An investigation of recast behavior in laser ablation of 4H-silicon carbide wafer

摘要

4H-silicon carbide (SiC) is a suitable semiconductor material for high-speed power devices but also a typical difficult-to-process and hard-brittle material. Laser ablation is an efficient approach to process SiC. However, recast layer is an inevitable thermal damage during laser ablation. The present study experimentally and numerically investigated the recast behavior in laser ablation of 4H-SiC. The recast humps are with height of 1.4-3.3 mu m and width of 12-22 mu m, consist of SiC, Si, and SiO2, where 15.08 wt% of oxygen is detected. Microcracks and pore clusters are observed on the recast humps under SEM. Recoil pressure and surface tension in the molten pool dominate the process of recast hump formation. The recoil pressure of Si vapor is 2.27 MPa at 5000 K, as high as 22 times of atmospheric pressure. The simulation results show that the fluid in the middle flows around while the surrounding fluid flows upwards. The upward flow is almost laminar while the flow at the bottom of the molten pool is turbulent entertaining air and forming bubbles. The vaporization-induced keyhole in the molten pool becomes deeper and the sidewall becomes more inclined over time, accompanying with the "growing" of the recast hump. There are mainly three reasons causing the increase of recast height and recast width with the increased average laser power: (a) increase of recoil pressure; (b) increase of temperature gradient in the molten pool, enhancing the Marangoni convection; and (c) enlarging the molten pool and leading to more materials melting. With the decrease of the laser scanning speed, more liquid molten material is evaporated in the molten pool instead of being ejected outward to form recast humps. The increase of the laser pulse frequency decreases the peak laser intensity and hence results in the decrease of the recoil pressure. Besides, it also leads to the weakening of the heating ability of laser and lessening of the liquid molten material that could be ejected.