Physical method solar cell polysilicon

Crystal cutting-inner circle cutting process and multi-wire cutting of single crystal silicon

admin on May 10, 2022 0 Comments • Tags: #innercirclecuttingprocess #multiwirecuttingofsinglecrystalsilicon

Crystal cutting-inner circle cutting process and multi-wire cutting of single crystal silicon

Yanmo Kaifang lays the foundation for the in-depth cutting of crystals, and crystal cutting is to use special equipment such as internal slicers or wire cutting to cut silicon crystals into thin slices that meet the requirements. At the same time, the silicon single crystal wafer used for device production also has certain requirements on its surface orientation. Therefore, when cutting, it must follow a certain direction, and select the appropriate silicon single crystal direction to meet the required crystallographic direction and degree of deviation. The most commonly used methods for directional cutting of silicon single products are light pattern orientation and X-ray diffraction orientation methods. The oriented silicon crystals are cut to obtain silicon wafers with dimensions similar to the actual application. Two cutting methods of crystalline silicon are introduced below: inner circle cutting and multi-line cutting.

  1. Inner circle cutting process

The inner circle cutting uses the inner circle blade as the cutting tool, and the inner circle is used as the knife edge, and the knife edge is inlaid with diamond particles, which are ground and cut piece by piece. The transformation of inner circle cutting varieties is simple, convenient, flexible, and low-risk, but the efficiency is low, the loss of raw materials is large, the deformation of the silicon wafer is large, and the consistency of processing parameters is poor.

Figure 1 shows a simple process flow for inner circle cutting. First, clarify the crystal cutting requirements and do the corresponding preparations, then bond and fix the tumbled body with a special cutting fixture, and then orient to determine and adjust the deviation angle and direction, and the cutting head will be automatically cut after proofreading , after the cutting is completed, the silicon wafer is removed, rinsed and degummed, and then sent for cleaning, so that the entire inner circle is cut.

Figure 1 - Simple Process Flow for Internal Circle Cutting
Figure 1 – Simple Process Flow for Internal Circle Cutting

(1) Preparations
The preparation for inner circle cutting includes reading the process sheet to clarify the processing instructions, checking the workpiece and checking the equipment and process conditions.

  1. Machining instructions. The command of silicon single crystal cutting refers to various parameter requirements related to silicon cutting, mainly including thickness, surface orientation of silicon wafer and so on.
  2. Check the workpiece. Before cutting, it is necessary to check whether the workpiece is correct, mainly from the number, diameter, length and appearance characteristics of the reference plane.
  3. Check the equipment and process conditions, check whether all parts of the equipment are normal, whether the compressed air, cooling water, indoor environment and labor protection supplies meet the requirements. The commonly used data standards are single-phase 220V and three-phase 380V, compressed gas pressure ≥ 0.5MPa, no moisture and impurities, and the indoor environment requires an appropriate constant temperature.

(2) Monocrystalline silicon bonding
First, determine the bonding direction of single crystal silicon, and pay attention to avoid the crystal cleavage plane and the bonding direction as much as possible when cutting out the knife, so as to facilitate directional cutting. Under normal circumstances, the bonding direction is: (111) the crystal bonding direction is perpendicular to the main reference plane; (110) the crystal bonding direction and the main reference plane are 25. After the crystal bonding direction is determined, the single crystal silicon, pallet and clamping tools are cleaned and then glued and reinforced with glue.

When the wafer is cut from the ingot, there needs to be something to prevent the wafer from falling loose. Typically, a carbon plate and the ingot are bonded together with an epoxy bond so that the wafer can be removed from the ingot. After being cut from the ingot, it remains glued to the carbon plate, which in many cases is trimmed, slipped, ground flat, and then bonded in the material preparation area.

Carbon board is not the only choice for bonding board, any kind of bonding board and epoxy bond must have the following characteristics: can support the silicon wafer, prevent it from falling during the slicing process and can be easily removed from the bonding board Peel-off on epoxy binders: also protects silicon wafers from contamination.

Graphite is a hard material used to support silicon wafers, and it is shaped to conform to the bonding site of the ingot. In most cases, the carbon plate should be bonded strictly along the reference surface of the ingot (Figure 2). ), so that the carbon plate can be processed into a rectangular strip. Of course, the carbon plate can also be bonded to other parts of the ingot, but it should also be consistent with the shape of this part. The shape of the carbon plate is very important, because it is required to use as little epoxy bonding agent as possible between the carbon plate and the ingot. and the shortest possible distance. This distance needs to be kept as short as possible because the epoxy bond is a fairly soft material and the carbon plate and ingot are very hard materials. Wafer chipping can occur as the blade cuts from hard material to soft material to hard material.
The carbon plate not only provides support for the silicon wafer during slicing, but also provides material to protect the blade after the blade has cut the silicon wafer.

Figure 2 - Bonding of carbon plate and ingot
Figure 2 – Bonding of carbon plate and ingot

(3) Orientation and proofreading
Single-crystal silicon dicing To obtain wafers with desired surface orientations, directional dicing techniques are usually used, that is, dicing along the crystallographic direction of the silicon single crystal axis.
After orientation, load the crystal into the cutting machine, adjust the deviation angle of the two-dimensional turntable on the machine according to the orientation result, cut the first piece for inspection and proofreading, if it meets the requirements, continue to cut, otherwise adjust until it meets the requirements.

