Texturing
The velvet production section (consisting of 6 lines) includes modules such as pre cleaning, pure water washing before velvet production, velvet production * 3, pure water washing after velvet production, post cleaning, pure water washing after post cleaning, acid washing, pure water washing after acid washing, slow pulling and pre dehydration, and drying * 5. The velvet production method of this project adopts automatic velvet production, and the entire operation process is carried out automatically. The pre cleaned silicon wafers are sent to the feeding area of the velvet machine by a conveyor arm. The silicon wafers pass through various corrosion and cleaning tanks in the automatic closed velvet machine through rollers. The equipment automatically controls the replenishment of acid, alkali and pure water in each module. The acid and alkali in the tanks are pumped in through pipelines, and the wastewater in the tanks is discharged regularly (with a single tank volume of 720L, replaced every 48 hours).
1) Pre cleaning
Pre cleaning purpose: To remove impurities (organic and metal impurities, etc.) adhered to the surface of silicon wafers, NaOH solution and H2O2 solution are used.
Immerse the loaded silicon wafers into a pre cleaning tank in sequence, add pure water to the tank, and add an appropriate amount of NaOH solution or cleaning solution according to the ratio (the mixed NaOH concentration is expected to be 0.6%, H2O2 concentration is expected to be 1.5%, automatically added) for high-temperature cleaning (60 ℃). Pre cleaning uses ultrasonic cleaning. Perform pure water cleaning after pre cleaning. Pure water cleaning is all overflow immersion cleaning, which is carried out at room temperature.
The chemical reactions that occur during the pre cleaning process are as follows:
Si+2NaOH+H2O=Na2SiO3+2H2↑
2) Alkali velvet
Objective: To perform anisotropic etching of silicon surface with alkaline solution, forming a 5um sized pyramid on the surface. The pyramid surface has excellent light trapping and anti reflection effects (10%). Alkali velvet uses NaOH solution and velvet additives.
Adding an appropriate amount of NaOH solution and velvet additive (NaOH solution concentration of about 0.6%, velvet additive concentration of about 0.4%) to the alkaline velvet tank can reduce the surface tension of silicon wafers, improve the wetting effect between silicon wafers and NaOH liquid, promote the release of hydrogen bubbles, enhance the anisotropy of corrosion, make the pyramid more uniform and consistent, and improve the production effect of velvet. The chemical reaction process for the formation of suede is as follows:
Si+2NaOH+H2O=Na2SiO3+2H2↑
The working temperature of the alkali velvet tank is 82 ℃, and the alkali velvet time is controlled at 420s.
3) After cleaning
After alkali velvet treatment, the silicon wafer enters the cleaning tank to remove residual organic matter and ensure the cleanliness of the silicon wafer surface, thereby improving the battery conversion efficiency to a certain extent. Immerse the loaded silicon wafers for cleaning, add pure water to the tank, and add an appropriate amount of NaOH solution or cleaning solution (NaOH concentration is expected to be 0.6%, H2O2 concentration is expected to be 1.5%) according to the ratio for high-temperature cleaning (60 ℃). Clean with pure water after post cleaning. Pure water cleaning is all overflow immersion cleaning, carried out at room temperature.
4) Acid washing
After post cleaning, a dilute acid solution (3.15% HCl and 7.1% HF) should be used for high-purity cleaning. The function of HCl is to neutralize residual NaOH, while the function of HF is to remove the oxide layer on the surface of the silicon wafer, making it more hydrophobic and forming the silicon complex H2SiF6. Through the complexation with metal ions, the metal ions are detached from the surface of the silicon wafer, reducing the metal ion content and preparing for diffusion bonding. Clean with pure water after acid washing.
The chemical reactions that occur during the pickling process are as follows:
HCl+NaOH=NaCl+H2O
SiO2+6HF=H2SiF6+2H2O
The working temperature of the pickling tank is at room temperature, and the pickling time is controlled at 120 seconds.
5) Slow pull pre dehydration
Purpose: Pre dehydration of the surface of crystalline silicon wafers is usually used as the last step in the pure water cleaning process.
Transfer the crystal silicon wafer cleaned with pure water to a slow pull groove. The silicon wafer first sinks into the pure water and is completely immersed. Then, it is slowly pulled upwards by a robotic arm and a basket, and the surface tension can pull down the water film on the silicon wafer.
The slow pull groove consists of a cleaning groove and a slow pull mechanism, and is semi enclosed. There is a serrated overflow port in the cleaning tank, and clean water continuously washes away the sewage in the cleaning tank during operation, keeping the water quality of the cleaning tank clean and achieving the cleaning effect; When the water is kept clean, there will be no water droplets on the working surface under slow pulling, and there will be no watermark during drying.
