Fracture strength and microstructural study of ultrathin Si die with Cu backside layer diced with picosecond laser

Mechanical and plasma dicing of Si wafers with a backside metal layer poses serious challenges in quality and manufacturing cost. Mechanical dicing through the metal layer causes blade clogging and damage, which results in severe Si die chipping and cracks. The cost of plasma dicing is high as additional photolithography steps are required for etching the metal layer, in addition to the extremely slow metal etch rate. Laser dicing is promising and is currently used to singulate thin Si wafers. In this paper, picosecond laser dicing of 20 mu m Si dies with 0-30 mu m backside Cu was found to be feasible. Utilizing an improved three-point bending test method, the frontside and backside characteristic fracture strengths of ultrathin Si dies with varying backside Cu thickness were measured. The die types with backside Cu show an average of 17% higher frontside characteristic fracture strength, and an average of 13% higher backside characteristic fracture strength, compared to the die type without backside Cu. The die sidewall microstructures, defects, and compositions have been characterized in detail by transmission electron microscopy, and their effect on mechanical strength is discussed.