(4) Operation process of inner circle cutting

  1. Blade installation and adjustment, select the appropriate blade according to the diameter and thickness of the silicon wafer to be cut and install it on the cutter head of the cutting machine. Pay attention to the centering and adjust the tension. The tension should be even and moderate.
  2. Turn on the power of the equipment and its cooling device. Open the air compression valve, protection valve and water valve on the air bearing and cutting machine, adjust the cooling water flow rate to 20mL/s, and then turn on the main power supply of the cutting machine.
  3. Adjustment of cutting parameters. Before cutting, it is necessary to set the feed index of the machine according to the thickness of the silicon wafer to be cut, and set the feed speed according to the condition of the crystal to be cut. Too slow will affect the cutting output, and too fast will affect the quality of the silicon wafer and consume the blade. Larger, usually set the cutting speed to 30~70mm/min.
  4. Initial. Cut and load the workpiece or special whetstone jig with the whetstone attached, and then cut the glued crystal after completion. The head piece for single crystal silicon cutting needs to be removed, and the crystal orientation, thickness, total thickness change and diameter should be measured, and the shape change of the silicon wafer should be observed.
  5. Spot check and knife selection. During the cutting process, both the blade and the knife edge may undergo some changes due to the internal stress of the crystal. For example, the change of the knife edge may cause cracks in the cut silicon wafer or damage to the knife edge, and the thickness of the silicon wafer will also be affected accordingly. If there is a problem, you must use a whetstone repair knife in time. If you can’t cut a qualified silicon wafer, you should stop the equipment and check the blade. During the cutting process, attention should be paid to safety, and the operator should be careful to prevent accidents when repairing the knife.
  6. Multi-wire cutting of single crystal silicon

Multi-wire cutting is an efficient cutting method that uses the high-speed reciprocating motion of metal wires to bring abrasives into the semiconductor processing area for grinding, and finally, hard and brittle materials such as silicon single crystals are simultaneously cut into hundreds of thin slices at a time. The CNC multi-wire cutting machine will gradually replace the traditional inner circle cutting machine and become the mainstream method of silicon wafer cutting. High-precision, high-speed, low-loss cutting of crystalline silicon materials and various hard and brittle materials, it can complete the division in both vertical and horizontal directions at one time, saving material, labor, and time, thus greatly improving production efficiency, grinding and cutting surface fine, No deep crack damage. The overall technology of high-precision CNC multi-wire high-speed cutting machine tools is comparable to the international advanced level, especially in terms of the tension control technology of the cutting line, the synchronization technology of the winding and pay-off motor and the main motor, which is far ahead in the world.

In general, the multi-wire cutting machine has the following parts: basic frame, cutting area, winding room, mortar system, temperature control system, electric control cabinet, power unit and measurement system. Each system is coordinated with each other, and each has precise requirements. For example, the groove distance of the guide wheel used in cutting has a clear specification, and the required temperature of the mortar is 20 °C. Only by maintaining these properties comprehensively can the silicon wafers that meet the requirements be cut.

Silicon wafer multi-wire cutting technology is a relatively advanced silicon wafer segmentation technology in the world. It is different from traditional cutting methods such as knives, saw blades and grinding wheels, and is different from advanced laser cutting and inner circle cutting principles. The principle is that a high-speed moving steel wire drives the cutting blade material attached to the steel wire to rub the silicon rod back and forth, so as to achieve high-efficiency cutting results. In the whole process, the steel wire is guided by more than a dozen wire pulleys to form a wire net on the main wire roller. When the workpiece to be processed is lowered through the worktable, the workpiece feeding is realized. Compared with other technologies, the multi-wire cutting technology of silicon wafers has the advantages of high efficiency, high productivity and high precision, and is currently the most widely used silicon wafer cutting technology.

(1) Preparations
As with inner circle cutting, preparation for multi-wire cutting also includes reading the process sheet and clarifying the processing instructions, checking the workpiece and checking the equipment and process conditions.

  1. Processing instructions: The processing instructions for single crystal silicon cutting refer to various parameters related to cutting, mainly including thickness, TTV BOW, WARP and wafer surface orientation.
  2. Check the workpiece: Before cutting, it is necessary to check whether the workpiece is correct, mainly from the appearance features such as number, diameter, length and reference surface.
  3. Check equipment and process conditions: Check whether all parts of the equipment are normal, whether electricity, compressed air, cooling water, mortar guide wheel, steel wire, indoor environment and labor protection supplies meet the requirements. The power supply is usually single-phase 220V and three-phase 380V; the compressed air pressure is greater than or equal to 5.5MPa, no moisture and impurities, the cooling water is primary pure water, the water pressure is 0.2~0.6MPa, the water temperature is below 15 ℃, no impurities; the indoor environment requirements are appropriate Constant temperature, 20~25℃ is appropriate.

(2) Monocrystalline silicon orientation
In the single crystal silicon cutting process, the orientation technology is used to realize the orientation cutting to obtain the wafer with the desired surface orientation.
The multi-line cutting of single crystal silicon generally does not use the light pattern method and X-ray diffraction method for crystal orientation, but adopts the following two methods. The first is to use the head piece calibration and adjustment method to adjust one end face of the crystal to the desired orientation by using the inner circle cutting method, and then use the end face as the benchmark to bond the crystal, so that the cutting is carried out along this datum plane to implement directional cutting. The second is to bond the crystal twice. The crystalline silicon of solar energy usually adopts P(100) crystal, and the crystal orientation of single crystal silicon is generally positive, so the orientation is easier, and it is often enough to cut perpendicular to the axial direction.

(3) Monocrystalline silicon bonding
The crystal bonding direction is determined according to the condition of the crystal to be bonded and its orientation result and identification. The bonding part of the crystal in multi-line cutting is subject to the orientation requirements. For solar-grade crystals, one of its planes can be used as the bonding direction. After the bonding direction of the single crystal silicon is determined, clean the single crystal silicon, place the pallet and the fixture on the part to be bonded, and then prepare the adhesive for bonding and curing. Special attention should be paid to scraping the edge of the adhesive. .

Read more: What is the Czochralski method for preparing single crystal silicon?

Comments are closed.