Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) "chips" such as computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and physico-chemical processing steps (such as thermal oxidation, thin-film deposition, ion-implantation, etching) during which electronic circuits are gradually created on a wafer typically made of pure single-crystal semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications. The fabrication process is performed in highly specialized semiconductor fabrication plants, also called foundries or "fabs", with the central part being the "clean room". In more advanced semiconductor devices, such as modern 14/10/7 nm nodes, fabrication can take up to 15 weeks, with 11–13 weeks being the industry average. Production in advanced fabrication facilities is completely automated and carried out in a hermetically sealed nitrogen environment to improve yield (the percent of microchips that function correctly in a wafer), with automated material handling systems taking care of the transport of wafers from machine to machine. Wafers are transported inside FOUPs, special sealed plastic boxes. All machinery and FOUPs contain an internal nitrogen atmosphere. The insides of the processing equipment and FOUPs is kept cleaner than the surrounding air in the cleanroom. This internal atmosphere is known as a mini-environment. Fabrication plants need large amounts of liquid nitrogen to maintain the atmosphere inside production machinery and FOUPs, which are constantly purged with nitrogen. Feature size A specific semiconductor process has specific rules on the minimum size and spacing for features on each layer of the chip. Normally a new semiconductor processes has smaller minimum sizes and tighter spacing. In some cases this allows a simple die shrink of a currently produced chip design to reduce costs, improve performance, and increase transistor density (number of transistors per square millimeter) without the expense of a new design. Early semiconductor processes had arbitrary names such as HMOS III, CHMOS V. Later each new generation process became known as a technology node or process node, designated by the process’s minimum feature size in nanometers (or historically micrometers) of the process's transistor gate length, such as the "90 nm process". However, this has not been the case since 1994, and the number of nanometers used to name process nodes (see the International Technology Roadmap for Semiconductors) has