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. |