Backside Chipping Investigation & Improvement on TiNiVAg Back Metal Silicon Die

As one of the constantly evolving semiconductor chipmaker company, producing products with high energy efficiency, quality, robustness with lower cost is mainly the focus. To enable high temperature requirement, silicon die are layered with back metallization to enable thermal conduction. In semiconductor, Titanium (Ti), Nickel (Ni) and Silver (Ag) are the common materials widely used for wafer metallization. Recent days, Vanadium (V) been added for wafer fab process efficiency. Addition of Vanadium sacrifices metal adhesion to the non-ultraviolet (non-UV) dicing tape type during back-end process. This paper outlines sawing process investigation of thin die with TiNiV Ag back metal to resolve a die crack issue. Aim of this paper is to investigate, identify root cause of die crack and implement robust manufacturing solution. Failure analysis inputs pointed crack die initiation start from back chipping and chip length issues. Design of experiment was performed to investigate sawing parameter, method and tooling optimization. Control baseline using step cut method and non UV tape. Since Z-Axis 2 (Z2) blade in step cut has strong correlation to backside chipping. Parameter optimization is focused to reduce the cutting load of Z2 blade and self-sharpening attributes. Second step of the study is to investigate effect of tape curing. There is no backside chipping improvement after implemented the best parameter with or without curing. The result from these combinations of non- cured tape, optimized Z2 sawing process, showed small improvements on back chipping. Final study is to investigate tape material combination to sawing parameter. Ultraviolet (UV) tape had the largest improvement on both chipping and chip length. The rigidity and high resistant to vibration characteristic of the tape during sawing process allows stable cutting throughout for biggest die size. To optimize the overall manufacturing cost, interaction of UV tape type were validated using best parameter onto full cut method. The blade with lower grit size were selected to perform the full cut with minimized stress during cut. The final results of this combination able to meeting the acceptance criteria. The finding of this new process window is capable to produce zero crack die with back chipping meeting 6 sigma process capability. In this study, impact of die strength is discovered where die is susceptible to crack when the back chipping exceeds half of its thickness and chipping length greater than half of the die length. The learning from this paper have enhanced wafer saw process design rule for selection of blade, tape and die characteristics for future package.