Additional Evaporative Vacuum Deposition Techniques

Other deposition processes that use the vacuum environment to control contamination and provide a long mean free path for vaporized material can be defined as vacuum deposition processes:

• Molecular beam epitaxy

• Ionized cluster beam deposition

• Ion beam sputter deposition

• Jet vapor deposition

Molecular Beam Epitaxy. Probably the most sophisticated vacuum deposition system/process is that used for molecular beam epitaxy (MBE), or vapor phase epitaxy (VPE) (Ref 103). Molecular beam epitaxy is used to form epitaxial films of semiconductor materials. A vacuum environment of better than 0.13 pPa (10-9 torr) is used to deposit atoms from carefully rate-controlled vapor sources (Knudsen-type sources) (Ref 9). The MBE deposition chamber can also contain a wide range of analytical instruments for in situ analysis of the growing film. These analytical techniques include methods for measuring crystal parameters, such as reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED).

Gaseous or vaporized metal-organic compounds can also be used as the source of film material in MBE. The molecular species are decomposed on the hot substrate surface to provide the film material. The use of metal-organic precursor chemicals is called metal-organic molecular beam epitaxy (MOMBE) (Ref 104). MOMBE is used in low-temperature formation of compound semiconductors with low defect concentrations.

Ionized Cluster Beam Deposition. Clusters of atoms (approximately 1000) can be charged and accelerated to such high velocities that the average kinetic energy of the depositing atom is greater than that associated with thermal vaporization. This is the basis of the ionized cluster beam (ICB) deposition process (Ref 105, 106). There is some controversy about how and if clusters are formed and what actually happens at the surface in this deposition process (Ref 107, 108, 109). There are numerous technical papers detailing the advantages of the ICB deposition process in forming epitaxial films at low temperatures that are defect free, have good adhesion, and have good surface coverage.

Ion-Beam Sputter Deposition. Ion guns can be used to sputter deposit films in a vacuum environment (Ref 110). This technique has limited applications because high-energy neutrals reflected from the sputtering target can bombard the growing film during deposition, giving rise to uncontrolled film property variations.

Jet Vapor Deposition. In the "jet vapor deposition" process, evaporated atoms/molecules are "seeded" in a supersonic jet flow of inert carrier gas into a rapidly pumped vacuum chamber (Ref 111, 112). The jet transports the atoms/molecules to the substrate surface, where they are deposited. The vapor source can be in the form of thermal evaporation or sputtering and is located in the jet nozzle. The deposition chamber pressure is about 130 Pa (1 torr) and is pumped using high-capacity mechanical pumps.

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