Kerf Loss Research Articles

Processing Characteristics Using Phosphorous Dielectric on Wire Electrical Discharge Machining of Polycrystalline Silicon

In this study, wire electrical discharge machining (WEDM) was implemented to process polycrystalline silicon ingot, and the influences on processing characteristics were examined. There are two different dielectrics, pure water and pure water with sodium pyrophosphate powder, that were experimented to compare their effects on cutting speed: kerf loss and surface roughness. In the experiment, sodium pyrophosphate powder has shown that it has enhanced process efficiency and has improved surface smoothness. From the results, it has been found that by using phosphorous dielectric, the cutting speed increased 2.4 times than pure water. Meanwhile, by using phosphorous dielectric, it caused high temperature and brought about the electrolysis effect, which improved its surface roughness enhanced 16% compared with pure water. This also decreased kerf loss and boosted up the material utilized. Through WEDM, high-temperature characteristic forces the phosphorous element of the dielectric to infiltrate on the cutting surface of the polycrystalline silicon ingot. The result proved that using phosphorous dielectric on WEDM could increase the efficiency and surface roughness, and could infiltrate phosphorous element on the surface of the polycrystalline silicon ingot. These findings could be future references on the researches of solar cell in both industrial and academic fields.

High aspect ratio grooving of Si using a wet etching assisted by wire-friction (A feasibility study for the applicaiotn for slicing Si ingots)

Silicon (Si) wafers for electronic or photovoltaic devices are fabricated by slicing a Si ingot using mechanical slicing with a diamond wire. Recently, the slicing method without generating damage on Si surface has been strongly required because of the increasing demand of ultra-thin Si wafers. In this study, we have developed a novel machining method for Si grooving based on a wet chemical etching. In this method, Si was processed by the chemical etching in HNO3 and HF mixture combined with an abrasion effect of metallic wires that contains no abrasives. The extremely low kerf loss with approx. 100 μm was achieved by optimizing the composition of etchant. SEM observation showed that Si surfaces processed by the proposed method had no crack and tool mark contrary to the mechanical slicing. Furthermore, Raman microscopy exhibited that the proposed method generated no disordered layer on Si surfaces, whereas the mechanical slicing caused amorphous layers. Surface roughness was improved by adding CH3COOH to the etchant.

Reorganization of Porous Silicon: Effect on Epitaxial Layer Quality and Detachment

Cost reduction is still a main goal in solar cell research and can be achieved by going towards thinner silicon bulk material. One way to avoid kerf loss and to reach thicknesses of less than 50 μm is a lift-off approach using porous silicon and epitaxial thickening for silicon foil fabrication. The porous layer structure requires a reorganization step that was varied in this work to optimize the detachment properties and the crystal quality of the epitaxial Si film. All processes were carried out in a quasi-inline Atmospheric Pressure Chemical Vapour Deposition (APCVD) reactor. Cross–sections were observed to see if the porous layer shows the desired structure. Stacking fault densities in epitaxial layers deposited on porous silicon layers significantly decrease with increasing reorganization time but are at least one order of magnitude higher than in epitaxial layers deposited on polished wafers. Microwave photoconductive decay (MWPCD) measurements and photo luminescence (PL) imaging were carried out to determine the effective carrier lifetimes of the detached foils and to correlate them with stacking faults and cracks. A detached 40 μm thin silicon foil with an averaged effective carrier lifetime of 22 μs is shown which corresponds to a diffusion length of over 200 μm. This investigation shows that silicon foils deposited in a quasi-inline APCVD reactor exhibit good detachment properties and a good crystal quality, which is both needed for high efficiency solar cell processing.

Recycling of kerf loss silicon derived from diamond-wire saw cutting process by chemical approach

Treatment processes of the kerf loss silicon using a chemical approach, hydrobromination reaction, have been investigated. Since the kerf loss silicon is generated in the cutting process of silicon ingot using a diamond wire saw, the sample in a form of wet cake was composed of fine particles of silicon kerf, cutting lubricant, and metallic impurities. The removal efficiency of lubricant and metal impurities using acetone and acid solutions, respectively, were examined by thermal and elemental analyses. The heat treatment was necessary, rather than the washing with acetone, for complete removal of the lubricant from the sample. After being treated successively with organic solvent (acetone) and acid (HNO3 and HCl), the dry sample obtained at each step was exposed to HBr gas in a flow reactor heated at 400°C. The reactivity was evaluated with respect to the conversion of HBr. The HBr conversion was much higher with the use of kerf silicon that did not experience acid treatment. Furthermore, the metal impurity concentration without acid treatment after the reaction was much lower than that with acid treatment. Further, to examine the impact of abrasive grain on the purity of the obtained products, diamond particles were intentionally added to the silicon sample. The presence of diamond gave little influence on the metal impurities in products, indicating that the operation of hydrobromination can eliminate the important steps adopted in the traditional purification process for kerf loss silicon with the acid treatment of metallurgical approach.

