Effect of Nanosecond Laser Dicing on the Mechanical Strength and Fracture Mechanism of Ultrathin Si Dies With Cu Stabilization Layer

Ultrathin dies require a Cu stabilization layer, which is essentially a backside Cu layer, to prevent warpage and cracks during solder die attach and wire bonding. The dicing of Si wafers with a backside Cu layer is challenging. Mechanical blade dicing through the Cu layer causes blade clogging and damage, which eventually results in severe die chipping and cracks. Plasma dicing is costly as it requires additional photolithography and etching steps. Laser dicing is promising and is currently used to singulate thin Si wafers. However, there is no reported work on its application for dicing ultrathin wafers with a backside Cu layer. In this paper, nanosecond laser dicing of 20 μm Si dies with 0-30 μm backside Cu was found to be feasible. The effect of nanosecond laser dicing on the die sidewall strength was evaluated with the three-point bend (3PB) test. Analytical and experimental results have shown that the Cu and AlCu layers have gone into plastic condition during the 3PB test. Comparison of the 3PB fracture loads indicates that the Si backside strength is higher than the Si frontside strength. Fractographic analysis has confirmed that the fracture initiation sites during the 3PB tests are at the die sidewall. The die sidewall defect morphologies, structures, and elemental compositions have been characterized in detail by transmission electron microscopy, and their effect on mechanical strength is discussed.