There are three conventional states of matter, namely solid state, liquid state, and gas state. Plasma is a special state of matter that exists under specific circumstances, which is different from the three conventional states, also known as the fourth state. In plasma, there are electrons in a high-speed motion state, neutral atoms, molecules, and atomic groups (free radicals) in an excited state, and ultraviolet radiation generated by molecular dissociation reactions. However, overall, the material is still in an electrically neutral state. The short wavelength and high or low energy of substances in plasma play an important role in the interaction between plasma and surface substances of objects. Plasma cleaning technology is based on the principle of generating plasma and utilizing active particles. By generating active ions, electrons, and excited atoms and molecules that can react chemically with surface pollutants or target substances, surface cleaning and modification can be achieved.

Plasma cleaning principle

1) The cleaning effect of free radicals on surface substances of objects

The atomic groups (free radicals) in plasma have a long lifespan due to their heavy electric charge, and therefore play an activating role in chemical reactions with surface substances of objects. The excited atomic groups (free radicals) are in an active state and undergo chemical reactions with the surface substances of the object, forming new atomic groups (free radicals). These newly formed atomic groups (free radicals) are also in an unstable state and will continue to undergo decomposition reactions, breaking down into smaller molecules and new unstable atomic groups (free radicals). This process will continue until they decompose into simple molecules such as water and carbon dioxide, thus cleaning the surface of the object.

2) The cleaning effect of electrons on surface substances of objects

Electrons in plasma are in a high-speed state, and their impact with the surface of an object is conducive to causing chemical reactions of surface substances, leading to decomposition or desorption reactions, thereby cleaning the surface of the object. Electrons carry a negative charge, causing the surface of an object to have a negative charge.

3) The cleaning effect of ions on surface substances of objects

Ions with positive charges in plasma are in a high-speed state and are attracted by electrons on the surface of an object, carrying negative charges. When positively charged ions rapidly rush towards the surface of an object, they gain greater kinetic energy and collide with the object's surface, removing the particles adsorbed on the surface and promoting the probability of chemical reactions on the object's surface. This phenomenon caused by the impact of ions on the object's surface is called sputtering.

4) The cleaning effect of ultraviolet radiation on surface substances of objects

During the molecular dissociation reaction in plasma, ultraviolet radiation with strong light energy is generated, which can cause the molecular bonds of substances on the surface of objects to break and undergo decomposition reactions, thereby cleaning the surface of objects. At the same time, this type of ultraviolet radiation also has strong penetration ability, which can penetrate the surface of objects by several microns and take effect.

From the above points, it can be seen that the principle of plasma cleaning is to utilize the characteristics of various substances in plasma, which undergo physical or chemical reactions with the molecules of substances attached to the surface of the object, thereby activating them. The substances and dirt attached to the surface of the object are cleaned and removed, achieving the effect of cleaning the surface of the object and improving its surface activity.

Reaction principle of plasma cleaning

There are two types of reactions that can occur between plasma and material surfaces: one is chemical reactions through atomic groups (free radicals), and the other is physical reactions through ions.

1) Chemical action

H2 and O2 are the two most commonly used gases in plasma cleaning processes. These two gases usually react under the action of electrodes to produce highly active free radicals, and their formulas are as follows:

e-+H2→2H；e-+O2→2O；O+O2→O3

These free radicals will further react with the surface of the material. The products of the reaction are volatile substances that can be extracted.

The principle of oxygen (O2) plasma cleaning is the use of chemical reactions for cleaning. It involves the oxidation reaction between the generated oxygen ions and organic pollutants attached to the workpiece, producing carbon dioxide gas (CO2) and water gas (H2O) that are discharged by the vacuum pump of the equipment to achieve the purpose of cleaning the workpiece;

2) Physical reactions

Ions with positive charges in plasma are in a high-speed state and are attracted by electrons on the surface of an object, carrying negative charges. The positively charged ions quickly rush towards the surface of the object, gaining greater kinetic energy and colliding with it to remove the particles adsorbed on the surface of the object, while promoting the probability of chemical reactions occurring on the surface of the object. Remove the surface molecular fragments and atoms by ion impact, remove the pollutants from the surface, activate the object surface, improve the wettability of the object surface, and improve the adhesion of the facial mask on the object surface. The commonly used gas is argon (Ar), which does not react with the surface and does not produce oxidation side effects, maintaining the chemical purity of the cleaned material.

The principle of argon (Ar) plasma cleaning is physical reaction cleaning, which uses the generated argon ions to bombard the surface of the workpiece to be cleaned at high speed, hitting the pollutants (mainly oxides) attached to the surface of the workpiece, thereby peeling off and cleaning the surface of the workpiece, achieving the purpose of cleaning the surface of the workpiece.