Rapid recovery of polycrystalline silicon from kerf loss slurry using double-layer organic solvent sedimentation method

The rapid development of photovoltaic (PV) industries has led to a shortage of silicon feedstock. However, more than 40% silicon goes into slurry wastes due to the kerf loss in the wafer slicing process. To effectively recycle polycrystalline silicon from the kerf loss slurry, an innovative double-layer organic solvent sedimentation process was presented in the paper. The sedimentation velocities of Si and SiC particles in some organic solvents were investigated. Considering the polarity, viscosity, and density of solvents, the chloroepoxy propane and carbon tetrachloride were selected to separate Si and SiC particles. It is found that Si and SiC particles in the slurry waste can be successfully separated by the double-layer organic solvent sedimentation method, which can greatly reduce the sedimentation time and improve the purity of obtained Si-rich and SiC-rich powders. The obtained Si-rich powders consist of 95.04% Si, and the cast Si ingot has 99.06% Si.

Effect of reciprocating wire slurry sawing on surface quality and mechanical strength of as-cut solar silicon wafers

Abstract This paper investigates reciprocating wire slurry sawing for photovoltaic (PV) silicon wafering and compares the resulting wafer surface quality and its mechanical strength to that obtained in unidirectional wire sawing. It is found that wire reciprocation creates two significantly different morphological or cutting zones on the wafer surface. The zone width varies with the distance travelled by the wires, the cutting location on the wafer surface, and direction of wire motion. The size of the morphological zone created during forward motion of the wire is larger than that created during its backward motion. The zone width is found to decrease along the wire cut direction. In addition, there appears to be greater kerf loss and increased surface roughness in the forward cutting zone. In general, results suggest a higher material removal rate during forward motion of the wire than during backward motion. Notwithstanding the surface morphology variations, the fracture strengths of reciprocating wire sawn wafers are found to be quite similar to that exhibited by wafers produced by unidirectional wire sawing.

Modeling of silicon ingot slicing process by wire–electrical discharge machining

Photovoltaic industry requires the cutting of silicon ingots into wafers in order to get minimum kerf loss, to facilitate manufacturing of thin wafers and to provide high productivity. Existing silicon ingot slicing processes like inner diameter saw and abrasive wire cutting have drawbacks of high kerf loss, lower finish and limitation on the thickness of wafer. In order to have economically viable photovoltaic technology, efficiency of wafer manufacturing process needs to be improved. The wire–electrical discharge machining technology is adopted as a new candidate for silicon slicing and is compared with the existing procedures and testified whether it can inevitably substitute the inner diameter saw or even compete with wire saws in some manufacturing orders. The main focus of this article is to understand the effect of processing parameters on the cutting process, so the parametric conditions delivering high slicing rate and low kerf loss can be achieved. Modeling of wire–electrical discharge machining is difficult as it involves several disciplines such as thermodynamics, hydrodynamics, electrodynamics and electromagnetics. Various researchers have proposed analytical models for electrical discharge machining and wire–electrical discharge machining through the use of electrothermal concepts. However, a comprehensive model that predicts the erosion rate in wire–electrical discharge machining process is needed. This article considers moving the line heat source nature of the wire to predict the temperature profiles of the workpiece in order to calculate the erosion rate. The results thus obtained have been analyzed experimentally, and conclusions have been derived enabling a better understanding of future research prospects and applications.

A wafer-based monocrystalline silicon photovoltaics road map: Utilizing known technology improvement opportunities for further reductions in manufacturing costs

