What Are Low-temperature Plasmas?

Low-temperature plasmas are partially ionized gases in which the electron temperature far exceeds the temperature of the heavier neutral and ion species, creating a thermally non-equilibrium state. In these plasmas, electrons can reach equivalent temperatures of tens of thousands of kelvin while gas-phase ions and neutrals remain near room temperature, a contrast that distinguishes them from the thermally equilibrated, high-energy plasmas found in stars and fusion reactors. This thermal non-equilibrium is the source of their practical utility: high-energy electrons drive chemical reactions and excite reactive species without heating the surrounding gas or substrate to damaging temperatures.

The field draws on atomic physics, plasma physics, gas discharge theory, and plasma chemistry. Low-temperature plasmas are generated by applying electrical power, usually in the radio-frequency or microwave range, to gas at low pressure or, increasingly, at atmospheric pressure. Glow discharges, dielectric barrier discharges (DBDs), and microwave-sustained plasma are among the most widely deployed reactor configurations. Grand challenges in low-temperature plasma research, published in Frontiers in Physics, surveys the open questions spanning fundamental plasma kinetics, plasma-surface interactions, and scale-up to industrial processes.

Physical Properties and Generation

Low-temperature plasmas are characterized by their degree of ionization, electron number density, and electron energy distribution function (EEDF). In weakly ionized industrial plasmas, ionization fractions range from parts per million to a few percent, yet the electron density is sufficient to sustain the discharge and drive chemistry. At low pressures (below 100 Pa), electrons gain energy between collisions and the EEDF is far from thermal; at atmospheric pressure, higher collision frequencies narrow the energy distribution and push the plasma closer to quasi-thermal equilibrium while still maintaining significant electron temperatures above the neutral gas. Generating atmospheric-pressure plasmas stably requires dielectric barriers, carefully shaped electrodes, or pulsed excitation to prevent the diffuse glow discharge from contracting into a high-current thermal arc. Power delivery methods, including capacitively coupled plasma (CCP), inductively coupled plasma (ICP), and helicon wave excitation, are selected based on the required electron density and the degree of independent control between plasma density and ion energy.

Plasma Chemistry and Reactive Species

The primary output of low-temperature plasmas is a mixture of reactive species: ions, electrons, photons, radicals, and metastable excited molecules. In air-fed plasmas, electron impact dissociation and ionization produce atomic oxygen, ozone, hydroxyl radicals, and a suite of reactive nitrogen species including nitric oxide and peroxynitrite. These species are chemically aggressive and can oxidize organic molecules, modify polymer surfaces, or inactivate microorganisms. NIH and PMC research on low-temperature plasma in biomedical applications reviews the reactive oxygen and nitrogen species responsible for plasma's antimicrobial, wound-healing, and anti-tumor effects, documenting mechanisms from lipid peroxidation to DNA strand breaks. In semiconductor manufacturing, low-temperature plasma chemistry is tuned for selective etching: fluorine radicals from CF4 or SF6 gases etch silicon and silicon dioxide at controlled rates while leaving masking materials and other device layers intact. Plasma-enhanced chemical vapor deposition (PECVD) uses plasma chemistry to deposit thin films at temperatures compatible with temperature-sensitive substrates.

Applications

Low-temperature plasmas have applications in a wide range of fields, including:

• Semiconductor device fabrication, where plasma etching and deposition are essential steps in integrated circuit manufacturing
• Surface treatment of polymers and metals to improve adhesion, wettability, and printability without bulk heating
• Biomedical and clinical applications, including wound disinfection, cancer cell treatment, and decontamination of medical instruments
• Environmental remediation, including non-thermal plasma catalysis for removing volatile organic compounds and odorous gases from air streams
• Gas discharge lighting, including fluorescent lamps, neon signs, and plasma display panels
• Agricultural applications, such as plasma-activated water for seed treatment and crop pathogen reduction