MEMS Capacitive Sensor Research Articles

Study on Static Deflection Model of MEMS Capacitive Microwave Power Sensors

Study on Static Deflection Model of MEMS Capacitive Microwave Power Sensors

Performance analysis of FEM simulated different shaped membranes based capacitive MEMS sensor

Performance analysis of FEM simulated different shaped membranes based capacitive MEMS sensor

Mems capacitive humidity sensors based on different structures for low humidity applications

Mems capacitive humidity sensors based on different structures for low humidity applications

A High-Resolution MEMS Capacitive Force Sensor With Bionic Swallow Comb Arrays for Ultralow Multiphysics Measurement

Precise force sensing is essential for the mechanical characterization and robotic micromanipulation of biological targets. In this work, a high-resolution MEMS capacitive force sensor was proposed for measuring ultralow multiphysics. A bionic swallow structure that contained multiple feathered comb arrays was designed for reducing chip dimension and eliminating undesirable mechanical cross-coupling effect. The comb structure was optimized for maximum sensitivity, linearity, and compact chip size. Utilizing a novel interconnection configuration, interferences derived from parasitic capacitance and electrostatic forces exerted negligible effects on the sensor output. The proposed bionic force sensor was fabricated following a simple three-mask process and integrated with ASIC readouts. Its measuring sensitivity was 7.151 fF/nm, 0.529 aF/nN, and 4.247 pF/g for displacement, force, and inclination measurements, respectively. The proposed sensor had a large measurement range of 1000.00 nm and 13.83 µN with a high linearity of 0.9998. The 1-σ resolution was 0.0328 nm and 0.4436 nN, and the noise floor resolution was 0.0044 nm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt {{\bm{Hz}}} $</tex-math></inline-formula> and 0.0597 nN/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sqrt {{\bm{Hz}}} $</tex-math></inline-formula> for displacement and force measurements, respectively. The bias stability of Allan deviance was 0.0050 nm and 0.0678 nN at an integration time of 0.65 s. The proposed bionic swallow sensor exhibited considerable improvement over existing capacitive sensors and feasibility for ultralow multiphysics measurement in biomedical applications.

Highly Sensitive Capacitive MEMS for Photoacoustic Gas Trace Detection.

An enhanced MEMS capacitive sensor is developed for photoacoustic gas detection. This work attempts to address the lack of the literature regarding integrated and compact silicon-based photoacoustic gas sensors. The proposed mechanical resonator combines the advantages of silicon technology used in MEMS microphones and the high-quality factor, characteristic of quartz tuning fork (QTF). The suggested design focuses on a functional partitioning of the structure to simultaneously enhance the collection of the photoacoustic energy, overcome viscous damping, and provide high nominal capacitance. The sensor is modeled and fabricated using silicon-on-insulator (SOI) wafers. First, an electrical characterization is performed to evaluate the resonator frequency response and nominal capacitance. Then, under photoacoustic excitation and without using an acoustic cavity, the viability and the linearity of the sensor are demonstrated by performing measurements on calibrated concentrations of methane in dry nitrogen. In the first harmonic detection, the limit of detection (LOD) is 104 ppmv (for 1 s integration time), leading to a normalized noise equivalent absorption coefficient (NNEA) of 8.6 ⋅ 10-8 Wcm-1 Hz-1/2, which is better than that of bare Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS), a state-of-the-art reference to compact and selective gas sensors.

An area-variant type MEMS capacitive sensor based on a novel bionic swallow structure for high sensitive nano-indentation measurement

In this paper, a high sensitive area-variant type MEMS capacitive sensor was proposed for nano-indentation measurement. The proposed bionic swallow structure processed a high mechanical sensitivity without influences from the load coupling effect. Six comb arrays were optimized and integrated into the proposed sensor with a novel comb sensing configuration for eliminating electrostatic interferences resulting from multiple combs and maintaining compact chip size. Based on the proposed micro-machined process, comb arrays have been fabricated with great consistency, which exhibited a static differential output of 0.0554 pF. The fabricated sensor has been characterized with rigid glass cube and AFM cantilevers, and the measured force sensitivity can reach 98.54 aF/nN for a larger measurement range of 142 µN with a linearity of 0.9996. The fabricated sensor also exhibited a high resolution of 0.9819 nN, indicating its great feasibility in nano-indentation measurement at the sub-nano-Newton level.

