Microelectromechanical Systems Capacitive Sensor Research Articles

Design and Modeling of a New MEMS Capacitive Microcantilever Sensor for Gas Flow Monitoring

A new capacitive microelectromechanical system (MEMS) microcantilever gas flow sensor is introduced for application in industrial gas flow measurement pipelines. The chip encompasses microcantilevers with different lengths (50, 100, 250, and 400 μm) and the same thickness and wideness of 2 and 50 μm, respectively. Besides MEMS sensor development, we have designed a customized bypass channel based on the orifice principle for the placement of the MEMS sensor chip. Accordingly, a minibypass has been designed, which alleviates the harsh flow condition on the sensor that will be placed in the flowmeter housing. Simulations and the experimental study revealed that the sensor is capable of measuring airflows from 0 to 25 m/s with a sensitivity of 2 x 10⁻⁴ pF/m/s, and this can be expanded to other gas flow measurements with a certain bandwidth. This measurement allows us to apply this sensor for measuring moderate flows up to ~200 m/s in ~10-cm diameter pipes based on the current design for bypass. According to experimental results, the sensor output capacitance varied from 3.3445 to 3.350 pF for a range of airflow between 0 and 25 m/s. We have shown that capacitive microcantilever MEMS flow sensors could be used for flow measurements in heavy-load industrial applications.

Dynamic Analysis of Viscoelastic Circular Diaphragm of a MEMS Capacitive Pressure Sensor using Modified Differential Transformation Method

In this paper, a dynamic analysis of the viscoelastic circular diaphragm of a Micro-Electro-Mechanical System (MEMS) capacitive pressure sensor using the Modified Differential Transformation Method (MDTM) is presented. The MEMS technology has been increasingly used to fabricate sensors and actuators and the MEMS capacitive pressure sensor is emerging in many high-performance applications. The deflection of these sensors diaphragm (plate) depends largely on the material of the diaphragm. In this study, a circular diaphragm of viscoelastic material is modeled using the classical plate theory. The governing differential equation is solved using Modified Differential Transformation Method (MDTM) and the result is validated with Finite Difference Method (FDM). The result shows excellent agreement with the numerical method. The effects of amplitudes, frequency, viscoelastic parameter, and time of applied pressure on deflection of the viscoelastic circular diaphragm are investigated. It is established from the results that the deflection of the sensor increases with an increase in the amplitude, frequency and time of the applied pressure. In addition, an increase in the viscoelastic parameters resulted in an increase the deflection of the diaphragm which consequently increases the capacitance and sensitivity of the sensor. Hence, the viscoelastic circular diaphragm of the MEMS capacitive pressure sensor exhibits better sensitivity performance when compared with that of elastic material. Finally, the Modified Differential Transformation Method applied in obtaining the solution of the developed model is effective in predicting sensor characteristics. The study will enhance the design of MEMS capacitive pressure sensor with viscoelastic circular diaphragm.

Thermal Drift Investigation of an SOI-Based MEMS Capacitive Sensor with an Asymmetric Structure.

High-precision, low-temperature-sensitive microelectromechanical system (MEMS) capacitive accelerometers are widely used in aerospace, automotive, and navigation systems. An analytical study of the temperature drift of bias (TDB) and temperature drift of scale factor (TDSF) for an asymmetric comb capacitive accelerometer is presented in this paper. A five-layer model is established for the equivalent expansion ratio in the TDB and TDSF formulas, and the results calculated by the weighted average of thickness and elasticity modulus method are closest to the results of the numerical simulation. The analytical formulas of TDB and TDSF for an asymmetric structure are obtained. For an asymmetric structure, TDB is only related to thermal deformation and fabrication error. Additionally, half of the fixed electrode distance is not included in the expressions of and for asymmetric structures, thus resulting in the TDSF of the asymmetric structure being smaller compared to a symmetric structure with the same structural parameters. The TDSF of the symmetric structure is [−200.2 ppm/°C, −261.6 ppm/°C], while the results of the asymmetric structure are [−11.004 ppm/°C, −72.404 ppm/°C] under the same set of parameters. The parameters of the optimal asymmetric structure are obtained for fabrication guidance using numerical methods. In the experiment, the TDSF and TDB of the packaged structure and the non-packaged structure are compared, and a significant effect of the package on the signal output is found. The TDB is reduced from 3000 to 60 μg/°C, while the TDSF is reduced from 3000 to 140 ppm/°C.

