What Are Microwave Plasmas?

Microwave plasmas are electrical discharges sustained by electromagnetic power in the microwave region, most often at the industrial, scientific, and medical frequency of 2.45 gigahertz and sometimes at 915 megahertz. Power is delivered through a waveguide, a coaxial line, or a resonant cavity rather than through electrodes in contact with the gas, so the discharge is electrodeless. That single property shapes the whole field: without electrodes there is no sputtering of metal into the process gas, no electrode erosion to limit source lifetime, and no sheath voltage large enough to bombard the substrate with high-energy ions unless one is deliberately applied through a separate bias.

Coupling is governed by the relationship between the driving frequency, the electron plasma frequency, and the electron collision frequency. Electrons oscillate in the applied field and gain energy through collisions, while the much heavier ions cannot follow the field, which is why low-pressure microwave discharges are strongly non-thermal, with electron temperatures of a few electronvolts and gas temperatures near ambient. Above the critical density at which the plasma frequency equals the driving frequency, roughly 7 times 10 to the sixteenth per cubic meter at 2.45 gigahertz, the wave no longer propagates into the bulk and deposits its energy in a thin skin layer, so overdense operation requires a different coupling mechanism.

Resonant and Surface Wave Coupling

Electron cyclotron resonance sources solve the density limit by adding a static magnetic field of about 87.5 millitesla, at which the electron gyrofrequency matches 2.45 gigahertz and electrons absorb energy continuously from the circularly polarized wave. This yields high electron densities at pressures of a few millitorr and a well-collimated downstream flux, and permanent-magnet designs have replaced bulky electromagnets in many systems. Characterization of such a source for deposition of diamond-like carbon films illustrates the operating regime and the transition into a high-density mode once absorbed power crosses a threshold. Surface wave sources take a different route: the wave propagates along the boundary between the plasma column and a dielectric tube, sustaining the discharge as it travels, which produces long uniform columns without any magnetic field. Compact surface wave devices can ignite atmospheric pressure discharges at input powers below ten watts.

Atmospheric Pressure Operation

Raising pressure to one atmosphere increases the collision rate, so electron and gas temperatures converge and the discharge becomes closer to thermal. Microwave torches and cavity-based plasma jets operating in air, argon, or nitrogen reach gas temperatures of several thousand kelvin, and they support chemistry that low-pressure sources cannot. Modeling these discharges requires coupling wave electrodynamics to particle kinetics and gas flow, since heating changes gas density and therefore the local dielectric properties that determine where the wave deposits power. Combined experimental and theoretical treatments of an atmospheric pressure microwave air plasma source derive wave dispersion characteristics from amplitude and phase measurements along the column.

Process Chemistry and Materials Growth

The high dissociation efficiency of microwave discharges makes them attractive for producing reactive species. Hydrogen plasmas generate atomic hydrogen that etches non-diamond carbon, which is what allows microwave plasma chemical vapor deposition to grow high-purity single-crystal and polycrystalline diamond. Similar reactors deposit silicon nitride, carbon nanotubes, and metal oxide nanostructures, and electron cyclotron resonance reactors have also been used for plasma-assisted deposition of zinc nanowires. Carbon dioxide and methane conversion has become an active application, since vibrational excitation in a non-thermal microwave discharge can drive dissociation at a lower energy cost than thermal routes.

Applications

Microwave plasmas have applications in a range of fields, including:

• Semiconductor etching and thin film deposition, including electron cyclotron resonance processing
• Diamond and diamond-like carbon synthesis
• Ion and electron source generation for particle accelerators
• Surface activation, sterilization, and biomedical plasma treatment
• Gas conversion and pollutant abatement, including carbon dioxide reforming
• Lighting, ozone generation, and analytical spectrochemical excitation sources