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Kinetics of transient reactive species in oxygen plasmas at intermediate pressure

Low-temperature plasmas in oxygen are extensively used in various processing applications, such as surface cleaning, sterilization of medical devices, plasma-based water treatment. Notably, RF (Radiofrequency) CCP (Capacitively Coupled Plasma) are commonly used on chemical vapor deposition of metal oxide films, material etching, and resist stripping in microelectronics.

However, unlike plasmas at low or atmospheric pressures, plasmas at intermediate pressures have been less studied and not fully understood due to the challenges associated with diagnostics and simulations in this pressure range. Experimentally, all existing electron density measurements are not very proper in such high-pressure, weakly-ionized plasmas. Specially, Langmuir probe become difficult to use due to the frequent collision in the probe sheath and strong rf oscillation. On modelling side, the PIC simulation becomes very costly as pressure increases, while fluid model also has some problem, we may talk about it later.

In this thesis, we investigate oxygen-argon plasmas using both experimental and modeling methods to gain a deeper understanding of the dynamics of neutral and charged particles in plasmas at this pressure range. The following research was conducted:

1. Investigation of DC Glow Discharges at Intermediate Pressure: This study involved measuring the density of oxygen atoms and the gas temperature using the CRDS (Cavity RingDown Spectroscopy) technique. The effects of plasma stabilization over time were examined, emphasizing the importance of achieving a stable plasma state before taking measurements. The impact of introducing argon gas, and the effects of small air leaks, were carefully analyzed. Recommendations for actinometry calibration involving argon gas were provided, highlighting the importance of maintaining good vacuum conditions for optimal plasma reproducibility. The study also discussed the mechanisms behind the surface loss of oxygen atoms and introduced a novel model that better aligns with our experimental observations. Additionally, the influence of varying wall temperatures on these processes was explored.

2. Experimental Study of RFCCP in O2-Ar Mixtures at Intermediate Pressure: This study focused on the effect of different power, pressure, and gas mixtures. Various electrical and neutral properties were measured, including plasma impedance (using an Octiv and Tektronix probes), electron density (using a hairpin probe), and surface ion flux (using an electrostatic probe array).
Additionally, oxygen atom density and gas temperature are measured by CW CRDS measurements. Lastly, the O atom loss rate, negative ion density and ozone concentration in the afterglow were obtained using the modulated CRDS measurements. The effects of pressure, power, and gas mixtures were discussed, and the mechanisms occurring in such plasmas for both charged particles and neutrals were demonstrated. Notably, the mode transition with power is observed and discussed. Meanwhile, the relevance between O atom surface loss rate and the ion bombardment towards boundary is revealed.

3. Modeling of RF CCP at Intermediate Pressure: The thesis also presents modeling efforts using both Particle-in-Cell (PIC) and fluid simulations. The study discusses the costs and limitations of the PIC (Particle-in-Cell) model, comparing it with the fluid model, and explains the conditions under which the fluid model becomes invalid. Ultimately, the thesis offers suggestions for choosing the appropriate modeling method for different pressure ranges and proposes a hybrid model that combines the strengths of both PIC and fluid simulations.