CMOS Microelectromechanical Systems Research Articles

Quality factor boosting of bulk acoustic wave resonators based on a two dimensional array of high-Q resonant tanks

An innovative resonance excitation methodology of a two-dimensional array of uniformly patterned structure into extremely high-quality factor (Q) mode using engineered frequency-matched thin piezoelectric film on substrate (TPoS) transducer is demonstrated in this Letter. The passive Q-enhancement can be attained lithographically without adding to the power requirement of the system. A 500 nm thick piezoelectric material facilitates the inter-electrical-mechanical domain coupling, and the displacement generated at the transducer periphery drives the patterned 10 μm thick single crystal silicon plate into various resonant modes. Using this topology, unexplored modes can be generated as well by the forced vibration feature of the interacting resonant tanks. Utilizing a multiport port excitation scheme, multiple modes are explored, while the primary focus is maintained on the Lamé mode of the array. In vacuum for an operational frequency of 58.56 MHz, an exceptionally high-quality factor of 58 276 is attained with a motional resistance of 1250 Ω and this is the record-high figure of merit (f × Q = 3.4 × 1012) attained for aluminum nitride-based TPoS based resonators. This successful demonstration of the energy coupling scheme paves a path for the study of existing and investigation of unexplored high-Q modes spanning over a wide range of frequency, making these devices a potential candidate for radio frequency front-end applications. The CMOS compatibility of aluminum nitride makes it an attractive solution for achieving low phase noise monolithically integrated CMOS–microelectromechanical systems oscillators.

Conceptual Design of a Resonant Pirani Gauge Toward Wide-Range Pressure Sensing

This letter reports a CMOS-microelectromechanical system (MEMS) pressure sensor comprising a double-ended tuning fork capacitive resonant transducer and a Pirani gauge thermal transducer in a single device configuration, which demonstrates a wide-range pressure sensing capability, small form factor, and sensor fusion. The device integrated with an interfaced circuit was fabricated through a 0.35- μ m CMOS foundry process, together with an in-house release process. Thanks to the resonant transducer which is sensitive to pressure due to squeezed-film damping effect, the high pressure sensing range is achieved by measuring resonant frequency shift, whereas the medium pressure region is sensed by quality factor ( Q ) change of the resonator. As a result, a full-scale range of 10–760 torr is realized by the proposed CMOS-MEMS pressure sensor using its resonant transducer. To further extend the measurement range to the lower pressure range, an embedded thermal transducer of the device is used to serve as Pirani gauge that is capable of detecting pressure from 0.02 to 10 torr by the use of thermal resistance $(R_{{\rm{th}}})$ change. Through a simple algorithm combining variations of Q , resonant frequency $f_{o}$ , and thermal resistance $R_{{\rm{th}}}$ , the proposed resonant Pirani gauge pressure sensor features a very wide-range pressure sensing capability from 0.02 to 760 torr.

A Comprehensive High-Level Model for CMOS-MEMS Resonators

This paper presents a behavioral modeling technique for CMOS microelectromechanical systems (MEMS) microresonators that enables simulation of an MEMS resonator model in Analog Hardware Description Language format within a system-level circuit simulation. A 100-kHz CMOS-MEMS resonant pressure sensor has been modeled into Verilog-A code and successfully simulated within Cadence framework. Analysis has shown that simulation results of the reported model are in agreement with the device characterization results. As an application of the proposed methodology, simulation and results of the model together with an integrated monolithic low-noise amplifier is exemplified for detecting the position change of the resonator.

Characterization of CMOS-MEMS Resonant Pressure Sensors

Comprehensive characterization results of CMOS-microelectromechanical systems resonant pressure sensor are presented. We have extensively evaluated the key performance parameters of our device in terms of quality factor ( $Q$ ) variations under variable conditions of temperature and pressure, characterized by Knudsen number ( $K_{n}$ ). The fundamental frequency of the reported device is 104.3 kHz. Over the full-scale pressure range of 0.1 to 100 kPa and a temperature range of −10 °C to 85 °C, $Q$ from 450 to 62.6 have been obtained. Besides, static variations of the device capacitance have been measured and analyzed with temperature to evaluate the spring softening and the pull-in effects. A nonlinearity analysis has been performed to assess the device stability. Furthermore, a statistical mismatch analysis has been carried out to determine the deviation of resonance with etching time and ascertain maximum device yield. With our in-house back-end of line metal-layer release, this sensor can be monolithically embedded in the same substrate as standard CMOS integrated circuits, resulting in a significant cost and area reduction.

