Theory and Simulations of Low Temperature Plasma Devices
Low Temperature Plasmas (LTP) are used in applications, because these plasmas are typically exist in non-equilibrium state, meaning that the electron temperature is much larger than the ion temperature, which is in turn larger than the gas temperature. Due to the partially ionized nature of LTPs, some of the particles that are energetic (i.e., the electrons and often the ions) while the specific energy content of the mixture is low because it is dominated by the far more abundant neutral gas. This provides a unique set of conditions wherein plasma species can be non-destructively and beneficially in contact with surfaces. For example, the entire microelectronics industry that forms the technological base of modern society is enabled by beneficial plasma-surface interactions, which deposit and remove materials with nm resolution in the fabrication of microprocessors.
Notwithstanding the significant improvements of plasma sources for technology applications during the past decade, the majority of commercial plasma devices have far from optimal performance due to the lack of a detailed understanding of the underlining physics and plasma chemistry. Therefore, the creation of low-temperature plasmas with controllable parameters, in particular, the plasma density, the electron temperature, and the electron and ion energy spectra, is one of the major tasks of modern plasma engineering. A promising approach to better control of the plasma parameters is connected with the utilization of peculiarities of plasmas with nonlocal electron energy distribution function (EEDF). Such plasmas have a remarkable property: changing the conditions at one location may lead to unexpected changes far away in another part of the plasma. The nonlocal nature of the electrons is important for low-pressure plasma, but can also be very important for atmospheric pressure discharges (for example, micro-discharges). Besides the interesting physics, the nonlocality of the EEDF gives the additional possibility of developing new ways of controlling plasma properties and, therefore, for further improvement of industrial devices.
The objective of future studies is to develop and perfect new methods of controlling plasma parameters based on the nonlocal nature of the EEDF in the presence of so-called “fast” electrons. In this context, the fast electrons have energy considerably greater than the average electron energy. The addition of even a small amount of fast electrons and/or negative ions in such plasmas may completely change the plasma properties. Therefore, in the proposed research the importance of taking into account nonlocal effects for technological plasmas will be demonstrated. Moreover, we shall explore the possibility of using fast electrons for controlling plasma properties.
The major effort is developing adequate computational tools, a suite of particle-in-cell codes that can model discharge in complex geometries and take into account relevant atomic physics. We use several particle-in-cell codes to simulate plasmas in various devices: RF-DC discharges for plasma etching in collaboration with Tokyo Electron America, Inc, gas plasma switch in collaboration with General Electric, Hall thrusters and micro discharges for two AFSOR projects.