Electrical Characterization of Microelectromechanical Silicon Carbide Resonators

Abstract

This manuscript describes the findings of a study to investigate the performance of SiC MEMS resonators with respect to resonant frequency and quality factor under a variety of testing conditions, including various ambient pressures, AC drive voltages, bias potentials and temperatures. The sample set included both single-crystal and polycrystalline 3C-SiC lateral resonators. The experimental results show that operation at reduced pressures increases the resonant frequency as damping due to the gas-rarefaction effect becomes significant. Both DC bias and AC drive voltages result in nonlinearities, but the AC drive voltage is more sensitive to noise. The AC voltage has a voltage coefficient of 1∼4ppm/V at a DC bias of 40V. The coefficient of DC bias is about -11ppm/V to - 21ppm/V for poly-SiC, which is more than a factor of two better than a similarly designed polysilicon resonator (-54 ppm/V). The effective stiffness of the resonator decreases (softens) as the bias potential is increased, but increases (hardens) as drive voltage increase when scan is from low to high frequency. The resonant frequency decreases slightly with increasing temperature, exhibiting a temperature coefficient of -22 ppm/°C, between 22°C and 60°C. The thermal expansion mismatch between the SiC device and the Si substrate could be a reason that thermal coefficient for these SiC resonators is about twofold higher than similar polysilicon resonators. However, the Qs appear to exhibit no temperature dependence in this range.

Electrothermally Actuated Silicon Carbide Tunable MEMS Resonators

“…When the AC amplitude is increased from 2 to 3 V, the RMS voltage increases from 1.41 V to 2.12 V hence leading to an increase of the power dissipated. Therefore, the frequency shifts shown in It is worth noting that the devices presented in this study exhibit a larger frequency variation as a function of AC drive voltage (-800 ppm/V) compared to reported results for a lateral resonator actuated electro-statically that showed shifts up to 4 ppm/V [30]. The smaller variation obtained with the lateral devices may be the result of the structure design, which features folded-beam anchors that may mitigate any changes in stress induced by the electrostatic force.…”

“…In particular, the measurements have shown a shift of -35,000 ppm with a rate of about -11,000 ppm/V with the u-shaped layout and a shift of -3,500 ppm with a rate of about -1,100 ppm/V with the slab electrode. The measured devices exhibit a greater frequency variation compared to previously reported results of -11 ppm/V that have been achieved actuating a lateral resonator electro-statically [30]. The greater variation indicates that DC thermal heating has a greater influence on the stress than an electrostatic force.…”

High Performance, Low Cost Electromechanical Systems Based on Electrostatically Actuated TiS2 Belts

“…To make sure of our speculation in attributing the peak to electromechanical response of the ribbons, the frequency response of the device was measured by applying different DC bias voltages. Up shifts in frequency as well as increase in signal ratio were observed by increasing the DC voltage due to electrostatic hardening effect where the effective stiffness of the resonator increases as the bias potential is increased as shown in Figure b . The measured resonance frequencies are shifted 400 kHz by increasing the DC voltage from 0.5 V to 7 V.…”

Applications of SiC-Based Thin Films in Electronic and MEMS Devices

“…In relation to choice of the insulator, Si 3 N 4 has shown more suitable than SiO 2 due to: (i) its higher dielectric constant that can reduce the leakage currents, and (ii) its thermal expansion coefficient is much closer to that of SiC than one of SiO 2 , thus the stress in SiC film grown on Si 3 N 4 will be lower than that on SiO 2 (Cheng, 2003). Nevertheless, most of the SiC thin-film MEMS devices reported in the literature use the SiO 2 as sacrificial layer and/or substrate electrical isolation (Chang, 2008;Mishra, 2009). One reason for this is the easy formation of SiO 2 achieved by thermal oxidation of Si substrates.…”