Recent development and futuristic applications of MEMS based piezoelectric microphones

This paper presents a comprehensive literature survey of MEMS based piezoelectric microphones along with the fabrication processes involved, application domains, and methodologies used for experimentations. Advantages and limitations of existing microphones are presented with the impact of process parameters during the thin film growth. This review identifies the issues faced by the microphone technologies spanning from the invention of microphones to the most recent state-of-the-art solutions implemented to overcome or address them. A detailed comparison of performance in terms of sensitivity and dynamic range is presented here that can be used to decide the piezoelectric material and process to be used to develop sensors based on the bandwidth requirement. Electrical and mechanical properties of different piezoelectric materials such as AlN, ZnO, quartz, PZT, PVDF, and other polymers that has great potential to be used as the sensing membrane in development and deployment of these microphones are presented along with the complications faced during the fabrication. Insights on the future of these sensors and emerging application domains are also discussed

Optimization and fabrication of MEMS based piezoelectric acoustic sensor for wide frequency range and high SPL acoustic application

This paper reports finite element model (FEM) simulation and fabrication of a square shaped diaphragm along with microtunnel for MEMS acoustic sensor which can be used for measurement of wide operational frequency range and high sound pressure level (SPL) 100 dB–180 dB measurement in launching vehicle and aircraft. The structure consists of a piezoelectric ZnO layer sandwiched between two aluminum electrodes on a thin silicon diaphragm. There is a microtunnel in the structure which relates the cavity to the atmosphere for pressure compensation. The microtunnel decides the lower cut-off frequency of device. Analytical and simulation approaches are used to optimize microtunnel dimension and simulation approach for diaphragm structure optimization. The change in displacement, stress, sensitivity and resonance frequency due to different diaphragm sizes with diaphragm thickness variation is also analyzed. The optimized diaphragm structure of 1750 × 1750 μm2 and microtunnel of 100 μm wide and 24 μm deep have been fabricated using bulk micromachining technique. The fabricated device response has been tested using LDV and sensitivity measurement system

Design and Development of an Array of Dielectric Suspended Membranes for Microhotplate Applications

The paper presents the design, fabrication and characterization of an array of suspended dielectric suspended membranes for microhotplate applications. A single cell membrane (100 µm ´ 100 µm) made of two different dielectric layers: SiO2 and Si3N4 separately, was designed and simulated using ANSYS 10.0. The simulation of stress generated in different dielectric membranes as a function of temperature is reported. The thickness of both layers was taken as 0.3 µm. The membranes of both SiO2 and Si3N4 dielectrics were fabricated on silicon substrate by bulk micromachining technique using TMAH solution. The buckling of the beam and breakage of membranes made of high-stress Si3N4 film are reported. The simulated results were verified by experiments. The membrane made of SiO2 layer was found to be more suitable in comparison to high-stress Si3N4 layer for microhotplate applications. The present approach provides high yield at low cost for fabrication of microhotplates for gas sensing applications

Design and Optimization of MEMS based AlN sensor for Acoustic Application

369-375The market for MEMS sensors based on Aluminum Nitride (AlN) is developing because of AlN material's capacity to produce CMOS-compatible, highly reliable, and self-powered devices. Utilizing the COMSOL software tool, the sensors parameters are designed and optimized in accordance with the dimension and thickness of AlN thin film layer. The proposed design technique is applicable to any piezoelectric diaphragm-based acoustic sensors, regardless of the cavity and hole structures in the silicon or SOI (silicon on insulator) based substrate. The diaphragm consists fixed 25 μm Si layer and variable (0.5 μm to 2.5 μm) Al/AlN/Al layer. The AlN layer is sandwiched between top and bottom Aluminum electrodes of thickness 0.3 μm. The diaphragm area is varying from 1.75 mm x 1.75 mm to 3.5 mm x 3.5 mm. Prior to engaging in expensive fabrication methods, this work optimizes the AlN layer with regard to resonance frequency, deflection at the diaphragm's center, and sensor response. The simulated results demonstrate the trade-off between the diaphragm deflection at the center and a workable frequency range in accordance with the design parameters that were specified. For a frequency range of 0.5 kHz to 18 kHz, the device's optimal design has a simulated sensitivity of 2.5 μV/Pa and at resonance the sensitivity is 200 μV/Pa

