ABSTRACT
In this article, the microfluidic channels that deliver liquid to a microscale thin-film piezoelectric-on-silicon (TPoS) gravimetric resonant sensor are incorporated into the backside of the silicon-on-insulator (SOI) wafer on which the resonator is fabricated. Specifically, a microwell is embedded at the bottom of the disk -shaped TPoS resonator, while a very thin layer of parylene covering the backside of the resonator and the microwell forms an isolation layer between the liquid and the top device-layer features. In this way, the liquid is in contact with the backside of the resonator, while the device-defining trenches and the electrical connections to the resonator stay clear, thus mitigating the acoustic energy loss and undesirable feedthroughs. The impact of the parylene layer thickness on a few symmetric ( S ) and antisymmetric ( A ) Lamb wave modes of the resonator is experimentally studied, and the performance of such modes in the liquid is characterized by filling the microwells through a PDMS-based microfluidic channel. The parylene layer, while marginally affecting the resonator in the air, is found to substantially enhance its performance in the liquid media. Strong resonance peaks with high quality factors ( Q ) are observed for the S modes, among which Q values above 400 are recorded for a specific mode named S (4, 2) (among the highest ever reported). This article can potentially facilitate the realization of highly stable and sensitive resonant mass sensors (i.e., microbalance) for real-time applications. Additionally, the effect of the acoustic energy radiation in the form of evanescent shear and longitudinal waves in liquid on the Q and resonance frequency of the disk resonators is experimentally validated.
ABSTRACT
In this paper, anisotropy of single-crystalline silicon (SCS) is exploited to enable side-supported radial-mode thin-film piezoelectric-on-substrate (TPoS) disk resonators. In contrast to the case for isotropic material, it is demonstrated that the displacement of the disk periphery is not uniform for the radial-mode resonance in SCS disks. Specifically, for high-order harmonics, nodal points are formed on the edges, creating an opportunity for placing suspension tethers and enabling side-supported silicon disk resonators at the very high-frequency band with negligible anchor loss. In order to thoroughly study the effect of material properties and the tether location, anchor loss is simulated using a 3-D perfectly matched layer in COMSOL. Through modeling, it is shown that eighth-harmonic side-supported SCS disk resonators could potentially have orders of magnitude lower anchor loss in comparison to their nanocrystalline diamond (NCD) disk resonator counterparts given the tethers are aligned to the [100] crystalline plane of silicon. It is then experimentally demonstrated that in TPoS disk, resonators fabricated on an 8- [Formula: see text] silicon-on-insulator (SOI) wafer, unloaded quality factor improves from ~450 for the second-harmonic mode at 43 MHz to ~11500 for the eighth-harmonic mode at 196 MHz if tethers are aligned to [100] plane. The same trend is not observed for NCD disk resonators and SCS disk resonators with tethers aligned to [110] plane. Finally, the temperature coefficient of frequency is simulated and measured for the radial-mode disk resonators fabricated on the 8- [Formula: see text]-thick degenerately n-type doped SCS, and the TFC data are utilized to guarantee proper identification of the harmonic radial-mode resonance peaks among others.
