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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 SiO2 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.