Spatial zigzag evolution of cracks in moving sapphire initiated by bursts of picosecond laser pulses for ultrafast wafer dicing.

Spatial zigzag evolution of cracks in moving sapphire wafer was observed after irradiation with sequences of picosecond laser pulses (bursts). The Gaussian beam was tightly focused inside the sapphire. The spatial position of laser initiated cracks moved in vertical and horizontal directions when a wafer was translated at a controllable speed perpendicular to the beam propagation direction. The cracking plane consisting of the periodically repeating inclined modifications and cracks was observed. The period of modifications and the inclination angle had a linear dependence on the wafer translation speed. The model of spatial zigzag crack evolution was created and the physical origin of modification growth at a measured speed of 1.3 ± 0.1 m s -1 is discussed. The zigzag cracking was applied for ultrafast stealth dicing and cleavage of the sapphire: dicing speed 300 mm s -1 , wafer thickness 430 μm, laser power 5.5 W, repetition rate 100 kHz, sub-pulse duration 9 ps, the temporal distance between sub-pulses in burst 26.7 ns, and the number of sub-pulses 13.