Impact of Light Element Impurities (O, C and N) on the Mechanical Properties and Crack Initiation/Propagation in Photovoltaic Silicon Wafers

High wafer breakage rate is a significant issue facing photovoltaic (PV) silicon industry and becomes more and more significant with the use of thinner and larger crystalline silicon wafers. Wafer breakage depends on factors such as surface/edge flaws, impurity precipitates, structural defects and their associated residual stresses. Successful prediction of wafer breakage requires knowledge of micromechanics of strength degradation, crack initiation/propagation associated with the impurities, structural defects and residual stresses. The goal of this work is to study the impact of light element impurities, their structural defects and residual stresses on the mechanical properties and crack initiation/propagation of PV silicon wafers.;Quantitative influence of oxygen concentration on the mechanical properties of Czochralski (CZ) silicon wafers grown at elevated rate was investigated. Nanoindentation and microindentation results indicated that both hardness and fracture toughness of CZ silicon decreases with increasing oxygen concentration. In addition, remarkable differences in the mechanical properties were found between the core and edge of the same wafer that has high oxygen concentration. Photoluminescence (PL) analysis and Nomarski optical microscopy identified ring-like distribution of oxidation induced stacking faults (OSFs) and other micro defect structures in those CZ silicon wafers. Localized nanoindentation and nanoscratch tests showed a softened zone near the oxygen precipitates and OSFs. Transmission electron microscopy (TEM) observation confirmed the formation of the platelet oxygen precipitates and large stress field around them. The softening effect is attributed to the stress profile gradients resulted from the dynamic interaction between oxygen and intrinsic point defect and the growth of structural defects developed in a fast growth regime.;Impact of single micro crack on the fracture strength of PV silicon wafer was also investigated based on a controlled flaw method. Results indicated that the fracture strength of PV silicon wafer is very sensitive to the microindentation load (radial crack scale). In addition, it was found that carbon impurity plays an important role in the contact cracking-fracture process in PV multicrystalline silicon (mc-Si). The fracture strength increased ∼21% with substitutional carbon concentration increased from 1.2×1018 to 6.4×10 18 atoms/cm3.;Silicon nitride inclusions in cast mc-Si wafers and the impact of nitrogen doping in CZ silicon wafers grown at elevated rate were studied. The rod-shape nitride inclusions were determined to be composed of β-Si3N 4 and single crystalline silicon phases according to the characterization by a scanning electron microscopy (SEM) with energy dispersive X-ray microanalysis (EDX) and micro-Raman spectroscopy. Large tensile stresses were found both at the vicinity and inside the silicon nitride rods. The large tensile stresses are believed to be resulted from large differences in the thermal expansion coefficient between silicon and silicon nitride. Nanoindentation measurements confirmed the silicon nitride structure and a reduced hardness zone. For the nitrogen-doping effect on the mechanical properties of CZ silicon wafers grown at elevated rates, it was found that nitrogen could modify the hardness of CZ silicon wafers in two ways, through the interaction between nitrogen, oxygen and intrinsic point defects and formation of associated structural defects during crystal growth.