Comparative Analysis of Fracture Strength of Slurry and Diamond Wire Sawn Multicrystalline Silicon Solar Wafers

This paper presents an analysis of fracture strength of multicrystalline silicon (mc‐Si) solar wafers produced by slurry and diamond wire sawing. The wafers were bent in two orthogonal orientations relative to the saw marks. The fracture strength of slurry sawn wafers increases gradually from wire entry to wire exit whereas the strength variation in the wire feed direction is small. The fracture strength of diamond wire sawn wafers is bi‐directional, with a higher strength if bent perpendicular to the saw marks and a lower strength if bent parallel to the saw marks. The fracture strength variation is related to the microcracks generated in the vicinity of grit‐induced surface damage.     

Fixed Abrasive Diamond Wire Saw Slicing of Single-Crystal Silicon Carbide Wafers

This article investigates the slicing of single-crystal silicon carbide (SiC) with a fixed abrasive diamond wire. A spool-to-spool rocking motion diamond wire saw machine using a 0.22 mm nominal diameter diamond wire with 20 µm average size diamond grit was used. The effect of wire downfeed speed on wafer surface roughness and subsurface damage was first investigated. The surface marks generated by loose diamond grit and stagnation of the wire during the change of the wire-cutting direction were studied. The use of scanning acoustic microscopy (SAcM) as a nondestructive evaluation method to identify the subsurface damage was explored. Effects of using a new diamond wire on cutting forces and surface roughness were also investigated. Scanning electron microscopy has been used to examine the machined surfaces and wire wear. This study demonstrated the feasibility of fixed abrasive diamond wire cutting of SiC wafers and the usage of a SAcM to examine the subsurface damage.     

On the scribing and subsequent fracturing of silicon semiconductor wafers

The integrated circuits deposited on silicon wafers are often separated by scribing with a diamond tool followed by bending to produce fracture. Using a commercial scribing tool we find permanent deformation and three types of crack. The median crack which propagates downwards is the objective of the scribing process. Lateral cracks which form, apparently following plastic deformation, may lead to chipping on either side of the scribing tool. These cracks and also the chevron cracks which form on the surface are very similar to cracks observed in scratching glass. However, in silicon, because of its anisotropy, the chevron cracks may be a serious problem since they can guide the median crack out of the scribing direction onto a preferred cleavage plane. This aspect leads to a brief discussion of the crystallography of silicon and recommendations for scribing configurations which should minimize undesired fracture. Finally, it is shown that the established methods of linear elastic fracture mechanics may be used to predict the maximum radius of curvature required to fracture a wafer containing a prescribed series of median cracks.     

Fracture Strength of Photovoltaic Silicon Wafers Evaluated Using a Controlled Flaw Method

Propagation of pre‐existing micro cracks and their associated residual contact stresses, generated from the wafer sawing process, is the leading cause for photovoltaic (PV) silicon wafer/cell breakage during handling and processing. In the current work, the impact of a single micro crack on the fracture strength of PV silicon wafer is investigated based on a controlled flaw method. Radial/median cracks with controllable scales are introduced through microindentation at the center of a PV silicon sample to simulate micro cracks resulting from wafer sawing, handling, or thermal processing. Results indicate that the fracture strength of PV silicon wafer decreases linearly with the increasing of the microindentation load (radial crack scale). In addition, it is found that the impurity carbon plays an important role in the contact cracking‐fracture process. The fracture strength increased ≈21% when the substitutional carbon concentration is increased from 1.2 × 10¹⁸ to 6.4 × 10¹⁸ cm^?3.     

The influence of backgrinding on the fracture strength of 100 mm diameter (1 1 1) p-type silicon wafers

The influence of grinding geometry and damage depth on the fracture strength of 100 mm diameter (1 1 1) p-type silicon wafers has been studied. The fracture strengths were measured in a biaxial flexure test after the wafers were ground to 0.36 mm from 0.53 mm thick, in a grinding apparatus that produces a swath of swirls on the silicon wafer surfaces. Analysis of orientations of the swirl geometries and fracture probability was used to deduce the fracture strength relative to the crystallographic orientation of the wafers. Optical and scanning electron microscopy of bevelled, and cleaved and etched samples was used to measure the damage depths from selected locations on the wafers. The depth of damage and fracture strengths were correlated to the geometry of the backgrind swirl pattern and the relative position of the orientation flat. The damage depth was smaller when the swirl path was parallel or at 45° to the orientation flat as compared to the swirl paths at 90° and 135° orientations. As a result, the wafers ground in the former orientations had a higher fracture strength than those of the latter orientations (136 and 124 MPa versus 100 and 103 MPa, for the four orientations, respectively). This revised version was published online in November 2006 with corrections to the Cover Date.     

