Gasphase deposition Cvd Pvd

Gas-phase deposition is an important class of processes, which are mainly used for the deposition of thin films from gaseous source materials; they are also used for the precipitation of monodisperse powders, quantum dots, etc. Gas-phase deposition can roughly be divided into chemical (CVD = chemical-vapor deposition) and physical (PVD = physical-vapor deposition) gas phase deposition. In all the methods the source materials from the gas phase are transported to the substrate surface where they form a thin film (Köhler 2001).

In the case of PVD a solid source material placed into a vacuum is transferred into the gas phase by physical means (e.g. thermal energy). The particles precipitate and form a thin film on a substrate. The different methods of PVD can be distinguished by the means of evaporation, i.e. the method of heating. In thermal evaporation, the conventional method, the starting material is heated in high vacuum in a crucible until it evaporates. Further methods include sputtering, arc-evaporation, molecular beam epitaxy (MBE) and ion plating (Fraunhofer IPA/IST & BAM 2004b). The general process of PVD is shown in figure 5.

Fig. 5. The PVD method36

CVD includes all methods that utilize the chemical reaction of a gaseous source material on or close to a substrate to precipitate a solid product. The gaseous source material is fed into a reaction chamber where it is then thermally decomposed. The energy is supplied either thermally, by excitation of the reactants with a plasma, or by electromagnetic radiation. A portion of the formed intermediate product is adsorbed at the substrate, where a heterogeneous reaction leads to the formation of a film. Those volatile reaction components that do not precipitate on the substrate are removed via the vacuum system (Fraunhofer IPA/IST & BAM 2004a). Important CVD methods include: Thermal CVD, plasma-activated CVD (PACVD), photo-assisted CVD and catalytic CVD, which is increasingly used in the production of carbon nanotubes.

36 Source: Fraunhofer IPA/IST & BAM (2004b)

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