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Electronics - Integrated Circuits

substrate surface silicon impurities

An integrated circuit consists of tens of thousands of transistors and other circuit elements that are fabricated in a substrate of inert material. That material can be ceramic or glass for a film-integrated circuit or silicon or gallium-arsenide for a semiconductor integrated circuit (SIC). These circuits are small pieces or chips that may be 0.08–0.15 sq in long (2–4 sq mm long). Designers are able to place these thousands of components on a chip by using photolithography to place the components and minute conducting paths in the proper patterns for the purpose of each type of circuit. Many chips are made simultaneously on a 4-sq-in (10-sq-cm) wafer.

Several methods are used to introduce impurities into the silicon in the planar process. A mask with some regions isolated is placed over the surface or plane of the wafer, and the surface of the silicon is altered or treated to modify its electrical character. Crystals of silicon are grown on the substrate in a process called epitaxy; another method, thermal oxidation, grows a film of silicon dioxide on the surface that acts as a gate insulator. During solid-state diffusion, impurities diffused as a gas or spread in a beam of ions are distributed or redistributed in regions of the semiconductor. The number of impurities diffused into the crystal can be carefully controlled so the movement of electrons through the chip will also be specific. Coatings can also be added by chemical vapor deposition, evaporation, and a method called sputtering used to deposit tungsten on the substrate; the results of all these methods are coatings on the substrate or disturbed surfaces of the substrate that are only atoms thick. Etching and other forms of lithography (using electron beams or x rays) are also used to pattern the wafer surface for the interconnection of the surface elements.

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