Current Research on Molecular Processing
Technological Applications
Electron induced reactions in both gaseous and condensed phases underpin many areas of applied and industrial research, and are fundamental to the operation of all plasma-based processing. For example, secondary electron cascades in mixed radioactive/chemical waste drive much of the chemistry that determines how those materials age, change, and interact with the natural environment. Electron collisions create the reactive molecular fragments in the plasma devices being used in clean technology e.g. for environmental remediation of NOx emission of combustion exhausts and the development of ozone generators to control bacterial growth in water treatment plants and retail food outlets.
Electron induced reactions also underpin the multibillion dollar modern semiconductor industry since it is the reactive fragments produced by electron impact of etchant gases that react with the silicon substrate rather than the parent compound. However despite its high costs (>$1 billion a plant) and technological importance most of the plasma processing protocols and equipment have been designed empirically. Indeed the 1996 US National Research Council Board report on plasma processing stated that ‘plasma process control remains largely rudimentary and is performed predominantly by trial and error’, an expensive procedure which limits growth and innovation of the industry. In the last eight years US and Japanese research communities (supported by their manufacturing industry) have been developing research programmes to understand properties and mechanisms of technological plasmas.
Major research effort has been directed towards (i) developing new techniques for in situ plasma diagnosis and (ii) employing large scale computer modelling to simulate conditions within such plasmas. Studies aim to elucidate the plasma characteristics through an understanding of the fundamental atomic and molecular physics and place this technology on a firm theoretical basis. The ultimate goal being to advance our understanding of plasma characteristics to such a level that it will be possible to custom design, through computer models plasma reactors for any specified commercial requirement (the ‘virtual factory’). To date these goals have only been partially met. While the diagnostics of such plasmas are increasingly well established, computer simulations remain inadequate and are unable to provide the technologist with any realistic predictions as to the operation of new reactor designs and processes.
Growing environmental concerns on the climatic effect of emissions of current plasma reactants (most of the compounds being strong greenhouse gases and/or ozone depletion compounds) have forced the industry to seek replacement etching gases, requiring the design of new reactors. The plasma industry has recently reinforced its call to the academic community to develop its ‘virtual factory’ programme allowing new reactors/plants to be developed computationally prior to the construction of commercial plants.
Models of technological plasmas require quantitative data on the reactions of all the constituent neutral species and ions. The 1996 US report stated ‘ The main road block to the development of plasma models is the lack of fundamental data on collisional, reactive processes occurring in the plasma. Among the most important missing data are the identities of key chemical species and the dominant kinetic pathways that determine the concentrations and reactivates of these key species, especially for the complex gas mixtures commonly used in industry’. Electron collisions initiate almost all of the relevant chemistry associated with the technology of plasma processing. Electron collision processes involving all possible reactants, products, and intermediates must be investigated. Dissociation, ionisation and attachment cross sections are of particular importance since they determine the ionisation balance within the plasma and thus influence plasma properties such as the electron energy distribution, which in turn influences etch and deposition characteristics of plasmas. Therefore an important part of this programme will be to study electron induced processes pertinent to the modern plasma processing industry in both the gaseous and condensed phase.