High sensitivity MEMS capacitive hydrogen sensor with inverted T-shaped electrode and ring-shaped palladium alloy for fast response and low power consumption

We report a novel palladium (Pd)-based micro electro-mechanical system (MEMS) capacitive hydrogen gas sensor that has an inverted T-shaped electrode and a ring-shaped Pd alloy layer, which enable high sensitivity and low power consumption. Thanks to these structures, deformation of the membrane as a result of hydrogen absorption can be efficiently transduced to capacitance change. The capacitance change of the proposed sensor is found to be three times larger than that of the conventional structure. Prototype sensors were fabricated by a CMOS-compatible surface micromachining process. The proposed sensor offers a broad design window for attaining high sensitivity. Finally, we show that our sensor provides fast response, is hysteresis-free, and has excellent hydrogen selectivity characteristics despite not employing a heater by using PdCuSi metallic glass.

(Invited) A 1-mG MEMS Sensor

This paper presents a 1-mG MEMS (microelectromechanical systems) capacitive inertial sensor developed by multi-layer metal technology. The sensor is designed to detect acceleration below 1 mG. A proof-of-concept device is fabricated by using a post-CMOS gold electroplating process. Measurement results suggest that the actual Brownian noise (BN ) is estimated to be lower than the target BN of 1 μG/Hz1/2. Acceleration responses are also experimentally evaluated, which shows the potential of sensing acceleration below 1 mG. Those results reveal that the proposed sensor design could be a promising way to improve the sensitivity and the sensing range of integrated CMOS-MEMS accelerometers

Systematic Development of Integrated Capacitance Measurement System With Sensitivity Tuning

Modern sensor systems integrate microelectromechanical system (MEMS)-based sensors with signal conditioning circuits. MEMS sensors are fabricated through micromachining technology, which is relatively new compared with the integrated circuit technology, used since last few decades to fabricate circuits. As a result, these microsensors experience significant variation after fabrication. Besides, packaging of the sensors and integration with the circuit are also nontrivial as the sensors are of micrometer dimensions and they may have movable structures. In this paper, we have developed an integrated capacitance measurement system with a fully on-chip signal conditioning circuit. A systematic method of integration and testing with a silicon-on-insulator (SOI) MEMS capacitive accelerometer sensor is also presented. The signal conditioning circuit incorporates an array of on-chip capacitors to nullify the sensor capacitance mismatch and a tunable square-wave generator to tune the sensitivity of the system to cope up with the variations after fabrication of the devices. The complete circuit is designed and fabricated in United Microelectronics Corporation 0.18- $\mu \text{m}$ CMOS process technology. This circuit is integrated with an SOI MEMS sensor structure and various test methods are discussed. The integrated system is then mounted on a vibrating shaker along with a reference accelerometer and the measurement results are provided.

A dual-axis MEMS capacitive inertial sensor with high-density proof mass

This paper reports a novel dual-axis microelectromechanical systems (MEMS) capacitive inertial sensor that utilizes multi-layered electroplated gold. All the MEMS structures are made by gold electroplating that is used as a post complementary metal-oxide semiconductor (CMOS) process. Due to the high density of gold, the Brownian noise on the proof mass becomes lower than those made of other materials such as silicon in the same size. The single gold proof mass works as a dual-axis sensing electrode by utilizing both out-of-plane (Z axis) and in-plane (X axis) motions; the proof mass has been designed to be 660 μm × 660 μm in area with the thickness of 12 μm, and the actual Brownian noise in the proof mass has been measured to be 1.2 $${\upmu}{\text{G/}}\sqrt {\text{Hz}}$$μG/Hz (in Z axis) and 0.29 $${\upmu}{\text{G/}}\sqrt {\text{Hz}}$$μG/Hz (in X axis) at room temperature, where 1 G = 9.8 m/s2. The miniaturized dual-axis MEMS accelerometer can be implemented in integrated CMOS-MEMS accelerometers to detect a broad range of acceleration with sub-1G resolution on a single sensor chip.

