Surface-Activated Direct Bonding of Diamond (100) and Sapphire with High Transparency for Quantum Devices

Diamond single crystals with color centers have attracted attention for their application in quantum photonic devices. Owing to the size limitation of diamond substrates, bonding a small diamond substrate to a large mother wafer is crucial for developing large-scale integrated devices. To access the quantum states of color centers using an optical technique, bonding the diamond with a transparent material with low optical loss in the visible region is desired. Sapphire is a promising candidate as a transparent material as it is a single crystal with a large optical bandgap. However, the direct bonding of a diamond substrate with (100) surface orientation is an unexplored challenge for sapphire wafers. In fact, such direct bonding has not been easy for Si wafers [1-5]. Herein, we report the successful direct bonding of diamond (100) substrates and sapphire (1000) wafers using the atom beam–assisted surface-activated bonding method and clarify its mechanism with detailed structural characterizations.<br/> CVD-grown diamond (100) substrates with a size of 4 × 4 mm² and a thickness of 0.5 mm were used in the experiments. We systematically investigated the relationship between the surface properties and the bonding strength. It was found that atomic-level surface flatness (roughness below 0.1 nm) in a micron-sized area and submicron-level global flatness (roughness below 0.2 μm) in the whole area are important to achieve stable bonding. The diamond and sapphire surfaces were activated using fast atom beam (FAB) irradiation in a vacuum chamber for 600 s, although the diamond/Si bonding studies used the wet-chemical activation process for the diamond surface [1-5]. Subsequently, the substrates were brought into contact under a pressure of 20 MPa at room temperature, resulting in full bonding. No graphite layers were observed on the activated diamond surface through Raman spectroscopy. Thus, a highly transparent bonding interface was realized. Cross-sectional transmission electron microscopy and energy dispersive X-ray spectroscopy revealed that an amorphous AlO_x layer with a thickness of 150–200 nm existed between the diamond and the sapphire single crystal. This intermediate amorphous layer was likely formed during the FAB treatment. The shape of the amorphous layer was deformed to fit the monolayer steps of the diamond surface, indicating its effectiveness as an adhesive layer. This bonding method was effective for the diamond substrates with various impurity concentrations, including very high–purity substrates suitable for quantum devices. Furthermore, it was confirmed that the light emitting from the color center was efficiently extracted from the sapphire side. The shear strength measurement showed that the diamond/sapphire-bonded wafer had a shear strength of 14.41 MPa, approximately nine times higher than that for diamond/Si wafers [1],[2], making it sufficient for device processes. To summarize, we have successfully bonded diamond (100) and sapphire with high transparency, realizing a promising technique for diamond integrated device applications.<br/> <br/>[1] T. Matsumae <i>et al</i>., Scr. Mater. 191, 52 (2021).<br/>[2] T. Matsumae <i>et al</i>., Scr. Mater. 175, 24 (2020).<br/>[3] T. Matsumae <i>et al</i>., Jpn. J. Appl. Phys. 59, SBBA01 (2020).<br/>[4] S. Fukumoto <i>et al</i>., Appl. Phys. Lett. 117, 201601 (2020).<br/>[5] J. Hermias <i>et al</i>., Hasselt Diamond Workshop 2022 - SBDD XXVI (2022).