SAW analysis of the MgxZn1-xO/SiO2/Si system

Magnesium zinc oxide (Mg_xZn_1-xO) is a new piezoelectric material formed by alloying ZnO and MgO. In this study, the SAW velocity dispersion and electro-mechanical coupling coefficients (K²) in the Mg_xZn_1-xO (x=0-30%)/SiO₂/Si system are analyzed using the transfer matrix method. Si is chosen as the substrate for potential integration of SAW devices with the main stream integrated circuits technology. The use of different Mg content in Mg_xZn_1-xO films leads to change in piezoelectric properties. The SAW characteristics of the system can be further tailored by varying the thickness ratio between the Mg_xZn_1-xO and SiO₂ layers. The effect of different Mg_xZn_1-xO to SiO₂ thickness ratios on SAW propagation in the multilayer structure is investigated. Four possible multilayer SAW device configurations, including IDT/Mg_xZn_1-xO/SiO₂/Si, Mg_xZn_1-xO/IDT/SiO₂/Si, IDT/Mg_xZn_1-xO/metal ground plane/SiO₂/Si, and metal ground plane/Mg_xZn_1-xO/IDT/SiO₂/Si, are studied. It is found that at the high frequency range, with each 10% increase of the Mg content in the Mg_xZn_1-xO, SAW velocity increases by 5∼8%, whereas K² decreases by around 30%. With same IDT configuration, V_SAW decreases as SiO₂ layer thickens. However, as Mg_xZn_1-xO thickness-frequency products hf reach high values, the SAW energy for the base wave mode is trapped in the Mg_xZn_1-xO layer and the thickness of SiO₂ no longer affects the SAW propagation. The multilayer configurations also play an important role. It is found that the Mg_xZn_1-xO/IDT/SiO₂/Si configuration with Mg_xZn_1-xO: SiO₂=2:1 yields the highest coupling coefficient (x=0) and the highest SAW velocity (x=0.3). The current study indicates that using Mg_xZn_1-xO-based multilayer structures will provide flexibility in SAW device design as well as the ability to tailor SAW properties.