oxygen plasmas

The development of industrial processes involving plasmas is in a phase of rapid expansion. Recently added to continuing development in areas where plasmas have been long used, such as integrated circuit fabrication, are many applications in new domains such as plasma medicine and plasma agriculture, which show tremendous potential for a broad range of societal benefits. [ADD REFERENCES]

The attraction of plasmas for these applications derives from the role of the free electrons present in all plasmas. A unique quality of plasmas critical to many applications is the capability of non-equilibrium chemistry; “high-temperature” gas phase reactions occur while substrates remain cool. These gas phase reactions, producing ‘radicals,’ highly reactive neutral and ionic species that react with exposed substrate surfaces, are enabled by an electron population selectively maintained at temperatures greater than 10,000 \(^\circ\)F through heating by electromagnetic fields. Collisions between molecules in the gas phase and highly energetic electrons cause negligible gas heating but, importantly, lead to significant rates for chemical reactions not possible at or near room temperature without a plasma. The electron temperature, \(T_e\), is therefore a key process parameter, since rates for these reactions are strong functions of \(T_e\). For some plasmas, \(T_e\) is not defined because electron energies do not follow a Maxwell-Boltzmann distribution, and in such cases, we must instead consider the so-called ‘non-Maxwellian’ electron energy distribution function (EEDF). Deviation from the Maxwell-Boltzmann form can have significant implications for the rates of electron-driven reactions, motivating the desire to quantify in detail the energy dependence of the EEDF.

The plan described herein to develop advanced diagnostic tools for low-temperature molecular plasmas will make a contribution toward the ultimate goal of “predictive design” of plasma processes, providing a scientific rather than empirical basis for process development and optimization. In particular, we seek to go beyond standard OES (optical emission spectroscopy) by making use of previously untapped potential of the glow emitted by the plasma. in short, our goal is to develop a non-invasive and easily-implemented diagnostic to quantify plasma properties, including electron temperature or EEDF, electron density and dissociation fraction, of central importance to many applications. MENTION community demand - cite plasma 2020, low temperature plasma report, etc.

Diagnostic development will follow two approaches to quantitative OES: molecular gas plasma spectroscopy (with an initial focus on pure oxygen plasmas), and rare-gas plasma spectroscopy in mixtures with molecular gases. Plasma properties measured with the proposed methods have utility for investigation of plasma phenomena, to provide benchmarking of process simulations and to guide development and optimization of plasma applications. Furthermore, fast data acquisition and analysis enabled by quantitative OES raises the possibility of use in a feedback loop for run-to-run or real-time process control.

While our proposed research builds on experience gained in developing OES-based diagnostics of rare gas plasmas, if successful, it will contribute non-invasive and easily implemented diagnostic tools in a new domain, that of molecular gas plasmas, applicable to a wide array of plasma process applications. The foundation of our method is a quantitative optical emission model to compute relative intensities of lines and bands as a function of plasma parameters. For each emitting state, the relative intensity computed by the emission model depends on the net rate of excitation to that state and other factors, such as the branching fraction describing the statistical distribution of wavelengths emitted. Central to the model are electron collisions with lower lying states that lead to excitation and have excitation probabilities that vary strongly with electron energy. In addition, excitation by photon reabsorption (radiation trapping) and deexcitation by electron collisions are also included. We determine the values of the plasma parameters as those for which the emission model produces a ‘best fit’ to measured intensities for a selected group of spectral features.

Developing an emission model for a molecular gas introduces both new scientific challenges and new potential rewards. Plasmas formed in diatomic gases contain many more species than rare gas plasmas, including atoms, diatomic molecules, larger molecules and metastable neutral species, as well as positive and negative atomic and molecular ions, some or all of which may undergo collisions leading to photon emission. As a result, emission models for such plasmas must include excitation mechanisms not relevant for rare gas plasmas, such as electron collisional dissociative excitation as well excitation resulting from recombination of positive and negative ions. Many of these species are critically important in industrial applications; our goal is an emission model including these mechanisms that enables determination of the concentrations of multiple species.

