Piezoelectric materials play a crucial role in a large number of devices and applications modern society would not like to miss. Mobile phones and ultrasonic imaging are just the most prominent ones. Since two decades, miniaturization of mechanical devices in silicon technology is a major research direction in engineering known under name of MEMS, which stands for micro-electro-mechanical systems. Piezoelectricity fits very well into this concept and was expected right from the beginning to play its role in MEMS. The breakthrough was made with RF filters in mobile phones working on the principle of standing thickness waves in AlN films. What counts here is acoustic quality and stability. The force champion among piezoelectric thin film materials, Pb(Zr,Ti)O3 gave more problems in processing, and requires more patience to meet requirements and needs for a mass applications. It seems, however, that the breakthrough is imminent. This article attempts to give an overview of the field, highlighting recent achievements, introduce operation principles, and describe some applications.

Recent Progress in Materials Issues for Piezoelectric MEMS

Piezoelectric microelectromechanical systems (MEMS) offer the opportunity for high-sensitivity sensors and large displacement, low-voltage actuators. In particular, recent advances in the deposition of perovskite thin films point to a generation of MEMS devices capable of large displacements at complementary metal oxide semiconductor-compatible voltage levels. Moreover, if the devices are mounted in mechanically noisy environments, they also can be used for energy harvesting. Key to all of these applications is the ability to obtain high piezoelectric coefficients and retain these coefficients throughout the microfabrication process. This article will review the impact of composition, orientation, and microstructure on the piezoelectric properties of perovskite thin films such as PbZr1−xTixO3 (PZT). Superior piezoelectric coefficients (e31, f of −18 C/m2) are achieved in {001}-oriented PbZr0.52Ti0.48O3 films with improved compositional homogeneity on Si substrates. The advent of such high piezoelectric responses in films opens up a wide variety of possible applications. A few examples of these, including low-voltage radio frequency MEMS switches and resonators, actuators for millimeter-scale robotics, droplet ejectors, energy scavengers for unattended sensors, and medical imaging transducers, will be discussed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/AC28E0B7FC52ADDE14E1F58F59416494/S0883769400003614a.pdf/div-class-title-piezoelectric-thin-films-for-sensors-actuators-and-energy-harvesting-div.pdf

Piezoelectric Thin Films for Sensors, Actuators, and Energy Harvesting

This article proposes a new, simple, and efficient strategy, allowing one to optimize the diffusion operator between a passive beam coupled to a like beam equipped with a periodically distributed network of shunted piezoelectric patch actuators. A multimodal wave dispersion model is used to compute the diffusion operator and analyze the stability properties of the combined system. Based on this mathematical tool, specific optimization procedures are introduced to allow maximization or minimization of the wave transmissibility between the passive and the active distributed beam. A specific example is used to demonstrate the capability of the shunted piezoelectric system to induce total reflection through the total absorption of incoming propagating flexural waves while guaranteeing the stability, robustness, and realizability of such a system.

Wave Motion Optimization in Periodically Distributed Shunted Piezocomposite Beam Structures

External mechanical disturbances including vibration and shock have profound impact on the performance and reliability of MEMS devices. This project seeks to develop generic technologies that protect microsystems against them. Vibration produces undesirable outputs that cannot be corrected with electronics. Tuning fork gyroscopes (TFG), which are known to be relatively immune to linear vibrations, cannot completely eliminate linear vibrations in several special situations. Simulations were conducted using two commonly used TFG designs. Both designs have decoupled sense and drive masses; however, one is anchored at its sense mass (Type-A), whereas the other is anchored at its drive mass (Type-B). The results demonstrate that both designs are affected by linear vibrations. However, Type-B design is more resistant to vibration-induced errors than Type-A (>99% reduction). Shock permanently damages MEMS devices and hard shock stops have been conventionally used to limit the damage. However, hard stops can generate subsequent impacts, which should also be minimized. This goal can be achieved by two novel shock protection technologies using nonlinear springs and soft coatings. Devices integrated with nonlinear spring (silicon) stops and soft coating (Parylene) stops were fabricated and tested together with hard-stop devices. Test results show that both stops provide superior shock protection. The device survival rates of nonlinear spring stops (87%) and soft coating stops (94%) are more than 10x better than hard stops (4%). This project is supported by the Defense Advanced Research Projects Agency HERMIT program under contract number W31P4Q-04-1-R001. Sang Won Yoon, Sangwoo Lee, Noel C. Perkins, and Khalil Najafi

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Vibration Isolation and Shock Protection for MEMS.

This article proposes a new, simple and efficient approach, allowing one to reduce and construct piezoelectric super elements guaranteeing an accurate representation of the electrical impedance without the need for static correction. This allows the electronic coupling to be fully addressed in the optimization of passive shunted piezoelectric transducers, energy harvesting piezoelectric systems or dense distributed transducers. The model obtained through this approach is also versatile, of small size, and is therefore quite tractable for use in intensive computation algorithms. Two example systems are used to demonstrate the numerical accuracy and convergence properties of the proposed approach.

Modal Synthesis and Dynamical Condensation Methods for Accurate Piezoelectric Systems Impedance Computation

Thin PZT films have a major interest for active control of mechanical structures. Precisely, it is an open field for the isolation of micro-components sensitive to dynamic effects. Indeed, the electronic components used, for example, in aircraft endure intense vibrations due to acceleration. These vibrations have some disturbing effects on the frequency stability and on the usable life of the electronic elements. The isolation of these elements becomes crucial to protect them from the vibrating environment. In order to manage this problem, it is advisable to isolate the electronic card either at the case level or at the card level or at the sensitive element level. The latter solution was chosen. Thus, we have direct access to the control electronics and the energy sources and the control energy is lower. An active suspension system is developed between the support and the sensitive element to be isolated. An original active suspension system is designed. Some modeling difficulties arise due to the existence of the inevitable bottom electrode common to the actuating layers and to the sensing layer.

J. Baborowski

Papers

Active isolation of electronic micro-components with piezoelectrically transduced silicon MEMS devices