The direct bonding process of a diamond-on-insulator (DOI) substrate enables monolithic integration of diamond photonic structures for quantum computing by improving photon collection efficiency and entanglement generation rate between emitters. It also addresses key fabrication challenges, such as robustness, bonding strength, and scalability. This study investigates strain effects in DOI substrates following direct bonding. Strain generation is expected near the diamond–SiO₂/Si interface due to the thermal expansion coefficient mismatch between the bonded materials. Strain-induced lattice distortions are characterized using nitrogen-vacancy (NV) centers in diamond via optically detected magnetic resonance (ODMR) and photoluminescence (PL) mapping. PL mapping reveals interference fringes in unbonded regions, indicating bonding irregularities. Depth-resolved ODMR measurements show a volumetric strain component increase of ≈0.45 MHz and a shear component increase of ≈0.71 MHz between the top surface and the DOI interface. However, ODMR signal contrast and peak linewidth remain largely unaffected, suggesting no visible deterioration in the optical properties of the emitters. By combining ODMR and PL mapping, this work establishes a robust methodology for assessing bonding quality and strain impact on NV centers, an essential step toward advancing scalable quantum technologies and integrated photonic circuits.

Diamond has emerged as a leading material for solid-state spin quantum systems and extreme environment electronics. However, a major limitation is that most diamond devices and structures are fabricated using bulk diamond plates. The absence of a suitable diamond-on-insulator (DOI) substrate hinders the advanced nanofabrication of diamond quantum and electronic devices, posing a significant roadblock to large-scale, on-chip diamond quantum photonics and electronics systems. In this work, we demonstrate the direct bonding of (100) single-crystal diamond plates to PECVD-grown SiO₂/Si substrates at low temperatures and atmospheric conditions. The surfaces of the SiO₂ and diamond plates are then activated using oxygen plasma and Piranha solution, respectively. Bonding occurs when the substrates are brought into contact with water in between and annealed at 200 °C under atmospheric conditions, resulting in a DOI substrate. We systematically studied the influence of Piranha solution treatment time and diamond surface roughness on the shear strength of the bonded substrate, devising an optimal bonding process that achieves a high yield rate of 90% and a maximum shear strength of 9.6 MPa. X-ray photoelectron spectroscopy was used for quantitative analysis of the surface chemicals at the bonding interface. It appears that the amount of -OH bindings increases with the initial roughness of the diamond, facilitating the strong bonding with SiO₂. This direct bonding method will pave the way for scalable manufacturing of diamond nanophotonic devices and enable large-scale integration of diamond quantum and electronic systems.

Quantum computer chip based on spin qubits in diamond uses modules that are entangled with on-chip optical links. This enables an increased connectivity and a negligible crosstalk and error-rate when the number of qubits increases onchip. Here, 3D integration is the key enabling technology for a large-scale integration of the diamond spin qubits with photonic and electronic circuits for routing, control and readout of qubits. There are several engineering challenges to integrate the large number of spins in diamond with the on-chip circuits operating at a cryogenic temperature. In this paper we will address challenges, present recent results and discuss future outlook of the integration technology for realization of a scalable quantum computer based on diamond spin qubits.

Surface-activated direct bonding of diamond (100) and c-plane sapphire substrates is investigated using Ar atom beam irradiation and high-pressure contact at RT. The success probability of bonding strongly depends on the surface properties, i.e, atomic smoothness for the micron-order area and global flatness for the entire substrate. Structural analysis reveals that transformation from sapphire to Al-rich amorphous layer is key to obtaining stable bonding. The beam irradiation time has optimal conditions for sufficiently strong bonding, and strong bonding with a shear strength of more than 14 MPa is successfully realized. Moreover, by evaluating the photoluminescence of nitrogen-vacancy centers in the diamond substrate, the bonding interface is confirmed to have high transparency in the visible wavelength region. These results indicate that the method used in this work is a promising fabrication platform for quantum modules using diamonds.

For quantum computing modules using diamond color centers, we propose an integrated structure of a quantum chip with photonic circuits and an interposer with electric circuits. The chip and interposer are connected via gold stud bumps using flip-chip bonding technology. For evaluating the proposed integrated structure, we bonded a test chip of 15 × 15 mm2, corresponding to the area that allows the allocation of color center qubits in the order of 102, with an interposer of 20 × 20 mm2, including test measurement lines. We confirm all connections of 16 lines with two bumps for each line at 10 K. The resistance of the lines with two bumps at 10 K is ~ 3.5O, These resistances are mainly attributed to the gold lines on the interposer, which is confirmed by simulations. The shear strength of the flip-chip bonded structure is 67 g/bump. It is larger than that of previous reports where the chips passed the standard temperature cycle test. Moreover, we integrate the flip-chip bonded structure with a printed circuit board (PCB). We confirm a connection between the connector terminal of the PCB and the test chip at 80 K. It is shown that the integrated structure using gold stud bumps has a potentially highly reliable connection at cryogenic temperature. These results will lead to realizing large-scale diamond spin quantum processors.

Quantum computer chip based on spin qubits in diamond uses modules that are entangled with on-chip optical links. This enables an increased connectivity and a negligible crosstalk and error-rate when the number of qubits increases on-chip. Here, 3D integration is the key enabling technology for a large-scale integration of the diamond spin qubits with photonic circuits and CMOS electronics for routing, control and readout of qubits. Several engineering challenges exist in order to integrate the large number of spins in diamond with the on-chip circuits operating at a cryogenic temperature. We will review trends, address challenges and discuss future outlook of the integration technology for realization of a scalable quantum computer based on diamond spin qubits.