Plasma processes, low temperature

Chang, J.-S. (1987) Proceedings of International Symposium on High Pressure Low Temperature Plasma Processing, IEE Japan Press, pp. 45-54. 

Low-temperature plasma processes, such as gas plasma treatment and plasma polymerization, have unique advantages in that active (depositing) species strongly interact with the surface of the substrate and modify the surface state. An ultrathin layer of plasma polymer, e.g., thickness less than 50 nm, can be viewed as a new surface state because such a thin layer does not develop a characteristic bulk... 

The most perspective are the methods based on application of nonequilibrium low-temperature plasma [3], They have a number of advantages smaller dimensions of the equipment, opportunity to automate both the process and quality control of processed environment, low involvement of human resources, opportunity to use new solutions, though poorly investigated, but having useful potential and properties. The basis for the process is the contact plasma discharge on the surface of a liquid phase formed between an electrode in gas phase and surface of the liquid, in which the second electrode is immersed. 

The Environmental Dynamics low-temperature plasma (LTP) process is an ex situ technology that treats soils contaminated with volatile organic compounds (VOCs). According to the vendor, the technology has the following advantages ... 

Plasma sources are also being introduced to produce plasmas at lower pressures and process temperatures. Inductively coupled plasma (ICP) and transformer-coupled plasma (TCP) are among the more commonly used sources, operating below 2.6 Pa (20 mTorr) (42). Low temperature RIE processing operates between 26—67 Pa (200—500 mTorr). 

The reactions underlying CVD typically occur both in the gas phase and on the surface of the substrate. The energy required to drive the reactions is usually supplied thermally by heating the substrate or, in a few instances, by heating the gas. Alternatively, photons from an ultraviolet (UV) light source or from a laser, as well as energetic electrons in plasmas, are used to drive low-temperature deposition processes. 

Fig. 5. Chemical evolution in a low temperature plasma. Illustrated are the types of charged particle reaction processes which may occur when ionization is created in a gas and which are terminated by recombination reactions, these returning the plasma to the neutral state... 

The electrodeposition of silver from chloroaluminate ionic liquids has been studied by several authors [45-47], Katayama et al. [48] reported that the room-temperature ionic liquid l-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4) is applicable as an alternative electroplating bath for silver. The ionic liquid [EMIM]BF4 is superior to the chloroaluminate systems since the electrodeposition of silver can be performed without contamination of aluminum. Electrodeposition of silver in the ionic liquids 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) and l-butyl-3-methylimidazoliumhexafluorophosphate was also reported [49], Recently we showed that isolated silver nanoparticles can be deposited on the surface of the ionic liquid Tbutyl-3-methylimidazolium trifluoromethylsulfonate ([BMIMJTfO) by electrochemical reduction with free electrons from low-temperature plasma [50] (see Chapter 10). This unusual reaction represents a novel electrochemical process, leading to the reproducible growth of nanoscale materials. In our experience silver is quite easy to deposit in many air- and water-stable ionic liquids. 

The properties of plasmas vary strongly with gas composition, pressure and the method and parameters of the plasma generation process. The charge carrier concentration depends on the pressure and the fractional ionization of the plasma, for instance basically on the power density. The mobility of the electrons depends on the electron temperature, which is typically several orders of magnitudes greater than the gas temperature or the temperature of the ionized species in non-thermal low temperature plasmas used for electrochemical purposes. 

Electron-impact electronic excitation, ionization and dissociation processes are among the most important collision processes that H2 molecules undergo in a low-temperature plasma. These include... 

Figure 1.3 clearly demonstrates the luminous gas phase created under the influence of microwave energy coupled to the acetylene (gas) contained in the bottle. This luminous gas phase has been traditionally described in terms such as low-pressure plasma, low-temperature plasma, nonequilibrium plasma, glow discharge plasma, and so forth. The process that utilizes such a luminous vapor phase has been described as plasma polymerization, plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), plasma CVD (PCVD), and so forth. 

An ultrathin layer of plasma polymer of trimethylsilane (TMS) has been utilized in the corrosion protection of aluminum alloys by means of system approach interface engineering (SAIE) [1 ]. SAIE by means of low-temperature plasmas utilizes low-temperature plasma treatment and the deposition of a nanolilm by luminous chemical vapor deposition (LCVD). This approach does not rely on the electrochemical corrosion-protective agents such as six-valence chromium, and hence the process is totally environmentally benign. 

V.N.Kondratiev, E.E.Nikitin, and V.L.Tal rose, Problems cormected with the investigation of elementary processes in low-temperature plasma. Pure and Applied Chemishy, 13, 367 (1966)... 

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