Thermal-piezoresistive Resonators Research Articles

MEMS thermal-piezoresistive resonators, thermal-piezoresistive oscillators, and sensors

MEMS thermal-piezoresistive resonators, thermal-piezoresistive oscillators, and sensors

CMOS-MEMS Thermal-Piezoresistive Resonators and Oscillators for Sensors

Microelectromechanical systems (MEMS) are in widespread commercial use due to their compact size, high performance, and low cost. MEMS resonators have emerged as front runners for sensing (accelerometers, gyroscopes, and particulate matter) and frequency (RF front-end, filters, timing, and frequency source) applications. The excellent stability, resolution, and accuracy of resonators lead them an ideal candidate for sensor implementation. The CMOS-MEMS technology allows for rapid, large-scale, and low-cost manufacturing. Thermal–piezoresistive resonators (TPRs) are promising candidates due to their favorable potential with scaling and robust performance in the ambient environment. A detailed finite element method (FEM) simulation flow is presented along with a mathematical model for device optimization. The devices were fabricated with the commercial CMOS technology utilizing the front-end-of-line (FEOL) polysilicon and back-end-of-line (BEOL) materials like silicon dioxide and interconnect metal. The flexibility of selective material placement in layout and complex routing using multi-metal interconnect is employed to develop a balanced TPR design at 2 MHz. A 5-MHz bulk mode TPR was designed for mass sensing application. The fabricated devices were characterized, and their performance is compared with other state-of-the-art works. Finally, the developed devices were used in real-world applications for mass sensing and pressure sensing. The device achieved 20 kHz/ng. The TPR devices combine principles of Pirani gauge and resonant sensors for improving the sensing range from 2 to 760 Torr (1 atm).

On the Dynamic Range and Resolution of Thermal-Piezoresistive Resonant Mass Sensors

Thermal-actuation and piezoresistive-detection effects have been employed to pump the effective quality factor of MEMS resonators, targeting better mass sensing performance in air. In this paper, frequency resolution (bias instability) of a thermal-piezoresistive resonator operating in air at room temperature is experimentally investigated. It is found that the dynamic range decreases when increasing the bias direct current whereas the effective quality factor rises. The measurement results indicate a maximum effective quality factor of 169k with a dynamic range of 47.8 dB for a bias current of 6.25 mA, and a minimum effective quality factor of 11.3k with a dynamic range of 70.1 dB for a bias current is 5.8 mA. Our work also shows that the frequency and amplitude bias instabilities are significantly lower due to the dynamic range decrease for a high bias current. [2021-0250]

Detection of Polystyrene Beads Concentration Using an SOI-MEMS Differential Rotational Thermal Piezoresistive Resonator for Future Label-Free Biosensing Applications

This work shows a differential rotational thermal piezoresistive resonator (DR-TPR) design, fabrication, and characterization. The resonator uses silicon on insulator (SOI) microelectromechanical systems (SOI MEMS) technology. The sensing of carboxyl-polystyrene beads requires estimating the variation in the frequency of the resonator. It can serve as an alternative enabling technology to detect small molecules having wide implementations in clinical and industrial applications. The resonator uses a negative frequency shift to find the change in the mass due to the chemical absorption of the carboxyl-polystyrene beads on the DR-TPR surface. The method of using DR-TPR can attain label-free detection for many future biomarkers. The use of carboxyl-polystyrene beads to showcase the potential of DR-TPR as a mass sensor for future chemical/biosensing applications is the highlight of the paper. It includes the characterization in different concentrations of immobilization on the resonator surface. The work shows the operation of the resonator in an ionic buffer solution which is crucial for its future applications. The Q value of 80 with differential feedthrough cancellation is measured in phosphate-buffered saline (PBS) using DR-TPR.