As an initial investigation into the current and potential economics of one of today's most widely deployed photovoltaic technologies, we have engaged in a detailed analysis of manufacturing costs for each step within the wafer-based monocrystalline silicon (c-Si) PV module supply chain. At each step we find several pathways that could lead to further reductions in manufacturing costs. After aggregating the performance and cost considerations for a series of known technical improvement opportunities, we project a pathway for commercial-production c-Si modules to have typical sunlight power conversion efficiencies of 19–23%, and we calculate that they might be sustainably sold at ex-factory gate prices of $0.60–$0.70 per peak Watt (DC power, current U.S. dollars).This may not be the lower bound to the cost curve for c-Si, however, because the roadmap described in this paper is constrained by the boundary conditions set by the wire sawing of wafers and their incorporation into manufacturing equipment that is currently being developed for commercial-scale production. Within these boundary conditions, we find that the benefit of reducing the wafer thickness from today's standard 180μm to the handling limit of 80μm could be around $0.05 per peak Watt (Wp), when the calculation is run at minimum sustainable polysilicon prices (which we calculate to be around $23/kg). At that minimum sustainable polysilicon price, we also calculate that the benefit of completely eliminating or completely recycling kerf loss could be up to $0.08/Wp.These downward adjustments to the long run wafer price are used within the cost projections for three advanced cell architectures beyond today's standard c-Si solar cell. Presumably, the higher efficiency cells that are profiled must be built upon a foundation of higher quality starting wafers. The prevailing conventional wisdom is that this should add cost at the ingot and wafering step—either due to lower production yields when having to sell wafers that are doped with an alternative element other than the standard choice of boron, or in additional capital equipment costs associated with removing problematic boron–oxygen pairs. However, from our survey it appears that there does not necessarily need to be an assumption of a higher wafer price if cell manufacturers should wish to use n-type wafers derived from the phosphorus dopant. And as for making p-type wafers with the traditional boron dopant, the potential price premium for higher lifetimes via the magnetic Czochralski approach is calculated to be very small, and can ostensibly be offset by the higher expected cell efficiencies that would result from using the higher quality wafers. With this final consideration, the projected minimum sustainable price requirements for three advanced c-Si solar cells are incorporated into a final bill of materials for a polysilicon-to-module manufacturing facility located within the United States.

Microgrooving Using an Ultrafine Wire Tool

We examined the slicing technique of silicon wafers with little kerf loss using an ultrafine wire tool. As the first step, we performed microgrooving with an ultrafine wire tool. The thinning of the wafer and the decrease in kerf loss can minimize production costs. We discuss the strength characteristics and the microgrooving performance of the ultrafine wire tool. The main results are as follows. The relative velocity greatly affects the grooving properties. The ultrafine wire tool easily breaks when the relative speed is high. When the ultrafine wire tool and abrasives with large diameter grains are used, the ultrafine wire easily breaks and chipping in the work material easily occurs; furthermore, the ultrafine wire strays.

Basic Experiment on Atmospheric-Pressure Plasma Etching with Slit Aperture for High-Efficiency Dicing of SiC Wafer

Silicon carbide (SiC) is a promising semiconductor material for high-temperature, high-frequency, high-power, and energy-saving applications. However, because the hardness and chemical stability of SiC are high, few conventional machining methods can handle this material efficiently. We previously developed a plasma chemical vaporization machining (PCVM) technique, which is an atmospheric-pressure plasma etching process, and investigated its application to the processing of SiC substrates. In this paper, we propose a novel style of PCVM technique for dicing, using slit apertures to confine the plasma. From experiments by means of an apparatus with a one-slit aperture formed by two masks, it was found that the kerf loss was almost proportional to the slit width, and that the etching depth increased with RF power. Furthermore, from experiments on a SiC wafer, we obtained a 130-μm etching depth and 300-μm kerf loss for an 11-min processing time and 200-μm slit width.

Slicing of Rotating SiC Ingot by Electric Discharge Machining

A new method of electric discharge machining (EDM) is proposed for slicing a large silicon carbide (SiC) ingot in order to realize low kerf loss and fast cut. This principle is based on the rotating ingot, and it is called the rotating slicing method (RSM). It would be defecate the cutting chip effectively and one-point discharge. In this paper, we reported results of examinations of the RSM experimentally. Unstable discharge was not observed. Discharge damages on the wire surface were fewer than those of the conventional method. Net cutting speed was almost the same as the present method for the 2-inches ingot. The rotation axis of the ingot should be perpendicular to the feed direction of the wire, and it is important to fix the performance of the EDM such as the kerf loss. Roughness of the cutting surface was 3.4 µm of Ra

S133012 Siのエッチング援用スライスにおける切断特性の向上([S133]先進砥粒加工技術)

Si wafers for solar cells are fabricated by slicing ingot using mechanical slicing method such as multi-wire sawing. Recently, the slicing method for achieving low kerf loss and thin wafer without generating subsurface damage is strongly required because of the increasing demand of low-cost Si wafers. The etching-assisted slicing method is abrasive free machining process employing chemical etching of Si. The method can achieve a lower kerf loss than the mechanical slicing and damage-free Si surface. In this paper, the removal characteristics of Si ingot are discussed under the different slicing conditions.

