Viscous and acoustic losses in length-extensional microplate resonators in liquid media

Abstract

Damping mechanisms in the length-extensional mode of rectangular, mid-point supported microplate resonators immersed in liquid are studied. Piezoelectrically excited structures with different lengths and thicknesses were designed, fabricated, and characterized both optically and electrically in isopropanol. The experimental quality factors were compared to the results of Finite Element Method (FEM) simulations and the two main mechanisms of losses, i.e., acoustic and viscous losses, were identified. Analytical models for those two mechanisms are presented and the effects of the geometry on the in-liquid performance of the resonators are discussed. By applying these models, we found that for a given thickness, a maximum quality factor is reached at a critical length, resulting from the balance between acoustic and viscous losses. To further increase quality factors, a quarter wavelength fluid cavity was implemented, thereby reducing acoustic losses; an increase over 40% in the quality factor was predicted by a 2D FEM model including the cavity, and a quality factor as high as 145 was measured for a 3 mm long and 93 μm thick resonator in this configuration.