Plasma cleaning is a process that removes contaminants from metal surfaces by bombardment or activation reactions using high-energy particles and active species in plasma. Since plasma cleaning does not use chemical reagents, it causes no secondary pollution. The cleaning equipment is highly reusable, so both equipment and operating costs are relatively low, and the operation is flexible and simple, enabling the cleaning of whole metal surfaces or partial and complex structures. Some metal surfaces after plasma cleaning can also have improved properties, which is conducive to subsequent processing and application of the metal. With the rapid development of high-tech industries, the application of plasma cleaning is becoming increasingly widespread, and it has now achieved an important position in high-tech fields such as electronics industry, semiconductor industry and optoelectronics industry.

Principles of Plasma Cleaning for Metal Surfaces

In plasma, in addition to a large number of gas molecules, electrons and ions, there are also large numbers of excited neutral atoms, atomic group radicals and light emitted by plasma. Plasma cleaning is a process that finally removes surface contaminants by the activation reactions respectively occurring between ions, electrons, excited atoms, free radicals and emitted light with contaminant molecules on the cleaned surface.
Role of Electrons in Metal Surface Cleaning

In plasma, collisions between electrons and atoms or molecules can generate excited neutral atoms or atomic groups (also called free radicals). These excited atoms or free radicals undergo activation reactions with contaminant molecules, causing the contaminants to detach.

Role of Ions in Metal Surface Cleaning

On the one hand, cations are accelerated by surfaces carrying negative charge to gain large kinetic energy, and pure physical collision occurs, which can strip off contaminants attached to the object surface. On the other hand, the impact of cations can also increase the probability of activation reactions occurring in contaminant molecules on the object surface.

Role of Free Radicals in Metal Surface Cleaning

Under normal circumstances, the number of free radicals in plasma is greater than that of ions. They are electrically neutral, have relatively long lifetimes, and possess relatively high energy. During cleaning, contaminant molecules on the surface readily combine with high-energy free radicals to produce new free radicals. These new free radicals also occupy a high-energy state and are extremely unstable, readily decomposing on their own into smaller molecules while generating new free radicals. This process continues ceaselessly until the contaminants are decomposed into stable, volatile simple small molecules, ultimately causing the contaminants to detach from the metal surface. In this process, the main role of free radicals is shown in the energy transfer in the "activation" process. During the combination of free radicals with surface contaminant molecules, a large amount of binding energy is released. This released energy acts as the driving force to promote new activation reactions in surface contaminant molecules, which is favorable for contaminants to be removed more thoroughly under the activation of plasma. It can thus be seen that the role of free radicals in plasma during the plasma cleaning process cannot be underestimated.

Role of Emitted Light in Metal Surface Cleaning

Plasma emits light while it is generated. This light has very high energy and strong penetrating power. Under the action of light, molecular bonds of contaminant molecules on metal surfaces are broken and decomposed, which helps to promote further activation reactions of contaminant molecules adhering to metal surfaces.

In summary, plasma cleaning mainly relies on the activation effect of active species such as electrons, ions, excited atoms and free radicals in plasma to gradually decompose large molecules of organic contaminants on metal surfaces step by step, ultimately producing stable, volatile simple small molecules and completely detaching and removing the contaminants adhering to the surfaces. At the same time, the adhesion and surface wettability of metal surfaces after plasma cleaning can be greatly improved, and these improvements in properties are also very favorable for further surface treatment of metal materials.