With the increasingly serious global energy crisis and the rapid development of international green energy projects, solar photovoltaic power generation has been recognized internationally due to its environmental protection, cleanliness, safety, and cost reduction. It has become the focus of competition in the world. Development and utilization is an effective way to finally solve the problems of conventional energy sources, especially petrochemical energy shortage, environmental pollution and greenhouse effect. In recent years, although the solar photovoltaic power generation industry has been booming, its large-scale utilization is still difficult to start in the short term. The cost problem is a bottleneck restricting the large-scale application of solar cells. To truly make solar energy an alternative energy source, solar cells The cost of generating electricity must be close to the cost of conventional power generation. Therefore, it is an urgent task for us to realize innovation and breakthrough in technology and develop a new generation of solar cells that are cheap and efficient.
According to the statistics of solar cells in the world in recent years, crystalline silicon solar cells are currently the mainstream of solar cells, and its market share is kept at over 90%. It is expected that crystalline silicon will still dominate in the next 10 years and will develop in the direction of high efficiency, low cost and thinning. In the entire industry chain, the cutting cost of solar wafers accounts for about 30% of the total manufacturing cost. The advantages and disadvantages of the cutting process not only affect the filming rate of the silicon wafer, but also affect the surface quality of the silicon wafer and the subsequent process. Processing volume and complexity. Therefore, under the premise of ensuring photoelectric conversion efficiency, in-depth study of advanced cutting technology and cutting process, reducing silicon wafer processing cost, improving raw material utilization rate and production efficiency, has important guiding significance and application for promoting the development of solar photovoltaic power generation industry. value.
1 solar wafer cutting process requires solar wafer processing process generally through crystal growth, cutting, outer diameter barreling, flat edge, slicing, chamfering, grinding, etching, polishing, cleaning, packaging and other stages, while wafer sliced ​​as The key process of the silicon wafer processing process, its processing efficiency and processing quality are directly related to the overall production of silicon wafers. The main parameters for determining the surface quality of the silicon slice are crystal orientation deviation, total thickness deviation TTV of the silicon wafer, bending degree Bow of the silicon wafer, warpage of the silicon wafer, Warp, and the like. The accuracy of these parameters plays a direct role in the processing of subsequent processes.
The deviation of crystal orientation refers to the angle at which the axis of the crystal does not coincide with the direction of the crystal; the total thickness deviation of the silicon wafer TTV refers to the difference between the maximum and minimum thickness of the silicon wafer; the curvature of the silicon wafer refers to the unevenness of the surface of the silicon wafer. The value of the deformation magnitude, the silicon wafer bending metric is defined as (ab)/2; the wafer warpage Warp refers to the difference between the maximum and minimum distances of the plane of the silicon wafer and the center plane of the silicon wafer.
The principle requirements for the slicing process technology are: high cutting precision, high surface parallelism, small warpage and thickness deviation; good cross-section integrity, elimination of wire drawing, knife marks and micro-cracks; improved yield, reduced knife (steel wire) Cutting, reducing the loss of raw materials; increasing the cutting speed and achieving automatic cutting.
Due to the high hardness and brittleness of the silicon wafer material, the slicing process is prone to chipping damage, and it is difficult to cut by ordinary processing methods; and because it is not electrically conductive, it cannot be processed by special processing methods such as electric spark.
2 Traditional solar wafer cutting technology In the processing of solar cells, silicon wafer cutting technology has always been a bottleneck. The current methods for solar wafer cutting are: external cutting, inner cutting and wire cutting.
2.1 External Circle Cutting Technology The outer circle cutting is the earliest developed process for cutting silicon wafers. This method is similar to the grinding of cylindrical grinding wheels. The thin diamond saw blades are clamped on the high-speed rotating spindle, and the diamond on the outer diameter is used. The abrasive grains cut the workpiece.
