- 1. Dephosphorylated silica glass
A layer of SiO2 containing phosphorus, also known as phosphorus silica glass, is removed before etching to prevent it from affecting the subsequent process, mainly by removing the SiO2 layer with HF. which relies on the process of HF reacting with SiO2 to form complexes.
- Wet acid etching
Because the wafer is placed back-to-back during diffusion, the front side and the edges of the wafer are diffused with a layer of P-N junction, and the surrounding diffusion layer is short-circuited because of the P-N junction, causing excessive leakage current, so it should be removed. The method of removal is divided into wet etching and dry etching, and the following will briefly introduce the wet acid etching, which is to remove the P-N junction at the edge by torturing off the silicon rot at the edge with a mixture of HF and HNO3, and the equation has equation (6.9)~equation (6.11), and the mechanism is the same as the principle of velvet making, except that the concentration is greater than the concentration of the solution in velvet making.
The etching equipment has many similarities with the fleece making equipment. The etching machine is also divided into etching tank, water tank, alkali tank (KOH), water tank, acid tank (HF), water tank, and drying tank, with the same loading and unloading machine at both ends. When the wafer travels in the etching tank, the absolute level of the rollers on the bottom side of the wafer must be ensured, because the liquid level of the solution must be precisely controlled to be flush with the wafer, so that the back and sides of the wafer can touch the etching solution and thus etch off the edges and back sides, so controlling the height of the etching solution so that it does not flood the front side of the wafer is also a key control point: in the etching tank, HNO3 and Si react first to produce SiO2 and NO NO2, and then SiO2 reacts with HF to etch off the edges of the wafer, thus removing the P-N junction at the edges.
- Industrial wet etching
Industrial wet etching
Wet etching is mainly used to remove the P-N junction on the back side of the cell by unprotected wet etching on the front side to separate the P N junction, so there will be some other reactions in the tank to hinder the effect of etching. The effect of etching is judged in industry mainly based on the following points.
1.Thinning amount:refers to the mass reduction of silicon wafer from before etching to after etching, generally 0.04~0.12g.
2. square Yang manipulation value, refers to the size of the square Yang uptake before and after etching, used to measure the concentration of phosphorus in the PN junction
3.edge etching effect, test the hot and cold effect by cold execution probe, the carrier of the hot side of the silicon wafer will diffuse to the cold probe direction, thus forming a current. The test should ensure that the edges are etched off to prevent short circuits on the upper and lower surfaces.
There are many ways to control the above three points, such as tank temperature, pump flow rate, drug addition, band speed (silicon wafer travel speed), etc.; and in the process of industrial production, engineers are using calculations and long-term experience to debug the parameters of them to achieve the desired effect.
In addition, etching requires extra attention to the size of the air extraction inside the tank, because the frontal unprotected method of etching is used in the etching tank. Because the volatilization of the liquid in the tank will form a local weather corrosion effect on the upper surface of the silicon wafer, which will destroy the originally diffused P-N junction, and this will change the unevenness of the silicon wafer surface, and after the PECVD will appear bad products such as color spots, bright spots, etc. Therefore, adjusting the air extraction system of the tank is the key to reduce the rework rate after etching.