Suppression of Spurious Modes in Aluminum Nitride S1 Lamb Wave Resonators Using A Mechanical Soft-Contact Scheme

In this study, a novel mechanical soft-contact (MSC) scheme is proposed, for the first time, to enable selective mode suppression in aluminum nitride (AlN) S
1
Lamb wave resonators (LWR). For MSC-LWRs, the degree of energy dissipation introduced at the contact point depends heavily on the vibrational modes. We have demonstrated that in MSC-LWR, the ratio of the quality factor (Q) for the S
1
and S
0
Lamb modes can be as high as 37.7 (944 for Q
S1
and 25 for Q
S0
) to effectively eradicate the S
0
mode. The proposed resonators are fabricated via surface micromachining and exploit residual stress-induced structural deformation to create a mechanically soft contact between the resonator plate and the substrate. As a result, a spectrum-clean MSC-LWR achieves Q
S1
of 1,381 and an effective electromechanical coupling coefficient $\left( {k_{{\text{eff}}}^2} \right)$ of 2.72% at 2.19 GHz. Furthermore, we show that Q
S1
can be further optimized by adjusting the tether width, which is presumably dominated by the normal force at the contact point induced by the supporting structure.