When the outer circle is cut, the noise is large, the saw blade is poor in rigidity, and the blade is too thin and radially receives the pressure of the crystal, and is easily deformed and laterally oscillated, so that the loss of the crystal material is large and the crystal face is not flat. The sawing depth of the outer circular saw is limited by the diameter of the saw blade, generally not exceeding three-thirds of the diameter. If the diameter of the saw blade is increased, the rigidity is inevitably reduced, so that the swing during the cutting process is difficult to ensure the straightness of the sawing. Although increasing the thickness of the saw blade by 2 degrees increases the rigidity, it increases the cost of the saw blade and increases the width of the kerf and reduces the yield. Therefore, the outer circular cutting method has been rarely used, and is mainly used for directional cutting and large-sized material shaping cutting of long crystals having large crystal deflection.
2.2 Inner Circle Cutting Technology At present, the medium and small size wafer cutting mainly adopts the inner circle cutting method. This method uses the inner diameter of the blade to plate the diamond abrasive as the cutting edge. When the inner circle is cut, the cutting blade is driven by the spindle to rotate at a high speed while the workpiece is relatively Radial feed. The internal circle cutting technology is mature, the blade stability is good, and it is widely used in the cutting field of hard and brittle materials, and is particularly suitable for cutting hard and brittle materials such as silicon crystal, ceramic and hard alloy. The advantages of the inner circle cutting technology are: high cutting precision, low thickness of the 200mm wafer, and low cost of slicing. The price of the inner circular slicer of the same specification is 1/3~1/4 of the price of the wire cutting machine; each piece can be used. Crystal orientation adjustment and slice thickness adjustment; flexible adjustability when processing in small batches and multiple specifications.
The disadvantage is that the damage surface of the wafer surface is large; the productivity is low, and only one slice is cut at a time.
2.3 Multi-wire saw cutting technology is accompanied by the increasing use of large-diameter silicon wafers. The inner circular saw blade is deformed by the tensioning force, and the surface of the cutting edge is wave-shaped, especially with the radial dimension of the inner circular blade. This increase is more pronounced, resulting in additional axial vibrations in addition to the spindle system when the inner circular saw rotates at high speeds. The structure of the inner circular saw limits the size of the workpiece to be cut. Generally, it can only cut silicon wafers with a diameter of not more than 200 mm, and it will produce large warpage deformation. The processing surface will leave large incisions and micro cracks, and the surface layer will be damaged. Layer depth can reach 10~50 claws. Therefore, for wafer processing of large diameter (200 claws in diameter) and small thickness (0.3 mm in thickness), the inner circular cutting is far from meeting the production requirements in terms of output and capacity, and multi-wire cutting has become the mainstream technology.
The multi-wire cutting technology currently used for silicon crystal slicing is only a reciprocating free abrasive wire saw. The principle is to use a wire wrapped around the set of wire guide rollers to form a row of cutting lines arranged at regular intervals. When cutting, the steel wire moves in a certain direction, and the workpiece and the steel wire are pressed against each other. When the cutting fluid containing free abrasive is poured between the workpiece and the wire, the rapid movement of the wire brings the cutting liquid into the workpiece slit to produce cutting. effect. Multi-wire saw cutting can cut 300~2000 square inches per hour (about 3~5 times of inner circle cutting), saw blade loss is only 0.2~0.3mm (about 60% of inner circle cutting), damage layer thickness is 5~15xm, see Table 1. However, due to the short history of development, the technology needs to be further matured. The main performances are: uneven thickness and large error; difficult to adjust slice thickness, difficult to achieve detection and control during cutting; Once the wire is broken, the whole crystal rod is scrapped; the cost is high, the metal wire is used once, and only a small part of the cutting fluid enters the cutting function; the equipment mainly depends on imports, and the price is expensive, and the import of one piece requires more than 3 million yuan. However, until the 1990s, Austrian GFM company actually applied this concept to the field of aerospace material processing, and began to ultrasonically cut a variety of composite materials.
The application of this technology has been used for nearly 20 years. Ultrasonic cutting equipment has been widely used in the production of large aircraft manufacturers and aerospace composites in Europe and the United States. CNC technology is a new machine tool control technology developed in the 1950s. It is one of the core technologies of automated manufacturing systems. If CNC technology is applied to traditional ultrasonic machining technology, it can control workpieces and tools through computers. The relative motion is automatically processed to produce the required workpiece, which improves the quality and efficiency of ultrasonic machining and further meets the needs of enterprise automation production.
