Design, modeling, and diagnostics of microplasma generation at microwave frequency.
Abstract: Plasmas are
partially ionized gases that find wide utility in the processing of materials,
especially in integrated circuit fabrication. Most industrial applications of plasma
occur in near-vacuum where the electrons are hot (>10,000 K) but the gas remains near
room temperature. Typical atmospheric plasmas, such as arcs, are hot and destructive to
sensitive materials. Recently the e... read moremerging field of microplasmas has demonstrated that
atmospheric ionization of cold gases is possible if the plasma is microscopic. This
dissertation investigates the fundamental physical properties of two classes of
microplasma, both driven by microwave electric fields. The extension of point-source
microplasmas into a line-shaped plasma is also described. The line-shape plasma is
important for atmospheric processing of materials using roll-coating. Microplasma
generators driven near 1 GHz were designed using microstrip transmission lines and
characterized using argon near atmospheric pressure. The electrical characteristics of
the microplasma including the discharge voltage, current and resistance were estimated
by comparing the experimental power reflection coefficient to that of an electromagnetic
simulation. The gas temperature, argon metastable density and electron density were
obtained by optical absorption and emission spectroscopy. The microscopic internal
plasma structure was probed using spatially-resolved diode laser absorption spectroscopy
of excited argon states. The spatially resolved diagnostics revealed that argon
metastable atoms were depleted within the 200μm core of the microplasma where the
electron density was maximum. Two microplasma generators, the split-ring resonator (SRR)
and the transmission line (T-line) generator, were compared. The SRR ran efficiently
with a high impedance plasma (>1000 Ω) and was stabilized by the self-limiting
of absorbed power (<1W) as a lower impedance plasma caused an impedance mismatch. Gas
temperatures were <1000 K and electron densities were ~1020 m-3, conditions which are
favorable for treatment of delicate materials. The T-line generator ran most efficiently
with an intense, low impedance plasma that matched the impedance of the T-line (35
Ω). With the T-line generator, the absorbed power could exceed 20W, which created
an electron density of 1021 m-3, but the gas temperature exceeded 2000 K. Finally,
line-shaped microplasmas based on resonant and non-resonant configurations were
developed, tested, and analyzed.
Thesis (Ph.D.)--Tufts University, 2012.
Submitted to the Dept. of Electrical Engineering.
Advisor: Jeffrey Hopwood.
Committee: Alan Hoskinson, Sameer Sonkusale, and Helen Maynard.
Keywords: Electrical engineering, and Plasma physics.read less