Plasma cleaning is a micro - or even nano level treatment that can modify the surface of materials without changing their original properties. The plasma treatment process is green, energy-saving, and environmentally friendly because plasma cleaning does not require any organic solvents or water, only a small amount of process gas required for cleaning, so there will be no environmental pollution problems. Plasma cleaning can react with pollutants in a short period of time and has high selectivity to meet the surface treatment requirements of the vast majority of materials, without damaging other parts. At present, the plasma cleaning technologies mainly used in industry are divided into two categories: vacuum low-pressure plasma cleaning technology and atmospheric pressure plasma arc discharge cleaning technology.

The plasma generation principle of vacuum low-pressure plasma cleaning technology is completely different from that of atmospheric pressure plasma discharge cleaning. According to the principle of plasma generation, vacuum low-pressure plasma cleaning technology can be mainly divided into three categories, namely capacitive coupling (CCP), inductive coupling (ICP), and spiral wave plasma (HWP). The discharge structures of CCP and ICP are shown in the following figures.

Figure 1.1 (a) Capacitive coupled plasma source, (b) Inductive coupled plasma source

The capacitive coupled plasma source is shown in Figure 1.1 (a), and the discharge device is equivalent to a flat capacitor. After being energized, an electric field is generated between the two plates, and plasma is generated between the two plates. One electrode is connected to the matching network of the RF power supply, while the other electrode is grounded. The main function of the two plates that make up a capacitor is to store charge. When it is in the positive half cycle of RF AC power, it is equivalent to the power source recharging the capacitor. When it is in the negative half cycle, the capacitor itself acts as a power source to discharge. This cycle of continuous charging and discharging is equivalent to a constant current flowing through the capacitor. In CCPs with the same electrode size specifications, electrons in the plasma flow directly from the ground electrode to the output electrode, and the current flowing to the sidewall is very small. Therefore, the thickness of the plasma sheath and the sheath voltage drop are symmetrical, and the current density flowing through the sheath is the same, without forming bias on any electrode. For CCP discharges with different electrode size specifications, the electrode area used as the output electrode is usually smaller than that used as the ground electrode. In order to maintain total current conservation, the current density flowing through the output electrode is higher. As a result, the charge density on the surface of the output electrode is higher than that of the ground electrode, and a negative DC bias is formed, that is, the average sheath voltage drop of the output electrode is greater than that of the ground electrode. This DC self bias helps to enhance the energy of ions, thereby regulating the cleaning process.

The inductively coupled plasma source is shown in Figure 1.1 (b), and the ICP body is a cylindrical antenna connected to an RF matching network. Typically, a medium such as a quartz tube is used to isolate the RF antenna from the plasma. The cylindrical antenna has a small structure and is relatively flexible to use. There are no special requirements for the shape of the chamber, and ICP discharge can be achieved by adding a quartz tube to the vacuum window. The ICP discharge mode is divided into E mode and H mode, usually experiencing E mode and H mode respectively as the power gradually increases. As the mode changes, the plasma density also gradually increases. ICP discharge solves the problem that ion flux and ion energy cannot be independently changed under CCP discharge, and ICP is more widely used in material surface treatment processes. The antenna of the spiral wave discharge plasma source is a spiral antenna, similar to the ICP antenna, with a small structure and relatively flexible use, but requires an external magnetic field to induce the discharge mode to transform into a spiral wave mode. As the power increases, the spiral wave discharge will undergo E-mode, H-mode, and W-mode (spiral wave mode) in sequence. In W-mode, the plasma density will increase by an order of magnitude relative to H-mode. Therefore, the spiral wave plasma cleaning method is more efficient than ICP cleaning. In addition, the movement speed of electrons in spiral wave plasma is much faster than that of ions, which is beneficial for electrons to reach the material surface in advance, making it negatively charged and providing a good environment for the subsequent activation reaction.