Silicon wafers are the main production materials in the semiconductor and photovoltaic fields. Silicon wafer multi wire cutting technology is currently one of the most advanced silicon wafer processing technologies in the world. It is different from traditional cutting methods such as saw blades and grinding wheels, as well as advanced laser cutting and inner circle cutting. Its principle is to use a high-speed moving steel wire to drive the cutting edge attached to the steel wire to friction the silicon rod, thereby achieving the cutting effect. Throughout the process, the steel wire is guided by more than ten wire wheels to form a wire network on the main rollers, while the workpiece to be processed is fed through the descent of the workbench. Compared with other technologies, multi wire cutting technology for silicon wafers has the advantages of high efficiency, high production capacity, and high precision. It is currently the most widely used silicon wafer cutting technology.
Multi wire cutting technology is a landmark innovation in the silicon processing industry and solar photovoltaic industry. It replaces the original inner circle cutting equipment, and the cut chips have many advantages compared to the inner circle slicing process, such as lower bending (BOW), warping (WARP), better parallelism (TAPER), smaller total thickness difference (TTA) discreteness, lower blade cutting loss, shallow surface damage layer, and lower surface roughness of the chips.
The wire cutting mechanism of solar silicon wafers is that the machine guide wheel drives the steel wire during high-speed operation, and the steel wire sends the mortar mixed with polyethylene glycol and silicon carbide powder to the cutting area. During the high-speed operation of the steel wire, it continuously rubs against the workpiece pressed on the wire mesh to complete the cutting process.
During the entire cutting process, the viscosity of the cutting fluid, the particle shape and size of silicon carbide micro powder, the viscosity of the mortar, the flow rate of the mortar, the speed and tension of the steel wire, and the feed rate of the workpiece play a major role in the quality and yield of the silicon wafer.
1、 The viscosity of cutting fluid (PEG)
Due to the fact that silicon carbide micro powder is suspended on the cutting fluid and cut through steel wire throughout the entire cutting process, the cutting fluid mainly plays a role in suspension and cooling.
1. The viscosity of the cutting fluid is an important guarantee for the suspension of silicon carbide micro powder. Due to the different system thinking in the development and design of different machines, the viscosity of mortar is also different, which requires different viscosity of cutting fluid. For example, Swiss wire cutting machines require the viscosity of the cutting fluid to be no less than 55, while NTC requires 22-25, and Ernst&Young requires it to be as low as 18. Only viscosity that meets the cutting standards required by the machine can ensure the uniform suspension distribution of silicon carbide micro powder and the stable entry of mortar into the cutting area through the mortar pipeline along with the steel wire during the cutting process.
2. Due to the high temperature caused by friction during the cutting of silicon material by steel wires with mortar, the viscosity of the cutting fluid plays an important role in cooling. If the viscosity does not meet the standard, it will lead to poor fluidity of the liquid, which cannot lower the temperature and cause burns or breakage. Therefore, the viscosity of the cutting fluid ensures temperature control throughout the entire process.
2、 The Particle Type and Size of Silicon Carbide Micropowder
The cutting of solar silicon wafers is actually carried out by steel wires carrying silicon carbide micro powder, so the particle shape and size of the micro powder are the key to the smoothness and cutting ability of the silicon wafer surface. Laser slicing machine, regular particle size, and cut silicon wafers indicate good smoothness; A uniform particle size distribution will improve the cutting ability of silicon wafers.
3、 The viscosity of mortar
The strength of wire cutting machine's ability to cut silicon wafers is closely related to the viscosity of mortar. The viscosity of mortar depends on the viscosity of the cutting fluid of the silicon wafer laser cutting machine, the compatibility between the silicon wafer cutting fluid and silicon carbide micro powder, the ratio of the silicon wafer cutting fluid to silicon carbide micro powder, and the density of mortar. Only when the mortar viscosity meets the machine requirements (such as NTC machine requirements of around 250) can the cutting efficiency and yield be improved during the cutting process.
4、 The flow rate of mortar
During high-speed movement of steel wire, in order to complete the cutting of silicon material, the mortar must be pumped from the storage box to the sandblasting nozzle by the mortar pump, and then sprayed onto the steel wire by the sandblasting nozzle. The uniformity of mortar flow rate and whether the flow rate can meet the cutting requirements play a crucial role in cutting ability and efficiency. If the flow rate cannot keep up, there will be a serious decrease in cutting ability, leading to wire marks, breakage, and even machine alarms.
5、 The speed of steel wire
Due to the fact that wire cutting machines can perform unidirectional and bidirectional wiring according to user requirements, the requirements for wire speed are also different in both cases. When running unidirectionally, the steel wire always maintains a speed (MB and HCT can be manually adjusted at different times according to the cutting situation), which is relatively easy to control. At present, the operation of one-way wiring is becoming increasingly rare, limited to MB and HCT machines.
When running in both directions, the speed of the steel wire starts to accelerate from zero in one direction for 2-3 seconds to the specified speed. After running for a period of time, it slowly decreases to zero in the original direction, pauses at zero for 0.2 seconds, and then slowly accelerates in the opposite direction to the specified speed. This is a periodic cutting process. In the process of bi-directional cutting, the cutting ability of the wire cutting machine increases within a certain range with the speed of the steel wire, but cannot be lower or higher than the cutting ability of the mortar. If it is lower than the cutting ability of mortar, there will be line marks or even broken lines; On the contrary, if the cutting ability of the mortar is exceeded, it may cause the mortar flow to not keep up, resulting in thick and thin pieces or even line marks.
At present, the average line speed of MB can reach 13 meters per second, and NTC can reach 10.5-11 meters per second.
6、 The tension of steel wire
The tension of steel wire is one of the core elements in the silicon wafer cutting process. Poor tension control is a significant cause of line marks, edge breakage, and even short lines.
1. If the tension of the steel wire is too small, it will lead to an increase in the bending degree of the steel wire, a decrease in sand carrying capacity, and a decrease in cutting ability. As a result, line marks and chips may appear.
2. If the tension of the steel wire is too high, the silicon carbide micro powder suspended on the steel wire will be difficult to enter the saw gap, reducing cutting efficiency, resulting in line marks and chips, and a high probability of wire breakage.
3. If there is a deviation from the zero point due to prolonged use of tension when cutting to the adhesive strip, there may be situations such as edge breakage.
The tension control of wire cutting machines such as MB and NTC is generally less than 1 difference between wire feeding and wire receiving, with only EY's difference of 7.5.
7、 The feed rate of the workpiece
The feed rate of the workpiece is related to the steel wire speed, the cutting ability of the mortar, and the shape of the workpiece at different feed positions. The feed rate of the workpiece is determined by the above related factors throughout the entire cutting process, and it is also the least quantitative factor. But poor control may also result in adverse effects such as line marks and chips, affecting cutting quality and yield.
In short, the operation of a solar silicon wafer wire cutting machine is a meticulous task that requires more experience than technical processes and standards. Only by constantly summarizing and exploring in practical operation can one master the machine with ease.