RESPONSE SURFACE ANALYSIS OF SLICING OF SILICON INGOTS WITH FOCUS ON PHOTOVOLTAIC APPLICATION

Polycrystalline silicon wafers are widely used in Photovoltaic (PV) industry as a base material for the solar cells. The existing silicon ingot slicing methods typically provide minimum wafer thickness of 300–350 μm and a surface finish of 3–5 μm Ra while incurring considerable kerf loss of 35–40%. Consequently, efficient dicing methods need to be developed, and in the quest for developing new processes for silicon ingot slicing, the wire-EDM (electric discharge machining) is emerging as a potential process. Slicing of a 3′′ square silicon ingot into wafers of 500 μm in thickness has been performed to study the process capability. This article analyzes the effect of processing parameters on the cutting process. The objective of the experimental study is improvement in slicing speed, minimization of kerf loss and surface roughness. A central composite design-based response surface methodology (RSM) has been used to study the slicing of polycrystalline silicon ingot via wire-EDM. A zinc-coated brass wire, 100 μm in diameter, has been used as an electrode in the slicing experiments. It has been observed that the optimal selection of the process parameters results in an increase of 40–50% in the slicing rate along with a 20% reduction in the kerf loss as compared to the conventional methods. The machined surfaces on the sliced wafer were free of micro-cracks and wire material contamination, thereby making it useful for electronic applications.

Challenge to Development of Functional Multi-Wire EDM Slicing Method Using Wire Electrode with Track-Shaped Section

Brittle materials, such as silicon, silicon carbide and sapphire have been conventionally sliced for wafers by a multi-wire saw method with slurry in industrial fields. Recently, the multi-wire saw method with a fixed diamond abrasive wire has been available as a commercial product at acceptable cost, and the high slicing performance is expected compared with the normal multi-wire saw method with slurry. However, there still remain some problems such as bad working environment with abrasives, cleaning cost of sliced wafers, crack generation on the sliced surface and a large kerf loss against a wafer thickness. On the other hand, the developed multi-wire EDM slicing method would accomplish the high performance slicing of silicon and silicon carbide with a narrow kerf width under a clean process environment. However, the thinner wire is challenging process with a normal round shape wire electrode. Therefore, the new wire electrode with track-shaped section was proposed in order to satisfy both the narrow kerf width and the high wire tension even in the case of thin wire electrode. In this study, the running control of wire electrode with a track-shaped section was experimentally investigated, and the possibility of proposed process was discussed.

Investigations on hydrobromination of silicon in the presence of silicon carbide abrasives as a purification route of kerf loss waste

Hydrobromination of three types of silicons (reagent grade silicon (RG-Si), metallurgical grade silicon (MG-Si), and kerf loss silicon (KL-Si)) has been investigated in a flow reactor with and without abrasive grains of silicon carbide (SiC) under various conditions. All the silicons reacted with HBr to form bromosilanes around 400°C, irrespective of the presence or absence of SiC. The reactivity of each Si sample was evaluated with respect to the conversion of HBr, while the purity of products was analyzed after hydrolyzing them to silicon dioxide. Metal impurities such as Al, Fe, and Ni in the starting materials were dramatically reduced in the hydrolyzed products. The addition of SiC did not cause any increase in impurities. Rather, it resulted in an improvement of the reaction kinetics in a temperature region of 350–400°C, probably as a result of better heat transfer in the reactant owing to the high thermal diffusivity of SiC. This demonstrates that the bromination reaction is effective to extract pure silicon from kerf loss waste even where SiC is present.

Fibre volume losses of eight softwood clearfell harvesting systems in South Africa

A study of both fibre volume loss and related opportunity cost was performed across the South African softwood sawtimber industry to gain information on the actual utilisation of useful fibre and potential loss or gain of opportunity in terms of potential revenue from both field practices and policy. The study quantified volume losses from high stumps, felling and crosscut saw kerf, log allowances, excessive removal of merchantable wood, incorrect log trimming allocation, and utilisable wood left in-field. Eight treatments were examined: four terminated with merchandising at roadside landing and four at merchandising yards. Within the treatments, felling was either motor-manual or mechanised and compartments were classed by average compartment tree size (less than or greater than 1 m3). Total wood utilisation was found to be 92.07%. Stumps were found to be 7 cm higher than necessary and volume losses because of high stumps were 0.79% and 0.07% for mechanical and motor-manual felling, respectively. For felling saw kerf losses, mechanical felling showed 0.92% loss, whereas motor-manual felling resulted in only 0.15% loss. Incorrect log trimming allowance provided for 0.60% loss. Excessive trimming of logs resulted in 2.02% loss. Volume lost because of crosscutting saw kerf was low at 0.20%, but tops not being extracted resulted in 2.57% loss. Using SIMSAW 6 simulations, these volume losses translated into an additional annual harvested volume of 421 722 m3 or additional 1 278 ha harvested area, R166 million log value and R393 million net lumber value loss. Total loss was lowest (6.49%) in the average tree size class less than 1 m3, which was mechanically felled regardless of merchandising location. For the majority of volume loss categories, motor-manual felling caused greater loss when compared to mechanical felling methods. Log trimming allowance consumed 80 604 m3 annually. The results indicate that the human element may have a greater impact on fibre and value losses than the system choice. Further research is recommended to quantify the effect of the human element.

