Study of the Edge and Surface Cracks Influence in the Mechanical Strength of Silicon Wafers

The objective of the present study is the estimation of the depth to which the wire sawing process causes damage to the wafer surfaces. Previous analyses were carried out by means of the four line bending test. The characteristic of this test implied that the failure could be due to surface cracks located in the central zone of the wafer or near the edges. In order to evaluate the influence of the edge or surface cracks a new study has been carried out using the ball/ring on ring test. Description and results of the tests are presented. The preliminary analysis of the failure stress using analytical methods confirms the expected results. A Finite Element model developed to get more information of the test results is also presented.

Impact of Impurities on the Mechanical Strength of Multicrystalline Silicon

The usage of more inexpensive silicon feedstock for crystallizing mc-Si blocks promises cost reduction for the photovoltaic market. For example, less expensive substrates of upgraded metallurgical silicon (UMG-Si) are used as a mechanical support for the epitaxial solar cell. This feedstock has higher content of impurities which influences cell performance and mechanical strength of the wafers. Thus, it is of importance to know these effects in order to know which impurities should be preferentially removed or prevented during the crystallization process. Metals like aluminum (Al) can decrease the mechanical strength due to micro-cracking of the silicon matrix and introduction of high values of thermal residual stress. Additionally, silicon oxide (SiOx) lowers the mechanical strength of mc-Si due to thermal residual stresses and stress intensification when an external load is applied in the surrounding of the particle. Silicon carbide (SiC) introduces thermal residual stresses and intensifies slightly the stress in the surrounding of the particle but can have a toughening effect on the silicon matrix. Finally, silicon nitride (Si3N4) does not influence significantly the mechanical strength of mc- Si and can have a toughening effect on the silicon matrix.