The parameters of industrial flocking monitoring mainly include the following: temperature, liquid medicine addition, pump flow and belt speed (the speed of silicon wafer in the flocking machine). According to the general experience in industrial production, the more obvious the surface reaction effect of silicon wafer is, the more obvious the suede will be, and the lower the reflectivity will be. But if it exceeds a certain limit, the reflectivity will also increase with the increase of suede. Here we only consider the general situation.
1 temperature. In process design, it is generally 6~8 ℃, and the higher the temperature, the more intense the reaction, the more obvious the surface suede of silicon wafer, and the lower the reflectivity. In industrial production, the temperature is usually adjusted according to the reflectivity of spot inspection to control the reflectivity.
- Dosage of liquid medicine. That is, the addition amount of HF and HNO3. When the cashmere machine is set, the automatic addition of liquid medicine is based on the number of polysilicon tablets entering the cashmere machine. Generally, the liquid medicine is supplemented every 200 or 400 tablets entering the cashmere machine. When the reflectivity is relatively high, the addition amount of these two liquids can be increased to adjust the reflectivity. Experimental research shows that, compared with the suede corrosion pit on the surface of the silicon wafer, HF has the downward corrosion effect, and HNO3 has the effect of corrosion around. Therefore, when you want to reduce the reflectivity, you should appropriately increase the amount of HF, so that the corrosion pit will be deeper and the reflectivity will be lower.
- Pump flow. First, understand the tank structure. In each tank of the cashmere machine, it will be divided into main tank and auxiliary tank. The main tank is the silicon wafer soaking tank, and the auxiliary tank is the liquid distribution tank, which will add the prepared liquid medicine to the main tank; There are two pumps in the cashmere machine. One is self circulation in the auxiliary tank, which is mainly used to stir the prepared liquid medicine in the tank evenly; The other pump sprays the prepared liquid medicine onto the surface of the silicon wafer and flows to the main tank. This pump is also the key to adjusting reflectivity. When the pump flow is relatively large, the amount of liquid medicine flowing through the silicon surface will be relatively large, the reaction will be more obvious, and the reflectivity will be relatively low.
- Belt speed, which is the speed of the silicon wafer in the cashmere machine, is generally 2.1~2.4m/s. The reflectivity will also be changed to a certain extent by adjusting the belt speed. Because when the belt speed is slow, the silicon wafer contacts the liquid medicine for a long time, and the reaction is relatively full, the reflectivity will be relatively low. Belt speed is usually an important parameter for industrial production improvement, but the adjustment of belt speed will cause a series of reactions. If the belt speed is adjusted, the above three parameters will be adjusted accordingly. In industrial production, the following parameters should be regularly and quantitatively detected:
- Process formula. Including temperature, automatic addition of HF and HNO3, pump flow and belt speed.
- Thinning amount: the weight reduction of a silicon wafer from before to after flocking should be within 0.3~0.6g. 3 reflectivity. The reflectivity of the middle area of the silicon wafer is measured to be between 27% and 29%.
4 suede. The suede size is normal, the depth is 3~5 µ m, and it covers the whole silicon surface
- Surface condition: a small amount of lint, no obvious brightness, fuzzy grain boundaries, and the silicon wafer is completely blown dry.
From the microscopic point of view, the reflectivity has an important relationship with the corrosion pits produced by acid corrosion. Experimental research shows that the antireflection effect of suede depends on the structural parameters of suede. The University of New South Wales has established a model diagram of the antireflection effect of acid etched suede by approximating the structure of the etched pit as an arc pit, and proposed the depth width ratio (h/d) of the etched pit. The greater the depth width ratio, the more obvious the antireflection effect is, as shown in Figure . Therefore, when controlling the suede reflectivity, we should not only look at the size of the suede, but also observe the depth of the suede. The ratio between the two will affect the size of the suede reflectivity. The suede structure of flanging is related to the formula of corrosive solution, corrosion time and temperature, but there are many factors to be considered in practice, such as the grain size of polysilicon itself, and the number of crystal defects will affect the reflectivity of silicon wafer surface. Therefore, the actual situation of industrial production should be considered when measuring reflectance and adjusting process parameters.
The overall thickness of silicon wafer corrosion is also one of the important parameters in the acid etching process. Considering the cost saving of silicon raw materials, the thickness trend of wire cut silicon wafer will be 100 µ m at present, but most silicon wafers in the market are more than 180 µ M. There will be 5-7 µ m damage layers on the two cutting surfaces of silicon wafer online cutting. In the process of polycrystalline silicon flocking, if the corrosion thickness is too small, the wire cutting damage layers and crystal defects on the surface of silicon wafer cannot be completely removed, the battery surface is prone to compound, the minority carrier life is shortened, and the short-circuit current and the conversion efficiency of solar cells are affected; The corrosion thickness is too large, or the silicon wafer is in the corrosive solution for too long, which will lead to the increase of the surface pore microstructure size, which will reduce the antireflection effect, and the silicon wafer becomes thinner, which will reduce the strength. The test results of a large number of samples in the factory show that the corrosion depth to achieve the optimal battery efficiency is 3~5 µ M.
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