CMOS MEMS Research Articles

Theoretical Modeling, Numerical Simulations and Experimental Study of Micro Thermal Convective Accelerometers

We present a one-dimensional (1D) theoretical model for the design analysis of a micro thermal convective accelerometer (MTCA). Systematical design analysis was conducted on the sensor performance covering the sensor output, sensitivity, and power consumption. The sensor output was further normalized as a function of normalized input acceleration in terms of Rayleigh number R $_{\mathrm {a}}$ (the product of Grashof number G $_{\mathrm {r}}$ and Prandtl number P $_{\mathrm {r}}$ ) for different fluids. A critical Rayleigh number ( R ac = 3,000) is founded, for the first time, to determine the boundary between the linear and nonlinear response regime of MTCA. Based on the proposed 1D model, key parameters, including the location of the detectors, sensor length, thin film thickness, cavity height, heater temperature, and fluid types, were optimized to improve sensor performance. Accordingly, a CMOS compatible MTCA was designed and fabricated based on the theoretical analysis, which showed a high sensitivity of 1,289 mV/g. Therefore, this efficient 1D model, one million times faster than CFD simulation, can be a promising tool for the system-level CMOS MEMS design.

Fully Integrated Bidirectional CMOS-MEMS Flow Sensor With Low Power Pulse Operation

Leveraging More-than-Moore technology, we demonstrate an integrated CMOS MEMS flow sensor via a very compact system on chip (SoC) that can sense the bidirectional N2 gas flow. Our SoC features a very low-noise instrumentation amplifier that is implemented on a CMOS wafer and serves as a readout integrated circuit for a thermoresistive micro-calorimetric flow sensor which is fabricated on a MEMS wafer. A compact heterogeneous integration is achieved by combining the two wafers via the proprietary InvenSense AlN process. The measured sensitivity of our CMOS MEMS flow sensor is 98 mV/sccm which is three times better than the state-of-the-art counterpart. The flow range measured by our SoC is from −26 to 26 m/s (from −50 to 50 sccm). Moreover, the pulsed heater operation makes this proposed SoC flow sensor a very low power (<9 mW) and, thus, a promising candidate for the Internet-of-Things applications in smart home/green buildings.

Ultrathin transmissive metasurfaces for multi-wavelength optics in the visible

Metasurface-based optical elements have the ability to shape wavefronts by locally changing the properties of incident illumination. They hold great potential to promote a new generation of wearable devices and thin optical systems for imaging and sensing. However, due to the diffractive nature of these metasurfaces, chromatic aberration represents a critical challenge toward practical implementations, especially for the visible spectrum. Existing solutions for multi-wavelength meta-optics inevitably increase the device thickness and system complexity, similar to the conventional refractive and diffractive counterparts. They are also limited to polarization sensitive lenses with a small size and numerical aperture (NA). In this manuscript, we present the design of single-layer multi-wavelength metasurfaces using ultrathin dielectric resonators (thickness ≪ wavelength) as structural elements. The electromagnetic response of these resonators can be tailored by the in-plane geometrical parameters and used to manipulate the transmitted light. We introduce a digital transmission modulation scheme which enables opportunities to design multi-wavelength optics beyond conventional binary diffractive optics. The design versatility of our approach is demonstrated by polarization-independent achromatic metalenses with a larger lens size and larger NA than existing approaches. The simplicity of the design and fabrication process makes these ultrathin metasurfaces good candidates for building flat optical elements that can be easily integrated onto CMOS electronics and MEMS devices.