Balanced Drive and Sense CMOS Thermal Piezoresistive Resonators and Oscillators

In this work, we report a novel fully balanced drive and sense thermal-piezoresistive resonator (TPR) in 2P-4M 0.35μm CMOS technology with maskless post-release process. Through the use of the Wheatstone bridge arrangement for both the thermal drive heater resistors and sense piezoresistors, the proposed design greatly simplifies the overall system by removing the need for bias-tee or noisy and power consuming current sink/source. This ensures that the differential terminals of drive and sense ports are at the same DC potential, thus allowing the use of standard differential amplifiers as interface circuits. In addition, this design utilizes bimorph like actuation which enables in-plane motion. An in-plane clamped-clamped beam (CCB) TPR was fabricated, demonstrating a quality factor (Q) of 1,600 at 2 MHz with transconductance (gm) of 46.8 μS for a dc-power of 1.7 mW in a vacuum (1 mTorr). To go one step further, a double-ended tuning fork (DETF) TPR was designed, featuring a transconductance of 62.7 μS with Q of 4,400 at 1.9 MHz under the same conditions as CCB. The CCB thermal-piezoresistive oscillator (TPO) implemented using a discrete interface circuit had phase noise of -89 dBc/Hz and -100 dBc/Hz at 1 kHz and 1 MHz respectively while that for DETF TPO was -100 dBc/Hz at 1 kHz and -100 dBc/Hz at 1 MHz in air. Frequency stability was found to be 180 ppb and 125 ppb for CCB and DETF TPOs respectively in air.

A CMOS-MEMS Thermal-Piezoresistive Oscillator for Mass Sensing Applications

We present a thermally driven piezoresistively sensed resonator primarily consisting of backend of line (BEOL) CMOS material for mass sensing applications. A detailed analysis of the operation mechanism has been performed to develop a mathematical and an electrical equivalent model. The thermal-piezoresistive resonator (TPR) is composed predominantly of low loss materials like silicon dioxide (SiO2) and polysilicon for drive and sense. A differential configuration with drive and sense isolation was employed to significantly reduce the feedthrough level. Theoretical and simulated values are validated with experimental data. The proposed design has a resonance frequency of 5.13 MHz with a transconductance ( ${g}_{m}$ ) of 7.8 $\mu \text{S}$ in the vacuum ( ${g}_{m}$ of ${5}~\mu \text{S}$ in the atmospheric pressure for 2.7-mW bias power at 5.09 MHz. One of the highest reported quality factors ( ${Q}$ ) of 2600 for CMOS-MEMS was achieved in air, while the same in the vacuum was >10 000. The thermal-piezoresistive oscillator (TPO) has Allan Deviation of 80 ppb and 20 ppm in ambient pressure using a lock-in amplifier with a phase-locked loop (PLL) and interface circuit using commercial amplifiers on printed circuit board (PCB), respectively. Using silver nanoparticles, the mass sensitivity of 24.96 kHz/ng was measured. The extracted mass resolution in air was 16.3 fg, thus having great potential to serve as an aerosol sensor.

A Fully Differential SOI-MEMS Thermal Piezoresistive Ring Oscillator in Liquid Environment Intended for Mass Sensing

In this paper, we present a silicon on insulator (SOI)-based MEMS oscillator that comprises a ring-shaped rotational mode resonator incorporating fully differential design for operation in aquatic environment using the thermal drive piezoresistive sense transduction. The differential scheme used in the resonator design enables high feedthrough cancellation, thus leading to floor value close to −62 dB with sufficient signal to feedthrough ratio (SFR) and allowing a later oscillator implementation. The proposed differential ring-shaped thermal piezoresistive resonator (DR-TPR) when made to operate in the rotational mode exhibits the special feature of minimizing the effect of viscous damping in liquid environment, attaining a quality factor value as high as 537 with a stopband rejection of 8.5 dB. The closed-loop measurement using the HF2LI lock-in amplifier with PLL from Zurich Instruments helps us to realize the differential ring-shaped thermal piezoresistive oscillator (DR-TPO) showing phase noise of −77.6 dBc/Hz at 1-kHz offset and −89.54 dBc/Hz at 10-kHz offset. The proposed DR-TPO in liquid environment can provide a frequency resolution of 6.95 Hz extracted from the measured Allan deviation, which corresponds to a minimum detectable mass of 4.11 pg, thus paving a way to both physical and biomedical detection in liquid.

