Fundamentals of non-equilibrium plasmas

Generation of gas plasmas is associated with production of energetic particles (e.g. electrons, ions, and photons), chemical reactive species (e.g. free radicals and metastables), and a myriad of transient fields (e.g. heat, shock and acoustic wave, electrostatic and electromagnetic fields). This is a highly coupled process of complexity and dynamics. For example, through Poisson’s equation, a non-uniform or non-neutral spatial distribution of charged particles can set up a local electrostatic field and its role near the electrodes can outweigh the contributions of the externally applied electric field. Photons and some charged particles (e.g. superoxide O2-) are also chemically reactive. In fact, production of different plasma agents and fields is closely coupled and it is usually not possible for one plasma agent to be produced without many other agents also being produced. When used for treating and processing materials, and biological materials in particular, these plasma agents and electric fields tend to work synergistically in their interaction with, and their effects on, the material that is brought in contact with the gas plasma. The synergistic effect is distinct in the extent and the richness of physiochemical functionalities facilitated and is not usually accessible with other techniques for materials treatment. This is an important aspect of gas plasmas from an application standpoint, and a key reason why gas plasmas have been used widely in our society, as discussed in Chapter 1. Yet it also brings with it the challenge of unraveling the associated complexity in plasma-material interactions in order to improve and optimize the efficiency of the material-processing application. Fortunately, the development of microelectronics industry since the 1970s has resulted in a considerable understanding of how plasma agents and fields interact with materials.