Temporary Protective Packaging for Vertical Integration of Optical-Window-Free TSV-Based Micromirror Array

Temporary protective packaging is a critical enabler to the development of micromirror array (MMA) module. Herein, aimed at the vertical integration of optical-window-free through-silicon-via-based (TSV-based) MMA and its corresponding application-specific integrated circuits (ASICs), this article proposes a temporary protective packaging method that is compatible with a series of back-end processing, including under-bumping-metallization (UBM), dicing, solder bumping, and flip-chip bonding. An ultraviolet-curing (UV-curing) strippable adhesive with tolerance to water, organic solvent, and high temperature is carefully selected and introduced into this temporary protective packaging method to bond caps to MMA wafer. A programmable dispenser is employed to inject the chosen adhesive into prefabricated trenches on MMA wafer, which enables patterning of this adhesive. In response to its contradiction between detachability and sufficient adhesion, a double-level temporary protective packaging is proposed, which is composed of wafer-level cap and die-level cap. Wafer-level cap not only focuses on adhesion to guarantee MMA wafer being sealed in UBM fabrication and to keep die-level cap free of mechanical influence before it comes into use, but also guarantees wafer profile quality in specific processing. Die-level cap focuses on its detachability, and comes into play after dicing to protect MMA in processing of solder bumping and flip-chip bonding. The effectiveness of the chosen adhesive is verified experimentally. Finally, an intact optical-window-free vertical integrated MMA module is achieved after all the back-end process and removal of die-level cap.