Strength of silicon wafers: fracture mechanics approach

This paper describes a model to predict mechanical strength distribution of silicon wafers. A generalized expression, based on a multimodal Weibull distribution, is proposed to describe the strength of a brittle material with surface, edge, and bulk flaws. The specific case of a cast, unpolished photovoltaic (PV) wafer is further analyzed. Assuming that surface microcracks constitute the dominant mechanism of wafer breakage, this model predicts the strength distribution of PV silicon that matches well the experimental results available in the literature.     

Rate effects on the mechanical response of magnesium aluminate spinel

The uniaxial compressive, biaxial flexural strength and fracture toughness of a polycrystalline transparent MgAl₂O₄ spinel were characterized over a wide range of loading rates. The flexural tests were carried out by means of ring-on-ring equibiaxial bending, while the fracture toughness was determined by four-point bending on samples with Chevron notch (CN) configuration. The surface crack (SC) method was also attempted in determining the fracture toughness. Quasi-static experiments were conducted on a servohydraulic testing machine, while the high-rate experiments were performed on a modified Kolsky bar. Results showed that both the failure strength and fracture toughness of the spinel were rate sensitive. Edge beveling in sample preparation did not affect the ring-on-ring flexural strength significantly, and the failure initiation sites were found to be inside the loading ring area regardless of edge conditions. Fracture toughness tests following ASTM standard were largely affected by the inherent coarse microstructure of this material.     

水导激光技术及其在半导体中应用

目前,随着晶圆厚度越做越薄和更多的三族、五族化合物半导体材料的应用,“钻石刀”技术越来越不能满足高质量的切割要求,晶圆切割开始使用激光技术。但传统的激光切割技术容易产生碎片,同时由于热影响在晶圆表面产生微细裂纹导致晶圆的断裂强度很低。为了解决这些问题,产生了一种新的切割技术——水导激光技术。这种技术采用头发丝细的水束流引导激光至工件表面,这就好比光纤一样。采用水导激光技术起初是为了解决切口热影响的问题,但该技术在克服光束发散和去除切割产生的熔融物质方面同样具有无可比拟的优势。全面介绍了水导激光切割技术的原理及其切割晶圆的应用,特别是在化合物半导体（砷化镓）晶圆、碳化硅晶圆方面的应用。     

材料特性对亲水性固结磨料研磨垫加工性能的影响

为研究材料特性对亲水性固结磨料研磨垫的加工性能影响,本文研究了K9玻璃和硅片两种材料在不同加工顺序下研磨过程中的声发射信号和摩擦系数特征,采用扫描电镜分析磨屑的尺寸与形态.结果表明:不同加工顺序下工件的材料去除速率差别很大.与直接研磨硅片相比,先研磨K9玻璃再研磨硅片,硅片的材料去除速率大幅下降;相反,先研磨硅片再研磨K9玻璃,与直接研磨K9玻璃相比,K9玻璃的材料去除速率变化不大.无论采用哪种加工顺序,后研磨的工件表面粗糙度均比直接研磨的同种工件要大.扫描电镜的分析表明,硅片的磨屑尺寸集中在600 nm~1.5μm,磨屑大部分都棱角完整;而K9玻璃的磨屑尺寸集中在300 nm~500 nm左右,无明显棱角.硅片磨屑较大的尺寸与完整的棱角促进了研磨垫的自修正过程,所以硅片这类脆性较大的材料有利于研磨垫的自修正过程.     

多晶硅表面金属催化化学腐蚀法制绒研究现状

在多晶硅太阳能电池的生产过程中,金刚线切割(Diamond wire sawing, DWS)技术具有切割速度快、精度高、原材料损耗少等优点,受到了广泛关注。金刚线切割多晶硅表面形成的损伤层较浅,与传统的酸腐蚀制绒技术无法匹配,金属催化化学腐蚀法应运而生。金属催化化学腐蚀法制绒具有操作简单、结构可控且易形成高深宽比的绒面等优点,具有广阔的应用前景。本文总结了不同类型的金属催化剂在制绒过程中的腐蚀机理及其形成的绒面结构,深入分析和讨论了具有代表性的银、铜的单一及复合催化腐蚀过程及绒面结构和电池片性能。最后对金刚线切割多晶硅片表面的金属催化化学腐蚀法存在的问题进行了分析,并展望了未来的研究方向。     

