## 6 Experimental Results  
Hybrid MEMS resonator devices have been characterized by on-wafer probing using an RF probe station under normal atmospheric pressure and ambient temperature. The RF probe testing utilized a CS\(-\)5 Short-Open-Load-Thru calibration standard kit from GGB Industries Inc., ensuring accurate de-embedding to bring the reference planes to the end of the probe tips. It was observed that some hybrid resonator devices experienced electrical short-circuiting at low bias voltages, which indicates particle intrusion into air gaps between surrounding capacitive electrodes and the resonator body, a common issue when drying MEMS devices with solvents under atmospheric pressure. To circumvent this issue and prevent capillary forces from drawing particles into these capacitive gaps, a critical CO\({}_{2}\) drying system is advised immediately after the HF or buffered oxide etch (BOE) wet-etching process for gap spacing oxide removal to avoid potential microfabrication complications, especially for small gap sizes, including stiction [9; 28; 33]. Figure 7: Illustration of signal paths for a hybrid capacitive/piezoelectric resonator with reduced feedthrough as the device operates under different configurations, including (**a**) piezoelectric drive and capacitive sense, and (**b**) capacitive drive and piezoelectric sense. Figure 8 shows a top-view SEM photo of a fabricated hybrid disk resonator, which consists of dual capacitive electrodes enclosing the resonator body to form a pair of lower-layer capacitive ports, with one or a pair of electrodes atop the piezoelectric layer above the disk-shaped resonator body functioning as one or two additional piezoelectric transducer ports. A versatile two-port measurement setup using a vector network analyzer (VNA) and a power supply were employed to evaluate the S-parameter frequency responses and investigate the quality factor and resonance frequency. The hybrid resonator device has been tested under two distinct measurement schemes: one that applies DC bias directly to the piezoelectric bottom electrodes, isolating the ground plane completely; and another that uses a bias tee to apply both AC and DC voltages to the capacitive electrodes, with the motional current detected from the top-layer piezoelectric electrode, as shown in Figure 8.  
Figure 8: Illustration of RF probe measurement set-up for hybrid resonator devices with a set of two capacitive ports and another one or a pair of piezoelectric transducer ports, fully isolated by the resonator body and ground, showing two different activation schemes to generate electrostatic force in the capacitive gap. (**a**) One applies an AC voltage to the capacitive electrodes and a DC voltage to the resonator body with two anchors to generate electrostatic force; and (**b**) The other applies both AC and DC voltages to the capacitive electrodes to induce electrostatic force in the capacitive gap for a resonator design with wider capacitive electrodes and one side-supporting anchor. Furthermore, feedthrough signal level measurements have been conducted, which are crucial for quantifying parasitic effects for limiting high-frequency port-to-port isolation within piezoelectric, capacitive, and hybrid resonators. Feedthrough occurs when an RF electrical signal leaks through the substrate or other parasitic couplings between the drive and sense ports of a resonator instead of undergoing electromechanical conversion. As seen in Figure 9, this was significantly suppressed in hybrid resonators, with more than 30 dB improvement, as compared to those of piezoelectric or capacitive resonators with identical geometry. For comparison, the measured feedthrough level of a CS\(-5\) calibration standard is included.