As the basis for making photovoltaic cells and integrated circuits, silicon wafer cleaning is very important. The effect of cleaning directly affects the final performance, efficiency and stability of photovoltaic cells and integrated circuits. Cleaning the silicon wafer not only removes impurities on the surface of the silicon wafer but also makes the surface of the silicon wafer passivated, thereby reducing the adsorption capacity of the surface of the silicon wafer. Commonly used wafer cleaning techniques include wet cleaning and dry cleaning. PAM-XIAMEN supplies silicon wafers, which have been cleaned during the manufacturing process.
1. Wet Cleaning for Silicon Wafer
Wet cleaning uses highly corrosive and oxidizing chemical solvents, such as H2SO4, H2O2, DHF, NH3·H2O and other solvents. The impurity particles on the surface of the silicon wafer react with the solvent to form soluble substances, gases or directly fall off. In order to improve the removal effect of impurities, megasonic, heating, vacuum and other technical means can be used, and finally ultrapure water is used to clean the surface of silicon wafers to obtain silicon wafers that meet the cleanliness requirements. The wet cleaning including technologies of RCA cleaning, ultrasonic cleaning, dual-flow spray cleaning, and ozone micro-bubble method.
Following we take the RCA cleaning for specific explanation
The current common RCA cleaning technologies include SPM, DHF, SC-1, SC-2. SPM is composed of H2SO4 with a volume fraction of 98% and 30% H2O2 in a ratio of 4:1. It has strong oxidizing properties between 120 °C and 150 °C, and can oxidize the organic matter adhered to the surface of the silicon wafer into H2O and CO2 , so as to effectively remove organic impurities. However, high concentrations of sulfuric acid tend to carbonize organic matter, and SPM solution cannot remove the carbonized organic matter.
DHF is a dilute HF solution. HF:H2O is between 1:100~1:250. It has strong corrosiveness between 20 and 25 °C, which can effectively remove the natural oxide layer on the surface of the silicon wafer. The metal elements (Al, Zn, Fe, etc.) in the oxide layer undergo a redox reaction to form metal ions and then be removed without affecting the silicon atoms on the surface of the silicon wafer. SC-1 is composed of NH3•H2O and H2O2 and H2O according to the ratio of 1:1:5. After cleaning at 70℃ for 10min, the thin layer of silicon atoms on the surface of the silicon wafer is corroded and peeled off by NH3•H2O, and the silicon atomic layer on the surface of the silicon wafer is associated with it. The granular impurities then fall off into the cleaning solution, thereby effectively removing the granular impurities. Experiments show that when the ratio of H2O:H2O2:NH3•H2O is 5:1:0.25, the removal rate of particles is the highest, but the roughness and defects of the silicon wafer surface are increased. SC-2 is composed of HCl, H2O2 and H2O in a ratio of 1:1:5. After cleaning at 70°C for 10 minutes, the metal and its compounds on the surface of the silicon wafer undergo redox reaction, forming metal ions into the cleaning solution. in order to effectively remove metal impurities. Experiments show that when the pH of the solution is between 3 and 5.6, not only metals and their oxides can be removed, but also the re-attachment of metal ions can be prevented.
The RCA cleaning technology in the order of SPM, DHF, SC-1, SC-2 basically meets the cleanliness requirements of most silicon wafers, and passivates the surface of silicon wafers. TMPan et al. added hydroxylated tetramethylamine (TMAH) and ethylenediaminetetraacetic acid (EDTA) in the SC-1 process of RCA cleaning, and cleaned the silicon wafer at 80 °C for 3 min. Since the combination of the hydroxylated tetramethylamine cation and Si shows hydrophobicity, and the adsorption of the hydroxylated tetramethylamine cation with the impurity particles shows hydrophilicity, the hydroxylated tetramethylamine cation gradually penetrates between Si and the impurity particles, carrying impurities away The wafer surface melts into the water. The measurement shows that the particle impurities and metal ions on the surface of the silicon wafer are basically removed and the effect is better than the traditional RCA cleaning, and the electrochemical performance of the silicon wafer is also improved.
This method eliminates the SC-2 cleaning process and simplifies the RCA cleaning technique. Using this method to clean the silicon wafer not only improves the cleaning efficiency, reduces the cost, saves time, and obtains excellent surface cleanliness, but also improves the electrochemical performance of the silicon wafer, which is suitable for comprehensive promotion.
2. Dry Cleaning for Silicon Wafer
Dry cleaning means that chemical solvents are not used in the cleaning process, such as vapor phase dry cleaning technology and beam cleaning technology. The vapor phase dry cleaning technology uses vaporized anhydrous HF to interact with the natural oxide layer on the surface of the silicon wafer, which can effectively remove the oxide on the surface of the silicon wafer and the metal particles in the oxide layer, and has a certain ability to inhibit the generation of oxide film on the surface of the silicon wafer. Vapor dry cleaning greatly reduces the amount of HF and speeds up the cleaning efficiency.
The dry cleaning techniques include dry ice cleaning, UV-ozone cleaning, gas phase cleaning and beam cleaning technology. Among all the dry cleaning technologies, the use of dry ice particle cleaning technology to clean silicon wafers is very effective, and it does not damage the surface of silicon wafers and does not pollute the environment. It is an ideal cleaning technology for silicon wafer. Specifically:
When the temperature exceeds 31.1 °C and the pressure reaches 7.38 MPa, CO2 is in a supercritical state, and the mutual conversion between gaseous and solid states can be realized. CO2 is suddenly ejected from the cylinder through the nozzle, the pressure drops, the body expands rapidly, and the isenthalpic change of CO2 occurs, and the CO2 mixed with gas and liquid generates solid dry ice particles, thereby realizing the cleaning of silicon wafers. Dry ice particles remove particles and organic impurities by different mechanisms. When removing particulate impurities, the dry ice particles collide elastically with the particulate impurities, resulting in momentum transfer, and the particulate impurities are crushed and taken away with the high-speed airflow. When removing organic impurities, the dry ice particles collide with the organic matter inelastically, and the dry ice particles liquefy and wrap the organic matter off the surface of the silicon wafer, and then solidify and are carried away by the high-speed airflow.