Atomistic Deposition

The microstmcture and imperfection content of coatings produced by atomistic deposition processes can be varied over a very wide range to produce stmctures and properties similar to or totally different from bulk processed materials. In the latter case, the deposited materials may have high intrinsic stress, high point-defect concentration, extremely fine grain size, oriented microstmcture, metastable phases, incorporated impurities, and macro-and microporosity. AH of these may affect the physical, chemical, and mechanical properties of the coating. 

Thin films formed by atomistic deposition techniques are unique materials that seldom have handbook properties. Properties of these thin films depend on several factors (4), including substrate surface condition, the deposition process used, details of the deposition process and system geometry, details of film growth on the substrate surface, and post-deposition processing and reactions. For some appHcations, such as wear resistance, the mechanical properties of the substrate is important to the functionaHty of the thin film. In order to have reproducible film properties, each of these factors must be controUed. 

An atomistic deposition process is one in which the overlay material is deposited atom-byatom. The resulting film can range from single crystal to amorphous, fully dense to less than fully dense, pure to impure, and thin to thick. Generally the term thin film is applied to layers which have thicknesses on the order of a micron or less (1 micron = 10 meters) and may be as thin as a few atomic layers. Thicker deposits are called coatings. The term thick film is usually not used for thick atomistically deposited vacuum deposits as that term is used for paint-on, fire-on types of deposition. 

Often the properties of thin films are affected by the properties of the underlying material (substrate) and can vary through the thickness of the film. Thicker layers are generally called coatings. An atomistic deposition process can be done in a vacuum, plasma, gaseous, or electrolytic environment. 

Physical vapor deposition processes (often just called thin film processes) are atomistic deposition processes in which material is vaporized from a solid or liquid source in the form of... 

Deposited thin films and coatings generally have unique properties compared to the material in bulk form and there are no handbook values for such properties. There have been many books and articles on film deposition and film properties but generally these treatments do not emphasize the importance of the substrate surface and deposition conditions on the film properties. The properties of a film of a specific material formed by any atomistic deposition process depend on five factors, namely ... 

Invariably, atomistically deposited films have a residual film stress that may be tensile or compressive and can approach the yield or fracture strength of the materials involved. 

Epitaxy Oriented overgrowth of an atomistically deposited film. See also Epitaxial growth Epitaxy, heteroepitaxy Epitaxy, homoepitaxy. 

Jet vapor deposition (film deposition) An atomistic deposition process where vaporized atoms are introduced into a supersonic jet flow of inert carrier gas that transports the atoms to the substrate surface. 

Figure 1 shows the family tree of the atomistic deposition processes. These are classified Into two groups known as Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) techniques as described In the following sections. 

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