A Three-Dimensional Integrated Micro Calorimetric Flow Sensor in CMOS MEMS Technology

This article presents a 3-D integrated molybdenum (Mo) thermoresistive microcalorimetric flow sensor in a 0.18-μm CMOS MEMS technology. The sensor consists of a MEMS structure which is fabricated inside a sealed microchannel and a constant temperature control circuit implemented on the CMOS wafer. The MEMS structure and the CMOS circuit are 3-D integrated at the wafer level. For the N2 gas flow, the proposed flow sensor achieves a high sensitivity of 0.71 mV/(m/s) and a wide bidirectional detection ability of −26–26 m/s. Moreover, an equivalent circuit model is proposed in this article, which depicts the nonlinear output/overheated temperature ( V out/Δ T h) sensor response to the input gas flow. This model would be an efficient tool for the design and optimization of high-performance system-on-chip calorimetric flow sensors.

Performance Optimization of Pressure Sensor Based on Suspended Gate MOSFET

This paper examines the modeling, simulation and optimization of CMOS–MEMS integrated pressure sensor based on suspended gate MOSFET. The pressure Sensor consists of a square poly silicone suspended membrane, which is the movable gate of the NMOS. This NMOS is designed using 2 μm CMOS technology. The mathematical model describing the complete behaviour of the PSFET pressure sensor has been described. Finite element method (FEA) based COMSOL Multiphysics is utilized for the simulation of pressure sensor. The simulation results show that, the output current of the pressure sensor varied from 355 to 3624 μA as the pressure changed from zero to 180 kPa and high pressure sensitivity of 15,18μA/kPa. Furthermore, this study emphasizes on the influence of the channel geometric parameters on the aforementioned characteristics to optimize the sensor performance.

A maskless etching technique for fabrication of 3D MEMS structures in SOI CMOS devices

A maskless etching technique for fabrication of 3D MEMS structures has been presented. The technique has been applied to micro structures embedded during a CMOS process in the dielectric membrane, which was realized by post-CMOS DRIE (deep reactive ion etching) back etch step. A comparison of the conventional etching technique with the developed maskless technique has been presented. The proposed technique takes advantage of the UV (ultra violet) transparent dielectric membrane supporting the CMOS-process based micro structures, where the UV light exposure of the photoresist on the top surface of the device is carried out from the reverse side through the UV-transparent membrane. The surrounding excess membrane is then etched away using suitable anisotropic etching technique such as RIE (reactive ion etching) to release 3D active structures in the CMOS MEMS device. The technique eliminates the requirement of a dedicated photo-mask as the CMOS metallization layer based micro structures are used to transfer the exact pattern on the device surface. Furthermore, expensive mask alignment equipment and skilled human resource is also not required. Thus the reported technique results in overall process simplification, fabrication process time and cost reduction and device yield enhancement. The proposed technique has also been successfully demonstrated to fabricate CMOS MEMS thermal wall shear stress sensors with significantly lower power consumption and stable performance.

Phase-Noise Reduction in a CMOS-MEMS Oscillator Under Nonlinear MEMS Operation

In this paper, we will present an all-CMOS oscillator based on monolithically integrated MEMS resonators fabricated from the VIA layer (tungsten) of a $0.35~\mu \text{m}$ CMOS technology process. With this CMOS MEMS closed-loop system, the nonlinear MEMS frequency response due to the electrostatic actuation has been reconstructed. Using a phase-controlled closed-loop the frequency stability of the self-sustained oscillator at different nonlinear operation points has been analyzed, showing an optimal point belonging to the unstable region of the MEMS resonator.

CMOS MEMS Hot-Film Thermoelectronic Flow Sensor

In this article, we present a complementary metal-oxide semiconductor microelectromechanical system (CMOS MEMS) thermoelectronic flow sensor and discuss the advantages of employing anemometric and calorimetric transduction principles simultaneously. This readout concept exploits the fact that anemometric flow sensors take advantage of the maximum temperature generated within the device and, thus, are provided with the maximum flow sensitivity, while calorimetric flow sensors have the temperature sensing elements placed away from this maximum temperature region and, thus, are intrinsically less sensitive. At the same time, fluid flow direction discrimination in bidirectional flow sensors can be achieved by simply looking at the polarity of the differential signal coming from the temperature sensors placed aside the heating element. We also show that thermoelectronic flow sensors can benefit by having temperature sensors formed by an array of diodes to further enhance the device sensitivity. Although we apply these concepts to a sensor with a nonoptimum design and lacking any support electronics (e.g., filters, amplifiers, constant temperature driving circuitry, etc.), results are among the best ever reported in literature (including non-CMOS technologies), thus showing the high potential of our technology.

