Plasma-Assisted Chemical Vapor Deposition Processes
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librium plasmas, the collisions of high energy electrons and gas molecules result in dissociation processes that would only occur at very high temperatures of more than 5,000 K in the case of thermal equilibrium. Therefore, nonisothermal plasmas allow the preparation of materials and compositions that are difficult to obtain using thermally activated, conventional CVD. Due to the initiation of chemical reaction by collisions with "hot" electrons rather than hot gas molecules, the processing temperature can, in many cases, be kept lower than in conventional deposition processes. These advantages—unusual material and lower processing temperature— along with the high reproducibility and good process control, make non-isothermal plasma CVD very appealing for industrial applications. In thermal "hot" plasmas, on the other hand, all the species in the gas are heated to more than 5,000 K and dissociated. In this case, the application of an electrical field is just a convenient way to supply the energy necessary to achieve high temperatures. Arc discharges and low (rf) frequency thermal plasmas at atmospheric pressure are typical examples. They are, however, much more difficult to control and to stabilize than low pressure plasmas. Thermal plasmas are used for deposition as well, but at a much lower level. These processes will, therefore, only be discussed briefly in this article. The possibility of circumventing the limitations of equilibrium processes, of course, inspired the minds of many scientists and helped develop a large variety of both deposition techniques and applications for various material sys-
tems. The techniques are best distinguished by the frequency of the electric field utilized to sustain the plasma. This frequency determines if only electrons or also heavier particles can follow the changing field direction and correlates with the degree of non-equilibrium achievable. As a rule of thumb, nonequilibrium is more likely at a higher frequency and a lower pressure in the gas phase. Ions become practically immobile at plasma excitation frequencies of more than ~1 MHz. For plasma CVD, dc glow discharges, 13.45 MHz radio frequency (rf), along with 900 MHz and 2.45 GHz microwave plasmas (mw) are commonly in use. This choice of very specific frequencies is mainly due to the availability of components and also must comply with federal regulations.
Plasma-Deposited Materials: Preparation and Applications To give an impression of the variety of materials investigated, Table I summarizes important examples and references of plasma deposited materials. This table is intended as an aid for the reader to get more details and additional references of other important contributions in the field but is by no means complete. It illustrates the impressive variety of practical applications of plasma-deposited materials, such as tribological, optical, electrical, and electronic applications. It is not the intention of this article to detail each and every one of these processes and materials, but to select a few typical topic
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