Oxygen has been selected as the primary focus for the development of the OES-based diagnostic. Methods developed will be adapted later in the study to other diatomic gas plasmas such as N\(_2\) and H\(_2\). Low pressure plasmas generated in pure oxygen have technological significance in a number of industrial applications. Oxygen and oxygen containing plasmas are in use or under development for a wide array of applications, including materials applications such as etching of polymers, photoresist removal in semiconductor manufacturing, ion implantation and surface modification. [Ozone? REFERENCES TO BE ADDED HERE] Such processes rely on the interaction of neutral radicals, O\(^+\) and O\(_2^+\) with substrate surfaces exposed to the plasma. In many applications, achievement of process goals is sensitive to the relative fluxes of the different gas phase species to the substrate surface. Production of ion and neutral radical species occurs primarily through gas phase reactions involving collisions with energetic electrons, so that production rates are sensitive functions of electron density and electron temperature.

Nitrogen plasmas are also of technological interest, due to the great importance of nitrogen atoms in material science applications. Nitrogen radical sources have a broad range of application, including in the growth of III–V nitride semiconductor devices such as GaN lasers, AlGaN/GaN and AlGaN/InGaN/GaN heterostructures for light emitting diodes. They are also used in p-type nitrogen doping of II–VI materials as well as in ‘plasma- assisted’ molecular beam epitaxy (MBE) of iron nitrides. Nitrogen plasmas are also used in plasma-based nitriding processes such as plasma source ion implantation (PSII) to improve surface wear properties of, among others, artificial hip joints fabricated from titanium alloys. [References]

We propose a new outreach activity to increase public awareness of plasmas and their application, in partnership with the Rocket Club at Madison West High School (letter of collaboration included as “supplementary documentation”). Our group will work with Rocket Club members to create a hands-on display with a demonstration and description of ion thrusters for spacecraft propulsion, to be integrated into the club’s existing outreach exhibit. As club participants, West High students engage in rocket competitions, local public outreach events and fund raising to support the club’s activities. The club, founded in 2003, regularly participates in national competitions, including the Team America Rocket Challenge and the NASA Student Launch program, and has won numerous awards. Student members interact with the public through their exhibit at annual public outreach events, including the Wisconsin Science Festival and the University of Wisconsin Physics Fair, as well as visits to schools and other groups. The current multifaceted exhibit includes rockets and payloads designed and built by the club as well as a “build your own” activity; designing and launching pneumatic paper rockets is a popular activity among visitors all ages.

The new display will add a working plasma thruster to the exhibit, one that operates at atmospheric pressure and has a modest tabletop footprint, adapted from a recently published design(citation not found: make2015). Operating at 300 watts, the thrust produced will be small, but observable through the deflection of a lightweight object such as a piece of paper in the path of the thruster “exhaust.” Visitors will also get a taste of plasma spectroscopy by looking at the thruster plasma through a diffraction grating; distinct spectral features are expected in the visible portion of the spectrum. Additional diagnostics will include thermal sensors, which we expect to show a comparatively cool exhaust. The diagnostics may be utilized to spark discussions about efficiencies in converting energy sources to thrust; a very important consideration for long distance space travel.

The UW plasma spectroscopy group will contribute parts and supplies and will work with Rocket Cub members to build the thruster. In addition, we will work with the high school students to design and print a companion poster to go with the thruster. With an emphasis on graphics and simple explanations, poster topics may include: a) what’s a plasma? b) electric propulsion and principles of plasma thruster operation (no moving parts, no exothermic reactions!), c) comparison between plasma thrusters, chemical rockets, etc., d) current and proposed uses of ion thrusters, such as satellite maneuvering and long-distance space flights, e) examples of other plasma applications.

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