3.1 Principle and structure of numerical control ultrasonic vibration cutting The basic principle of ultrasonic cutting is to use an electronic ultrasonic generator to generate ultrasonic waves with a frequency of 20~30kHz. Ultrasound-mechanical transducers placed in the ultrasonic cutting head will be used for ultrasonics with small amplitude and energy. The vibration is converted into mechanical vibration of the same frequency, and then amplified by resonance to obtain a large enough amplitude and energy (power) to meet the requirements of cutting the workpiece, and finally transmitted to the tool at the top of the ultrasonic cutting head, and the workpiece is subjected to the ultrasonic cutting head. Cutting processing.
According to the principle of ultrasonic vibration cutting, the ultrasonic cutting device generally comprises an ultrasonic generator, an ultrasonic vibration system, an ultrasonic cutting machine tool, an abrasive suspension cooling circulation system and the like.
The ultrasonic generator mainly generates ultrasonic vibration of a certain power to provide reciprocating motion of the tool and the energy required for cutting the workpiece mainly includes three parts: an ultrasonic transducer, a horn and a cutter head, and the main function thereof is to change the high frequency oscillation. Into the required mechanical vibration. For ultrasonic cutting heads for wafer cutting, the head 1. Super Lubo generator Z ultrasonic vibration system consists of a set of thicknesses of 0.127 3. Cooling loop system 4, T. mm of soft steel sheet rivetingç½”3 ultrasonic cutting principle, each piece spacing 1.14mm (can be adjusted according to the thickness of the silicon wafer), the tool extension height should consider the number of times the blade is worn after re-grinding, the outermost blade should be 0.5mm higher than other blades, as Guided for cutting. The connection between the tool head and the horn is welded.
The structure is relatively simple, mainly realizes the feed motion of the tool and adjusts the relative position movement between the tool and the workpiece. For the purpose of automatic machining, it can be modified on the existing CNC machine tool.
However, the circulation system adopts the pump supply to achieve a good circulation of the suspension in the cutting area. If the hole can be opened in the middle of the cutter and the horn, the suspension flows from the hole, which has a good effect on improving the cutting quality.
3.2 Numerical control ultrasonic vibration cutting characteristics The main cutting object is hard and brittle material. Because ultrasonic cutting is realized by the continuous impact and cavitation of abrasive and liquid molecules, the main cutting object is hard and brittle material, especially the material that is difficult to cut by electric machining.
Since the cutting process relies on the action of tiny abrasives, the surface to be cut has no structural change and no residual stress, and the cutting dimensional accuracy can reach the force of the workpiece during the cutting process. The ultrasonic cutting process does not exist, and there is no lateral friction force, which is beneficial to the processing. Thin-walled, narrow-slit and low-rigidity workpieces.
The cutting efficiency is between the inner and the multi-wire saw, which is equivalent to three times the normal vibration-free cutting.
4 Conclusion In summary, the main features of the current solar wafer precision cutting technology can be summarized as follows: For medium and small diameter (with diameter within 200mm) wafer processing, generally use internal circle cutting, and is suitable for small batches and multiple specifications. Processing.
For wafer processing with a large diameter (200 mm diameter) and a small thickness (0.3 mm thickness), a multi-wire saw is usually used for cutting. However, multi-wire saw cutting machines rely on imports more, are expensive, and have higher cutting costs. Generally, small-scale enterprises are unbearable.
In contrast, ultrasonic vibration cutting has a great advantage in terms of cost performance. Its cutting efficiency is equivalent to three times that of ordinary vibration-free cutting, and it can reduce cutting force, improve cutting precision and extend tool life.
With the maturity of ultrasonic processing technology and numerical control technology, the advantages of CNC ultrasonic cutting technology will become more and more obvious, and will be highly valued by international and domestic experts and scholars. Looking into the future, I believe that with the development of modern industry and solar energy industry, with the unique advantages of ultrasonic processing technology in the processing of non-metallic hard and brittle materials such as crystalline silicon, the numerical control ultrasonic cutting technology of solar wafers will have a good development prospect.
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