Capacitance-to-voltage converter employing parallel-series passive charge integrator for low noise power efficient MEMS capacitive sensor

Parallel-series passive charge integrator (PSPI) based capacitance-to-voltage converter (CVC) is proposed in this paper to reduce the noise floor of readout circuit for MEMS capacitive sensor with high power efficiency in terms of the figure of merit (FoM). For MEMS capacitive accelerometer, open-loop structure is preferred for its low power consumption. However, in the open-loop structure, the capacitance variation of sensing element is limited to femto-farad level in order to overcome nonlinearity. As a result, the thermal noise from parasitic capacitance becomes significant, which leads to low SNR. Traditional oversampling method can reduce the parasitic-induced noise, but leads to low power efficiency. In this work, the PSPI-CVC is proposed to reduce the parasitic-induced noise with high power efficiency. The PSPI-CVC is designed by a commercial 0.18 μm CMOS process. The transistor-level simulation results show that, compared to the traditional CVC, the PSPI-CVC can improve the SNR by 24 dB without increase of the power consumption. Compared with the other similar works, the proposed readout circuit achieves competitive FoM1 (FoM1 =20.2pJ)and the best FoM2 (FoM2).

Reliability Investigation of Bioinspired Hair Flow-Sensor

In capacitive MEMS sensors, the excitation voltage is necessary for operating the device and increasing the sensitivity. For reliable operation, the amplitude of the excitation voltage is restricted by the pull-in voltage of the sensor. This paper investigates the pull-in phenomenon associated with the capacitive sensing mechanism of artificial hair flow-sensor (HFS), as an important design parameter to improve reliability, robustness, and sensitivity of the sensor. The pull-in voltage of the HFS is investigated theoretically and verified experimentally. The electrostatic force acting on the membrane, the pull-in voltage, and the deflection of the membrane for different excitation voltages are considered. The theoretical model together with the HFS design parameters is used to inspect the possible origin for the deviation between the theory and the experiments. Furthermore, we pointed out and stated the applicable conditions of the HFS based on the experimental results and theoretical analysis. Such a study forms a step further to improve the reliability of the HFS to be a suitable candidate for the utilization at different conditions and in various applications.

A Sawtooth MEMS Capacitive Strain Sensor for Passive Telemetry in Bearings

The development of economical and high-sensitive strain sensor is crucial for precise measurement of strain in industrial applications, such as bearings. During operation, bearing itself is a harsh environment, which usually has large temperature variations and large centrifugal forces. This paper presents a sawtooth MEMS capacitive strain sensor for passive telemetry in bearings. The interdigital fingers are designed into sawtooth shape to decrease the influence of centrifugal acceleration. The strain sensor chip is 4mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times4$ </tex-math></inline-formula> mm in size, and is fabricated by a simple SOI process. Connecting the sensitive capacitor with an inductor, an LC resonant tank is formed, and the strain signal can be read out wirelessly and passively through a readout coil. Measurement results show that, in the range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0\sim 1000~\mu \varepsilon $ </tex-math></inline-formula> , the sensitivity of the LC strain sensor reaches 34kHz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> , and the gauge factor (GF) of the strain sensor reaches 430. The influence of thickness of adhesive layer on the strain sensitivity is investigated. When the thickness of the adhesive layer reaches <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$500~\mu \text{m}$ </tex-math></inline-formula> , the applied stress cannot be transferred at all. Finally, the LC strain sensor is tested on a rotating platform. The measured resonant frequency shows only 0.4% deviation under 1000m/s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> centrifugal acceleration.

Active Temperature Compensation for MEMS Capacitive Sensor

Temperature variations are one of the most crucial factors that need to be compensated for in MEMS sensors. Many traditional methodologies require an additional circuit to compensate for temperature. This work describes a new active temperature compensation method for MEMS capacitive strain sensors without additional circuits. The proposed method is based on a complement 2-D capacitive structure design. It consumes zero-power, which is essential toward the realization of a low-power temperature-compensated sensor in battery-powered or energy-harvesting applications. The gauge factor of the developed MEMS capacitive strain sensor is 7. The best result showing a capacitance variation of 1 ppm/°C compared with 13 ppm/°C on conventional design.

A Wireless Artificial Mechanoreceptor in 180-nm CMOS.

This article presents an implantable low-power wireless integrated system for tactile sensing applications. The reported ASIC utilizes a low-loss magnetic human body communication channel for both wireless power and data transfer. The chip is hybrid-integrated with an in-house fabricated MEMS capacitive force sensor to form an implantable artificial mechanoreceptor. An on-chip correlated double sampling capacitance to time converter consumes 750nW from a 1.2V on-chip regulated supply, achieving a 2.0fF resolution for an input capacitance of 14pF and a FoM of 49 fJ/c-s while occupying an area of only 0.04 mm2. The capacitance to time converter has a time-multiplexed mode to interface with 4 input capacitors. Wireless power management feedback is utilized to ensure robust operation in the presence of hand gesture changes and process-voltage-temperature variations. The on-chip data transmitter can operate in on-off keying or frequency-shift keying modulation formats, where it consumes only 7.8 in the on-off keying mode. The 1.62mm2 chip is fabricated in a standard 180nm CMOS process and consumes 110.3 .

