Low-Temperature Oxide Wafer Bonding for 3-D Integration: Chemistry of Bulk Oxide Matters

The effect of bulk chemistry of deposited oxide materials on the eventual wafer bonding energy was fundamentally studied. Although low-temperature silicon oxide (LTO) and tetraethyl orthosilicate (TEOS) exhibited the same bulk density, and nitrogen plasma generated a higher degree of surface activation for TEOS than LTO, using LTO as the bonding oxide resulted in a much higher bonding energy than TEOS. This was attributed to the relatively high percentage of hydrogen-bonded silanol groups in LTO, which pointed to the existence of fine defect areas in LTO that would better accommodate the water molecules generated later by the interfacial condensation reactions. A pre-bonding baking step was found favorable for LTO wafer bonding.