Radical-controlled plasma processes

In plasmas, a variety of radicals which are defined as electrically neutral radicals in this article are efficiently produced by collisions between electrons and gas molecules. These radicals can subsequently undergo gas phase reactions with solids, liquids and living organisms that result in non-equilibrium surface/interface physicochemical processes. The specific phenomena produced by these reactions remain largely unknown, even though these plasma-based processes could lead to disruptive technological innovations. As an example, in the case of semiconductor microfabrication processes, the density, energy and lifetime of individual radicals, as well as the reaction time constants of these species with various materials should be ascertained. This would allow the identification and control of the effective radical species during processes, such as the high-precision etching and deposition of functional thin films. In addition, the type of reactions occurring between radicals generated in plasmas with liquids or living organisms is still an unexplored area. Establishing a theoretical system for these radical reactions and controlling the associated mechanisms could lead to innovations in the fields of functional devices and materials as well as in the areas of environmental protection, medicine and agriculture/fisheries. Focusing on the non-equilibrium surface/interface physicochemical reactions between radicals and solids occurring in semiconductor plasma processing, this paper describes the formation of nanostructured thin films by top-down mechanisms based on controlled radical production and bottom-up processes involving radical-induced self-organization. As well, this review examines next-generation medical and agricultural applications, such as the selective killing of cancer cells and plant growth promotion and functionalization. These systems result from the interactions of radicals generated in atmospheric-pressure, low-temperature plasmas with liquids, or the interactions of gas or liquid phase radicals with biological species. Finally, the importance of academic research into radical-controlled plasma processes and potential future technologies based on this interdisciplinary field are examined.