page_title: Chemistry of photolithography
section_title: Silicon wafer
hierarchical_section_title: Chemistry of photolithography / Silicon wafer
context_page_description: Photolithography is a process in removing select portions of thin films used in microfabrication. Microfabrication is the production of parts on the micro- and nano- scale, typically on the surface of silicon wafers, for the production of integrated circuits, microelectromechanical systems, solar cells, and other devices. Photolithography makes this process possible through the combined use of hexamethyldisilazane, photoresist, spin coating, photomask, an exposure system and other various chemicals. By carefully manipulating these factors it is possible to create nearly any geometry microstructure on the surface of a silicon wafer. The chemical interaction between all the different components and the surface of the silicon wafer makes photolithography an interesting chemistry problem. Current science has been able to create features on the surface of silicon wafers between 1 and 100 μm.
context_section_description: Silicon wafers are cut from a solid ingot of nearly-pure (99.9999999%) silicon. This is done through the process of Czochralski growth, which is diagramed in the adjacent image, and produces a single intact diamond cubic silicon crystal. Due to its structure, monocrystalline silicon is anisotropic, which gives it different structural and electrical properties in different plane directions. Using miller indexes to denote the different plane orientations, the (1,0,0) and the (1,1,1) faces are typically used in silicon wafers (see image). The silicon ingot is oriented and cut along one these planes to expose that surface for processing through photolithography. The reason to use either of these planar faces depends on the application for which the silicon wafer will be used, or how it will be processed. At any rate, this depends of the use of etchants, photoresist, and acids to treat the surface, and the chemical interactions of these chemicals with the crystal surface depends on the surface properties of that crystalline face. A table on the right depicts the surface energies, atomic densities, and interatomic spacing of the three planes for a silicon crystal.
caption: Silicon ingot formation using the Czochralski process