Modular quantum computer chip based on spin qubits in diamond will enable an increased connectivity, a high fidelity, and a low error-rate when the number of qubits increases. The high-quality diamond substrates have the maximum size in the order of mm2, preventing use of advanced semiconductor manufacturing line. Therefore, for a large-scale integration of qubits, there is a need for a wafer-scale diamond for scalable nanofabrication. Here, diamond-on-insulator structure will enable self-standing diamond structure by under-etching the insulator. Hydrophilic direct bonding of single crystalline diamond substrate on silicon is a promising approach.

In this research, a process is developed and optimized for hydrophilic direct bonding of (100)-oriented diamond on a low-temperature deposited SiO2 on silicon substrate. The process condition involves treating a (100) CVD diamond substrate in a Piranha solution. A 300-nm-thick SiO2 is PECVD deposited on a (100) silicon wafer which afterwards undergoes a surface activation process by oxygen plasma. The two substrates were contacted in an ambient without applying pressure, cooled, and then annealed. It should be noted that there was no pressure applied during the annealing. By die shear testing, a strong shear strength was measured and further elongating the Piranha cleaning time provided more shear strength bonding.