What Is Plasma Chemistry?

Plasma chemistry is a branch of physical chemistry and plasma science concerned with the chemical reactions that occur in partially or fully ionized gases, including the generation, interaction, and transformation of electrons, ions, excited atoms and molecules, and free radicals under plasma conditions. It underlies a broad range of industrial processes, from the etching of semiconductor devices to the treatment of biological tissue, because plasmas can drive chemical reactions that are inaccessible or energetically prohibitive under ordinary thermal equilibrium conditions.

The field combines elements of gas-phase kinetics, surface science, and plasma physics. It draws from work in discharge physics initiated by Irving Langmuir and colleagues in the early twentieth century, and it expanded significantly with the growth of semiconductor manufacturing in the 1970s and 1980s, when engineers required precise models of how reactive gases behave under radio-frequency plasma excitation. Plasma chemistry today encompasses both thermal equilibrium plasmas (such as plasma torches operating above 10,000 K) and non-equilibrium low-temperature discharges, which are the more common subjects of engineering research.

Electron-Driven Reactions

In most plasma chemistry contexts, free electrons are the primary initiators of chemistry. Energetic electrons collide with neutral gas molecules through a set of fundamental processes: elastic collisions that transfer momentum without changing molecular structure; inelastic collisions that excite vibrational, rotational, or electronic states; ionization collisions that strip electrons from neutral atoms to produce ion pairs; and dissociative attachment, where an electron is captured and the resulting negative ion immediately fragments. These electron-impact cross sections depend strongly on electron energy and are tabulated in databases such as the LXCat cross-section compilation used widely in plasma modeling.

The electron energy distribution function (EEDF) determines the relative rates of these processes. In a non-equilibrium plasma, the EEDF can be far from a simple Maxwellian, shaped instead by the applied electric field waveform, gas pressure, and the energy-loss channels available in the particular gas mixture. Accurate chemical kinetics modeling therefore requires solving for the EEDF alongside the reaction rate equations, typically using Boltzmann solvers or particle-in-cell Monte Carlo codes.

Reactive Species and Their Roles

The products of electron-impact processes are the reactive species that drive plasma chemistry: ground-state and excited atoms, molecular radicals, and ions. In oxygen-containing plasmas, atomic oxygen (O), ozone (O3), and hydroxyl radicals (OH) are central species, each with distinct reactivity toward organic compounds and biological targets. In nitrogen-containing plasmas, nitric oxide (NO) and nitrogen dioxide (NO2) are key, particularly in the plasma medicine context where RONS (reactive oxygen and nitrogen species) govern biological effects.

Radical densities in plasma typically exceed ion densities by one to two orders of magnitude, making radical chemistry the dominant pathway for most surface modification and gas-phase conversion applications. Research on radical-controlled plasma processes has examined how independently adjusting radical fluxes relative to ion fluxes allows precise control over etch selectivity and deposition composition in semiconductor manufacturing.

Plasma-Surface Chemistry

At plasma-material interfaces, gas-phase species impinge on the solid surface and participate in adsorption, reaction, recombination, and desorption events that are collectively described by plasma-surface interaction models. Ion bombardment supplies directional kinetic energy that activates surface reactions and sputters material, enabling anisotropic etching profiles unachievable by purely chemical means. The synergy between ion flux and radical flux, where neither alone produces the desired etch rate but the combination does, is a defining feature of reactive ion etching (RIE) in microelectronics fabrication.

Plasma-enhanced chemical vapor deposition (PECVD) exploits plasma-surface chemistry to grow films at substrate temperatures 200 to 300°C below those required in thermal CVD, by generating precursor radicals in the gas phase rather than relying on thermal dissociation at the surface. Frontier research on neutral reactive species generation continues to refine understanding of how process conditions translate into specific surface film compositions and properties.

Applications

Plasma chemistry has applications in a wide range of industries and research fields, including:

• Semiconductor device manufacturing through plasma etching and thin-film deposition
• Plasma medicine including wound sterilization and cancer cell treatment
• Environmental remediation of volatile organic compounds and nitrogen oxides
• Synthesis of nanomaterials and surface-functionalized particles
• CO2 conversion to synthetic fuels and chemical feedstocks using non-thermal plasmas