Suan Hui Pu

A Three Degree-of-Freedom Weakly Coupled Resonator Sensor With Enhanced Stiffness Sensitivity

This paper reports a three degree-of-freedom (3DoF) microelectromechanical systems (MEMS) resonant sensing device consisting of three weakly coupled resonators with enhanced sensitivity to stiffness change. If one resonator of the system is perturbed by an external stimulus, mode localization occurs, which can be detected by a change of modal amplitude ratio. The perturbation can be, for example, a change in stiffness of one resonator. A detailed theoretical investigation revealed that a mode aliasing effect, along with the thermal noise floor of the sensor and the associated electrical system ultimately limit the dynamic range of the sensor. The nonlinearity of the 3DoF sensor was also analyzed theoretically. The 3DoF resonator device was fabricated using a silicon on insulator process. Measurement results from a prototype device agreed well with the predictions of the analytical model. A significant, namely 49 times, improvement in sensitivity to stiffness change was evident from the fabricated 3DoF resonator sensor compared with the existing state-of-the-art 2DoF resonator sensors, while the typical nonlinearity was smaller than ±2% for a wide span of stiffness change. In addition, measurements indicate that a dynamic range of at least 39.1 dB is achievable, which could be further extended by decreasing the noise of the device and the interface electronics.

A sensor for stiffness change sensing based on three weakly coupled resonators with enhanced sensitivity

This paper reports on a novel MEMS resonant sensing device consisting of three weakly coupled resonators that can achieve an order of magnitude improvement in sensitivity to stiffness change, compared to current state-of-the-art resonator sensors with similar size and resonant frequency. In a 3 degree-of-freedom (DoF) system, if an external stimulus causes change in the spring stiffness of one resonator, mode localization occurs, leading to a drastic change of mode shape, which can be detected by measuring the modal amplitude ratio change. A 49 times improvement in sensitivity compared to a previously reported 2DoF resonator sensor, and 4 orders of magnitude enhancement compared to a 1DoF resonator sensor has been achieved.

Finite Element Model of a Bilayered Gold-Coated Carbon Nanotube Composite Surface

Vertically aligned multiwalled carbon nanotubes (MWCNTs), with a gold (Au)-coated surface, have been shown to provide a stable contact resistance for electrical contact switching applications under low force conditions (micronewton to millinewton), with the MWCNT surface providing a compliant support for the conducting Au layer. In this paper, nanoindentation results are used in the development of a finite element contact model for the composite, referred to as Au/CNT. The results show that the surface is best modeled as a bilayered structure, in which the top layer is modeled as an elastic-plastic layer of the Au/CNT mixed material and the under layer as a CNT forest. The resultant model matches the experimental results for a range of samples with different thickness configurations.

Stable zipping RF MEMS varactors

Novel zipper varactors with the potential for achieving large tuning ranges have been fabricated and characterized. These varactors have a curved cantilever electrode that is actuated by a single pull-down electrode. The shape of the cantilever is designed such that its local stiffness is tailored to enable extended stable zipping. In a series-mounted varactor, the measured capacitance ratio was 16.5 for actuation voltages between 0 and 46 V. However, the presence of an unexpected tuning instability at 32 V bias limited the practical tuning range in this varactor. This behaviour was attributed to fabrication imperfections. The first electrical self-resonant frequency of the same varactor was extrapolated to be 72.6 and 17.2 GHz at 20 and 329 fF, respectively. In a different shunt-mounted varactor, the quality factor (Q) was measured to be 91 (60 fF) and 176 (600 fF) at 2 GHz. Including the anchor, the varactors have a small device footprint and fit within an area of 500 by 100 µm.

