Silicon full wafer bonding with atomic layer deposited titanium dioxide and aluminum oxide intermediate films

Silicon-on-insulator (SOI) wafers made by direct wafer bonding are widely used as starting substrates for microelectromechanical systems (MEMS) fabrication. Adding another layer next to the SiO2 SOI, or replacing it with another material, will be a way to tailor the SOI wafers further. Atomic layer deposition (ALD) can be used to deposit pinhole-free nanometer-thin conformal and smooth inorganic films at low temperatures, making it of interest for many applications in MEMS. In this work, the direct wafer bonding of ALD TiO2, and Al2O3 for reference, is investigated, eventually in order to fabricate SOI wafers with buried ALD oxides. Finding suitable process conditions for TiO2 bonding was challenging: bonding could not be made for TiO2 deposited directly on SiO2 or Si, and annealing at 1100 °C gave non-continuous Ti-containing layers. Using a 2-nm Al2O3 under-layer and annealing at 700 °C gave void-free bonding with continuous TiO2 and a bond strength of approximately 1600 mJ/m2, enabling the fabrication of SOI wafers with buried TiO2.