Study of the Failure Mechanism of Crystalline Silicon: Relation between Crack Orientation and Failure Stress [Poster]

Abstract

In the PV industry, cracking of solar cells is one of the main causes of failure and demotion. Most cracks are generated in the cutting process to obtain the silicon wafers. For this reason, the characterization of the mechanism of breakage and the behaviour of strength of silicon wafers is highly important in order to minimize the fracture rate and to optimize the process steps. The strength characterization involves fracture tests. Depending on the purpose, different fracture tests can be used: Ring-on-ring, 4-line bending and twist test are some of the most tests used. The high scatter observed in the strength characterization of brittle materials is represented by means of statistical distributions getting a more accurate failure prediction. Based on the weakest link theory, Weibull introduced a probabilistic approach in 1939 that is widely used in the strength characterization of brittle materials. Weibull approach is valid for an area subjected to a uni-axial tensile stress. This assumption is generally not true. There are different models to consider the multi-axial stress state. The most used is the Principle of Independent Action (PIA) which consider each point subjected to a different uni-axial stress for each principal stress. More accuracy models requires a better knowledge of the failure mechanism and the relation between the stress and the cracks length and orientation. This work intends to study this relation in order to stablish an accuracy failure criteria for silicon wafers. To this end, samples without cracks from wafering are the starting point. Controlled cracks are generated and after that, the strength is determined. Moreover, the test are recorded with an HS camera in order to analyse the crack propagation and the failure mechanism. With all these results, an accuracy model to take into account the multiaxial stress state is presented.