The plasma is an ionized gas composed of ions, electrons and neutral particles. The neutral particles mainly include molecules, atoms and free groups, and the plasma is electrically neutral. In the plasma surface cleaning process, neutral particles such as ions, electrons and free groups contained in the plasma will produce different effects in the process. The mass of the electron is small, and the moving speed of the electron is much faster than that of the ion. The electron reaches the surface to be cleaned earlier than the ion and makes the surface of the object negatively charged, which is conducive to further activation of the reaction. On the other hand, the collision between the electron and the material on the surface to be cleaned will produce excited neutral atoms or free groups to achieve the purpose of cleaning ; the role of  ions usually refers to the role of cations. The cations accelerate in the electric field and the kinetic energy increases. The kinetic energy of the cations reaching the surface to be cleaned is large enough to impact and remove the material attached to the surface to be cleaned. The surface plays a cleaning role ; the free group in the excited state has higher energy, and it is easy to react with the material on the surface to be cleaned to form new substances or free radicals. The new free radicals further react with the material on the surface to be cleaned to achieve the cleaning effect. In addition, the emitted light in the plasma has strong light energy, which can break the molecular bonds on the surface of the object to be cleaned to achieve the effect of cleaning.

In conventional plasma cleaning, the distance between the workpiece and the plasma generator is very close, and the electrons, ions and excited state free groups contained in the plasma have an effect on the processing of the workpiece.

Principle of remote plasma cleaning

The neutral particles such as electrons, ions and excited free groups contained in the plasma will disappear during the recombination process. The recombination process includes the recombination of positive ions and negative ions, the recombination of electrons and positive ions, and the recombination of free groups and free groups. The recombination rate of electrons and ions, negative ions and positive ions is 10-7cm3 / s, and the recombination rate of excited free groups is 10-33cm3 / s. The remote plasma cleaning technology uses the different recombination speeds of different particles in the plasma to place the workpiece away from the plasma generator. During the process of the plasma from the plasma generation area to the surface of the workpiece to be cleaned, the electron and ion recombination speed is fast, and the concentration is gradually reduced. The recombination speed of neutral particles such as free groups is slower and the concentration is gradually increased. The plasma that reaches the surface of the workpiece mainly reacts with the surface of the workpiece through chemical reactions between neutral particles such as free groups and the surface of the workpiece to achieve the cleaning effect, which can reduce the damage to the surface of the workpiece caused by ion bombardment ultraviolet radiation. The reaction is mild, the side reactions are few, and the controllability is strong. 

The most significant feature of remote plasma cleaning is that the plasma generation area and the reaction area are not in the same cavity. The cleaning platform used by the remote plasma source will separate the cavity from the reaction cavity, and its structure is shown in Figure 1. The generation of plasma and the reaction with the sample to be cleaned are completed in different chambers. The reaction gas enters the plasma excitation chamber and is ionized to produce plasma under the action of an external electric field, and then enters the reaction chamber through a pipe or a specific filter device. Because the charged particles will be filtered out by the pipe wall or a specific device during the transmission process, neutral free radicals will enter the reaction chamber and react with the contaminants on the surface of the cleaned sample. Because there are no charged particles, the whole reaction process will not cause damage to the reaction chamber.

Fig. 1 Schematic diagram of remote plasma cleaning principle