Etching
1) Go to BSG
Silicon wafers are washed in a chain cleaning machine by floating on water (with the backside in contact with acid solution) to remove BSG from the backside. The main component of the acid solution is 24.5% HF, and the main chemical reaction equation includes:
HF+SiO2→SiF4+H2O
SiF4+HF→H2SiF6
After washing with water and drying with a wind knife, it enters the next process. The BSG cleaning machine equipment is a semi sealed device that includes an acid tank, a pure water cleaning tank, and an induced draft system to create a micro negative pressure environment inside the equipment and collect volatile gases.
The main pollution in this process includes acidic waste gas (G4) containing HF, which is collected through pipelines and sent to the acidic waste gas scrubbing tower for treatment. High concentration acidic wastewater containing hydrofluoric acid (W10) and general acidic cleaning wastewater (W11).
2) Back etching
To improve the reflectivity of the silicon wafer backside, the backside of the silicon wafer is polished with alkali and polishing agent.
The alkali polishing section (6 lines) includes modules such as pre cleaning, water washing, alkali polishing * 2, hydrogen peroxide cleaning (reserved), micro velvet (reserved), pure water cleaning, post cleaning, pure water cleaning, acid washing * 2, pure water washing after acid washing, slow pulling pre dehydration, drying * 5, etc. The entire process of back etching is automatically carried out, using a transfer arm to send the pre cleaned silicon wafers to the feeding area of the alkali polishing machine. The silicon wafers pass through various corrosion and cleaning tanks in the automatic closed alkali polishing machine through rollers. The equipment automatically controls the replenishment of acid, alkali solution, and pure water in each module. The acid and alkali solution in the tank are pumped in through pipelines, and the wastewater in the tank is discharged regularly.
3) Pre cleaning
After processing, 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 in pre cleaning, add pure water to the tank, and add an appropriate amount of NaOH solution or cleaning solution (NaOH concentration is expected to be 0.39%, H2O2 concentration is expected to be 0.61%) according to the ratio for high-temperature cleaning (60 ℃). Perform pure water cleaning after pre cleaning. Pure water cleaning is all overflow immersion cleaning, carried out at room temperature for 100 seconds.
4) Alkali polishing and washing
The alkaline polishing tank is equipped with pure water, and an appropriate amount of NaOH solution and polishing additives (NaOH solution is about 1.6%, polishing agent concentration is 0.97%) are added. Then, the back surface of the silicon wafer is polished at an operating temperature of 65 ℃. Wash with alkali before rinsing with pure water. The chemical reactions that occur during the alkali throwing process are as follows:
Si+2NaOH+H2O=Na2SiO3+2H2↑
The working temperature of the alkali washing tank is 65 ℃, and the alkali washing time is controlled to be 220s.
5) Post cleaning and micro velvet production
Add pure water to the tank, and add appropriate amounts of NaOH solution and hydrogen peroxide (NaOH solution about 0.55%, hydrogen peroxide concentration 0.25%) according to the ratio for room temperature cleaning. After cleaning, perform pure water cleaning.
The chemical reactions that occur during the micro velvet process are as follows:
Si+2NaOH+H2O=Na2SiO3+2H2↑
6) Acid washing
After post cleaning, a dilute acid solution (0.9% HCl and 0.23% 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 100 seconds.
7) Drying
Transfer the slowly pulled pre dehydrated crystalline silicon wafer to a drying tank, and blow hot air at 90 ℃ up and down on the wafer for drying, using electric heating.
In the above-mentioned back etching process, the pre cleaning, alkali polishing, and post cleaning processes generate high concentration alkaline wastewater containing sodium hydroxide (W12, W14, W16) and general alkaline cleaning wastewater (W13, W15, W17). The acid washing process generates high concentration acidic wastewater containing hydrochloric acid and hydrofluoric acid (W18) and general acidic cleaning wastewater (W19, W20). The above operation is carried out in a closed alkali throwing machine. The acid washing process will volatilize and produce acidic waste gas (G5) containing HCl and HF, which will be collected through pipelines and sent to the acidic waste gas washing tower for treatment.
In situ doping of POPAID deposition
The POPAID process is a key technique for preparing plate coatings by integrating tunneling oxide layers and doped crystalline silicon layers.
Firstly, the silicon wafer enters the loading chamber under atmospheric conditions, is transported into the 300 ° preheating chamber, and then enters the PO process chamber. At this time, O2 is transported to the gas separation block through the trachea, and is activated by the RF power supply to ionize. The ions oxidize on the surface of the silicon wafer, forming a tunneling oxide layer; Then the silicon wafer passes through a transition and buffer chamber and is transferred into the paid chamber. The paid source deposits a certain thickness of amorphous silicon on the back of the substrate, and PH3 gas is introduced during the deposition process. Gaseous phosphine enters the machine and is excited into a state of phosphorus ions by 10kev and 0.5-2kev high-voltage radio frequency. A direct current high voltage is added between the ion source and ground, so that the phosphorus ions obtain energy through the high-voltage electric field. The beam width is 420mm, and then the silicon wafer is transferred to the bottom of the beam. During the process of the atoms of the paid source flying towards the substrate, they carry P ions or react with P ions to achieve in-situ phosphorus doping.
The main reaction equation is: PO+PAID＝POPAID
Plasma oxidation (PO): SiH4+O2 → SiO2
Plasma assisted in-situ doping (PAID): Si (source)+PH3 → n-Si
After the reaction is completed, nitrogen is blown and the ion is injected into the self-contained adsorbent, with a treatment efficiency of up to 100%. The concentration of phosphine before entering the adsorption tower is 179.05ppm, and no PH3 is detected after adsorption. This project plans to connect this exhaust gas to the DA003 waste gas tower for treatment and then discharge it. At the same time, the enterprise plans to install an automatic alarm for phosphorus leakage with a detection limit of 0.1mg/m3.
Analysis of Pollution Production Processes: The main pollution processes in this process are Ar, PH3, and N2 introduced during the process, which are collected by dedicated pipes and sent to the acid waste gas scrubbing tower for treatment.