6) Drying
Transfer the crystalline silicon wafer to the drying tank and blow hot air at 90 ℃ up and down the wafer for drying, using electric heating.
The pre cleaning and alkaline velvet making processes mentioned above will produce high concentration alkaline wastewater containing sodium hydroxide (W1, W3, W5) and general alkaline cleaning wastewater (W2, W4, W6). The acid washing process will produce high concentration acidic wastewater containing hydrochloric acid and hydrofluoric acid (W7) and general acidic cleaning wastewater (W8, W9). The above operation is carried out in a closed velvet making machine. The acid washing process will volatilize and produce acidic waste gas (G1) containing HF and HCl, which will be collected through pipelines and sent to the acidic waste gas washing tower for treatment.
Boron diffusion
The purpose of the diffusion process is to form a PN junction on the silicon wafer to achieve the conversion of light energy into electrical energy. The PN junction manufacturing equipment is a diffusion furnace, and the project uses gaseous boron trichloride to diffuse silicon wafers in the diffusion furnace. Boron atoms diffuse into the silicon wafer and form a layer of borosilicate glass on the surface of the silicon wafer. The main reaction equation is:
4BCl3+3O2→2B2O3+6Cl2↑
2B2O3+3Si→3SiO2+4B
The diffusion furnace is a closed negative pressure equipment equipped with an inlet and outlet, using electric heating, and the equipment comes with an oil-free dry mechanical vacuum pump. The specific process is as follows: first, a large flow of N2 is introduced to drive away the air in the quartz tube of the diffusion furnace, and the diffusion furnace is heated up. After the furnace temperature reaches 1050 ℃ and remains constant, the chip is placed in a quartz boat and sent to the furnace mouth for preheating for 20 minutes, then pushed into the constant temperature zone. Oxygen is first introduced, and then boron trichloride is introduced for diffusion. The overall process time is 180 minutes. During the reaction, both Si and O2 were excessive, and BCl3 reacted completely, resulting in the production of C12. After the reaction is complete, use N2 to clear the equipment and automatically discharge the material.
Analysis of Pollution Production Process: The main pollution process in this process is the diffusion process, where BCl3 is introduced and reacts to produce chlorine gas (G2) mixed with residual oxygen, nitrogen, etc., which is collected by a dedicated pipe and sent to the acid waste gas scrubbing tower for treatment. After being collected through pipelines, it is sent to the acid waste gas scrubbing tower for treatment.
SE laser re doping
Laser doping technology involves heavy doping at the contact area between the metal gate line (electrode) and the silicon wafer, while light doping (low concentration doping) is maintained outside the electrode. Pre diffusion is carried out on the surface of silicon wafers through thermal diffusion to form light doping; At the same time, the surface BSG (borosilicate glass) serves as a local laser re doping source, and through the local thermal effect of the laser, the atoms in BSG rapidly diffuse into the interior of the silicon wafer, forming a local re doping region.
The SE laser process generates dusty exhaust gas (G3), which is treated by the equipment's built-in dust collector and discharged through the workshop's top exhaust system (at a height of approximately 15 meters).
Post oxidation
The oxide layer on the boron diffusion surface (incident surface) of the silicon wafer surface treated by laser SE is destroyed by the energy of the laser spot. During alkaline polishing and etching, a layer of oxide is required as a mask layer to protect the phosphorus diffusion surface (incident surface) of the silicon wafer. Therefore, it is necessary to perform oxide layer repair on the surface scanned by laser SE.
This project uses the method of thermal oxidation to prepare SiO2 oxide layer. The entire oxidation process is carried out in an oxidation furnace, which is a closed atmospheric pressure equipment and heated by electricity. Firstly, the silicon wafer is loaded onto the quartz boat using an automatic wafer loading machine. Then, the automatic robotic arm places the quartz boat on the silicon carbide cantilever slurry of the oxidation furnace. The silicon carbide slurry sends the quartz boat loaded with silicon wafers into the high-temperature quartz furnace tube. After the quartz boat enters the furnace tube, close the furnace door, start the oxidation program, and the oxidation furnace will run automatically. The main chemical reactions that occur during the thermal oxidation process are:
Si+O2=SiO2
O2 reacts with the surface of silicon wafers at high temperatures to generate SiO2, and a certain amount of nitrogen gas is introduced to maintain a constant furnace tube pressure. Maintain high temperature oxygen flow for a period of time to form a certain thickness of SiO2 thin layer on the surface of the silicon wafer. The process parameters are: oxidation temperature of 750 ℃, nitrogen flow rate of 12L/min, oxygen flow rate of 5L/min, and 25 minutes of oxidation time. This process generates oxidation waste gas (hot air) containing oxygen and nitrogen, which is discharged through the exhaust port of the oxidation furnace and then discharged through the workshop top hot exhaust system.