Design and Development of Polysilicon-based Microhotplate for Gas Sensing Application

The paper presents the design and development of a polysilicon-based microhotplate (MHP) on a SiO2 membrane formed by bulk micromachining in orientation P-type silicon. The chip comprises four microheater cells, which can be used separately or in series combination. The chip size is 2.1 × 2.1 sq. mm. The design and simulation of a single-cell microhotplate is carried out using ANSYS. The complete fabrication process is described in this paper. The temperature coefficient of resistance (TCR) of polysilicon resistors of values 5.7 kW and 3.36 kW has been measured as 0.69 × 10-3 and 0.5 × 10-3 per °C respectively. These values are used to estimate the temperature of the polysilicon heater by measuring the change in resistance value of the resistor on applying a voltage to it. Temperatures up to 367 °C have been calculated at low bias voltages. As the sensitivity of the gas sensing film is temperature dependent, the developed hotplate will be used as a platform for fabricating the gas sensors

Design and Simulation of Double-spiral Shape Micro-heater for Gas Sensing Applications

The paper presents the design and simulation of double spiral shape micro-heater using ANSYS 10.0 and MATLAB, which requires 12.5 mW-78.3 mW powers to create the temperature 181 °C-1002 °C for gas sensing applications. The results obtained from ANSYS simulation were verified using MATLAB Tool. A platinum-based bulk micro-machined hotplate of size 500 mm × 500 mm has been designed for fabrication as a multi-layer structure on a silicon substrate with thermal silicon dioxide as the supporting membrane, followed by LPCVD (Low pressure chemical vapor deposition) silicon nitride film. Gas sensing film (SnO2) will be deposited on the interdigitated Pt electrodes formed on the PECVD oxide layer. The temperature uniformity of microhotplate (as it is essential for better sensing mechanism) based on double spiral heater has been reported in this paper. To estimate the resistance of the Pt heater, a 2000 Aº thick platinum film has been deposited by sputtering on silicon and its sheet resistance has been measured as 2.5 Ohm/□. We have used this value to calculate the resistance of Pt resistor, which was found 319 Ohm

Design and electro-thermal simulation of a polysilicon microheater on a suspended membrane for use in gas sensing

332-335A microhotplate array comprising four 100 μm × 100 μm unit cells has been designed. It has a multi-layer structure: Si-thermal oxide-polySi-PECVD (plasma enhanced chemical vapour deposition) oxide-Sputtered Pt-Gas sensing film (SnO₂). Electro-thermal simulation of the unit cell has been carried out using ANSYS. The simulations show that for an applied bias of 4 V a mean temperature of 706ºC is obtained at the centre of the heater area. The heat is assumed to flow from the hot plate to the surrounding air at an ambient temperature of 30ºC by conduction. The power dissipation is less than 50 mW. The paper presents the design methodology of the hotplate

MEMS-based piezoresistive and capacitive microphones: A review on materials and methods

Microelectromechanical systems (MEMS)-based piezoresistive and capacitive microphones have gained significant attention due to their miniaturization, high performance, and diverse applications. This review paper provides a comprehensive overview of the materials and methods employed in these microphone technologies. We discussed various transduction mechanisms, including electrostatic, piezoresistive, and piezoelectric, along with their working principles and advantages. Additionally, we explored the utilization of surface acoustic wave (SAW) and bulk acoustic wave (BAW) resonators in microphone design. Performance characteristics such as sensitivity, noise floor, linearity, dynamic range, and bandwidth are analyzed, highlighting the key factors influencing microphone performance. Furthermore, we delve into the application areas of these microphones, ranging from aircraft design and satellite launching to biomedical fields and audio engineering. Lastly, we discuss the materials used for MEMS microphones, focusing on substrate materials, etchant materials, and the specific requirements for piezoresistive and capacitive materials based microphones. Further, this review paper explores the emerging trends of graphene-based microphones, MEMS/NEMS hybrid devices, and the integration of artificial intelligence and signal processing techniques, as well as the potential applications in biomedical and healthcare fields, providing a comprehensive overview of the materials and methods employed in MEMS-based capacitive and piezoresistive microphones