Surface pretreated low-temperature aluminum\u2013aluminum wafer bonding

Aluminum–aluminum wafer bonding is becoming increasingly important in the production of CMOS microelectromechanical systems. So far, successful bonding has required extreme processing temperatures of 450 °C or more, because the chemically highly stable oxide layer acts as a diffusion barrier between the two aluminum metallization layers. By using the ComBond® system, in which a surface treatment and subsequent wafer bonding are both performed in a high vacuum cluster, for the first time successful Al–Al wafer bonding was possible at a temperature of 150 °C. The bonded interfaces were characterized using C-mode scanning acoustic microscopy and transmission electron microscopy, and featured areas of oxide-free, atomic contact.

Design and Sensitivity Improvement of CMOS-MEMS Scanning Microwave Microscopes

In this paper, we present the performance and imaging results of an integrated single-chip CMOS-microelectromechanical systems (MEMS) scanning microwave microscope (SMM). A systematic analysis for sensitivity improvement is described in detail. We first explain why it is important to have high sensitivity for this type of microscope and then propose a systematic method to analyze and design the entire structure for better sensitivity. For this, accurate lumped models are derived for each section of the system and comparisons are made between different designs. Furthermore, a new concept based on the quality factor of the individual sections is described to give the designer a tool to improve the sensitivity of individual sections without the need to simulate or model the entire system. A high-sensitivity measurement system is also explained. Finally some measurement results are presented. While the analysis presented in this paper is for CMOS-MEMS SMM, it is applicable to any type of SMM.

0.35 μm CMOS–MEMS low-mechanical-noise micro accelerometer

A CMOS–microelectromechanical systems (MEMS) accelerometer chip produced using Taiwan Semiconductor Manufacturing Company 0.35 μm CMOS technology and the Chip Implementation Center micromachining postprocess is proposed in this study. The chip integrates a MEMS gravity sensor and a readout circuit on a silicon substrate. The measured results indicated that the resonant frequency, mechanical noise, static capacitance, sensitivity, and noise floor characteristics were 3.1 kHz, 6.673 μg/√Hz, 120 fF, 2.4 mV/g, and 29.447 μV/√Hz, respectively.

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]

A Surface Micromachined CMOS MEMS Humidity Sensor

This paper reports a CMOS MEMS (complementary metal oxide semiconductor micro electromechanical system) piezoresistive humidity sensor fabricated by a surface micromachining process. Both pre-CMOS and post-CMOS technologies were used to fabricate the piezoresistive humidity sensor. Compared with a bulk micromachined humidity sensor, the machining precision and the sizes of the surface micromachined humidity sensor were both improved. The package and test systems of the sensor were designed. According to the test results, the sensitivity of the sensor was 7 mV/%RH (relative humidity) and the linearity of the sensor was 1.9% at 20 °C. Both the sensitivity and linearity were not sensitive to the temperature but the curve of the output voltage shifted with the temperature. The hysteresis of the humidity sensor decreased from 3.2% RH to 1.9% RH as the temperature increased from 10 to 40 °C. The recovery time of the sensor was 85 s at room temperature (25 °C).

Integrated chip of capacitive three‐axis accelerometer and sensing circuit

A CMOS-microelectromechanical system 0.18 µm process of the Taiwan Semiconductor Manufacturing Company is used to design an integrated chip of a capacitive three-axis accelerometer and a sensing circuit. The sensing architecture uses capacitive sensing principles, and the three axes are integrated on the same structure and share the same mass. The three-axis design method can not only reduce the chip area but also has good sensing ability. The readout circuit architecture combines two capacitor voltage converters and a subtraction amplifier, which can change the sensing signal into voltage amplitude values. As a result, the total chip area is 1.6 × 2.0 mm2, and the sensing range is from 1 to 4 G. The simulated sensitivities of X, Y, and Z are 34.8, 20.0, and 13.7 mV/G, respectively.

