Femtosecond laser dicing of ultrathin Si wafers with Cu backside layer - A fracture strength and microstructural study

Abstract

Previous studies on nanosecond and picosecond laser dicing have shown that the Si die sidewall microstructures and fracture strengths are significantly different when dicing simultaneously through Si and a Cu backside layer, in contrast to dicing solely through Si. This work systematically investigated the effect of femtosecond laser dicing on the fracture strength and sidewall microstructure of 20 μm Si dies with 0−30 μm Cu backside layer. Using an improved three-point bending (3PB) test method, the intrinsic fracture strengths of the die sidewall were measured. The die types with Cu backside show an average of 46.5 % higher backside characteristic fracture strength, but an average of 6.6 % lower frontside characteristic fracture strength, compared to the die type without Cu backside. Fractographic analysis by scanning electron microscopy was carried out to determine the fracture initiation behaviour in the 3PB test samples. The microstructures, phases, and defects at the sidewall were characterized by transmission electron microscopy, and their effect on fracture strength is discussed. Based on the observed microstructural features, some suggestions in relation to the femtosecond laser processing parameters are made to improve the die sidewall frontside and backside fracture strengths, in comparison to previous results from nanosecond and picosecond laser dicing.