Abstract Abstract: In the production process of solar wafers, the existence of thin and thick sheets will affect the yield of products, and will affect the production process of battery sheets. This paper mainly analyzes the reasons why solar wafers produce thin slabs to better reduce and avoid the generation of thin slabs. 1. lead...
Abstract: In the production process of solar wafers, the presence of thin and thick sheets will affect the yield of products, and will affect the production process of battery sheets. This paper mainly analyzes the reasons why solar wafers produce thin slabs to better reduce and avoid the generation of thin slabs. 1 Introduction
The slicing process is an important process for the preparation of solar wafers. The principle of solar wafer cutting is that the rotating steel wire carries a large amount of silicon carbide particles, and the position of the table is slowly decreased, because the hardness of silicon carbide is greater than that of polysilicon (crystal silicon The hardness is 6.5, the Mohs hardness of silicon carbide is 9.5), and the silicon block is continuously ground by the angular shape of the silicon carbide to serve as a cutting action. Thin slabs are a very important indicator of the quality of silicon wafers. The presence of thin slabs will affect the pass rate of the wafer and the production process of the cell, so this puts more stringent requirements on the quality of the wafer.
2. Principle of variation in thickness of silicon wafer
The cutting process of the silicon wafer is completed on the guide wheel, and the steel wire is wound on the guide wheel to form a parallel net line parallel to each other, and moves at a speed of 10-15 m/s, and the mortar flows evenly through the slurry nozzle to the wire mesh. The silicon carbide in the mortar is wrapped on the steel wire due to the suspension of the suspension, and the silicon block is cut. However, as the cutting progresses, both the steel wire and the silicon carbide will wear to different degrees, the ellipticity of the steel wire increases, the sand carrying capacity decreases, and the roundness of the silicon carbide becomes larger, the average particle diameter decreases, and the cutting ability also has Therefore, generally, in the direction in which the parallel table moves, the thickness of the silicon wafer entrance point is smaller than the thickness of the exit point; and in the direction perpendicular to the moving direction of the silicon block, the thickness of the silicon wafer entrance side is smaller than the outlet side thickness. The thickness of the silicon wafer has a certain range of deviation. For a silicon wafer having a thickness of 180 μm, the deviation range is ±20 μm. If it exceeds this range, it becomes a defective product-thin thick sheet. Fundamentally speaking, the production of thin slabs is caused by various network problems caused by wire mesh jitter.
3. Analysis of the reasons for thin and thick film
Thin slabs can be divided into two categories: (1) TV (ThicknessVariation thickness deviation), which mainly refers to the thickness deviation between the same position between the silicon wafer and the silicon wafer, and is usually present in the same sawing silicon wafer. (2) TTV (Total Thickness Variation) refers to the deviation between the thickest position and the thinnest position on the same silicon wafer. Thin slabs can be divided into four categories according to their distribution position in the silicon wafer: the whole film thickness (TV); the thickness of the entrance point (TTV); the thickness of the silicon wafer to the exit point (TTV); the thickness of the single piece is uneven ( TTV). The reasons for its analysis are as follows:
(1) The whole piece is thin:
a. The guide wheel groove pitch is not uniform. Wafer thickness = slot pitch - steel wire diameter - 4 times (silicon carbide) D50, according to the required thickness of the silicon wafer, the optimal groove pitch can be calculated. In addition, since the steel wire is worn during the cutting process, the diameter of the steel wire becomes small, and the port is changed from a circular shape to an elliptical shape, so the groove pitch of the guide wheel needs to be compensated according to the line loss condition to ensure uniform thickness of the silicon wafer.
b. No zero is set before cutting. The correct way to set the zero point is (take the HCT machine as an example): After loading the ingot into the machine, manually lower the table so that the guide bars of the four ingots just touch the net and click the touch screen main interface to set the zero button, then slow Raise the workbench to the -1.5mm position and set the zero point and name the cut number. If the zero position is set improperly and the guide strip contacts the wire mesh, the tension of the steel wire due to the frictional force after the start of the cutting is unstable, resulting in a thin thickness from the beginning of the cutting.
c. There is a gap between the guiding strip and the silicon block. After the cutting starts, as the steel wire runs, part of the broken guiding strip is brought into the wire mesh, and the steel wire is misaligned, because the steel wire is instantaneously positioned during the cutting process, so that This causes the whole wafer to be thin and thick.
d. The guide wheel groove is seriously worn. The coating of the guide wheel is made of polyurethane material. After cutting a certain number of knives, the root of the guide wheel groove is seriously worn, and the guide wheel groove is cut off. During the cutting process, the steel wire is swayed in the guide wheel groove to cause a whole thin thickness.
