V. Seena

This manuscript presents the design, process integration, fabrication, and characterization of an out-of-plane suspended gate field-effect transistor (SGFET)-based parylene-C MEMS accelerometer. A comprehensive analysis and simulation were conducted in semiconductor, mechanical, electromechanical, and circuit domains. The multidisciplinary analysis was aimed to optimize key specifications of the SGFET and MEMS structures, including substrate doping, pull-in voltage and range, supply voltage, parylene-C polymer thickness, load resistance, and other critical parameters. The parylene-C polymer is used as the MEMS structural material, providing low Young’s modulus, better thermal stability, and chemical inertness. Surface micromachining process is developed to realize parylene MEMS suspended gate of SGFET. Plasma doping process was adopted to realize shallow source and drain junction at low doping energy. The threshold voltage ( V TH ) of SGFET with 1 µm air-gap ( t air ) was 1.7 V and the effective charge density ( Q eff ) was 1.26 × 10⁻⁸ C cm⁻². The laser Doppler vibrometer and nanoindenter based experiments were carried out on the fabricated accelerometer shows a resonant frequency of 7.2 kHz and a stiffness of 6 N m⁻¹, respectively. A common source amplifier with SGFET realized on a printed circuit board followed by mounting the complete sensor on a custom fixture for a tilting experiment to observe the response of the accelerometer in the dynamic range of ±1 g. The sensitivity and the non-linearity at the output were 4.67 mV g⁻¹ and ∼5%, respectively.

In this letter, we present a comprehensive study on the design, simulation, and modeling of nano-Newton (nN) force sensor using 2-D molybdnem disulphide (MoS$_{2}$)-based suspended body dual-gate field-effect transistor (2D SB-DG-FET) with integrated piezoresistor. The sensor uses the hybrid transduction scheme involving suspended body (SB) FET and piezoresistive load resistors in common source amplifier (CSA) configuration. The sensor consist of a MoS$_{2}$-based FET integrated on a suspended microelectromechanical systems (MEMS) structure with MoS$_{2}$ piezoresistors acting as a load. The choice of MoS$_{2}$ allows the use of same functional material as both FET channel and piezoresitive load. During force sensing, MEMS structure ensures the constant gate capacitance change leading to an output current change of the SB-DG-FET. Simultaneously the applied force also causes resistance change in the piezoresistors. COMSOL Multiphysics 6.0 and CoventorWare MP 10.3 have been used for the design and simulation of the MEMS structure. The design and simulation of the 2D SB-DG-FET and its application in CSA configuration with piezoresistive load have been carried out in COMSOL using a lookup table. The CSA exhibits the linear response with output sensitivity 1.15 $\upmu \text{V}/\text{nN}$ and maximum detection range upto 2 $\upmu \text{N}$. This letter demonstrates the advantage of this hybrid transduction scheme due to the response of SB-FET and piezoresistor in CSA circuit.

This paper reports the development and characterization of a piezoresistive silicon MEMS Nanomechanical Membrane-Flexure (NMF) sensor and its application for the detection of hydrogen gas. The electromechanical transduction of the NMF sensor is achieved with an integrated Indium Tin Oxide (ITO) thin film piezoresistor with a high negative gauge factor of -430. The sensors are fabricated using a standard SOI-based silicon MEMS process, facilitating batch production. A novel high-resolution hybrid optical-electromechanical method was used for the strain sensing study of the NMF sensor with in-situ optical surface profiling for curvature measurement through white light interferometry. The optimized sensor design showed a deflection sensitivity of 2.76 ppm[nm] ⁻¹ . This work also reports the ability of the NMF sensor with signal conditioning circuit in detecting hydrogen gas in parts-per-million (ppm) concentration with good sensor response of 520 mV for 100 ppm and quick recovery using low background flow cell temperature supporting their suitability for reusable biochemical sensing applications demanding low detection limits.

This work reports the design, modelling, and simulation of a novel polymer MEMS gas sensor platform called a ring–flexure–membrane (RFM) suspended gate field effect transistor (SGFET). The sensor consists of a suspended polymer (SU-8) MEMS based RFM structure holding the gate of the SGFET with the gas sensing layer on top of the outer ring. During gas adsorption, the polymer ring–flexure–membrane architecture ensures a constant gate capacitance change throughout the gate area of the SGFET. This leads to efficient transduction of the gas adsorption-induced nanomechanical motion input to the change in the output current of the SGFET, thus improving the sensitivity. The sensor performance has been evaluated for sensing hydrogen gas using the finite element method (FEM) and TCAD simulation tools. The MEMS design and simulation of the RFM structure is carried out using CoventorWare 10.3, and the design, modelling, and simulation of the SGFET array is carried out using the Synopsis Sentaurus TCAD. A differential amplifier circuit using RFM-SGFET is designed and simulated in Cadence Virtuoso using the lookup table (LUT) of the RFM-SGFET. The differential amplifier exhibits a sensitivity of 2.8 mV/MPa for a gate bias of 3 V and a maximum detection range of up to 1% hydrogen gas concentration. This work also presents a detailed fabrication process integration plan to realize the RFM-SGFET sensor using a tailored self-aligned CMOS process adopting the surface micromachining process.