Post micromachining of MPW based CMOS–MEMS comb resonator and its mechanical and thermal characterization

In recent years micro electro mechanical system (MEMS) based micro resonant sensors have been given a lot of attention due to their potential as a platform for the development of many novel physical, chemical, and biological sensors. That is why this paper covers post processing of the structures fabricated through Multi-Project-Wafer using 0.35 µm MIMOS CMOS technology with particular focus on dry etching of Si and SiO2 from the front side of CMOS---MEMS chip that is optimized using aluminum coated carrier wafer and achieved results are debris free as compared to photoresist coated carrier wafer. The device is etched through from the front side to avoid parasitic capacitances and squeeze film damping by keeping minimum size of the die. The etching of SiO2 as well as deep Si etch-through using the same plasma etcher (SS110A Tegal) is successfully demonstrated in this work. Finally, after the successful post CMOS micromachining of the device, resonance frequency i.e. 8164 Hz and quality factor i.e. 51.34, is determined. The joule heating effect due to the passing of current through the central shuttle of the device is characterized. The maximum temperature close to the anchors of the comb resonator where the piezoresistors are located is determined through temperature coefficient of resistance measurement using PE-4RF type probe station and it is found to be 37.62 °C.

A Vertically Coupled MEMS Resonator Pair for Oscillator Applications

This letter presents the design of a micromechanical oscillator based on a vertically coupled (VC) CMOS-microelectromechanical systems (MEMS) resonator pair for phase noise reduction. The prototyped resonator pair consists of two vibrating plates coupled with each other, thus providing enhanced power handling while keeping compact footprint. The proof-of-concept oscillator based on a 6.5-MHz, VC mode resonator achieves a phase noise of −97 dBc/Hz at 1-kHz offset and −118 dBc/Hz at 1-MHz offset from carrier with a resonator size of $30\times 30\mu \text{m}^{\mathrm {\mathbf {2}}}$ . Compared with its nonresonance-coupled saddle mode counterpart, the VC-mode-based oscillator features 10-dB phase noise reduction using the same oscillator circuit setup. The design concept of this letter can be extended to three dimensional (3-D) mechanically coupled resonator designs in the future. [2015-0053]

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.

Thermo-optical characterization of fluorescent rhodamine B based temperature-sensitive nanosensors using a CMOS MEMS micro-hotplate

A custom designed microelectromechanical systems (MEMS) micro-hotplate, capable of operating at high temperatures (up to 700°C), was used to thermo-optically characterize fluorescent temperature-sensitive nanosensors. The nanosensors, 550nm in diameter, are composed of temperature-sensitive rhodamine B (RhB) fluorophore which was conjugated to an inert silica sol–gel matrix. Temperature-sensitive nanosensors were dispersed and dried across the surface of the MEMS micro-hotplate, which was mounted in the slide holder of a fluorescence confocal microscope. Through electrical control of the MEMS micro-hotplate, temperature induced changes in fluorescence intensity of the nanosensors was measured over a wide temperature range. The fluorescence response of all nanosensors dispersed across the surface of the MEMS device was found to decrease in an exponential manner by 94%, when the temperature was increased from 25°C to 145°C. The fluorescence response of all dispersed nanosensors across the whole surface of the MEMS device and individual nanosensors, using line profile analysis, were not statistically different (p<0.05). The MEMS device used for this study could prove to be a reliable, low cost, low power and high temperature micro-hotplate for the thermo-optical characterisation of sub-micron sized particles. The temperature-sensitive nanosensors could find potential application in the measurement of temperature in biological and micro-electrical systems.

Study of Current Femto-Satellite Approches

The success of space technology evolves according to the technological progression in terms of density of CMOS integration (Complementary on - Silicon Metal) and MEMS (Micro-Electro-Mechanical System) [4]. The need of spatial services has been a significant growth due to several factors such as population increases, telecommunication applications, climate changes, earth control and observation military goals, and so on. To cover this, spatial vehicle generations, specific calculators and Femto-cell systems have been developed. More recently, Ultra - Small Satellites (USS) have been proposed and different approaches, concerning developing of these kind of spatial systems, have been presented in literature. This miniature satellite is capable to fly in the space and to provide different services such as imagery, measures and communications [4, 9, 10]. This paper deals with the study of two different USS Femto-satellite architectures that exist in literature in order to propose a future architecture that can provide an optimization of power supply consumption and ameliorate service communication quality.