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]

Inertial Impaction on MEMS Balance Chips for Real-Time Air Quality Monitoring

This paper reports on integration of microelectromechanical systems (MEMS) balance chips with aerosol inertial impactors for real-time monitoring of airborne particulate matter. Cascade inertial impactors have been extensively used for sampling and size separation of micro to nano-size airborne particles since they were first used in 1945. To introduce the real-time measurement capability to such tools, herein, MEMS resonator chips are employed as their impaction substrates. Dual-plate thermal-piezoresistive resonators (TPRs), that are shown to operate as promising mass balances, are integrated within a two-stage custom made aerosol impactor capable of size segregating particles down to a few tens of nanometers. Unlike cantilever-based microbalances or resonators operating in their bulk mode, TPRs provide uniform mass sensitivity over a big portion of their surface area. Upon deposition of airborne particles on the resonant sensors, the mass loading on each sensor and as a result the mass concentrations of size-segregated particles is measured in real-time. The air quality of different environments, including a class 10,000 cleanroom, was analyzed and monitored via the real-time impactor over a four day period. Comparison of the results with those of an optical particle counter demonstrates a clear correlation between the system response and the expected particle counts. Compared to existing optical particle counters, the proposed system offers the added advantage of detecting sub-100nm particles.

Micromachined Frequency-Output Force Probes

This letter presents a new class of microelectromechanical systems-based frequency output force and displacement probes with sub-nanometer displacement resolution. The force probe is a single layer monolithic crystalline silicon micro-structure comprised of a thermal-piezoresistive resonator embedded within a micro-cantilever. Deflection of the cantilever due to the applied force mechanically distorts the resonator modulating its resonance frequency. Such devices can be used as atomic force microscope probes or high-resolution surface profilometers with fully electrical operation eliminating the bulky and complex optical detectors typically used in such systems. A prototype sensor, operating at 7.5MHz, shows a displacement sensitivity of 2.5 Hz/nm. With a typical Allen deviation of 0.1-0.2 ppm for similar resonators operated as self-sustained oscillators, frequency resolution in the order of 1 Hz is expected to be achievable. This translates to 0.4 nm of displacement and 11 nN of force resolution for such sensors.

A Low-Voltage CMOS-Microelectromechanical Systems Thermal-Piezoresistive Resonator With $Q > 10\,000$

We report a thermally driven and piezoresistively sensed CMOS-microelectromechanical systems (MEMS) resonator with quality factor Q >10000 and stopband rejection of 15 dB under CMOS-compatible bias voltage. The bias voltage requirement of this letter is two orders of magnitude lower than that of the previous CMOS-MEMS capacitively transduced resonators. In addition, the combination of the bulkmode resonator design and highQ SiO2/polysilicon structural material leads to resonator Q >10000, a key index for low-phasenoise oscillators and low-insertion-loss filters. The resonator with a center frequency at 5.1 MHz was fabricated using a standard 0.35 μm 2-poly-4-metal CMOS process, featuring low cost, batch production, fast turnaround time, easy prototyping, and MEMS/IC integration. To resolve the feedthrough issue often seen in conventional thermal-piezoresistive resonators: 1) separation of the heater and piezoresistor is first adopted because of the routing flexibility of the structural configuration offered by CMOS back-end-of-line materials and 2) fully differential measurement scheme is then applied to the proposed device, both of which enable a low-feedthrough level with 65-dB improvement as compared with its single-ended counterpart.

Thermal-Piezoresistive Resonators and Self-Sustained Oscillators for Gas Recognition and Pressure Sensing

This paper presents experimental and theoretical investigation of the response of microscale dual-plate thermal-piezoresistive resonators (TPRs) and self-sustained oscillators (TPOs) to different gases and pressures. It is demonstrated that the resonant frequency of such devices follow particular trends in response to the changes in the surrounding gas and its pressure. A mathematical model is derived to explain the damping dependent frequency shift characteristic of the TPO. The solution of the model indicates that the stiffness of the actuator beam decreases as the value of the damping coefficient drops at lower gas density caused by the change in the gas molecular mass or pressure. When operated in the TPR mode (linear operation), however, the frequency shift of the same silicon structure is mainly a function of gas thermal conductivity. The two different sensing mechanisms are confirmed by the measurement results showing opposite frequency shift for the TPR and TPO in helium-nitrogen mixtures. In pressure tests, frequency shifts as high as -2300 ppm are measured for a TPO by changing the air pressure from 84 to 43 kPa.