Mechanical testing of directionally solidified eutectic ceramics (DSECs): specific problems and limitations

The specific problems of DSECs mechanical testing result from the particularities of these 3-D interconnected eutectic ceramics. First of all, 4-point bending tests ensure pure bending loading, whereas 3 PB tests only lead to a tensile and shear stress combination. Consequently, due to the 3-D microstructure of DSECs, interfaces between the various phases are subjected to a mixed (tensile and shear) loading which makes the interpretation of the results (strength) and of the fracture surfaces, rather difficult. For usual ceramics, biaxial flexure testing offers many advantages over 3- or 4-point beam-bending testing. The coaxial-ring test is free of edge influences (flaws): cracks initiate in the central area and propagate outwardly. However, in the case of DSECs, due to the presence of high internal thermal stresses (especially for ternary eutectics), interfaces can be subjected to a strong radial tensile and shear (near the free surface) stress combination. In the presence of the radial tensile stress resulting from biaxial loading, this internal thermal stress combination can lead to premature crack initiation leading to failure. Specimen machining through grinding leads to the formation of a strongly damaged layer. Annealing of this layer leads to the formation of a rough surface: slightly protruding phases and stress concentrations at the interfaces. The measured strength is ≈20% lower after annealing than that directly after grinding. Concerning the effect of the microstructure size, four representative sizes have been selected in the ≈10 µm to submicrometre range. A classical crack propagation criterion has allowed explaining the corresponding strength values.     

Shear ligament phenomena in Al–Si alloys

Abstract

Fracture problems in Al–Si alloys involve mixed mode (shear and opening) displacements in the aluminium matrix along the crack surface. Subject to such displacements, fracture must be influenced by the non-planarity of microcracks which depends on many factors including particle and slip orientation. Instead of producing brittle cracks along silicon particles clusters, a number of shear bands are formed in the aluminium matrix ligaments between microcracks. Shear ligaments, which are ligamentlike structures connected between microcracks, were observed on the tensile specimens. They undergo ductile fracture by shearing and enhance fracture toughness. This toughness was estimated by a micromechanical model. The amount of toughening depends on the ligament length, a ligament toughness parameter representing the work to fracture, the area fraction of the ligament, and the area fraction of frictional contact.     

Formation of subsurface cracks in silicon wafers by grinding

Single-crystal silicon is an important material in the semiconductor and optical industries.However,being hard and brittle,a silicon wafer is vulnerable to subsurface cracks(SSCs)during grinding,which is detrimental to the performance and lifetime of a wafer product.Therefore,studying the formation of SSCs is important for optimizing SSC-removal processes and thus improving surface integrity.In this study,a statistical method is used to study the formation of SSCs induced during grinding of silicon wafers.The statistical results show that grinding-induced SSCs are not stochastic but anisotropic in their distributions.Generally,when grinding with coarse abrasive grains,SSCs form along the cleavage planes,primarily the{111}planes.However,when grinding with finer abrasive grains,SSCs tend to form along planes with a fracture-surface energy higher than that of the cleavage planes.These findings provide a guidance for the accurate detection of SSCs in ground silicon wafers.     

Crack healing and fracture strength of silicon crystals

The influence of annealing at 700 to 1100° C on fracture strength of pre-cracked silicon wafers was examined by four-point bending tests at room temperature. The fracture strengths of the specimens annealed in oxygen increased significantly with increasing annealing temperature. On the other hand, annealing in vacuum showed little influence on the fracture strength. The strength increase by the annealing in oxygen was found to be caused by crack healing. Utilizing transmission electron microscopy, it is suggested that the crack surfaces were rebonded by the formation of a thin oxide layer at the crack interface. The activation energy for the crack healing was determined to be 2.0±0.1 eV, which was consistent with that of the reaction-limited growth of thin oxide film.     

Interface fracture surface energy of sol–gel bonded silicon wafers by three-point bending

To probe the interface of silicon sol–gel bonded wafers we developed in–situ micromechanical bending test coupled with optical microscopy. The silicon wafers were bonded together at room temperature using sol–gel silica and dried at 60 °C and sintered at 600 °C. Beam specimens were cut from the bonded wafers, then notched and tested in three-point bending. During bending the crack opening from a notch and the deviation along the interface was observed with an optical microscope. To quantify the interfacial debonding from considering the experimental results, a simple energy balance allows an apparent interfacial fracture surface energy to be determined. Experiments and the determined interfacial surface energies show that the bonding of the silicon wafers depends on the silica sol–gel chemistry and on the temperature of the thermal treatment during the bonding process.     