Design and analysis of a boosted pierce oscillator using MEMS SAW resonators

This paper highlights the design and analysis of a pierce oscillator circuit for CMOS MEMS surface acoustic wave resonators. The boosted pierce topology using two, three-stage cascode amplifiers provides sufficient gain to counteract the high insertion losses of − 65 dB at 1.3 GHz of the SAW resonator. For accurate prediction of the oscillator’s performance before fabrication, circuit design utilized touchstone S2P measurement results of the MEMS SAW resonator, which provides better results compared to the conventional method of using equivalent circuit simulations. This circuit was designed using Silterra’s 0.13 μm CMOS process. It has low power consumption of 1.52 mW with high voltage swing 0.10–0.99 V. All simulations were conducted using Cadence Design Systems and results indicate that phase noise of 92.63 dBc at 1 MHz.

Optimization of the Close-to-Carrier Phase Noise in a CMOS-MEMS Oscillator Using a Phase Tunable Sustaining-Amplifier.

In this paper, the phase noise of a 24-MHz complimentary metal-oxide-semiconductor microelectromechanical systems (CMOS-MEMS) oscillator with zero-level vacuum package is studied. We characterize and analyze the nonlinear regime of each one of the modules that compose the oscillator (CMOS sustaining-amplifier and MEMS resonator). As we show, the presented resonator exhibits a high nonlinear behavior. Such a fact is exploited as a mechanism to stabilize the oscillation amplitude, allowing us to maintain the sustaining-amplifier working in the linear regime. Consequently, the nonlinear resonator becomes the main close-to-carrier phase noise source. The sustaining amplifier, which functions as a phase shifter, was developed such that MEMS operation point optimization could be achieved without an increase in circuitry modules. Therefore, the system saves on area and power, and is able to improve the phase noise 26 dBc/Hz (at 1-kHz carrier frequency offset).

Automated Resonance Matching for CMOS MEMS Micro-Resonators

This paper reports the design and characterization of a phase-locked loop-based control scheme implemented in a complementary metal–oxide–semiconductor process for resonance matching between capacitive micro-resonators. The system-on-chip implementation can be utilized for calibrating individual resonance in a large sensor array or matching the drive and sense modes for micromachined vibratory gyroscopes. The target micro-resonator acts as a voltage-controlled oscillator in the control loop with the resonance being adjusted by the electrostatic spring-softening effect. Sinusoidal test inputs applied to the control loop are shown to provide better resonance matching than square-wave inputs. This paper successfully demonstrates automated matching by pulling the resonance of a target micro-resonator within $2.4\times 10^{-2}$ % of the reference resonance under atmospheric pressure.

Theoretical and Experimental Investigations of Thermoresistive Micro Calorimetric Flow Sensors Fabricated by CMOS MEMS Technology

A general 1-D model was presented to predict the characteristics of CMOS thermoresistive micro calorimetric flow (TMCF) sensor with two types of packaging, i.e., open-space type and channel type, for both gases and liquids. The 1-D model was first validated by a numerical computational fluid dynamics (CFD) model and was subsequently normalized for different fluids flow. Notably, the model proposed by Nguyen and Dotzel is a special case of our 1-D model. The normalized output of TMCF sensor is a function of normalized input parameters of Reynolds number Re and Prandtl number Pr. The scaling analysis of the sensor output, sensitivity, and power consumption was performed to optimize the design of TMCF sensors in terms of key design parameters, including the thin film thickness, the height of bottom cavity, and so on. Accordingly, three pairs of TMCF sensors were designed and fabricated by using a 0.35 $\mu \text{m}$ 2P4M CMOS microelectromechanical systems technology. The fabricated sensors showed a normalized sensitivity of 230 mV/(m/s)/mW for nitrogen gas flow, which was two orders of magnitude higher than the previous CMOS flow sensors. Therefore, the proposed 1-D model is a promising tool for the sensor’s system-level design with the on-chip microelectronics for the Internet of Things. [2016-0098]

