Plasma is a conductive fluid composed of electrons, ions, atoms (excited or ground state), molecules (excited or ground state), and free radicals formed by the dissociation and ionization of gas molecules excited by heat or external electric fields and radiation. In a plasma generator, in addition to positively and negatively charged particles, there are also many chemically active substances and photons of different wavelengths emitted by radiation. The energy of plasma can be applied to the surface of materials through light radiation, neutral molecular flow, and ion flow. Charged and neutral particles moving at high speeds within the plasma transfer their energy directly to the material surface through collisions, resulting in a series of complex physical and chemical reactions such as heating, etching, formation of free radicals, polymerization, crystallization, and cross-linking.

The physical process of plasma acting on the surface of materials

The physical processes of the interaction between plasma and material surfaces mainly include the following aspects:

(1) Desorption: When plasma interacts with material surfaces, energy can be transferred to atoms or molecules adsorbed on the material surface through ions, electrons, neutral particles, and photons, causing them to overcome adsorption forces (van der Waals forces or chemical bonding forces) and desorb from the solid surface, namely ion desorption, electron desorption, neutral particle desorption, and photodesorption.

(2) Composite: Due to the negative electrode or rapid electron bombardment on the wall surface, the wall establishes a layer potential and becomes negatively charged. Particles with positive charges are attracted to move towards the negatively charged surface, which promotes the recombination process between electrons and positive ions. In a three body collision, positive and negative charged particles collide and recombine, and the excess energy in the recombination is carried away by a third party. The wall often acts as a third party and accelerates the recombination process.

(3) Excitation: The collision of incident electrons with electrons on the surface of a solid can excite the electrons to higher energy levels or cause ionization. Electrons incident on a solid surface can generate Auger electrons. Photon bombardment on solid surfaces produces photoelectron emission. In addition, electrons can also excite electrons in solids to produce collective oscillations.

(4) Sputtering: When ions or neutral particles interact with a solid surface, the kinetic energy of the incident particles is transferred to the surface atoms through collision, allowing the surface atoms to acquire kinetic energy exceeding the binding energy and sputter out. Some sputtered particles return to the solid surface and also undergo a self sputtering process. Its effect is to reduce the molecular weight of the material surface.

(5) Injection: Electrons, ions, and neutral particles with a certain energy bombard the surface of a solid, enter the interior of the solid and combine with atoms inside the solid, causing changes in the structure of the solid and increasing the molecular weight of the material surface.

(6) Etching: Particles in plasma combine with surface atoms or molecules to generate volatile products, which evaporate from the surface and cause plasma etching on the material surface.

Chemical processes of plasma on material surfaces

The chemical process of plasma on solid surfaces is essentially the generation of new chemical bonds or the breaking of existing chemical bonds by surface atoms or molecules induced by particles in the plasma. It includes chemical reactions in the solid surface layer, as well as chemical reactions between the solid surface and the plasma boundary layer. The specific processes are listed below.

(1) Oxidation: Oxygen is a strong oxidant. When oxygen is present in the plasma, it will oxidize the surface of the material, generating oxides or peroxides.

(2) Reduction: Oxygen is a reducing agent. The oxygen in plasma can not only reduce the oxides on the surface of the material, but also often penetrate deeper into the material, reducing its oxides, such as reducing the metal in metal oxides.

(3) Catalyst: In the process of plasma chemical reactions, the presence of a certain material (such as platinum electrode or vessel wall) plays a promoting role in some chemical reaction processes.

(4) Decomposition and cracking: Under the action of plasma, molecules on the surface of materials will decompose, large molecules will undergo cracking, molecular chains will break, molecular weight will decrease, and volatile substances will be produced.

(5) Aggregation: When monomers are introduced into the plasma, polymerization reactions can occur on the surface of the material. This reaction can generate polymers under plasma conditions and deposit them on the surface of materials, or induce plasma to generate free radicals on the surface of materials, which then bind with monomers through molecular chain crosslinking, grafting or functional group displacement from side chains, and block polymerization.