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 SiO 2 layer at the sidewall. The mechanisms behind the SiO 2 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 Cu 3 Si layer, a SiO 2 layer mixed with Cu 3 Si, and finally a thick SiO 2 layer. The Cu 3 Si catalyzes the growth of the SiO 2 through an oxidation step of the Cu 3 Si on the sidewall surface as well as at the SiO 2 /Si interface. In the lower region of the sidewall, the microstructure is similar to the upper region, but there is a separation of the SiO 2 layer from the crystalline Si. The SiO 2 undergoes a decomposition reaction at the SiO 2 /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 SiO 2 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 SiO 2 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 SiO 2 layer. In the sidewall region near the die frontside, the SiO 2 thickness is more than regions near the die backside, and no microvoiding and separation at the SiO 2 /Si interface were detected. This is hypothesized to be due to a higher O 2 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 O 2 deprivation and lower O 2 pressure.