Dynamic Analysis of Wafer Slicing by Using Multi Wire Saw

Study on the subsurface damage depth of monocrystalline silicon in ultrasonic vibration assisted diamond wire sawing

Recycling and reuse of kerf-loss silicon from diamond wire sawing for photovoltaic industry

In recent years, wire sawing is considered to be a potentially better technology of slicing brittle crystals especially semiconductor, owing to the unmatched advantages of slicing large ingot into thin wafers with high efficiency, fine quality and less kerf loss. This paper analyses the dynamic effect on wire from a variety of excitation and mechanical structure in the process of cutting on the basis of the effects of vibration on slicing wafer quality. A suitable model is built according to Hamilton principle, closed form expressions is present for the steady state response using finite element method. The relation between inherent characteristics of wiresaw and different system parameters is obtained.

The silicon slicing efficiency and quality of silicon wafer seriously affect the cost of solar power generation and the development of photovoltaic energy industry. This paper proposed an improved cooling and lubrication method using a water tank bath for diamond wire sawing. Compared with the traditional jet sawing with array nozzles, it can be found that the surface roughness is better, no matter the average roughness value, the skewness coefficient, or the kurtosis coefficient, especially in high production efficiency processing. Similar conclusions can also be obtained from the comparative analysis of surface profile distribution and SEM micrographs. This is because the bath sawing can make full use of the cooling and lubricating capability of the cutting fluid, so that the diamond wire can achieve better fluid-carrying effect with the same directionality as the sawing direction into the cutting zone. Furthermore, the service life of diamond wire in bath sawing is longer and wire consumption is lower.

The great growth of the photovoltaic panels industry generates an important amount of slurry during the sawing step, two techniques are used: metallic wire sawing and diamond wire sawing. The metallic wire sawing slurry can mainly contain particles of pure silicon up to 40% Wt, particles of silicon carbide (SiC) which are used as an abrasive element during sawing, metallic impurities based on iron (Fe) which come from the wire, polyethylene glycol (PEG) which is used as a lubricant. However, the sawing stage determines the cost of producing photovoltaic cells, so its recovery has become a necessity. For this purpose, we are interested to study the chemical behavior of the input elements involved in the composition of the slurry. For that, the pH of the slurry was measured for 7 days during its storage. Then, it was washed with acetone to dissolve the polyethylene glycol and separate the liquid phase from the solid one. Solid phase was dried at 70 °C to obtain a slurry powder. This powder has been characterized by ATR-FTIR spectroscopy, Raman spectroscopy and the X-ray diffractometer. The liquid phase has been characterized by ATR-FTIR, Raman and TGA analysis. The results confirm the chemical instability of the slurry and the appearance of nanoparticles that have been identified.

By decreasing the subsurface damage in silicon wafers produced by diamond wire sawing, the mechanical strength of wafers can be increased, and the amount of silicon to be etched in subsequent manufacturing steps can be minimised, enhancing both product and process sustainability. Apart from the sawing process parameters, the subsurface damage in as-sawn silicon wafers is influenced by the design of the diamond wire. We present a fracture mechanics approach for the design of fixed abrasive diamond wires used in wire sawing of silicon wafers for solar cells. Starting from an allowable damage (crack) depth, indentation fracture mechanics and contact analysis are used to determine the wire design parameters, namely the grit protrusion and peripheral distribution of diamond abrasives. The improved wire design can reduce subsurface damage and thereby improve the surface integrity (product sustainability), and reduce the processing time and chemicals used in the subsequent saw-damage removal step (process sustainability).

Recycling of silicon from waste PV diamond wire sawing silicon powders: A strategy of Na2CO3-assisted pressure-less sintering and acid leaching

At present, the fixed abrasive wire sawing (FAWS) technology is gradually used in the photovoltaic industry to cut polycrystalline silicon slices. However, there are obvious directional wire marks, parallel grooves, and amorphous silicon layer on the surface of the slices formed by the FAWS, which leads to a high optic reflectivity of the textured surface obtained after the mature acid etching texturization technology. So the slices cannot meet the requirements of the photovoltaic cell. In the paper, a novel fixed-free abrasive combined wire sawing (FFACWS) technology for cutting PV polycrystalline silicon is presented to solve this problem, by adding loose SiC abrasives to cooling lubricant during the fixed abrasive wire sawing. A single-factor and orthogonal experimental study on sawing characteristics was carried out. The effect of size and mass fraction of SiC abrasives in the slurry, workpiece feed speed and wire speed on the surface morphology, roughness, and kerf loss were studied. The results show that within the range of the processing parameters in the paper studied, the obvious wire marks, parallel grooves, and ductile layers on the surface of the slices can be removed by the FFACWS. The surface roughness of the slices along the wire movement direction and the workpiece feed direction increases with the increase of the mass fraction of SiC abrasives in the slurry and workpiece feed speed and it decreases with the increase of wire speed. But the effect of the size of SiC abrasives is related to the matching of the protruding height of the fixed abrasives on the wire surface along the workpiece feed direction. In the wire movement direction, it increases with the size of SiC abrasives. The kerf loss increases with the increase of size and mass fraction of SiC abrasives in the slurry and the wire speed but has little effect with the change of workpiece feed speed.

