Joshua E.-Y. Lee

Study of lateral mode SOI-MEMS resonators for reduced anchor loss

MEMS resonators fabricated in silicon-on-insulator (SOI) technology must be clamped to the substrate via anchoring stems connected either from within the resonator or through the sides, with the side-clamped solution often employed due to manufacturing constraints. This paper examines the effect of two types of commonly used side-clamped, anchoring-stem geometries on the quality factor of three different laterally-driven resonator topologies. This study employs an analytical framework which considers the relative distribution of strain energies between the resonating body and clamping stems. The ratios of the strain energies are computed using ANSYS FEA and used to provide an indicator of the expected anchor-limited quality factors. Three MEMS resonator topologies have been fabricated and characterized in moderate vacuum. The associated measured quality factors are compared against the computed strain energy ratios, and the trends are shown to agree well with the experimental data.

Low loss HF band SOI wine glass bulk mode capacitive square-plate resonator

This paper reports on the design and electrical characterization of a single crystal silicon micromechanical square-plate resonator. The microresonator has been excited in the anti-symmetrical wine glass mode at a resonant frequency of 5.166 MHz and exhibits an impressive quality factor (Q) of 3.7 × 106 at a pressure of 33 mtorr. The device has been fabricated in a commercial foundry process. An associated motional resistance of approximately 50 kΩ using a dc bias voltage of 60 V is measured for a transduction gap of 2 µm due to the ultra-high Q of the resonator. This result corresponds to a frequency-Q product of 1.9 × 1013, the highest reported for a fundamental mode single-crystal silicon resonator and on par with some of the best quartz crystal resonators. The results are indicative of the superior performance of silicon as a mechanical material, and show that the wine glass resonant mode is beneficial for achieving high quality factors allowed by the material limit.

Ultrasensitive mass balance based on a bulk acoustic mode single-crystal silicon resonator

A single-crystal silicon resonant bulk acoustic mass sensor with a measured resolution of 125pg∕cm2 is presented. The mass sensor comprises a micromachined silicon plate that is excited in the square-extensional bulk acoustic resonant mode at a frequency of 2.182MHz, with a quality factor exceeding 106. The mass sensor has a measured mass to frequency shift sensitivity of 132Hzcm2∕μg. The resonator element is embedded in a feedback loop of an electronic amplifier to implement an oscillator with a short term frequency stability of better than 7ppb at an operating pressure of 3.8mTorr.

A Single-Crystal-Silicon Bulk-Acoustic-Mode Microresonator Oscillator

A timing reference incorporating a single-crystal-silicon micromechanical resonator with a quality factor of larger than one million and a resonant frequency of 2.18 MHz is demonstrated. The resonator is excited in the square extensional bulk acoustic mode at 4 mtorr, and it has been fabricated in a foundry SOI MEMS process. The silicon microresonator is adapted as a timing element for a precision oscillator with a measured short-term Allan deviation of 0.6 ppb.

Single-Device and On-Chip Feedthrough Cancellation for Hybrid MEMS Resonators

Microelectromechanical systems (MEMS) resonators typically exhibit large parasitic feedthrough where the input drive signal is directly coupled to the output ports, presenting a challenge to full electrical characterization of resonators where the output is heavily embedded in feedthrough. We here present an on-chip solution that significantly mitigates the undesirable effects of parasitic feedthrough but using only a single device. We have demonstrated its use in a symmetrical mode of vibration (the extensional mode of a square-plate MEMS resonator) to show its applicability to most generic resonator mode shapes. In our measurements, we show that the proposed method for feedthrough cancellation provides a 40-dB common-mode rejection compared to when no feedthrough cancellation is implemented. The necessary matching of drive circuit capacitances is achieved by properly sizing and placing a dummy pad in the vicinity of each drive pad. The studies reported herein demonstrate that the integrity of the output signal from a MEMS resonator is not only determined by device dimensions but also strongly influenced by the interaction between fringing fields radiating from electrodes in proximity. These results could open up a new avenue in the design of hybrid MEMS resonant devices where the issue of feedthrough can be both effectively and cheaply addressed.

Diameter dependence of electron mobility in InGaAs nanowires

In this work, we present the diameter dependent electron mobility study of InGaAs nanowires (NWs) grown by gold-catalyzed vapor transport method. These single crystalline nanowires have an In-rich stoichiometry (i.e., In0.7Ga0.3As) with dispersed diameters from 15 to 55 nm. The current-voltage behaviors of fabricated nanowire field-effect transistors reveal that the aggressive scaling of nanowire diameter will induce a degradation of electron mobility, while low-temperature measurements further decouple the effects of surface/interface traps and phonon scattering, highlighting the impact of surface roughness scattering on the electron mobility. This work suggests a careful design consideration of nanowire dimension is required for achieving the optimal device performances.

Micromachined Resonators: A Review

This paper is a review of the remarkable progress that has been made during the past few decades in design, modeling, and fabrication of micromachined resonators. Although micro-resonators have come a long way since their early days of development, they are yet to fulfill the rightful vision of their pervasive use across a wide variety of applications. This is partially due to the complexities associated with the physics that limit their performance, the intricacies involved in the processes that are used in their manufacturing, and the trade-offs in using different transduction mechanisms for their implementation. This work is intended to offer a brief introduction to all such details with references to the most influential contributions in the field for those interested in a deeper understanding of the material.

AlN piezoelectric on silicon MEMS resonator with boosted Q using planar patterned phononic crystals on anchors

We report an approach to suppress anchor loss in thin-film piezoelectric-on-silicon (TPoS) micromechanical (MEMS) resonators by patterning 2D phononic crystals (PnCs) externally on the anchors. The PnCs serve as a frequency-selective reflector for outgoing acoustic waves through the tethers of the TPoS resonator. According to our experimental results, combining the PnCs with the conventional TPoS resonator significantly enhances the quality factor (Q) and correspondingly lowers the insertion loss (IL). The measured improvement is reproducible over multiple samples and consistent with the simulations by tuning the PnC bandgaps, suggesting significant reduction of acoustic leakage to the substrate by adopting the PnCs.

System-level simulation of a micromachined electrometer using a time-domain variable capacitor circuit model

A micromachined electrometer, based on the concept of a variable capacitor, has been designed, modeled, fabricated and tested. The electrometer has an equivalent noise floor of , which is over two orders of magnitude better than the best line of commercial instruments currently available for room temperature and ambient pressure operation. As with all other capacitive MEMS applications comprising both mechanical and electrical domain subsystems, system-level equivalent circuit simulation cannot be implemented in most commercial circuit simulators for they do not accept in-line equations for a variable capacitor. This paper uses a voltage source representation method creating a nonlinear time-domain variable capacitor model in SPICE. We show that a circuit model can be realized for any arbitrary nonlinear capacitive MEMS element for embedded co-simulation with interface electronics. The results in the case of a variable capacitor based MEMS electrometer are in good agreement with experimental data. This simple technique is proposed as a method to provide the missing link in aiding design optimization of both electro-mechanical structures and sensor electronics within the same simulation interface.

Room temperature electrometry with SUB-10 electron charge resolution

We have fabricated and demonstrated a micromachined electrometer with a charge resolution of 6 e/√Hz, operating at room temperature and ambient pressure. We thus show that high-resolution electrometry is realizable at room temperature and competitive with alternative low temperature (<10 K) solutions. The device presented in this paper functions as a modulated variable capacitor, wherein a dc charge to be measured is modulated and converted to an ac voltage output. By reviewing a selection of different devices based on this concept, we show that single-electron charge resolution at room temperature and ambient pressure is achievable with the aid of practical design considerations.