With the fast development of the fifth-generation cellular network technology (5G), the future sensors and microelectromechanical systems (MEMS)/nanoelectromechanical systems (NEMS) are presenting a more and more critical role to provide information in our daily life. This review paper introduces the development trends and perspectives of the future sensors and MEMS/NEMS. Starting from the issues of the MEMS fabrication, we introduced typical MEMS sensors for their applications in the Internet of Things (IoTs), such as MEMS physical sensor, MEMS acoustic sensor, and MEMS gas sensor. Toward the trends in intelligence and less power consumption, MEMS components including MEMS/NEMS switch, piezoelectric micromachined ultrasonic transducer (PMUT), and MEMS energy harvesting were investigated to assist the future sensors, such as event-based or almost zero-power. Furthermore, MEMS rigid substrate toward NEMS flexible-based for flexibility and interface was discussed as another important development trend for next-generation wearable or multi-functional sensors. Around the issues about the big data and human-machine realization for human beings’ manipulation, artificial intelligence (AI) and virtual reality (VR) technologies were finally realized using sensor nodes and its wave identification as future trends for various scenarios.

Development Trends and Perspectives of Future Sensors and MEMS/NEMS.

Energy harvesting is the most effective way to respond to the energy shortage and to produce sustainable power sources from the surrounding environment. The energy harvesting technology enables scavenging electrical energy from wasted energy sources, which always exist everywhere, such as in heat, fluids, vibrations, etc. In particular, piezoelectric energy harvesting, which uses a direct energy conversion from vibrations and mechanical deformation to the electrical energy, is a promising technique to supply power sources in unattended electronic devices, wireless sensor nodes, micro-electronic devices, etc., since it has higher energy conversion efficiency and a simple structure. Up to now, various technologies, such as advanced materials, micro- and macro-mechanics, and electric circuit design, have been investigated and emerged to improve performance and conversion efficiency of the piezoelectric energy harvesters. In this paper, we focus on recent progress of piezoelectric energy harvesting technologies based on PbZrxTi1-xO3 (PZT) materials, which have the most outstanding piezoelectric properties. The advanced piezoelectric energy harvesting technologies included materials, fabrications, unique designs, and properties are introduced to understand current technical levels and suggest the future directions of piezoelectric energy harvesting.

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Recent Progress on PZT Based Piezoelectric Energy Harvesting Technologies

This paper discusses piezoelectric acoustic devices based on widely used piezoelectric materials. Commonly used piezoelectric thin film deposition techniques and the influence of process parameters on the growth, texture and orientation of the films are discussed. Etching techniques are also outlined. A comparative study of different devices developed previously is given. Also, applications of developed devices in aero-acoustic and medical fields have been briefly discussed. Flow charts of various techniques of deposition along with a combined one for full acoustic device fabrication are given. Various techniques, frequently used for thin film characterization have been discussed. The testing and measurement techniques to determine the responses of acoustic devices such as sensitivity, resonance frequency, frequency response, piezoelectric co-efficient etc. have been briefly illustrated. This paper discusses common failure modes with respect to the field of use of acoustic devices. It also concisely discusses various reliability tests done in industries to assess the quality of the developed devices. This review has also suggested directions for future development of thin film acoustic sensors.

Piezoelectric MEMS based acoustic sensors: A review

This paper describes the development of aluminum nitride (AlN) resonant accelerometers that can be integrated directly over foundry CMOS circuitry. Acceleration is measured by a change in resonant frequency of AlN double-ended tuning-fork (DETF) resonators. The DETF resonators and an attached proof mass are composed of a 1-mum-thick piezoelectric AlN layer. Utilizing piezoelectric coupling for the resonator drive and sense, DETFs at 890 kHz have been realized with quality factors (Q) of 5090 and a maximum power handling of 1 muW. The linear drive of the piezoelectric coupling reduces upconversion of 1/f amplifier noise into 1/f 3 phase noise close to the oscillator carrier. This results in lower oscillator phase noise, -96 dBc/Hz at 100-Hz offset from the carrier, and improved sensor resolution when the DETF resonators are oscillated by the readout electronics. Attached to a 110-ng proof mass, the accelerometer microsystem has a measured sensitivity of 3.4 Hz/G and a resolution of 0.9 mG/radicHz from 10 to 200 Hz, where the accelerometer bandwidth is limited by the measurement setup. Theoretical calculations predict an upper limit on the accelerometer bandwidth of 1.4 kHz.

Post-CMOS Compatible Aluminum Nitride Resonant MEMS Accelerometers.

Energy harvesting is one of the most promising research areas to produce sustainable power sources from the ambient environment. Which found applications to attain the extensive lifetime self-power...

