A Sub-1 Hz Resonance Frequency Resonator Enabled by Multi-Step Tuning for Micro-Seismometer

Abstract

We propose a sub-1 Hz resonance frequency MEMS resonator that can be used for seismometers. The low resonance frequency is achieved by an electrically tunable spring with an ultra-small spring constant. Generally, it is difficult to electrically fine-tune the resonance frequency at a near-zero spring constant because the frequency shift per voltage will diverge at the limit of zero spring constant. To circumvent this issue, we propose a multi-step electrical tuning method. We show by simulations that the resonance frequency can be tuned by 0.008 Hz/mV even in the sub-1 Hz region. The small spring constant, however, reduces the shock robustness and dynamic range of the seismometer. To prevent this, we employ a force-balanced method in which the mass displacement is nulled by the feedback force. We show that the displacement can be obtained from the voltage that generates the feedback force.

Fabrication of low-resonant-frequency inertial MEMS using through-silicon DRIE applied to silicon-on-glass

“…The methodology to attain the ultra-low resonant frequency is explained in our previous work. 10) The essence is that the total spring constant k is formed by a positive mechanical spring k m and an electrically tunable spring k .…”

“…A multi-step tuning method described in our previous paper ensures finetuning of f res even at the low-frequency regime. 10) Our target is to attain f 1 res < Hz. The design parameters of our device are listed in Table I.…”

“…9) Along with this trend, we have previously proposed a method to lower the resonant frequency using negative electrical stiffness. 10) However, in the manufacturing process, the weak mechanical springs and large mass cause an issue with respect to heat dissipation. 11) Due to insufficient cooling during the throughsilicon DRIE (TSD) process, the topside temperature of the wafer goes up.…”