Wire sawing is widely used for wafer cutting in the photovoltaic and microelectronics industries. The quality of cutting wafers is affected by the wire bow deflection. In this study, wire bow deflections in a crystal sawing process were systematically measured with a high-speed camera. A simulation model of wire bow deflection was established using the Adams software on the basis of experimental conditions. It is shown that the curve of the wire bow deflection was a quadratic curve. Wire bow deflection was obviously affected by the wire tension force and the feeding speed. However, wire speed had little effect on the wire bow deflection. The experimental and the simulation results of wire bow deflection were similar for different process parameters. The force ratio played a key role in influencing the wire bow deflection.

The epitaxial wafer equivalent concept promises to reduce the cost for photovoltaic solar energy conversion. For this purpose, less expensive substrates of upgraded metallurgical silicon (UMG-Si) are used as a mechanical support for the epitaxial cell. In the presented paper the mechanical strength of substrates coming from different positions of an UMG-Si ingot was investigated. The investigation of mechanical strength was performed after block crystallization, multi-wire sawing of the wafers and after depositing a 20 µm crystalline thin film until the production of an epitaxial wafer equivalent. UMG-Si substrates and epitaxial wafer equivalents were tested with the ring on ring bending test. To interpret the results IR-transmission and scanning electron microscopy, impurity concentration measurements and residual stress measurements were done on selected wafers. It could be shown that the presence of precipitates in the silicon matrix has a strong influence on the mechanical strength of the wafers. UMG-Si as well as industry standard Cz-Si and mc-Si showed less mechanical strength after epitaxial processes. This could be due either to a change in microstructure of the substrate or to thermal stresses introduced during the high temperature process. The deposition of the crystalline silicon thin film did not decrease the mechanical strength of the epitaxial wafer equivalent. It could be concluded that the UMG-Si investigated in this work shows sufficient mechanical strength for manufacturing epitaxial wafer equivalents.

The wire bow angle is an important factor that affects the shape precision of an ingot after the diamond wire sawing process. In this research, the wire bow angles of the inside and outside of an ingot were recorded with a high-speed camera. The effects of the processing parameters such as the wire tension force, feed speed, and wire velocity on the wire bow angles inside and outside the ingot were analyzed. A numerical simulation model of the wire bow in the wire sawing process is presented in this paper to describe the wire bow angle inside the ingot. It was shown that the wire bow angle inside the ingot was smaller than that of outside the ingot for all of the processing parameters. The wire bow angles improved with the increase of the feed speed and decrease of the wire velocity and the wire tension force. The results of the wire bow angle measurement of the inside ingot and the simulation calculation were similar for the process parameters.

A comprehensive review of diamond wire sawing process for single-crystal hard and brittle materials

The surface quality of diamond wire sawing (DWS) wafers directly affects the efficiency and yield of subsequent processing steps. This paper investigates the motion trajectory of abrasives in ultrasonic-assisted diamond wire sawing (UADWS) and its mechanism for improving surface quality. The influence of ultrasonic vibration on the cutting arc length, cutting depth, and interference of multi-abrasive trajectories was analyzed through the establishment of an abrasive motion trajectory model. The ultrasonic vibration transforms the abrasive trajectory from linear to sinusoidal, thereby increasing the cutting arc length while reducing the cutting depth. A lower wire speed was found to be more conducive to exploiting the advantages of ultrasonic vibration. Furthermore, the intersecting interference of multi-abrasive trajectories contributes to enhanced surface quality. Experimental studies were conducted on monocrystalline silicon (mono-Si) to verify the effectiveness of ultrasonic vibration in improving surface morphology and reducing wire marks during the sawing process. The experimental results demonstrate that, compared with DWS, UADWS achieves a significantly lower surface roughness Ra and generates micro-pits. The ultrasonic vibration induces a micro-grinding effect on both peaks and valleys of wire marks, effectively reducing their peak–valley (PV) height. This study provides a theoretical basis for optimizing UADWS process parameters and holds significant implications for improving surface quality in mono-Si wafer slicing.

Freestanding and supported processing of sub-70 μm kerfless epitaxial Si and thinned Cz/FZ Si foils into solar cells: An overview of recent progress and challenges

Slicing parameter optimization based on wire net dynamic reliability constraint in multi-wire saw