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The ejection of material from a solid or liquid surface following the impact of energetic ions, atoms, or molecules. Sputtering is the basis of a large variety of methods for the synthesis and analysis of materials.
Sputtering can be classified according to the mode of energy loss of the incident (primary) particle. Nuclear stopping involves billiard ball-like atomic collisions in which a significant momentum transfer occurs; it dominates for incident ion energies below about 1–2 keV per nucleon. Electronic stopping involves collisions in which little momentum is transferred, but significant electronic excitation is caused in the target; it dominates for energies above about 10 keV per nucleon.
Sputtering has also been classified into physical and chemical sputtering. Physical sputtering involves a transfer of kinetic energy from the incident particle to the surface atoms leading to ejection, while chemical sputtering occurs when the incident species react chemically with the target surface leading to the formation of a volatile reaction product which evaporates thermally from the surface.
Sputtering of complex materials—metal alloys, inorganic and organic compounds and polymers, and minerals—can produce complex results. The relative efficiencies with which different elemental species are ejected following ion impact can differ, giving rise to preferential sputtering. When preferential sputtering occurs, the species sputtered with the lower efficiency accumulates to a higher concentration at the surface. Subsurface collisions of the incident ion cause atomic motion leading to atomic mixing of surface and subsurface layers over the ion penetration depth. Chemical bonds can be broken, and sometimes new bonds can be formed. Sputtering of solids which have multiple phases, or which are polycrystalline, leads to the development of surface roughness due to the differences in sputtering yields between different regions. See Ion beam mixing
Sputtering is widely used in the manufacture of semiconductor devices; sputter deposition is used to deposit thin films with a high degree of control by sputtering material from a target onto a substrate; sputter etching is used to remove unwanted films in a reversal of this process. Reactive ion etching is a chemical sputtering process in which chemically active sputtering species form volatile compounds with the target material leading to significantly higher etch rates and great selectivity. For example, fluorine-containing compounds etch silicon rapidly by forming volatile silicon tetrafluoride but do not etch aluminum or other metals used to make electrical interconnections between devices on a semiconductor chip because the metal fluorides are involatile. Sputter etching and reactive ion etching have the useful advantage of being anisotropic—that is, they etch only in one direction so that very fine surface features can be delineated. See Integrated circuits
In materials characterization, sputtering is used to remove surface material controllably, allowing in-depth concentration profiles of chemical composition to be determined with a surface-sensitive sampling technique.