Back-via 3D integration technologies by temporary bonding with thermoplastic adhesives and visible-laser debonding

Back-via three-dimensional (3D) integration using multiple thin-wafer transfer processes has been developed at GINTI, Tohoku University, where visible laser was employed for wafer debonding. The potential advantages of laser debonding are (i) the realization of ultra-thin wafer releasing with less stress as compared to the conventional thermal and chemical debonding methods, and (ii) no adhesive residues were left on the thinned wafer surface owing to their excellent solubility in solvents. The edge-trimming width and depth for Si before temporary bonding and the temporary bonding parameters using thermo-plastic adhesives were carefully investigated and optimized, in order to avoid any undesirable effects in background thin wafers. Through-Si-Vias with a diameter of 5-15 μm were formed by masking the via patterns (using i-line, back-side-alignment) on the SiO ₂ surface of the back-ground side of 30 - 50 μm-thick LSI wafer that was temporarily bonded to the support glass, followed by selective deep-reactive-ion-etching of SiO ₂ , Si, and bottom SiO ₂ , and subsequently barrier and seed layers deposition and via filling. Using laser debonding technique, the thinned Si wafers with Cu-vias were transferred to the other glass with different temporary adhesive. The observed low resistance values from the I-V data for 5000 Cu-via daisy chain reveals that the proposed back-via 3D integration using laser debonding is now ready for industrial use.