Low temperature plasma is commonly used for material surface cleaning, with high strength and low penetration ability. It has the characteristics of simple and effective process treatment, environmental protection and energy saving. It can undergo a series of complex physical or chemical reactions with the material surface. The principle of plasma cleaning is that plasma collides with the surface of the material to be cleaned through particles with specific reaction activity or high energy inside, such as ions, electrons, excited atoms and molecules, and free radicals. The pollutants or micro particles attached to the surface are stripped off or react with organic matter on the product surface to generate volatile gases, which are discharged with a vacuum pump to achieve the purpose of cleaning the material surface. It belongs to dry cleaning technology, which effectively removes pollutants on the surface of the material to be cleaned without damaging the surface characteristics of the material. Its advantages are obvious and can replace traditional wet cleaning technology.

Radio frequency excitation generates plasma by cleaning gas; Particles in an excited state undergo sputtering and etching on the surface of the material to be cleaned, leading to plasma cleaning reactions. Because plasma contains active particles such as electrons, ions, and free radicals, it is easy for them to undergo physical or chemical reactions with solid surfaces. (1) The interaction between free radicals such as atomic groups and the surface of the product to be cleaned: Plasma free radicals are mostly electronic and exhibit electrical neutrality. Free radicals in an excited state have high energy and are easily combined with surface molecules of the product to continuously generate new free radicals. The newly formed free radicals also have high energy, which can easily trigger decomposition reactions to produce small molecules and new free radicals, thereby triggering continuous chemical reactions on the surface of the product and ultimately decomposing into volatile substances such as water and carbon dioxide; (2) The role of electronics on product surfaces: Electrons impact the surface of the product, causing chemical decomposition or desorption of gas molecules attached to the surface, and a large number of electrons accumulate on the surface of the product, which can easily trigger chemical reactions; (3) The interaction between ions and product surfaces: Typically, positively charged cations accelerate towards negatively charged surfaces, causing atoms or molecules on the product surface to gain greater kinetic energy and ultimately escape from the product surface, a phenomenon known as sputtering; (4) The interaction between ultraviolet radiation and product surface: It has strong light energy and strong penetration ability, which can break the molecular bonds of substances attached to the surface of the product, penetrate the surface of the product and react up to several micrometers deep. In short, by utilizing the activation properties of various high-energy substances within the plasma, surface cleaning of products can be carried out to desorb oxides and pollutants attached to the surface of the product to be cleaned.

Plasma cleaning can be divided into three categories: chemical cleaning, physical cleaning, and physicochemical cleaning.

(1) Chemical reaction based plasma cleaning

The highly active free radicals excited by radio frequency in plasma can react chemically with organic pollutants on the surface of the material to be cleaned, forming specific functional groups such as nitrogen-containing and oxygen-containing functional groups, which can greatly improve the adhesion and wetting properties of the material surface. Using oxygen plasma for cleaning, organic compounds that are not easily volatile react chemically with oxygen to form volatile forms, which are then expelled from the equipment chamber through vacuum aspiration. The advantages of chemical cleaning are fast cleaning speed, good selectivity for organic pollutants, and good treatment effect. Its main disadvantage is that the generated oxides may re contaminate the surface of the material to be cleaned. During wire bonding, it is least desirable to have oxide on the surface of the solder pad, and this phenomenon can be avoided by optimizing the cleaning parameters appropriately.

(2) Plasma cleaning dominated by physical reactions

Sputtering Etching refers to the physical impact of highly active ions excited by radio frequency in plasma to remove atoms attached to the surface of the material to be cleaned. Using argon plasma cleaning, which is an inert gas and does not undergo chemical reactions with the surface of the product, it removes surface pollutants by impacting the surface of the material to be cleaned with sufficient energy from its internal ions. Moreover, after the argon plasma cleaning is completed, the microstructure of the material surface will become rougher, the surface activity and wetting performance will be effectively improved, and the surface adhesion performance will also be greatly enhanced. The advantage of argon plasma cleaning is that no oxides will form on the surface of the material after cleaning. The drawback is that excessive cleaning or the re accumulation of pollutant particles on other unwanted product surfaces may occur. Cleaning parameter optimization can be carried out to overcome these drawbacks.

(3) Plasma cleaning with simultaneous physical and chemical reactions

In certain cleaning situations, both physical and chemical reactions are crucial. When using a mixture of argon and oxygen in an appropriate ratio for plasma cleaning, the reaction rate is faster compared to using argon or oxygen alone. The kinetic energy generated by the accelerated argon ions will enhance the chemical reactivity of oxygen ions, so physical and chemical methods are used to remove severely polluted material surfaces. The selection of plasma cleaning method mainly depends on the surface characteristics and pollutant types of the product to be cleaned, as well as the surface requirements of the product in the subsequent process. Choosing appropriate cleaning processes and parameters for different occasions can effectively improve product reliability and yield.

Advantages of plasma cleaning:

Compared with traditional wet cleaning, plasma cleaning has the following advantages: (1) plasma cleaning has a good cleaning effect under normal or low temperature conditions, without the need for preheating; (2) Plasma cleaning does not require the use of chemical solutions such as acids, alkalis, or other organic solvents, and has no hidden dangers such as corrosion, making it safe and reliable; (3) No waste liquid is generated after plasma cleaning, which is beneficial for environmental protection; Plasma cleaning also does not distinguish the material type of the product to be cleaned. It can effectively clean metal materials, wafers, or most polymer materials on the surface of chip substrates, including various polymers, and can achieve cleaning of complex structures. Its unique advantages are increasingly being applied in various production industries in China.