The unbiased propagation mechanism in laser cutting silicon wafer with laser induced thermal-crack propagation

In the silicon dicing processes, the laser induced thermal-crack propagation (LITP) has been reported about its superiority over traditional dicing methods in aspect of crack surface quality and mechanical properties. However, this process may cause deviation of its propagation path especially under the condition of asymmetric linear cutting. Aiming to solve the path-deviation problem, a method of pre-crack on the surface of the silicon wafer has been proposed. A mathematical model which consists absorption of laser, heat transfer, generation of thermal stress, and crack initiation and propagation, has been established. Numerical simulation in the LITP of silicon wafer with respect to two pre-crack modes: pre-crack on the end and pre-crack on the surface were conducted to study the high-quality crack propagation mechanism. Unbiased crack propagation was found in the simulation of pre-crack on the surface process and the mechanism of it was revealed through the analysis of the tensile stress distribution. It was found that the three-dimensional shape of the tensile stress at the front of the crack tip would result in an uncertain propagation in conventional LITP of pre-crack on the end. Conversely, the slice shape of the tensile stress would induce a certain propagation direction that was along the plane of the pre-crack in the simulation of pre-crack on the surface. The experimental results verified the unbiased propagation mode and showed good crack surface quality.