A Three-Axis CMOS-MEMS Accelerometer Structure With Vertically Integrated Fully Differential Sensing Electrodes

This study presents a novel CMOS-microelectromechanical systems (MEMS) three-axis accelerometer design using Taiwan Semiconductor Manufacturing Company 0.18-μm one-poly-Si six-metal/dielectric CMOS process. The multilayer metal and dielectric stacking features of the CMOS process were exploited to vertically integrate the in-plane and out-of-plane capacitive sensing electrodes. Thus, the three-axis sensing electrodes can be integrated on a single proof mass to reduce the footprint of the accelerometer. Moreover, the fully differential gap-closing sensing electrodes among all three axes are implemented to increase the sensitivities and decrease the noise. The in-plane and out-of-plane sensing gaps are respectively defined by the minimum metal line width and the thickness of one metal layer by means of the metal wet-etching post-CMOS process. Thus, the capacitive sensitivities are further improved. The fully differential gap-closing sensing electrodes also bring the advantage of reduced cross talks between all three axes. As a result, the footprint of the presented three-axis accelerometer structure is only 400 × 400 μm2. Compared with existing commercial or CMOS-MEMS studies, the size is significantly reduced. The measured sensitivities (nonlinearities) are 14.7 mV/G (3.2%) for the X-axis, 15.4 mV/G (1.4%) for the Y-axis, and 14.6 mV/G (2.8%) for the Z-axis.

Development of a CMOS-Based Capacitive Tactile Sensor With Adjustable Sensing Range and Sensitivity Using Polymer Fill-In

This paper reports a capacitive-type CMOSmicroelectromechanical system tactile sensor containing a capacitance-sensing gap filled with polymer. Thus, the equivalent stiffness of the tactile sensor can be modulated by the polymer fill-in, so as to further tune its sensing range. Moreover, the polymer fill-in has a higher dielectric constant to increase the sensitivity of the tactile sensor. In short, the sensing range and sensitivity of the proposed tactile sensor can be easily changed by using the polymer fill-in. In application, the tactile sensor and sensing circuits have been designed and implemented using the 1) TSMC 0.35 μm 2P4M CMOS process and the 2) in-house post-CMOS releasing and polymer-filling processes. The polydimethylsiloxane (PDMS) material with different curing agent ratios has been exploited as the fill-in polymers. The experiment results demonstrate that the equivalent stiffness of tactile sensors can be adjusted from 16.85 to 124.43 kN/m. Thus, the sensitivity of the tactile sensor increases from 1.5 to 42.7 mV/mN by varying the PDMS filling. Moreover, the maximum sensing load is also improved.

Process Development for CMOS-MEMS Sensors With Robust Electrically Isolated Bulk Silicon Microstructures

This paper presents a deep reactive-ion etching (DRIE)-based post-CMOS micromachining process that provides robust electrically isolated single-crystal silicon (SCS) microstructures for integrated inertial sensors. Several process issues arise from previously reported three-axis CMOS microelectromechanical system (MEMS) accelerometers, including sidewall contaminations of SCS microstructures in plasma etch and a severe silicon undercut caused by overheating of suspended microstructures. Solutions to these issues have been found and are discussed in detail in this paper. In particular, a lumped-element model is developed to estimate the temperature rise on suspended microstructures in a silicon DRIE process. Based on the thermal modeling and experiments, a thick photoresist layer has been used as a thermal path to avoid the severe silicon undercut. The sidewall contamination problem is also eliminated using the modified CMOS-MEMS process. A three-axis accelerometer with a low-noise, low-power on-chip amplifier has been successfully fabricated using the new process. Footing effect was observed on the backside of the sensor microstructure, but it has little effect on the structural integrity and sensitivity of the sensor.

Integrated CMOS-MEMS with on-chip readout electronics for high-frequency applications

A bridge-shaped first-lateral-mode 60-MHz mechanical resonator, which is monolithically integrated with capacitive CMOS readout electronics, is presented. The resonator is fabricated directly on a commercial CMOS technology using the top metal level as a structural layer. A maskless single-step wet-etching process for mechanical structure release after the standard CMOS integration process is the only postfabrication requirement. Electrical characterization of the electromechanical device demonstrates the feasibility of implementing a CMOS-microelectromechanical system for high-frequency applications using a standard conventional CMOS technology.

MEMS digital-to-acoustic transducer with error cancellation

An acoustic transducer comprising a substrate; and a diaphragm formed by depositing a micromachined membrane onto the substrate. The diaphragm is formed as a single silicon chip using a CMOS MEMS (microelectromechanical systems) semiconductor fabrication process. The curling of the diaphragm during fabrication is reduced by depositing the micromachined membrane for the diaphragm in a serpentine-spring configuration with alternating longer and shorter arms. As a microspeaker, the acoustic transducer of the present invention converts a digital audio input signal directly into a sound wave, resulting in a very high quality sound reproduction at a lower cost of production in comparison to conventional acoustic transducers. The micromachined diaphragm may also be used in microphone applications.