PVD

Fig. 21. Schematic illustration of the four primary vapor-phase deposition processes used in optical-fiber fabrication outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), plasma vapor deposition (PVD), and vapor axial deposition (VAD) (115).

Physical Vapor Deposition Processes. The three physical vapor deposition (PVD) processes are evaporation, ion plating, and sputtering... 

The materials deposited by PVD techniques include metals, semiconductors (qv), alloys, intermetaUic compounds, refractory compounds, ie, oxides, carbides, nitrides, borides, etc, and mixtures thereof. The source material must be pure and free of gases and inclusions, otherwise spitting may occur. 

GVD Coatings. As in PVD, the stmcture of the deposited material depends on the temperature and supersaturation, roughly as pictured in Figure 8 (12). In the case of CVD, however, the effective supersaturation, ie, the local effective concentration in the gas phase of the materials to be deposited, relative to its equiUbrium concentration, depends not only on concentration, but on temperature. The reaction is thermally activated. Because the effective supersaturation for thermally activated reactions increases with temperature, the opposing tendencies can lead in some cases to a reversal of the sequence of crystalline forms Hsted in Figure 8, as temperature is increased (12). 

Another growing apphcation that overlaps the electrically functional area is the use of transparent conductive coatings or tin oxide, indium—tin oxide, and similar materials in photovoltaic solar ceUs and various optic electronic apphcations (see Photovoltaic cells). These coatings are deposited by PVD techniques as weU as by spray pyrolysis, which is a CVD process. 

Vacuum Deposition. Vacuum deposition, sometimes called vacuum evaporation, is a PVD process in which the material is thermally vaporized from a source and reaches the substrate without coUision with gas molecules in the space between the source and substrate (1 3). The trajectory of the vaporized material is therefore line-of-sight. Typically, vacuum deposition takes place in the pressure range of 10 10 Pa (10 10 torr), depending on the level of contamination that can be tolerated in the resulting deposited film. Figure 3 depicts a simple vacuum deposition chamber using a resistively heated filament vaporization source. 

Surface Coverage. The surface-covering abiHty of deposition techniques is best when the materials are deposited from a vapor or from a fluid having no need for an appHed voltage. The macroscopic and microscopic surface coverage of a thin film deposited by PVD techniques on a substrate surface may be improved by the use of gas scattering and concurrent bombardment during film deposition. 

In many of the thin coatings on cemented carbide, either single or multiple coatings of single phase materials, such as TiC, are used. It would appear that extending the use of soHd solutions of multicarbides of W, Ti, and Ta or Nb for coatings may further enhance the performance of the coated carbides. It would not be difficult to accomplish this either by CVD or PVD techniques. 

There are several vacuum processes such as physical vapor deposition (PVD) and chemical vapor deposition (CVD), sputtering, and anodic vacuum arc deposition. Materials other than metals, ie, tetraethylorthosiHcate, silane, and titanium aluminum nitride, can also be appHed. 

Fig. 4.37. Depth (temporal) profile obtained on a multilayer coating produced by plasma vapor deposition (PVD) using optimized rf-glow discharge conditions. Layer thickness ...

---