'writing' respectively. As suggested by the name compounded from them, photolithography is a printing method (originally based on the use of limestone printing plates) in which light plays an essential role. In the 1820s, Nicephore Niepce invented a photographic process that used Bitumen of Judea, a natural asphalt, as the first photoresist. A thin coating of the bitumen on a sheet of metal, glass or stone became less soluble where it was exposed to light; the unexposed parts could then be rinsed away with a suitable solvent, baring the material beneath, which was then chemically etched in an acid bath to produce a printing plate. The light-sensitivity of bitumen was very poor and very long exposures were required, but despite the later introduction of more sensitive alternatives, its low cost and superb resistance to strong acids prolonged its commercial life into the early 20th century. In 1940, Oskar Süß created a positive photoresist by using diazonaphthoquinone, which worked in the opposite manner: the coating was initially insoluble and was rendered soluble where it was exposed to light. In 1954, Louis Plambeck Jr. developed the Dycryl polymeric letterpress plate, which made the platemaking process faster. In 1952, the U.S. military assigned Jay W. Lathrop and James R. Nall at the National Bureau of Standards (later the U.S. Army Diamond Ordnance Fuze Laboratory, which eventually merged to form the now-present Army Research Laboratory) with the task of finding a way to reduce the size of electronic circuits in order to better fit the necessary circuitry in the limited space available inside a proximity fuze. Inspired by the application of photoresist, a photosensitive liquid used to mark the boundaries of rivet holes in metal aircraft wings, Nall determined that a similar process can be used to protect the germanium in the transistors and even pattern the surface with light. During development, Lathrop and Nall were successful in creating a 2D miniaturized hybrid integrated circuit with transistors using this technique. In 1958, during the IRE Professional Group on Electron Devices (PGED) conference in Washington, D.C., they presented the first paper to describe the fabrication