Mechanism study of SiO2 layer formation and separation at the Si die sidewall during nanosecond laser dicing of ultrathin Si wafers with Cu backside layer

Abstract

Laser dicing of ultrathin dies is promising and is gaining importance because of its cost and quality advantages over mechanical and plasma dicing. However, the effects of laser dicing on the mechanical strength and microstructure of ultrathin Si dies need to be further understood, especially when dicing through Si wafers with backside Cu layer. A critical phenomenon effecting the Si die sidewall strength after nanosecond laser dicing of Si wafers with backside Cu is the formation and separation of a SiO2 layer at the sidewall. The mechanisms behind the SiO2 layer formation and separation were studied in this work. Si wafer samples without and with backside Cu layer were prepared by dicing with nanosecond laser using standard production parameters. The microstructure and phases formed were investigated by energy dispersive spectroscopy and nanobeam diffraction in a transmission electron microscope. In die samples without backside Cu, the sidewall consists of a thin surface layer of amorphous Si, followed by a polycrystalline Si layer, and finally an epitaxial Si layer. In die samples with backside Cu, the sidewall microstructure was observed to be vastly different. At the upper region of the sidewall, a surface layer of polycrystalline Cu was found, followed by a polycrystalline Cu3Si layer, a SiO2 layer mixed with Cu3Si, and finally a thick SiO2 layer. The Cu3Si catalyzes the growth of the SiO2 through an oxidation step of the Cu3Si on the sidewall surface as well as at the SiO2/Si interface. In the lower region of the sidewall, the microstructure is similar to the upper region, but there is a separation of the SiO2 layer from the crystalline Si. The SiO2 undergoes a decomposition reaction at the SiO2/Si interface, releasing volatile SiO which causes microvoids to form and grow laterally at the interface. The growth and coalescence of the microvoids eventually lead to the separation of the SiO2 layer from the crystalline Si, leaving behind a clean and rough crystalline Si surface with a peak-to-peak roughness of 100–200 nm. In the areas where the SiO2 layer has separated from the Si die sidewall, the fracture strength of the sidewall is dependent on the material property and surface roughness of the crystalline Si, and not on the SiO2 layer. In the sidewall region near the die frontside, the SiO2 thickness is more than regions near the die backside, and no microvoiding and separation at the SiO2/Si interface were detected. This is hypothesized to be due to a higher O2 pressure at the upper region of the narrow dicing trench which is open to the atmosphere compared to the lower regions where there could be O2 deprivation and lower O2 pressure.