Publication: Fabrication and characterization of copper-stabilized ultrathin silicon dies using ultrashort-pulse laser dicing
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Date
2021-11-01
Authors
Marks. Michael Raj
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Abstract
Ultrathin dies require a copper (Cu) stabilization layer, which is essentially a backside Cu layer, to prevent warpage and cracks during subsequent packaging assembly process. The dicing of silicon (Si) wafers with a backside Cu layer by mechanical blade dicing and plasma dicing pose various quality and cost challenges. Laser dicing is a promising method for dicing thin and ultrathin Si wafers. The feasibility of dicing ultrathin Si wafers with backside Cu layer using nanosecond pulse laser has been reported in the literature. However, there is no reported work on the application of ultrashort pulse lasers in the picosecond and femtosecond range for dicing ultrathin Si wafers with backside Cu layer. Theoretically, ultrashort pulse lasers could reduce the thermal effects during laser dicing. The main aim of this research is to systematically investigate the dicing of 20 µm ultrathin Si wafers with 10 to 30 µm backside Cu with picosecond and femtosecond pulse lasers. In order to study the effect of the ultrashort pulse lasers on the die sidewall fracture strength accurately, a novel procedure for three-point bend (3PB) test was developed. Fractography analysis was performed to determine and study the fracture initiation sites of 3PB test samples.
During laser dicing of ultrathin Si wafers with backside Cu, a Si oxide layer is formed between the crystalline Si substrate and a layer of polycrystalline Cu silicide. It was observed that the Si oxide layer detaches from the crystalline Si at the backside region of the sidewall. The detachment of the Si oxide from the crystalline Si is through a microvoiding mechanism which has a significant effect on the fracture strength of the die sidewall. The die sidewall defect morphologies, microstructures, and elemental compositions after picosecond and femtosecond pulse laser dicing have been characterized in detail by transmission electron microscopy equipped with energy dispersive spectroscopy and nano beam diffraction. Picosecond laser dicing on ultrathin Si wafers with Cu backside layer up to 30 µm thickness seems to be the best laser dicing approach in terms of die sidewall quality and fracture strength. In general, without backside Cu layer, the sidewall backside strength is approximately 9 to 125% higher than the frontside strength, and with 10 to 30 µm Cu backside Cu layer, the average sidewall backside strength is approximately 66 to 71% higher than the average frontside strength.