Elucidating Electron Kinetics in Low Temperature Plasmas with Non-Invasive Optical Diagnostics

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Low-temperature plasmas (LTP) have had a tremendous societal impact by enabling a wide range of technologies that improve quality of life, such as computer chips, energy efficient light sources and wear-resistant artificial joints. Fundamental plasma science is instrumental in continued LTP application development for meeting basic human needs in domains including energy, materials and healthcare. Specifically, many applications rely on manipulation of energy flow and particle fluxes in the plasma, an area lacking a complete science foundation. The quantities of ions, neutral radicals, and photons, which all play central roles in technological outcomes, depend on reactions in the plasma driven by collisions involving energetic electrons. Process results are thus critically sensitive to the spectrum of electron energies, known as the "electron energy distribution function" (EEDF), which in turn depends on pressure, power, gas mixture,flow rates, system geometry and method of power input. "Predictive plasma design" is an as yet unfulfilled goal of LTP modeling with transformational potential for emerging applications. Diagnostics play an essential role in identifying the physical processes that dominate plasma behavior, and in confirming that models accurately represent that behavior. We bring the field closer to the goal of predictive plasma design by applying a new method to experimentally capture electron kinetics using non-invasive optical diagnostics to address fundamental science questions for LTP systems. Intellectual Merit- Electron collisions are responsible for the electronic excitation leading to the plasma glow, and the spectrum of emitted light carries an encoded mapping of the EEDF. Probing the EEDF from the spectrum of the light emitted by the plasma is thus possible, but only with a thorough understanding of processes leading to the excitation and de-excitation of photon emitting species. The efficiencies of the electron-driven excitation processes are expressed as "cross sections," and a set recently measured at UW forms the foundation of an emission model to probe the EEDF using the recorded plasma optical emission spectrum. Here we apply the non-invasive optical technique to examine fundamental behavior of two or more of the following types of radio frequency (rf) LTPs, each of which is distinguished by phenomena involving the high energy region of the EEDF: 1) rf LTPs augmented with dc power, 2) rf LTPs in electronegative gases, and 3) dual-frequency rf capacitively-coupled plasmas. EEDF characterization in space and time will target pecific science questions about the form of the high energy region of the EEDF and its impact on discharge properties. Broader Impacts- The potential impact of the proposed research has several distinct components. 1) If successful, research outcomes will advance understanding in an area critical to establishing predictive and control capabilities for LTP technological applications. 2) The diagnostic approach is widely applicable and easily implemented by others. 3) The interdisciplinary nature of this research effort (PIs from Physics and Engineering) will provide a wide range of teaching and learning experiences and will provide excellent training in both basic and applied physics for the students involved. 4) Plasma technology will also be highlighted in a new program designed to introduce the "grand challenges" in middle school science and math courses, an effort to increase interest in engineering careers among a more diverse student population through an emphasis on humanitarian applications.