Knowledge | 2022-05-16
Introduction of various silicon wafer cleaning methods and their latest development direction
At present, our country's photovoltaic industry is developing rapidly, and the annual production volume of solar batteries in my country ranks first in the world. The cleanliness of solar silicon wafers has a great impact on the continued development of solar cells, so people put forward more demanding requirements for the method of cleaning solar silicon wafers. During the process of the section, chamfering, grinding, polishing, etc., the surface of solar silicon wafers will be contaminated to varying degrees, such as particles, metal ions, and organic matter. If this pollution is not handled, the performance of the device will be greatly affected. At present, half of the total loss of manufacturing is due to the failure of the device caused by improper silicon wafers. Therefore, people's research on solar silicon wafer cleaning technology is continuously deepened. This article briefly introduces the type of pollution of silicon wafers, and based on this, several commonly used solar silicon wafer cleaning methods and their latest development direction.
1. The type of pollution of the processing surface of silicon wafers
The probability of silicon wafers pollution is relatively high because, during the processing of silicon wafers, it is impossible to achieve no pollution. The pollution path may be derived from the atmosphere, water, chemical reagents, people, and processing. Pollutants can be divided into the following types: particles, organic matter, and metals. 1) Granules: The type of particles is mainly some polymers, optical resistances, etc.; 2) Organic objects: These can have many ways exist, such as lubricating oil, muck, wax, etc. If these substances are not effectively cleaned, the subsequent processing process will be greatly affected; 3) Metals: It exists mainly in three forms such as covalent bonds, Van Dehua gravitation, and electronic transfer on the silicon wafer. The existence of metal will destroy the oxidation layer, causing fog-like defects or micro-structural defects.
2. Several methods commonly used in solar silicon wafer cleaning
2.1 RCA cleaning method
The RCA cleaning method is a typical wet chemical cleaning. Although it has been invented earlier, the technology at that time was not now developed, but it still occupies an important position in various cleaning methods. RCA cleaning is very effective when removing pollutants such as organic surface masks, particles, and metals, which is why it can be widely used. However, there are many disadvantages of the cleaning method: for example, a large number of different chemical reagents should be used during the cleaning process, which is serious to environmental pollution; and because the cleaning process is mainly in a high-temperature environment, this requires a lot of liquid chemicals to use a lot of liquid chemicals. He is water; at the same time, to inhibit the evaporation of chemical reagents, a large amount of air is needed; the use of chemical reagents will increase the roughness of the silicon wafer. Therefore, the use of large chemicals, large emissions, and a polluted environment have restricted the continued application of the RCA cleaning method, and it is necessary to improve or adopt other cleaning methods.
2.2 ultrasonic cleaning
Ultrasonic cavitation effect, radiation pressure, and sound flow are the main principles of ultrasonic cleaning. In the process of cleaning, the silicon wafer is placed in the liquid in the tank, and then the energy is transferred to the liquid by using the ultrasonic vibrator at the bottom of the tank and passed through the liquid in the form of the acoustic wavefront. When the vibrations are strong, the liquid is torn apart, creating many bubbles called cavitation bubbles. These bubbles are the key to ultrasonic cleaning. They store the energy for cleaning. Once the bubbles touch the surface of the silicon, they explode, releasing a huge amount of energy to clean the surface. Add the appropriate Surfactant to the cleaning solution to increase the ultrasonic cleaning effect. Ultrasonic cleaning has many advantages: the speed of cleaning; the cleaning effect is better; can clean a variety of complex shapes of silicon surface; easy to achieve remote control and automation. It has the following defects: the use of volatile organic solvents in the cleaning process, the need to increase the recovery equipment, increasing the cost of cleaning; The larger the particle size, the better the cleaning, but the smaller the particle size, the less effective the cleaning; the Surfactant used in the cleaning process is organic, and when the inorganic material is removed, the particles of the chemical itself are left behind to pollute; when the cavitation bubbles burst, the enormous energy can cause inevitable damage to the wafer.
2.3 gas-phase dry cleaning
In gas-phase dry cleaning, the film is rotated at a low speed and then dried at a higher speed. At this time, HF steam can be used to remove oxide film stains and metal contaminants. The method can effectively clean the deep parts of the structure, such as the groove. The cleaning effect of the particles on the surface of the silicon wafer is also better, and there is no secondary pollution. Although HF steam can remove natural oxides, it can not remove metal contamination effectively.
2.4 alkaline detergent and hydrogen peroxide
In the method, the first step is to clean the solar wafers with alcohol; the second step is to clean the wafers in turn with an alkaline cleaning solution and water, and the third step is to clean the wafers in turn with hydrogen peroxide and water, finally, cleaner solar wafers can be obtained. In this method, the alkaline cleaning agent solution reacts with and dissolves the metal powder on the surface of the silicon wafer, and then the metal powder can be removed by washing with water. The hydrogen peroxide solution oxidizes and dissolves organic solvents on the surface of the solar wafers, which can then be removed by washing with water to produce cleaner solar wafers. The method can effectively reduce the residual metal powder and organic solvent on the surface of the solar energy silicon wafer and clean the silicon wafer.
3. Latest developments in solar wafer cleaning
With the development of science and technology, the latest development direction of solar silicon wafer cleaning is the laser cleaning method, which is favored by the industry. Its principle is mainly the instantaneous thermal expansion mechanism. The advantages of this cleaning technology are mainly shown in the following aspects: (1) laser cleaning is not directly carried out in close range, it can also be used to clean places that were previously difficult to reach; (2) no chemical solution is used, therefore, the pollution to the environment is relatively small; (3) laser cleaning can not only choose different pollutants on the surface of cleaning materials, but also the materials will not be harmed in the process of cleaning; (4) laser cleaning has a relatively wide range, it is useful for different types of pollutants, and the degree of cleaning is also high. (5) laser cleaning can not only clean the big particle pollutants but also clean the micron size of the pollution particles effectively; (6) the equipment used for laser cleaning has the advantages of high operation efficiency, low operation cost, and automatic operation.
There are many ways to clean solar wafers nowadays, but each method has its advantages and disadvantages, enterprises can choose according to their actual needs. The development of solar silicon wafer cleaning should be directed in the direction of resource-saving, high cleaning efficiency, and environmental protection, such as the popular laser cleaning method. This has an extremely vital role and significance to environmental protection and the national economy's sustainable development.