The structure of slow crack interfaces in silicon nitride

Fracture interfaces formed in silicon nitride at high temperatures were studied using light and electron microscopy. The structure of the fracture interface depended on the type of silicon nitride fractured. High-purity, reaction-bonded silicon nitride always formed flat, relatively featureless, fracture surfaces. Fracture occurred by a brittle mode even at the highest temperature (1500° C) studied. The critical stress intensity factor for reaction-bonded silicon nitride ( 2.2 MN m^–3/2) is relatively low and is insensitive to temperature. By contrast, hot-pressed silicon nitride gave evidence of plastic flow during fracture at elevated temperatures. Crack growth in magnesia-doped, hot-pressed silicon nitride occurs by creep, caused by grain boundary sliding and grain separation in the vicinity of the crack tip. As a consequence of this behaviour, extensive crack branching was observed along the fracture path. The primary and secondary cracks followed intergranular paths; sometimes dislocation networks, generated by momentary crack arrest, were found in grains bordering the crack interface. As a result of the high temperature, cracks were usually filled with both amorphous and crystalline oxides that formed during the fracture studies. Electron microscopy studies of the compressive surfaces of fourpoint bend specimens gave evidence of grain deformation at high temperatures by diffusion and dislocation motion.     

Deformation and fracture of cast iron with an optimized microstructure

An optimized microstructure of cast iron has been developed. In situ studies with the use of scanning electron microscopy have been made of the microprocesses of deformation and fracture in tension. The surface microstructure and the corresponding fracture surface and matching fracture were observed. The optimized nodular graphite-martensite interface, in combination with the high yield strength martensite shell, has been found to delay the initiation and propagation of interfacial cracks. Subsequent propagation and the formation of cast iron cracks is hindered by the ferrite-plus-martensite matrix. During crack propagation, shear deformation, the fracture of ligaments between the crack tip and the microcracks ahead, crack deflection, and bridging take place. With the use of these observations, it is possible to explain why cast iron with this optimized microstructure has excellent strength and toughness.     

Development of fatigue cracks from mechanically machined scratches on 2024‐T351 aluminium alloy—part I: experimentation and fractographic analysis

Clad and unclad 2024‐T351 aluminium alloy sheets, weakened by mechanically machined scratches, were fatigued to investigate the effect of small surface damage, like scribe marks, on aircraft fuselage joints. The role of scratch cross section geometry on fatigue life of scribed components was analysed. Scratches between 25 and 185 µm deep, with 5, 25 and 50 µm root radii, were cut on sample surface by using diamond‐tipped tools. After testing, fracture surfaces were examined using a scanning electron microscope, and crack growth rates were measured by striation counting. Scratches reduced aluminium fatigue life under tensile and bending load up to 97.8% due to multiple crack nucleation at their roots. Short cracks nucleated from sharp scratches coalesced to form unique elongated cracks growing through sample thickness. Cracks initiated from scratches were typical short cracks, growing faster than conventional long cracks. Despite the different scribing process, fatigue data of regular diamond tool cut scribes can be used to conservatively predict life reduction owing to ploughed in‐service scribe marks on fuselage joints. Finite element analyses on scribed samples and the fatigue life prediction models are described in Part II of this paper.     

磨削表面损伤对RBSiC弯曲强度的影响

借助X射线衍射方法测量了反应烧结碳化硅(RBSiC)材料的磨削表面的残余应力状态,并根据断裂力学评价了磨削引入的裂纹尺寸,分析了RBSiC的弯曲强度受磨削引入裂纹和残余应力的影响.研究表明,由于磨削过程中与磨削方向有关的机械载荷占主导作用,使磨削后的表面残余应力具有方向依赖性.砂轮轴向进给从0.90μm/s增加到1.35μm/s,磨削表面的残余压应力数值降低,计算得到的磨削引入的裂纹尺寸增大,导致强度下降.     

挖掘机销轴断裂分析

利用宏观检验、断口分析、化学成分分析、金相检验以及硬度检测等方法,对42CrMo钢挖掘机销轴的断裂原因进行了分析。结果表明:销轴断裂为双向弯曲疲劳断裂。由于销轴表面存在较脆的白亮层ε相,且白亮层分布有较严重的疏松,增加了销轴表面的脆性,使销轴表面形成了较多的微裂纹,导致了疲劳裂纹的萌生;销轴的渗氮层深度和硬度偏低也降低了销轴的疲劳强度,加速了疲劳裂纹的扩展,最终使销轴发生早期疲劳断裂。