Design of sensor integrating gears: methodical development, integration and verification of an in-Situ MEMS sensor system

State of the art vibration-based condition monitoring at gearbox housings faces uncertainties in the interpretation of measurement data due to signal transformations and noise. The state of research shows that direct measurements at the source of vibrations with integrated sensors provide higher quality data. Capacitive MEMS sensors seem predestined for integration, but there is limited research covering compactly integrated MEMS sensor systems for condition monitoring by vibration measurement. In this contribution an integrated MEMS sensor system is designed methodically based on VDI 2206. A sensor system is selected based on requirements extracted of previous contributions and verified on a rotational shaker test rig. Afterwards it is integrated on a gear wheel in a gear test bench. Several verification measurements using different principles and locations are performed to verify the measurands. Results show that the gear mesh vibrations including the sidebands can be measured with the integrated sensors which provide superior signal-noise-ratios compared to other locations. This proofs that the sensor integrating gear system is principally able to perform high quality condition monitoring.

A 15.4V Fully-Integrated Energy-Efficient Pulse Generator in Standard 0.18 μm CMOS

This brief presents the design and implementation of an energy efficient high-voltage (HV) pulse generator for MEMS capacitive sensors and actuators. The core circuit of the proposed HV pulse generator consists of four dual-mode switched-capacitor voltage doublers (D-SCVDs). Dynamic output impedance (DOI) method is proposed and implemented to improve energy efficiency during the rising edge generation. Load energy recycling (LER) method is also used during the falling edge generation to reduce the overall energy consumption. The proposed circuit achieves an end-to-end energy efficiency of 43.5% with 250 kHz driven frequency under 550 pF capacitive load and 3.3V supply. The maximum pulsing voltage achieves 15.4V and standby power is measured as 9.8 nW. The proposed design is implemented in SMIC standard $0.18\boldsymbol{\mu }\text{m}$ CMOS process occupying a core area of $700\boldsymbol{\mu }\text{m}$ by $800\boldsymbol{\mu }\text{m}$ .

A 200 μg/√Hz, 2.7 milli-g Offset Differential Interface for Capacitive Micro Accelerometer

This brief presents a switched-capacitor based low offset differential output sensor interface for MEMS capacitive accelerometers. The proposed interface employs an improved demodulation scheme by using a fully differential op-amp and a modified clocking scheme to reduce the zero-g offset. The interfacing Application Specific Integrated Circuit (ASIC) is designed and fabricated using UMC 180 nm CMOS process technology. The fabricated ASIC is integrated with a MEMS capacitive acceleration sensor and the measured results are presented. The proposed configuration achieves maximum sensitivity of 1.45 V/g with less than 2% non-linearity error. The zero-g offset is less than 3 milli-g while the noise floor is 200 μg/√{Hz} approximately.

Electric Modulation on the Sensitivity and Sensing Range of CMOS-MEMS Tactile Sensor by Using the PDMS Elastomer Fill-In

This study presents the design and implementation of a MEMS capacitive tactile force sensor with polydimethylsiloxane (PDMS) filler for in-process and in-use modulation of the sensing range and sensitivity performance. The presented MEMS tactile force sensor consists of a loading unit with driving electrodes, PDMS filler, and a sensing unit. By varying input voltages on the driving electrodes (either in curing process or in-use), the stiffness of the PDMS filler as well as the loading unit can be modulated. Thus, the sensitivity and sensing range of the presented tactile force sensor was modulated accordingly. Since each sensor has its own driving electrodes and electrical routings, different input voltages can be applied to modulate the PDMS for each individual sensor. As a result, tactile sensors of different sensitivities and sensing ranges can be simultaneously fabricated and monolithically integrated on the same chip using the presented approach. The tactile sensors were implemented using TSMC 0.18 μm 1P6M standard CMOS process together with the in-house post-CMOS releasing and the polymer filling processes. Experiments demonstrate the capabilities of implementing various tactile sensors with different sensing ranges and sensitivities on a single chip by the in-process and in-use modulation techniques. Through the in-process and in-use modulation, the presented CMOS-MEMS tactile sensors show the sensing window: (1) for sensitivity modulation: 8.6 fF/N to 219.1 fF/N, and (2) for sensing range modulation: 30 mN to 270 mN.