Cutting Speed of Electric Discharge Machining for SiC Ingot

A new method based on electric discharge machining (EDM) was developed for cutting a silicon carbide (SiC) ingot. The EDM method is a very useful technique to cut hard materials like SiC. By cutting with the EDM method, kerf loss and roughness of sample are generally smaller than those obtained by cutting with a diamond saw. Moreover, the warpage is smaller than that by the diamond saw cutting, and the cutting speed can be 10 times faster than that of the diamond saw at the present time. We used wires of 50 mm and 100 mm diameters in the experiments, and the experimental results of the cutting speed and the kerf losses are presented. The kerf loss of the 50 mm wire is less than 100 mm, and the cutting speed is about 0.8 mm/min for the thickness of a 6 mm SiC ingot. If we can maintain the cutting speed, the slicing time of a 2 inches diameter ingot would be about seven hours.

Magnetically Guided Nano–Micro Shaping and Slicing of Silicon

Silicon is one of the most important materials for modern electronics, telecom, and photovoltaic (PV) solar cells. With the rapidly expanding use of Si in the global economy, it would be highly desirable to reduce the overall use of Si material, especially to make the PVs more affordable and widely used as a renewable energy source. Here we report the first successful direction-guided, nano/microshaping of silicon, the intended direction of which is dictated by an applied magnetic field. Micrometer thin, massively parallel silicon sheets, very tall Si microneedles, zigzag bent Si nanowires, and tunnel drilling into Si substrates have all been demonstrated. The technique, utilizing narrow array of Au/Fe/Au trilayer etch lines, is particularly effective in producing only micrometer-thick Si sheets by rapid and inexpensive means with only 5 μm level slicing loss of Si material, thus practically eliminating the waste (and also the use) of Si material compared to the ~200 μm kerf loss per slicing and ~200 μm thick wafer in the typical saw-cut Si solar cell preparation. We expect that such nano/microshaping will enable a whole new family of novel Si geometries and exciting applications, including flexible Si circuits and highly antireflective zigzag nanowire coatings.

Removal of silicon carbide from kerf loss slurry by Al–Si alloying process

A novel method to separate silicon and silicon carbide from kerf loss slurry by Al–Si alloying process has been reported in this paper. The kerf loss slurry was washed and dried, and then aluminum was added on the top of these dry powders with silicon and silicon carbide. The Al–Si alloying process was performed in argon atmosphere using a vacuum carbon tube furnace at 1773K. In this way, an Al–Si ingot was obtained, on the surface of which a lot of hexagonal crystals were observed. The Al–Si ingot was characterized by X-ray diffraction, scanning electron microscopy, X-ray fluorescence and electron probe micro-analyzer. The X-ray results indicated that the Al4C3 phase was obtained on the top of the cast. The scanning electron microscopy, X-ray diffraction and electron probe micro-analyzer results revealed that the Al–Si alloy without silicon carbide phase formed in the cast, which indicated that silicon and silicon carbide can be separated from slurry by this alloying process.

Selective hydrobromination of metallurgical-grade silicon in a flow reactor system

Reactions of metallurgical-grade silicon (MG-Si) with gaseous hydrogen bromide (HBr) has been monitored by means of online gas chromatography in a flow reactor system. The formation of tri-bromosilane started to occur at 380 °C, accompanied by the consumption of HBr. The conversion of HBr into bromosilanes increased with an increase in reaction temperature and reached a maximum at 440 °C. An increase of the HBr concentration in a HBr-N2 mixed gas led to an increase in the consumption of MG-Si, while it reduced the selectivity of the tri-bromosilane formation. An increase in total flow rate of the reaction gas caused a dramatic decrease in the HBr conversion and enhanced the selectivity of the tri-bromosilane formation. The rate constant for overall bromination reaction at 400 °C was measured to be 0.46 s−1. Concentrations of impurities in the product were much less than those in MG-Si. Moreover, kerf loss silicon was subjected to the bromination reaction under the optimized conditions.