PECVD oxide as intermediate film for wafer bonding: Impact of residual stress

The deposition rate, the etch rate in a HF-based solution and the residual internal stress of PECVD oxides are systematically analysed for various deposition conditions and post-anneal treatments. Rapid thermal anneal (RTA) at a temperature over 900 °C for 15 s is proven to be the most efficient to reduce the residual stress in the film and its etch rate in BHF solution, as well as to enhance its long term stability. The reduction of the internal stress in PECVD oxide is mandatory to minimize the wafer bow which degrades the wafer bonding quality. Bonded samples show that the resulting surface energy tends to vary inversely with the elastic energy stored by the conformation of the wafers during the direct bonding. About 45 μm wafer bow (3 inch wafer, 380 μm-thick) comes out as an upper bow limit, preventing direct bonding to occur. The use of a RTA step following the PECVD oxide layer deposition is demonstrated to be an efficient technological solution to minimize the wafer bow and thus maximize the bonding surface toughness. The experimental results presented in this paper highlight the importance of monitoring the residual stress in intermediate oxide layers to assure high quality and reliable bonding and thus future three-dimensional integration.