Solution: a. Check whether the groove distance is uniform after the guide wheel is slotted, and compensate the guide wheel groove distance according to the line loss condition.
a. When setting the zero point, control the position of the guide bar from the wire net.
b. Standardize the adhesive operation. When bonding the guide strip on the surface of the silicon block, pay attention to check whether the guide strip is bent, whether the glue is evenly applied, and ensure that there is no gap between the guide strip and the silicon block after the bonding guide strip.
c. During the use of the guide wheel, periodically use the optical projection profiler to detect the guide wheel groove, observe the groove depth and angle of the guide wheel, and find the guide wheel in time when the guide wheel groove is seriously worn.
(2) Thickness of the entry point: (The position where the thickness is thick on the silicon wafer is usually the area from the entrance point to the upward extension of 6 mm) a. The replacement of the table pad is wrong. When replacing the new guide wheel, replace the table pad according to the diameter of the guide wheel (the thickness of the pad is 305mm minus the diameter of the guide wheel). If the thickness of the pad is wrong, the two ingots of the upper table will be set when the zero point is set. The height of the wire mesh is inconsistent with the height of the two crystal rods of the lower workbench, resulting in an unstable wire mesh when entering the knife.
b. The mortar flow rate is large during the cutting process. Due to the unstable state of the steel wire when entering the knife, the mortar flow assembly will impact the wire mesh, and the steel wire will be shaken to produce a thin plate.
Remedy: a. According to the diameter of the guide wheel, replace the pad with the same thickness value.
b. Adjust the mortar flow process in the cutting stage, control the mortar temperature and mortar flow, and reduce the impact on the wire mesh.
(3) Thickness from the middle of the silicon wafer to the exit point:
a. During the cutting process, impurities such as fragments and broken guide strips are mixed into the mortar. As the flow of the mortar and the rotation of the steel wire are drawn into the wire mesh on the guide wheel, the jumper and the skew are caused, resulting in silicon in the area. The middle of the film is thin and thick.
b. The mortar flow rate is not suitable. Whether the mortar flow rate is uniform and whether the flow rate can meet the cutting requirements plays a key role in wafer cutting. If the flow rate of the mortar is interrupted during the cutting process, the cutting force will be uneven, resulting in a thin slab.
Solution: a. Before starting the cutting, clean the debris in the machine cutting room, replace the cutting chamber filter and the filter barrel and filter bag in the slurry tank to ensure the cleanliness of the machine.
b. Clean the slurry nozzle in the cutting chamber, open the mortar, and see if the sand curtain sprayed from the slurry nozzle is intact.
(4) Uneven thickness of single piece:
a. The steel wire tension is abnormal and the fluctuation is large. Steel wire tension is the core element of the silicon wafer cutting process. During the cutting process, the steel wire of the retracting spool must maintain a certain tension (usually 0.12mm steel wire release shaft tension setting range is 22N-24N, 0.13mm steel The wire tensioning shaft tension setting range is 21N-23N). If the tension is too large or too small, it is not conducive to the silicon carbide particles suspended on the steel wire entering the kerf to cut. The normal tension of the steel wire is in the range of ±1N. The factors that cause the abnormality of the steel wire tension are: cylinder leakage, tension arm pulley or cutting chamber inlet pulley sway or pulley jacket deflection.
b. The guide wheel jumps loudly. Generally, the radial runout of the guide wheel is <30μm, and the axial runout is <20μm. If the guide wheel jumps too much, the wire mesh will be shaken, resulting in uneven thickness of the silicon wafer.
c. Machine level problems. If the machine is not level, it will cause a large vibration during the cutting process, which will cause the net to shake.
d. Guide wheel problem. Mainly in the case of severe wear of the new guide wheel or the guide wheel groove, the diameter of the steel wire and the diameter of the root of the guide groove cannot be completely matched.
solution:
a. Use a tensiometer to check the tension of the steel wire, check the air leakage of the cylinder, the sway of the pulley and the wear of the pulley casing, and replace the abnormality in time.
b. After each change of the guide wheel, check whether the guide wheel jump is within the normal range. If it does not meet the requirements, reinstall the guide wheel.
c. Regularly check the level of the machine and adjust it in time.
d. Replace the new guide wheel. Before cutting the first saw silicon wafer, first perform the air-cutting operation to set the steel wire tension high to achieve the purpose of the guide wheel, so that the steel wire can completely match the root of the guide wheel groove. Replace the guide wheel with severe wear and tear in time.
4. Conclusion
By analyzing the type and cause of the thin slab, it is possible to better reduce the generation of thin slabs and improve the yield of finished products.
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