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.
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 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 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.
The energy of plasma electrons and ions is sufficient to ionize neutral atoms, break molecules apart to form reactive radical species, generate excited states in atoms or molecules, and locally heat the surface. Depending on the process gases and parameters, plasmas are capable of both mechanical work, through physical ablation and high-energy ion bombardment of the surface, and chemical work, through the interaction of reactive radical species with the surface. In general, plasmas can interact with and modify a surface through several mechanisms: ablation, chemical etching, activation, deposition, and cross-linking.
Plasma ablation involves the mechanical removal of surface contaminants by energetic electron and ion bombardment. Ablation affects only the contaminant layers and the outermost molecular layers of the substrate material. Argon gas is often used for its high ablation efficiency and chemical inertness with the surface material.
Chemical etching involves the chemical reaction of surface organic contaminants or films with highly reactive free radicals in the plasma to form volatile byproducts that are released from the sample surface. By proper selection of the gas chemistry, various types of materials can be selectively etched. while minimizing etching of other materials on the sample surface. Oxygen (O2) is commonly used to chemically etch and remove organic materials from sample surfaces.
Plasma surface activation involves the creation of surface chemical functional groups through the use of plasma gases - such as oxygen, hydrogen, nitrogen and ammonia - which dissociate and react with the surface. In the case of polymers, plasma breaks down weak surface bonds in the polymer and replaces these surface polymer groups with chemical groups from the plasma gas, such as carbonyl, carboxyl, and hydroxyl groups. Such activation alters the surface chemistry and wettability of the surface, which can greatly enhance adhesion and bonding to other surfaces.
Plasma deposition involves the formation of a thin polymer coating on the substrate surface through polymerization of the process gas. The process gas is a mixture of a vaporized monomer and inert carrier gas to be used for plasma formation. Plasma polymerization may be applied to deposit a polymer layer with unique chemical functional groups.
Cross-linking is the bonding and linkage of molecular chains in a polymer. Plasma processing with inert gases can be used to cross-link polymers and produce a stronger and harder substrate microsurface. Under certain circumstances, crosslinking through plasma treatment can also lend additional wear or chemical resistance to a material.