Influence of the failure effect of MEMS capacitive high g acceleration sensor on the limit range and sensitivity

With the development of MEMS capacitive accelera-tion sensor to a higher measurement range (tens of thousands of g or even hundreds of thousands of g), the failure of the device microstructure under extreme mechanical impact has become a key factor restricting its performance improvement. Existing research mainly focuses on material stress failure and device structure optimization. Aiming at the failure of micro structure under impact, a simulation model of impact response of micro structure is established in this paper. Based on the existing over stress failure and device structure optimization, the stress failure effect and short-circuit failure effect of MEMS capacitive acceleration sensor under strong impact are proposed, and the influence rules of the two failure mechanisms on the sensor limit range and sensitivity are obtained. At the same time, the optimization design of the key structural parameters of the sensor is discussed.

Efficiency‐enhanced and high‐precision of input common‐mode feedback control using OSA–CLS technique

Efficiency-enhanced and high-precision input common-mode feedback control (ICMFB) is proposed for a fully differential readout circuit of the MEMS capacitive sensor. This ICMFB improves the efficiency and performance of input common-mode voltage control using a smaller feedback capacitor. The noise performance of the readout circuit is improved as a decrease of a feedback capacitor. The correlated level shift (CLS) technique is used to improve the output swing of the amplifier therefore reduce the value of a feedback capacitor. Also, the oversampling successive approximation (OSA) technique is used to improve the accuracy of input common-mode voltage control using a low-gain op-amp. Simulation results show that the input common-mode voltage error decreases to 0.01% when the value of the feedback capacitor is the same as the MEMS sensor. Also, the noise power spectral density of the readout circuit increases by 10 dB.

Development of capacitive temperature sensors with high sensitivity using a multiuser polycrystalline silicon process

A MEMS capacitive temperature sensor is presented that displays high sensitivity over a specific temperature range using out-of-plane thermal actuators and capacitive electrodes. The sensor is suitable for applications that require high precision in relatively small temperature range, such as biomedicine, food processing and food safety. The device uses a combination of bilayer, rigid and flexible beams to create a nonlinear capacitance-temperature response over a short temperature range. The bilayer beams act as the out-of-plane thermal actuator and move the top electrode of the sensor up and down to change the capacitance. The capacitance is inversely proportional to the electrodes' gap and a decrease in the gap increases the sensitivity of the capacitance-temperature (C-T) response. Sensors with different geometries have been simulated using ANSYS to show the possibility of geometric modification to shift the temperature range with high sensitivity. Fabricated sensors are used in an experimental set up to demonstrate the applicability of the proposed design concept. The measured data verify the simulation results and demonstrate that the proposed sensor design can provide high sensitivity of 20 fF/°C in a desired range of ambient temperature where low sensitive region display noticeably smaller sensitivity of 5 fF/°C.

Modeling and Characterization of a Pull-in Free MEMS Microphone

In this study, we examine the feasibility of designing a MEMS microphone employing a levitation based electrode configuration. This electrode scheme enables capacitive MEMS sensors that could work for large bias voltages without pull-in failure. Our experiments and simulations indicate that it is possible to create robust sensors properly working at high DC voltages, which is not feasible for most of the conventional parallel plate electrode-based-microscale devices. In addition, the use of larger bias voltages will improve signal-to-noise ratios in MEMS sensors because it increases the signal relative to the noise in read-out circuits. This study presents the design, fabrication, and testing of a capacitive microphone, which is made of approximately 2 μm thick highly-doped polysilicon as a diaphragm. It has approximately 1 mm 2 surface area and incorporates interdigitated sensing electrodes on three of its sides. Right underneath these moving electrodes, there are fixed fingers being held at the same voltage potential as the moving electrodes and separated from them with a 2 μm thick air gap. The electronic output is obtained using a charge amplifier. Measured results obtained on three different microphone chips using bias voltages up to 200 volts indicate that pull-in failure is completely avoided. The sensitivity of this initial design was measured to be 16.1 mV/Pa at 200 V bias voltage, and the bandwidth was from 100 Hz to 4.9 kHz.

Novel capacitive MEMS sensor for monitoring in-stent restenosis.

Cardiovascular is a disorder of the heart and its blood vessels that causes millions of deaths around the world. Implanting stent in patient's artery is a typical procedure of treatment. However, a major problem associated with stents is the buildup of plaque/fatty acids deposits, a condition known as in-stent restenosis. Therefore, one of the preventive solutions is the development of "smart" stents that contain pressure sensors for monitoring stent blockage at early stages. Such sensors need to be associated with high sensitivity for early detection of flow abnormalities within the stent. Here, we propose several capacitive pressure sensor plate designs and analyze their sensitivities. Our results show that a square-shaped plate with straight slots on its edges can drastically improve sensitivity. Several analyses were carried out to optimize dimensions of the proposed design for monitoring in-stent restenosis.