Silicon wafer is the basic material for making crystalline silicon solar cells, and it is also the most expensive part of crystalline silicon solar cells. Wafer cutting is a key process in solar cell manufacturing. This process is used to process solid silicon ingots of monocrystalline or polycrystalline silicon. At present, a wire saw is used for slicing. The wire saw first cuts the silicon rod (ingot) into squares, and then cuts into very thin silicon wafers (Fig. 1), including the rolling and squaring of silicon crystals, crystal cutting, silicon wafer grinding, Wafer polishing and cleaning in five steps.
- Basic properties of crystals
Generally speaking, what we call a crystal is a solid with many particles, like atoms and molecules, which are regularly, ordered and periodically arranged in three-dimensional space. In view of the above unique arrangement structure, crystalline silicon has Many features that contribute to industrial production.
- The shape of the product is regular and symmetrical. Due to the regular arrangement of atoms in the crystal, there is a repeated arrangement. The appearance of the crystal has the characteristics of regular symmetry, which is obviously different from the amorphous one.
- The crystal has a fixed melting point. Different types of crystals have different melting temperatures, but as long as the crystals only melt when the temperature reaches a fixed melting point, the amorphous ones do not.
- The crystal has anisotropy. There are different crystallographic directions in the crystal, and the crystals show different physical properties in different directions, such as the anisotropy of graphite conduction.
- Cleavage, when the crystal is subjected to external force, the crystal will crack along a specific face, such as mica. Different crystals have different cleavage planes, and the study of cleavage planes is of great help for the tumbling of silicon crystals.
- Crystal rolling mill and square equipment
Single crystal silicon wafers are usually rolled and squared by a single crystal square mill, a band saw, or a wire saw.
The single crystal square root mill has higher requirements on the mechanical transmission, hydraulic operation, electrical system and cooling system inside the equipment. The grinding process includes: the reciprocating motion of the longitudinal table; the longitudinal motion and rotational motion of the workpiece; the back-and-forth and rotational motion of the rolling grinding wheel; the vertical up-down motion and rotational motion of the square saw blade. The band saw firstly divides a certain aspect of the crystal with spacing, and then rotates 90″ before dividing. Compared with the tumbling square machine, it has higher production capacity, smaller sawing gap, low raw material loss and high production efficiency. Wire sawing Fang is more efficient than the above two methods, and the cutting accuracy is higher.
- Crystal grinding and square rooting process
Roll grinding is a mechanical grinding process, through the relative movement of the grinding wheel and the workpiece, so that the diamond particles on the grinding wheel grind the workpiece to achieve the purpose of processing. Grinding processing is usually divided into two categories: fixed abrasive processing and free abrasive processing according to grinding tools. Different forms of processing have very different uses, working principles and movement conditions. In silicon wafer processing, single crystal silicon barrel grinding, inner circle cutting, diamond wire cutting and chamfering belong to fixed abrasive processing, while grinding, sandblasting multi-wire cutting and polishing belong to free abrasive processing. Figure 2 shows the barrel grinding wheel.
The three stages of crystalline silicon grinding are as follows:
The first stage: elastic deformation stage. The abrasive grains begin to contact with the crystalline silicon, and the abrasive grains are not cut into the crystalline silicon but only rub on the surface, resulting in thermal stress on the surface, which is the elastic deformation stage of the crystalline silicon.
The second stage: the scoring stage. With the deepening of the grinding process, the amount of abrasive grain cutting increases, and the abrasive grains gradually cut into the silicon crystal, causing the silicon crystal material to bulge on both sides to form a notch on the crystal surface, which is the scribing stage.
The third stage: the cutting stage. The abrasive grains have been cut into a certain depth, and the longitudinal cutting has reached a certain degree. When the temperature reaches a certain temperature, this part of the crystalline silicon slips with the cutting surface and forms chips that flow out. The following describes the tumbling prescribing. The tumble grinding process is designed according to the crystal condition and user requirements. For crystalline silicon of solar grade silicon, rounding and squaring are required. If it is a solar-grade ingot polycrystalline block, it only needs to be squared. When single crystal silicon is rolled and cut, it should be oriented first. The main purposes of orientation are two:
- Check whether the axial direction of single crystal silicon meets the requirements;
- Determine the position of the single crystal silicon reference plane or the solar quasi-square ingot, and then round the shape to make the reference plane, and then the machine will complete the fixed size of the single crystal silicon cut by the reference plane. For polycrystalline silicon ingots, due to the large block size, it is necessary to use a band saw or a wire saw for slicing. With the advancement of the cutting process, the cutting of larger silicon blocks will no longer be difficult. If you need to make a reference plane, first turn the crystal so that the side of the reference plane is at the grinding table and make the reference plane marked on the cross section of the crystal perpendicular to the horizontal plane. Then calculate the feed amount according to the width of the required reference plane, and then adjust the upper and lower positions of the grinding wheel appropriately according to the size of the single crystal, and then turn on the grinding wheel spindle and the water pump switch or start the automatic program to make the reference plane.
After the silicon crystal is rolled and squared, a layer of damage will be formed on the surface, which will affect the quality of the subsequent processed products. Therefore, certain chemical corrosion or mechanical polishing is adopted to make the surface of the grain boundary smooth and reduce the probability of the crystal being damaged again.
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