g-Chi Chen

A graphene-based microelectrode for recording neural signals

We designed, fabricated and tested a novel prototype neural interface using two-dimensional single-atom-thick graphene as microelectrode for neural applications. The cytotoxicity indicated a satisfactory biocompatibility and non-toxicity. The treatment graphene with steam plasma improved the biological properties. The action potentials of lateral giant (LG) nerve fiber of the escape circuit of an American crayfish had a ratio of signal to noise (SNR) as great as 30.2±2.45 dB.

Foundry-CMOS integrated oscillator circuits based on ultra-low power ovenized CMOS-MEMS resonators

A cutting-edge ovenized micromechanical resonator circuit comprised of a double-ended tuning fork (DETF) resonator and serpentine-shaped heaters to enable ultra-low heater power of only 0.47 mW over the entire temperature range (-40°C to 85°C) has been reported for the first time in a low-cost, foundry CMOS-based fabrication platform. The combination of low thermal conductivity materials (i.e., SiO2 and poly-Si) and high thermal isolation designs is key to attaining low heater power consumption in a sub-mW level. An ovenized 1.2-MHz CMOS-MEMS oscillator with a phase noise lower than -103 dBc/Hz at 1-kHz offset and -110 dBc/Hz at 1-MHz offset was also demonstrated in this work, verifying the great potential of low power oven-controlled MEMS oscillators realized using the well-established CMOS-MEMS technology.

Enhancement of temperature stability via constant-structural-resistance control for MEMS resonators

In this work, we proposed a methodology to enhance thermal stability of MEMS resonators by the use of a constant-structural-resistance control where temperature coefficient of resistivity (TCR) of the MEMS resonator serves as an instrinsic temperature sensor, thus leading to a constant structural temperature to greatly alleviate the frequency drifts due to change of ambient temperature. As a proof of concept, the effective temperature coefficient of frequency (TCf) was measured under a manual control of bias power with one-point calibration to maintain constant structural resistance, ultimately demonstrating up to 46 times improvement of temperature stability as compared to uncompensated counterparts. This technology prevents the use of external temperature sensors or reference resonators with which errors occur due to indirect temperature measurement of the active MEMS resonator. Notably, this technology is well suited for all kinds of MEMS resonators featuring proper temperature-dependent structural resistance, such as capacitive, piezoresistive, piezoelectric, and thermal-piezoresistive silicon-based resonators.

A balanced measurement and characterization technique for thermal-piezoresistive micromechanical resonators

A novel balanced two-port measurement technique capable of adjusting feedthrough signal has been proposed to demonstrate both cancellation and tuning of feedthrough levels for one-port thermally actuated resonator with piezoresistive sensing (thermal-piezoresistive resonator), therefore attaining clean frequency spectra and real resonance characterization. Conventional one-port thermal-piezoresistive resonators significantly suffer high transmission feedthrough and often necessitate post-data processing and de-embedding to extract pure resonance behavior. For the first time, the one-port thermal-piezoresistive resonators with longitudinally vibrating mode shape were tested using the proposed technique to offer the directly measurable resonator Q, motional impedance, and resonance frequency with an improvement of around 80 dB feedthrough reduction. This approach could be easily applied to any one-port thermal-piezoresistive resonator with much higher frequency, showing great potential to enable future sensor and RF applications.

Thermally-actuated and piezoresistively-sensed CMOS-MEMS resonator array using differential-mode operation

A very-high-frequency (VHF) bulk-mode II-BAR CMOS-MEMS resonator array with electrothermal actuation and piezoresistive sensing (thermal-piezoresistive transduction) under a novel differential mode of operation has been demonstrated for the first time using a standard 0.35μm CMOS process. The proposed array consists of SiO2 and embedded polysilicon, the former of which provides high-Q structural material while the latter serves as both joule-heating elements and piezoresistors. Such a thermal-piezoresistive II-BAR array technique not only inherits all advantages from the single II-BAR but also shows great potential for low feedthrough, low motional impedance, and high power efficiency in VHF or ultra-high-frequency (UHF) operation using advanced down scaling and large-scale integrated (LSI) arrays.

Hydrophilic modification of multi-walled carbon nanotubes based neural microelectrode array

This paper reports an improved method for reducing the impedance of microelectrode array (MEA). The impedance reduction is generally required by increasing the effective surface area of electrode. We have fabricated and treated the multi-walled carbon nanotubes (MWCNTs) based MEA for neuroscience application. The effect of plasma treatment on the surface wettability of MWCNTs was examined and characterized. The H2O plasma treatment was utilized to modify the surface of MWCNTs from superhydrophobic to superhydrophilic. The treated recipe of power under 25 W for 10 sec could improve the electro-chemical and biological properties efficiently.

