Ultrafast pulsed laser stealth dicing of 4H-SiC wafer: Structure evolution and defect generation

Ultrafast pulsed laser for wafer processing has been promising in the semiconductor industry due to its high precision and low thermal effect. The microstructure and defect during the stealth dicing process is critical to controlling the desired quality of the wafer. The structural evolution and defect formation mechanisms during stealth dicing of 4H-SiC with femtosecond and picosecond lasers were investigated. In order to understand the ultrafast pulsed laser interaction with 4H-SiC wafer, we studied the laser intensity dependence of the nonlinear refractive index, the propagation of laser beam focusing in 4H-SiC, the laser absorption, electron concentration evolution and energy deposition. The effect of two significant processing factors was discussed, including laser fluence and laser scribing times. The propagation paths and energy deposition of femtosecond and picosecond lasers in the internal processing of 4H-SiC were calculated by finite element method. The evolution of electrons temperature, lattice temperature and free electron concentration in the material during laser processing was analyzed. Femtosecond laser stealth dicing had higher processing accuracy due to minimal thermal effects. The defect induced by femtosecond laser was dominated by nanovoids and micro-cracks. The defect structure was affected by laser processing parameters such as scanning passes and laser power density. Multi-focusing of ultrafast pulsed lasers in transparent materials was generally considered to be a dynamic balance of nonlinear self-focusing and plasma defocusing. Picosecond laser processing resulted a higher processing efficiency under the same laser energy density comparing with femtosecond laser processing. However, defects during picosecond laser dicing were dominated by thermal ablation, which impaired the processing accuracy.