Pick-and-place process for sensitivity improvement of the capacitive type CMOS MEMS 2-axis tilt sensor

This study exploits the foundry available complimentary metal-oxide-semiconductor (CMOS) process and the packaging house available pick-and-place technology to implement a capacitive type micromachined 2-axis tilt sensor. The suspended micro mechanical structures such as the spring, stage and sensing electrodes are fabricated using the CMOS microelectromechanical systems (MEMS) processes. A bulk block is assembled onto the suspended stage by pick-and-place technology to increase the proof-mass of the tilt sensor. The low temperature UV-glue dispensing and curing processes are employed to bond the block onto the stage. Thus, the sensitivity of the CMOS MEMS capacitive type 2-axis tilt sensor is significantly improved. In application, this study successfully demonstrates the bonding of a bulk solder ball of 100 µm in diameter with a 2-axis tilt sensor fabricated using the standard TSMC 0.35 µm 2P4M CMOS process. Measurements show the sensitivities of the 2-axis tilt sensor are increased for 2.06-fold (x-axis) and 1.78-fold (y-axis) after adding the solder ball. Note that the sensitivity can be further improved by reducing the parasitic capacitance and the mismatch of sensing electrodes caused by the solder ball.

Simulation Study of Sensitivity Performance of MEMS Capacitive Bending Strain Sensor for Spinal Fusion Monitoring

This study evaluates the sensitivity of microelectromechanical system (MEMS) capacitive bending strain sensor with a double layer cantilever designed to meet the requirements of spinal fusion monitoring. The cantilever structure of the sensor consists of two parallel substrate plates which constitute the electrodes, attached to an anchor made of silicon dioxide. The sensor was able to monitor bending strain value ranging from 0 to 1000 με. In order to evaluate the sensitivity of the sensor, parametric study was carried out by varying electrode gap, anchor length, and dielectric coverage between the electrodes. The nominal capacitive strain sensor for various applications has sensitivity ranging from 255 aF/μεto 0.0225 pF/με. An increase in the sensitivity was observed on reducing the electrode gap and the anchor length and increasing the dielectric coverage, resulting in a highest sensitivity value of 0.2513 pF/με. It was also observed that dielectric constant has a significant effect on the sensitivity behavior of the sensor.

Nanoprecision MEMS Capacitive Sensor for Linear and Rotational Positioning

This paper presents a microelectromechanical systems (MEMS) capacitive position sensor for nanopositioning applications in Probe storage systems. The objective of the sensor system design is to develop a high-precision <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> linear and rotational position sensor with a minimum sensor area and a large range of movements at high speed. To achieve this, first, a simple sensor noise model scalable with a sensor area was developed, in which all the parasitic capacitances are taken into account. Furthermore, a signal-processing solution was developed to compensate for the nonlinearities caused by rotational disturbances and, at the same time, to generate a rotational position signal for active rotation-control purposes. A MEMS capacitive sensor prototype was constructed with the design of a 13-mum pitch, a 300-mum peak-to-peak linear stroke, and a 3.46-mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> sensor area at a 3-mum gap. The measured sensor noise was 0.2 nm 1sigma, which corresponds to 12 mudeg 1sigma for the fabricated prototype sensor, at 25-kHz bandwidth. Furthermore, the signal linearity was significantly enhanced by the proposed sensor signal processing, with a measured sensor signal nonlinearity of 0.78% for an 80-mum peak-to-peak stroke at 200 Hz. Finally, the capacitive sensor-based dynamic closed-loop <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</i> linear and rotational position control of an electromagnetic scanner was successfully demonstrated.