Performance evaluation of CMOS-MEMS thermal-piezoresistive resonators in ambient pressure for sensor applications

In this work, we report a thermally driven and piezoresistively sensed (a.k.a. thermal-piezoresistive) CMOS-MEMS resonator with high quality factor in ambient pressure and with decent power handling capability. The combination of (i) no need of tiny capacitive transducers gap spacing thanks to thermal-piezoresistive transduction, (ii) the use of high-Q SiO2/polysilicon structural materials from CMOS back-end-of-line (BEOL), and (iii) the bulk-mode resonator design leads to resonator Q more than 2,000 in ambient pressure and 10,000 in vacuum. Key to attaining sheer Q in ambient pressure relies on significant attenuation of the air damping effect through thermal-piezoresistive transduction as compared to conventional capacitive resonators which necessitate tiny transducers gap for reasonable electromechanical coupling. With such high Q and inherent circuit integration capability, the proposed CMOS-MEMS thermal-piezoresistive resonators can potentially be implemented as high sensitivity mass/gas sensors based on resonant transducers. The resonators with center frequency around 5.1 MHz were fabricated using a standard 0.35 μm 2-poly-4-metal (2P4M) CMOS process, thus featuring low cost, batch production, fast turnaround time, easy prototyping, and MEMS/IC integration.

A mode-matching 130-kHz ring-coupled gyroscope with 225 ppm initial driving/sensing mode frequency splitting

A degenerate mode 130-kHz ring-coupled gyroscope with auxiliary transducer array is designed to enhance the sensitivity as well as the mode-matching feature. The proof-of-concept device with 3 μm transducers gap is fabricated using a conventional (100) silicon-on-insulator (SOI) wafer process with only two lithography steps. The auxiliary parallel-plate transducer array is located at the maximum displacement of the vibrating ring to enhance the electromechanical coupling while reducing the sensing noise. The in-plane trefoil mode (n=3) is adopted to alleviate the initial frequency splitting in (100) crystalline silicon device layer. The average frequency split for the drive/sense modes over multiple tested devices is only 225 ppm with the mean resonance frequency of 130 kHz. The measured Q-factor is 50 in atmospheric pressure and up to 10,000 in vacuum. Owing to the larger transduction area benefitting from the transducer array design, a low dc-bias voltage (VP) of 3 V in vacuum (21 V in air) is sufficient to sustain the driving loop oscillation. As integrated with the sensing circuits to operate the proposed gyroscope, a scale factor of 2.2 mV/°/s and resolution of 0.26 °/s, respectively, are characterized in atmospheric pressure.

A self-sustained nanomechanical thermal-piezoresistive oscillator with ultra-low power consumption

This work reports wing-type thermal-piezoresistive oscillators operating at around 840 kHz in vacuum with ultra-low power consumption of only 70 μW for the first time. The combination of N-type heavily doped silicon with negative piezoresistive coefficients and sub-micron cross-sectional dimensions of the thermally actuated beams is key to ensuring self-sustained oscillation under sub-100μW level. In particular, shrinking thermal beam cross-sections in 2D is a novel and effective approach to achieve better figure of merit (e.g., FOM defined by the ratio of transconductance gm to dc power consumption PDC) of the self-sustained oscillators. By using proper control of silicon etching (ICP) recipe, the sub-micron cross-sectional dimensions of the thermally actuated beams can be easily and reproducibly fabricated in one process step. The phase noise of the proposed wing-type oscillators is also investigated in this work with -93.41 dBc/Hz at 1-kHz offset and -97.95 dBc/Hz at 100-kHz offset in air, and -95.9 dBc/Hz at 1-kHz offset and -95.7 dBc/Hz at 100-kHz offset in vacuum, respectively.

Thermal-Piezoresistive SOI-MEMS Oscillators Based on a Fully Differential Mechanically Coupled Resonator Array for Mass Sensing Applications

A mechanically coupled array technique to enable a fully differential operation of single-crystal silicon thermal-piezoresistive resonators (TPRs) has been demonstrated to alleviate resistive feedthrough issues often seen in TPRs, therefore, featuring clear resonance behavior with decent signal-to-feedthrough ratio. The proposed thermal-piezoresistive dual-II-BARs exhibits feedthrough reduction of more than 67 dB, and no spurious mode, is found within a wide frequency span. The resonator array together with board-level sustaining circuitry also works as a thermal-piezoresistive oscillator (TPO) and demonstrated its performances for real-time mass sensing. Finally, the proposed TPO mass sensor possesses several key features, including real-time monitoring, fast response time, and most importantly, excellent mass resolution of only 83 fg extracted from the measured TPO’s Allan deviation. [2017-0085]