Recent advances in flexible PVDF based piezoelectric polymer devices for energy harvesting applications

Taking advantage of good hermeticity, tiny parasitic capacitance, batch mode fabrication, and compatibility with multiple bonding techniques, the glass-silicon composite substrate manufactured by the glass reflow process has great potential to achieve 3D wafer-level packaging for high performance. However, the difference in etching characteristics between silicon and glass inevitably leads to the formation of the undesired micro-protrusions near the silicon-glass interface when preparing a shallow cavity etched around a few microns in the composite substrate. The micro-protrusions have a comparable height with the depth of the cavity, which increases the risks of damages to sensitive structures and may even trigger electrical breakdown, resulting in thorough device failure. In this paper, we studied the characteristics of the chemical composition and etching mechanisms at the interface carefully and proposed the corresponding optimized solutions that utilized plasma accumulation at the interface to accelerate etching and bridge the gap in etching rates between different chemical compositions. Finally, a smooth transition of 131.1 nm was achieved at the interface, obtaining an ideal etching cavity surface and experimentally demonstrating the feasibility of our proposal. The micromachining solution is beneficial for improving the yield and structural design flexibility of higher performance micro-electromechanical systems (MEMS) devices.

https://www.mdpi.com/2072-666X/10/6/420/pdf

Research on the Protrusions Near Silicon-Glass Interface during Cavity Fabrication

This paper reports a novel design for the decoupling of microelectromechanical systems (MEMS) gyroscopes. The MEMS gyroscope is based on piezoelectric aluminum nitride (AlN) film, and the main structure is a mass hung by T-shape beams. A pair of parallel drive electrodes are symmetrically placed on the surface of the vertical bar for driving the oscillating mass. A serpentine sense electrode is placed on the lateral bar. When the gyroscope is oscillating in drive mode, charges with equal quantity and opposite sign will be polarized and distributed symmetrically along the lateral bar. These charges neutralize each other at the sense electrode. Therefore, no coupling signals can be detected from the sense electrode. This design can realize the decoupling between the drive mode and sense mode. In this work, the T-shape decoupled structure was designed as the key component of an AlN piezoelectric gyroscope and the whole structure was simulated by COMSOL Multiphysics 5.2a. The working principle of the decoupling is described in detail. Electrical properties were characterized by the dynamic signal analyzer. According to the test results, the drive mode and the sense mode are decoupled. The coefficient of orthogonal coupling is 1.55%.

https://www.mdpi.com/2072-666X/10/4/244/pdf

A Decoupling Design with T-Shape Structure for the Aluminum Nitride Gyroscope.

A stripping method for preparing metal graph with another kind of relatively-easily-corroded metal comprises the steps that (1) an easily-corroded metal layer grows on a substrate through a steaming or sputtering technology; (2) photoresist is arranged on the surface of the easily-corroded metal in a spin-coating mode, a conventional photolithography technique is used for photoetching, and a graph is made; (3) an ICP etching technology is used for etching the easily-corroded metal, a groove is formed in the easily-corroded metal, and by adjusting technology parameters, the horizontal undercutting of the etched groove is enlarged; (4) through the steaming or sputtering technology, metal to be stripped grows on the substrate exposed out of the groove and the surface of residual photoresist, and a sample wafer is formed; (5) the sample wafer is placed into corrosive liquid where the metal can be easily corroded, the easily-corroded metal is corroded, and the metal graph on the substrate is obtained; (6) deionized water is used for washing, and preparing is completed. The stripping method has the advantages of being simple in technology and good in repeatability, requirements for the photolithography technique are low, fracture forming is easy, and stripping is easy.

Stripping method for preparing metal graph with another kind of relatively-easily-corroded metal

The invention discloses a piezoelectric MEMS decoupling structure and an MEMS gyroscope. The piezoelectric MEMS decoupling structure comprises a T-shaped beam structure, a second top electrode, a first top electrode and a third top electrode, wherein the T-shaped beam structure comprises a transverse beam and a longitudinal beam which are integrated, the T-shaped beam structure sequentially comprises a substrate, a bottom electrode layer and a piezoelectric material layer from bottom to top, the second top electrode is adopted as a detecting electrode, is in a T shape, is located above the T-shaped beam structure and is symmetric about the center line of the longitudinal beam, and the first top electrode and the third top electrode are adopted as drive electrodes, located above the longitudinal beam, symmetrically distributed at the two sides of the center line of the longitudinal beam and dependent from the portion, on the longitudinal beam, of the second top electrode. According to the decoupling structure, the whole shape is set as the T-shaped beam structure including the integrated transverse beam and longitudinal beam, the T-shaped detecting electrode symmetric about the center line of the longitudinal beam and the two drive electrodes symmetrically distributed on the longitudinal beam are arranged on the T-shaped beam structure, complete decoupling is achieved, and the detection precision of the gyroscope can be effectively improved.

Jian Yang

Papers

Research on the Protrusions Near Silicon-Glass Interface during Cavity Fabrication

A Decoupling Design with T-Shape Structure for the Aluminum Nitride Gyroscope.

Stripping method for preparing metal graph with another kind of relatively-easily-corroded metal

Piezoelectric MEMS decoupling structure and MEMS gyroscope