LowK wafer dicing robustness considerations and laser grooving process selection

Abstract

Zero defect in semiconductor packaging is key especially for high demanding reliability applications (automotive, spatial…) combined with high performance technologies (Silicon ultra lowK wafers 40nm and beyond). The most complex challenge is the Chip Package Interconnect which requires an optimized and controlled singulation process to prevent weaknesses occurring during the process (ILD crack/ delamination, side wall crack, metal burrs, humidity propagation and die strength weakness). Laser grooving prior mechanical dicing offers a good seal ring protection of the chips, ie use of a laser to perform ablation and remove the sawing street contents (dielectric / metallization / patterns) in order to make a clean ‘path’ for the mechanical sawing step afterward. Degradation of the blade condition is also reduced with this approach. This article describes the interactions between laser grooving process and the lLD BEOL delamination prevention by applying a perfect U shape profile.Laser Grooving process selection is a complex combination of different requirements: (i) U shape profile geometry tradeoff to guarantee integrity of BEOL layers during mechanical dicing, while keeping a high level of miniaturization of the sawing street not to increase wafer cost. (ii) Laser technology and process to match the required U-shape geometry minimizing redeposition of residues (recast), avoid pitting (voids) and absorb the heterogenous content of a scribe line with a minimum level of energy applied. This is achieved thanks to an accurate selection of the laser source (wavelength, beam size), accuracy of the beam position (lens optical system), ablation method (multi pass) and process parameter optimization (power, speed, frequency…). (iii) Advanced characterization technics are required to ensure the required quality criteria are matched, like 3D profilometer, FIB cross sections, SEM controls, AOI and 3 points die strength.STMicroelectronics is deploying this quality driven mindset to high volume manufacturing environment in all BE manufacturing plants. Emerging technologies like ultrafast laser wavelength or replacement of mechanical dicing by plasma dicing are next steps of interest further enhanced the robustness of more and more fragile wafer FE technologies.