Mechanical characterization of a Au coated carbon nanotube multi-layered structure

Multiwalled Carbon Nanotube (MWCNT)-coated surfaces have been proved to be able to provide stable resistance for the electrical contact under low force conditions since they can offer a compliant support for the conducting gold layer. However, the contact mechanics of the Au/MWCNTs composite have not been understood. In this study, a finite element multilayered contact model was developed, in which the top layer was modeled as a composite and under layer was modeled as CNT forest. The study shows that this complex surface is best modeled as a multi-layered structure. The model is optimized and validated with nano-indenter data. The model can help to better understand the configuration and material properties of Au/CNTs surfaces, and can provide guidance to optimize the surface in terms of contact resistance performance in MEMS switches.

A Comparative Study of Output Metrics for an MEMS Resonant Sensor Consisting of Three Weakly Coupled Resonators

This paper systematically investigates the characteristics of different output metrics for a weakly coupled three degree-of-freedom microelectromechanical systems resonant sensor. The key figures-of-merit examined are sensitivity and linear range. The four main output metrics investigated are mode frequency shift, amplitude difference, amplitude ratio, and eigenstate shift. It is shown from theoretical considerations, equivalent RLC circuit model simulations and electrical measurements, that there is a strong tradeoff between sensitivity and linear range. For instance, the amplitude difference has the best sensitivity but the worst linear range, whereas frequency shift has the widest linear range but the lowest sensitivity. We also show that using the vibrational amplitude ratio as an output metric provides the best balance between sensitivity and linear range. [2016-0077].

Comparative study of different output metrics for a three weakly coupled resonator sensor

This paper, for the first time, investigates the characteristics of different output metrics for a three degree-of-freedom (DoF) coupled resonator sensor. The main aspects examined are sensitivity and linear range. It is shown from theoretical estimations, equivalent RLC circuit model simulations and electrical measurements that using the vibration amplitude ratio as an output signal provides improved sensitivity and linearity range, compared to other methods such as shift in eigenstate, mode frequency or amplitude difference.

Development of a MEMS test platform for investigating the use of multi-walled CNT composites electric contacts

The use of gold-coated multi-walled carbon nanotube (Au/MWCNT) composites have been shown to extend the life of electrical contacts, in previous work. Due to the long lifetimes (which are of the order of 106 up to 108 cycles) the lifetime testing tends to be highly time consuming. In this work we discuss the design and development of an electrostatically actuated MEMS cantilever beam which enables testing at higher frequencies than our previous experimental rig. Following calculations using fundamental cantilever beam equations, a computational model of the designed beam was developed to accurately predict the characteristics of the beam, including the resonant frequency, pull-in voltage and contact force. Where possible the values from the model have been compared with the fabricated MEMS cantilever beam. A MEMS-based electrostatically actuated cantilever beam has been fabricated and incorporated with Au/MWCNT composite surfaces to form a MEMS switch test platform. Initial results show the improved performance over a PZT based test platform.

Electrical behavior of nanocrystalline graphite/p-Si Schottky diode

The electrical characteristics of nanocrystalline graphite (NCG) on p-type Si Schottky diodes were investigated. The NCG/p-Si Schottky diodes were fabricated on a 6-inch wafer by metal-free catalyst plasma enhanced chemical vapour deposition (PECVD) and photolithography pattern transfer method. The NCG film consists of nanoscale grains of ~35 nm in size. The NCG/p-Si Schottky diode shows rectifying behavior with Schottky barrier height of 0.58 eV. This result in addition to nanosized grains can be exploited towards various chemical and gas sensor applications.

RF MEMS Zipping Varactor With High Quality Factor and Very Large Tuning Range

An RF MEMS zipping analog varactor with a high-permittivity bismuth zinc niobate dielectric has been fabricated and characterized. The varactor can be continuously tuned from 10 to 280 fF, with a very large tuning range of 2700%. By use of gold electrodes to ensure a low series resistance, the high quality factors of 317 and 63 at 1 GHz were measured for the minimum and maximum capacitances, respectively. The self-resonance frequency of the varactor is estimated to be 16.6 GHz at the maximum capacitance and 87.6 GHz at the minimum capacitance. With a small device footprint of 540 × 130 μm2, the zipping varactor is suitable for application in portable reconfigurable RF systems.