Molecular beam epitaxy
Molecular beam epitaxy (MBE) is an epitaxy method for thin-film deposition of single crystals. It was invented in the late 1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho. MBE is widely used in the manufacture of semiconductor devices, including transistors for cellular phones and WiFi.
Molecular beam epitaxy takes place in high vacuum or ultra-high vacuum (10−8 - 10−12Torr). The most important aspect of MBE is the deposition rate (typically less than 3,000 nm per hour) that allows the films to grow epitaxially. These deposition rates require proportionally better vacuum to achieve the same impurity levels as other deposition techniques. The absence of carrier gases as well as the ultra high vacuum environment result in the highest achievable purity of the grown films.
In solid source MBE, elements such as gallium and arsenic, in ultra-pure form, are heated in separate quasi-Knudsen effusion cells or electron beam evaporators until they begin to slowly sublime. The gaseous elements then condense on the wafer, where they may react with each other. In the example of gallium and arsenic, single-crystal gallium arsenide is formed. When evaporation sources such as copper or gold are used, the gaseous elements impinging on the surface may be adsorbed (after a time window where the impinging atoms will hop around the surface) or reflected. Atoms on the surface may also desorb. Controlling the temperature of the source will control the rate of material impinging on the substrate surface and the temperature of the substrate will affect the rate of hopping or desorption. The term "beam" means that evaporated atoms do not interact with each other or vacuum chamber gases until they reach the wafer, due to the long mean free paths of the atoms.
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