Characterization of embedded microheater of a CMOS–MEMS gravimetric sensor device

A CMOS–MEMS device for mass detection has been designed using 2008 CoventorWare software and fabricated using 0.35µm CMOS technology. This paper reports the characterization of the microheater and the temperature sensor embedded in the device. The measured resistances of the microheater and the temperature sensor were found to be close to the modeled values within ~4.2% error. The average temperature coefficient of resistance (TCR) of the temperature sensor of five dies was determined by increasing or decreasing the temperature in a range of 25°C–100°C. The resistance of the temperature sensor was found to increase with either an increase in ambient temperature or the voltage applied to the microheater, with a correlation factor of 0.99. The average TCR was found to be 0.0034/°C for the increasing temperature and 0.0036/°C for the decreasing temperature as compared to 0.0037°C reported in the literature, indicating an error of 8.1% and 3.5%, respectively. These differences between the measured and reported values are believed to be due to fabrication tolerances in the design dimensions or the material properties. The humidity was found to have a negligible effect on the resistance of the temperature sensor for increasing humidity levels from 40% to 90%. The repeatability of the measurements has shown low standard errors, which gives confidence in the reliability of the fabricated device.

Characterization of 0.18-μm CMOS MEMS Capacitive Ultrasonic Sensors for Fast Photoacoustic Imaging

This work presents a 4 × 4 capacitive ultrasonic sensor array fabricated in a 0.18-μm CMOS process. Compared to prior CMOS MEMS ultrasonic sensors, the smaller electrode separation in the new platform significantly improves the capacitive sensitivity. The sensor has a measured center frequency at 3.8MHz with a fractional bandwidth of 74%. The measured sensitivity is 499 mVpp/MPa/V. With the advance in technology, individual readout circuit can be placed directly beneath each sensor to allow unlimited scalability. Feasibility for fast 3D photoacoustic imaging is demonstrated with the sensing array presented in this work.

Development of a CMOS MEMS pressure sensor with a mechanical force-displacement transduction structure

This study presents a capacitive pressure sensor with a mechanical force-displacement transduction structure based on the commercially available standard CMOS process (the TSMC 0.18 μm 1P6M CMOS process). The pressure sensor has a deformable diaphragm to support a movable plate with an embedded sensing electrode. As the diaphragm is deformed by the ambient pressure, the movable plate and its embedded sensing electrode are displaced. Thus, the pressure is detected from the capacitance change between the movable and fixed electrodes. The undeformed movable electrode will increase the effective sensing area between the sensing electrodes, thereby improving the sensitivity. Experimental results show that the proposed pressure sensor with a force-displacement transducer will increase the sensitivity by 126% within the 20 kPa–300 kPa absolute pressure range. Moreover, this study extends the design to add pillars inside the pressure sensor to further increase its sensing area as well as sensitivity. A sensitivity improvement of 117% is also demonstrated for a pressure sensor with an enlarged sensing electrode (the overlap area is increased two fold).

