Plasma, the fourth state of matter, is a distinct processing medium for surface treatment and surface modification. This note discusses the nature of plasma and how plasma is formed, its unique advantages, and the types of surface interactions that are possible during plasma treatment.

Nature of Plasma

Plasma is a partially ionized gas consisting of electrons, ions and neutral atoms or molecules. Although the plasma electrons are at a much higher temperature (around 104 K ) than the neutral gas species, the plasma as a whole is at near-ambient temperature. The plasma electron density is typically around 1010 cm-3.

Plasma Formation

Plasma is generated when a radio frequency (RF) oscillating electric field is generated in the gas, either through the use of capacitive plates or through magnetic induction. At sufficiently low pressures, the combined effect of the electric field acceleration of electrons and elastic scattering of the electrons with neutral atoms or field lines leads to heating of the electrons. When electrons gain kinetic energy in excess of the first ionization threshold in the neutral gas species, electron-neutral collisions lead to further ionization, yielding additional free electrons that are heated in turn.

Plasma Advantages

Plasma treatment mainly affects the near surface of a material without altering the bulk material properties. In addition, the plasma forms at near-ambient temperature, minimizing the risk of damage to heat-sensitive materials. Depending on process gases and usage configuration, plasma treatment can be used to clean, activate, or chemically modify surfaces. As such, plasma treatment can be applied to many different materials as well as complex surface geometries, including glass coverslips and slides, semiconductor wafers, polymer fibers and fibrous scaffolds, oxide and metal nanoparticles, and porous membranes.

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