A Review of the Approaches to Improve The Effective Coupling Coefficient of AlN based RF MEMS Resonators

This work reviews various methods which improve the effective coupling coefficient (${k^{2}}_{eff}$) of non-bulk acoustic wave (BAW) aluminum nitride (AlN) based RF MEMS resonators, mainly focusing on the innovative structural design of the resonators. ${k^{2}}_{eff}$ is the key parameter for a resonator in communication applications because it measures the achievable fractional bandwidth of the filter constructed. The resonator's configuration, dimension, material stack and the fabrication process will all have impact on its ${k^{2}}_{eff}$. In this paper, the authors will review the efforts in improving the ${k^{2}}_{eff}$ of piezoelectric MEMS resonators from research community in the past 15 years, mainly from the following three approaches: coupling lateral wave with vertical wave, exciting two-dimensional (2-D) lateral wave, as well as coupling 2-D lateral wave with vertical wave. The material will be limited to AlN family, which is proven to be manageable for manufacturing. The authors will also try to make recommendations to the effectiveness of various approaches and the path forward.