A CMOS–MEMS arrayed resonant-gate field effect transistor (RGFET) oscillator

A high-frequency CMOS–MEMS arrayed resonant-gate field effect transistor (RGFET) fabricated by a standard 0.35 μm 2-poly-4-metal CMOS–MEMS platform is implemented to enable a Pierce-type oscillator. The proposed arrayed RGFET exhibits low motional impedance of only 5 kΩ under a purely capacitive transduction and decent power handling capability. With such features, the implemented oscillator shows impressive phase noise of −117 dBc Hz−1 at the far-from-carrier offset (1 MHz). In this work, we design a clamped–clamped beam (CCB) arrayed resonator utilizing a high-velocity mechanical coupling scheme to serve as the resonant-gate array. To achieve a functional arrayed RGFET, a corresponding FET array is directly placed underneath the resonant gate array to convert the motional current on the resonant-gate array into a voltage output with a tunable transconductance gain. To understand the behavior of the proposed device, an equivalent circuit model consisting of the resonant unit and FET is also provided. To verify the effects of the post-CMOS process on device performance, a conventional MOS ID current measurement is carried out. Finally, a CMOS–MEMS arrayed RGFET oscillator is realized by utilizing a Pierce oscillator architecture, showing decent phase noise performance that benefits from the array design to alleviate the nonlinear effect of the resonant gate.

A double-sided, single-chip integration scheme using through-silicon-via for neural sensing applications.

We present a new double-sided, single-chip monolithic integration scheme to integrate the CMOS circuits and MEMS structures by using through-silicon-via (TSV). Neural sensing applications were chosen as the implementation example. The proposed heterogeneous device integrates standard 0.18 μm CMOS technology, TSV and neural probe array into a compact single chip device. The neural probe array on the back-side of the chip is connected to the CMOS circuits on the front-side of the chip by using low-parasitic TSVs through the chip. Successful fabrication results and detailed characterization demonstrate the feasibility and performance of the neural probe array, TSV and readout circuitry. The fabricated device is 5 × 5 mm(2) in area, with 16 channels of 150 μm-in-length neural probe array on the back-side, 200 μm-deep TSV through the chip and CMOS circuits on the front-side. Each channel consists of a 5 × 6 probe array, 3 × 14 TSV array and a differential-difference amplifier (DDA) based analog front-end circuitry with 1.8 V supply, 21.88 μW power consumption, 108 dB CMRR and 2.56 μVrms input referred noise. In-vivo long term implantation demonstrated the feasibility of presented integration scheme after 7 and 58 days of implantation. We expect the conceptual realization can be extended for higher density recording array by using the proposed method.

Dip pen nanolithography-deposited zinc oxide nanorods on a CMOS MEMS platform for ethanol sensing

Zinc oxide nanorods integration with CMOS MEMS substrate using dip pen nanolithography for low cost, low power ethanol sensor development.

A CMOS micromachined capacitive tactile sensor with integrated readout circuits and compensation of process variations.

This paper presents a capacitive tactile sensor fabricated in a standard CMOS process. Both of the sensor and readout circuits are integrated on a single chip by a TSMC 0.35 μm CMOS MEMS technology. In order to improve the sensitivity, a T-shaped protrusion is proposed and implemented. This sensor comprises the metal layer and the dielectric layer without extra thin film deposition, and can be completed with few post-processing steps. By a nano-indenter, the measured spring constant of the T-shaped structure is 2.19 kNewton/m. Fully differential correlated double sampling capacitor-to-voltage converter (CDS-CVC) and reference capacitor correction are utilized to compensate process variations and improve the accuracy of the readout circuits. The measured displacement-to-voltage transductance is 7.15 mV/nm, and the sensitivity is 3.26 mV/μNewton. The overall power dissipation is 132.8 μW.

Gas chemical sensitivity of a CMOS MEMS cantilever functionalized via evaporation driven assembly

This work demonstrates an electrostatically actuated resonant microcantilever fabricated in a complementary metal oxide semiconductor process and functionalized with a chemically sorbent polymer layer for the detection of volatile organic compounds. Deposition of the chemically sorbent layer is controlled through evaporation-driven assembly. Analytical and finite element analysis models of the deposited polymer layer on the microcantilever resonant frequency and mass sensitivity are presented. Fabrication of the chemical sensor, including a description of polymer deposition through evaporation-driven assembly within a capillary, is detailed. The completely functionalized resonator demonstrates a limit of detection of 1.6 ppm for toluene. An optimal polymer sensitive layer deposition of 42% of the total beam length is measured from frequency instability and sensitivity tests.