This article proposes a single-axis, highperformance Complementary Metal Oxide Semiconductor- Microelectromechanical System (CMOS-MEMS) closed-loop accelerometer, implemented using in-plane movable suspended gate-field-effect transistor (IP-SGFET). The IPSGFET’s gate is embedded within a movable proof mass supported by flexible folded beams. The external acceleration displaces the proofmass, which changes the drain current of IP-SGFET. The change in current is read out with the help of a simple differential amplifier which is constructed with the IP-SGFET. The differential amplifier’s output voltage is used througha closed-loopintegral controland a high-voltage dynamically reconfigurable charge pump, which generates a high-voltage differential drive of up to 10.5 ± 5.12 V, to drive an actuator that brings the gate back to its resting position. The actuator used is a comb-drive type electromechanical actuator. In this work, two tools, viz., MEMS+ and Technology Computer Aided Design (TCAD) device simulation tools are used in a sequential manner to handle the electrostatics coupled with the micro-mechanical domain present in the suspended gate field effect transistors (SGFET). The results obtained are then used as look-up-tables for the design and simulation of the entire system, including the control circuits, in SPECTRE. Appropriate design of the MEMS suspended structure, the field effect transistor (FET) device, and interface circuit results in a open-loop sensitivity of 927 mV/g with a near-zero cross-axis sensitivity. Using the actuator and the reconfigurable charge pump, closed-loop operation is realized with a dynamic range of ±4 g at a supply voltage of 3.3 V.

Introduction of structural modification and novel transduction materials to nanomechanical cantilever (NMC) sensor can bring considerable improvement in sensor performance leading to ultra-sensitive nanomechanical sensor platforms. This work reports development of an optimized and highly sensitive silicon MEMS Nanomechanical Membrane-Flexure (NMF) piezoresistive surface stress sensor. This MEMS structure consists of a circular adsorbate membrane suspended by four inverse trapezoidal flexures. The electromechanical transduction of the sensor is performed by integrating a high gauge factor low temperature sputter deposited Indium Tin Oxide (ITO) thin film as piezoresistor. The ITO thin film was experimentally characterized for extracting its electrical, mechanical, and electromechanical properties to assess its candidature in integrating as strain sensing element with nanomechanical sensors. The gauge factor of ITO thin film was measured using a high precision four-point bending fixture experimental setup. An ultra-thin ITO film deposited at room temperature with no oxygen flow and post annealing treatments exhibited a high negative gauge factor of −430, making it an excellent candidate in nanomechanical sensor platform. The incorporation of sputter deposited ITO thin film as piezoresistive layer obviated the need for doping in silicon which further reduced the fabrication process complexity for NMF sensor. The NMF sensor fabrication following SOI based silicon MEMS process along with device characterization have also been carried out as part of this work. The optimized design of piezoresistive NMF sensor exhibited an improved surface stress sensitivity of nearly 15 times higher than conventional cantilever sensor and thereby opening new possibilities in bio-chemical and environmental sensing applications. [2021-0127]

This article presents a novel stepped suspended gate field-effect transistor (SSGFET) array-based ${z}$ -axis accelerometer with an enhanced detection range. The stepped gate electrode structure of SGFET aids in extending the stable driving range beyond 33.33% of the initial air gap. The stable driving range is extended to 50% of the initial air gap with ~90% increase in the pull-in voltage. Mechanical, electrical, and electromechanical analytical models are developed. These models are validated through microelectromechanical systems (MEMS) simulations using CoventorMP and transistor simulations in Synopsys TCAD. An SSGFET-based common source (CS) amplifier with diode-connected p-MOSFET load is designed and simulated in Cadence Virtuoso using the lookup table (LUT) approach. The $z$ -axis accelerometer exhibits a sensitivity of 38 (mV/g) with a supply voltage of 3.3 V for a dynamic range (DR) of ±6 g with the nonlinearity of about 5.3% comparedtoSGFETswith a planar gate electrode,whichcan detect up to ±4 g with the same sensitivity. The 3-dB bandwidth of the accelerometer is 412 Hz with a noise-limited resolution of 109.31 $\mu$ g/(Hz) ^1/2 . This article also presents a detailed analysis of the relation between the number of gate fragments, the pull-in voltage, stable driving range, and the DR along with a feasible fabrication integration plan.

MEMS inertial sensors are used in handheld smart devices such as smartphones and smart watches to detect and monitor physical activities. The most common transduction techniques used are based on piezoelectric [1], piezoresistive [2] or capacitive [3] techniques. The performance of these sensors is limited by passive detection, the need of a functional material, area of electrode, converter and amplifier circuits in the front end of read-out network. Suspended gate FET (SGFET) based sensors are being investigated as an alternative type of inertial sensor that offers in-built amplification as well as eliminates the aforementioned limitations. The suspended gate SGFET is an active transducer and has the potential for monolithic integration with CMOS.

There have always been demands for the development of microscale sensors with integrated miniature electronics circuitry. Requirement of external amplification for the conventional MEMS accelerometer poses limitation on scaling, design, fabrication and on-chip Integrated Circuit (IC) compatibility. These limitations could be surpassed by the implementation of integrated CMOS-MEMS architectures for sensors and actuators. Here a novel U-channel suspended gate silicon on insulator field effect transistor (USG-SOIFET) is proposed to circumvent the pseudo short channel effects (P-SCE) present in the suspended gate field effect transistor (SGFET). In the proposed USG-SOIFET, the source and drain are separated by an air-gap and thereby ensuring effective screening of drain electric field penetration into source region. The gate length (L) of USG-SOIFET is 4 times lower than the conventional planar channel SGFET for the same performance of the device. Since the same performance is achieved with a smaller length, this results in a smaller device. Finite Element Modeling (FEM) of USG-SOIFET z-axis accelerometer using CoventerMP1.3 and TCAD process, device modeling and simulation of the USG-SOIFET using Synopsys Sentaurus are presented here. The sensitivity of the accelerometer is 4.185 μA/g with a non-linearity of 4.96% for ± 5 g detection range. The bandwidth of the accelerometer is 100 Hz and the cross-axis sensitivity is found to be 3.52%. A fabrication process integration scheme for realizing USG-SOIFET has also been discussed along with process simulation to demonstrate the feasibility of realizing this new device architecture. This device platform is a potential candidate for monolithic integration with CMOS.