Fundamental investigations of cutting of silicon for photovoltaic applications

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Wu, Hao
Melkote, Shreyes N.
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Crystalline silicon (Si) wafers used as substrates in the semiconductor and photovoltaic (PV) industries are traditionally manufactured using a multi-wire slurry sawing (MWSS) technique. Due to its high productivity potential, the fixed abrasive diamond wire sawing (DWS) technique is of considerable interest to Si wafer producers. Although both sawing techniques are currently used in the industry, a fundamental understanding of the underlying process is still lacking, particularly for diamond wire sawing. Consequently, optimization of the wire sawing process is carried out largely based on experience and trial and error. This thesis aims to develop a systematic fundamental understanding of diamond wire sawing of Si materials used for PV applications. First of all, a comparative analysis of the characteristics of silicon wafers cut by slurry and fixed abrasive diamond wire sawing is presented. The analysis results indicate that fixed abrasive diamond wire sawing may be a viable alternative to slurry wire sawing. Modeling and experimental studies of single grit diamond scribing of Si are proposed to shed light on the basic cutting mechanisms. Although Si is brittle at room temperature, it is possible to properly control the cutting conditions to obtain a completely ductile mode of material removal. The effects of material anisotropy, abrasive grit shape, friction condition and external hydrostatic pressure on the ductile-to-brittle mode transition in cutting of single crystal Si (sc-Si) are systematically investigated. Multicrystalline Si (mc-Si) based solar cells take up the majority of the global PV market. Hard inclusions (Silicon carbide and Silicon nitride) in multicrystalline Si (mc-Si) ingots may cause wire breakage and negatively impact the process, surface/subsurface morphology and mechanical properties of the resulting wafer. Their effects are experimentally studied through the single grit diamond scribing on the mc-Si sample with high density of inclusions. Finally, it is identified that there is a correlation between the high dislocation density and the increase of fracture toughness in mc-Si. The increase in fracture toughness leads to greater capability of ductile mode of cutting and higher specific scribing energy in the brittle fracture regime. Results of these fundamental investigations are expected to generate useful knowledge for optimizing the diamond wire sawing process in order to achieve high productivity and minimum surface/subsurface damage.
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