lithium niobate thin films on diamond (LNOD)

lithium niobate thin films on diamond (LNOD)

Lithium niobate (LiNbO₃) is a key piezoelectric material widely used in surface acoustic wave (SAW) filters due to its high electromechanical coupling coefficient (k² = 10%–25%), enabling broad bandwidth and low insertion loss. Compared to materials such as aluminum nitride (AlN) and zinc oxide (ZnO), LiNbO₃ exhibits superior piezoelectric constants and higher shear wave velocities. However, bulk LiNbO₃ suffers from relatively low acoustic velocity and limited support for multiple wave modes, restricting performance in high-frequency applications.

To address these limitations, integration with high-velocity substrates such as silicon carbide, silicon, and diamond has been explored. Diamond, with the highest known acoustic velocity and excellent thermal conductivity, is particularly promising. Yet, heteroepitaxial growth of LiNbO₃ on diamond is hindered by significant lattice and thermal expansion mismatch, often resulting in poor crystalline quality and limited mode excitation.

Bonding techniques offer a viable alternative, circumventing lattice-matching constraints. Room-temperature surface activation bonding (RT-SAB), in particular, enables integration of materials with large thermal expansion mismatches, such as diamond (1.0 × 10⁻⁶ K⁻¹) and LiNbO₃ (6.5 × 10⁻⁶ K⁻¹). However, RT-SAB requires stringent surface conditions, which are difficult to achieve on hard diamond substrates. Additionally, mechanical mismatch increases the risk of film damage during bonding.

A nanometer-scale silicon interlayer was employed to facilitate room-temperature surface activation bonding (RT-SAB) between single-crystal lithium niobate (LiNbO₃) and diamond. By applying the smart-cut technique, the first successful fabrication of a LiNbO₃-on-diamond (LNOD) heterostructure was achieved. Structural and interfacial characterizations confirmed high crystalline quality and a defect-free bonding interface. Residual stress measurements, in conjunction with finite element simulations, further validated the mechanical robustness of the integration. This method offers a promising platform for high-frequency, thermally stable surface acoustic wave (SAW) devices and significantly expands the potential of diamond-piezoelectric integration.