Development of Micrometer-Thick Bonding Material for Wafer-On-Wafer (WOW) Applications

We have developed permanent and temporary adhesives that are suitable for fabricating multilevel stacks using ultra-thin wafers of several micrometers or less [1]. In this paper, we describe a hot-melt type temporary adhesive layer with a thickness of 10 μm formed on a device wafer using a spin-on technique for bumpless TSV interconnect applications. After stacking the device wafer on a carrier, the device wafer was thinned to approximately 10 μm with a grinding and polishing processes. In the permanent bonding process, an approximately 2.5 μm-thick permanent adhesive layer was formed on the thinned wafer by the spin-on technique, and a thin wafer was stacked on top. There were no voids between the stacked wafers. After the carrier was removed, residual temporary adhesive was removed by using a conventional solvent. The permanent adhesive coating had excellent thermal stability, low warpage due to the thinness of the layer and the low curing shrinkage, strong adhesion strength to SiO
₂
and Si, and a good etching profile. Moreover, low contact resistivity and fine uniformity of the vertical TSV interconnects were achieved in a stack of 300 mm wafers. By using our material as an adhesive, efficient stacking of devices will be achieved.