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 paper describes fabrication arid characterization results of piezoelectric micromachined ultrasonic transducers (pMUTs) based on 2-/spl mu/m-thick Pb(Zr/sub 0.53/Ti/sub 0.47/O/sub 3/) (PZT) thin films. The applied structures are circular plates held at four bridges, thus partially undamped. A simple analytical model for the fully clamped structure is used as a reference to optimize design parameters such as thickness relations and electrodes, and to provide approximate predictions for coupling coefficients related to previously determined thin film properties. The best coupling coefficient was achieved with a 270-/spl mu/m plate and amounted to k/sup 2/=5.3%. This value compares well with the calculated value based on measured small signal dielectric (/spl epsi/=1050) and piezoelectric (/spl epsi//sub 31,f/=15 Cm/sup -2/) properties of the PZT thin film at 100 kV/cm dc bias. The resonances show relatively large Q-factors, which can be partially explained by the small diameters as compared to the sound wavelength in air and in the test liquid (Fluorinert 77). A transmit-receive experiment with two quasiidentical pMUTs was performed showing significant signal transmission up to a distance of 20 cm in air and 2 cm in the test liquid.

Piezoelectric micromachined ultrasonic transducers based on PZT thin films

A gas detector (54) and process for detecting a fluorine-containing species in a gas containing same, e.g., an effluent of a semiconductor processing tool undergoing etch cleaning with HF, NF3, etc. The detector in a preferred structural arrangement employs a microelectromechanical system (MEMS)-based device structure and/or a free-standing metal element (8) that functions as a sensing component and optionally as a heat source when elevated temperature sensing is required. The free-standing metal element can be fabricated directly onto a standard chip carrier/device package (6) so that the package becomes a platform of the detector.

Apparatus and process for sensing fluoro species in semiconductor processing systems

Vibration Energy Harvesting with PZT Micro Device

Novel, highly sensitive piezoelectric acoustic sensors based on partially unclamped Pb(Zr-x, Ti1-x)O-3 (PZT) coated cantilever and bridge have been fabricated by silicon micromachining. High sensitivity at low frequencies (5-100 Hz) has been achieved by patterning very narrow slits (3 to 5 mum) around the structures. A typical response of 100 mV Pa-1 and a noise equivalent pressure of 1.6 mPa Hz(1/2) at 20 Hz have been measured using a 10 pF charge amplifier. Stress compensation, dry etching and integration of high performance piezoelectric thin films were the key issues. PZT/Pt/SiO2 stacks have been patterned by reactive ion etching and stress compensation has been achieved by compensating the PZT film's tensile stress by adjusting the thickness of a thermal SiO2 layer. The integration of sol-gel PZT films with a transverse piezoelectric coefficient e(31,f) of -12.8 C m(-2) has been realized without any degradation of the properties. The microphones were successfully integrated into a miniature photoacoustic detector and tested for CO2 detection. Concentrations down to 330 ppm could be measured with significant signals.

https://iopscience.iop.org/article/10.1088/0960-1317/14/12/008/pdf

Piezoelectric Pb(Zrx, Ti1−x)O3 thin film cantilever and bridge acoustic sensors for miniaturized photoacoustic gas detectors

A photo-acoustic gas sensor for measuring a gas concentration, e.g., of an explosive gas, consists of a cylindrical sensor body (1) which includes a cylindrical measurement chamber (13) whose longitudinal axis extends perpendicular to the longitudinal axis of the cylindrical sensor body. Without changing the outer dimensions of the sensor body, the length of the measurement chamber can be changed in accordance with a desired measurement sensitivity and a desired measurement range. A light source (5) is disposed such that it irradiates the measurement chamber without irradiating a gas-permeable membrane (2), so that the membrane will not produce an interference signal. The gas sensor is rendered explosion-proof as the light source and a photo-diode (6) which monitors the light source are tightly sealed from the environment. Proper functioning of a microphone (4), the light source and the photo-diode are monitored by analyzing a background signal.

Photo-acoustic gas sensor and its manufacture and use

The measuring arrangement contains a measuring cell and a reference cell (9, 10) to which microphones (11 or 12) are allocated, an evaluating electronics unit (7, 8) being connected to said microphones. In said evaluating electronics unit, the signals of the microphones (11 or 12) are subtracted. The inventive arrangement also comprises a source of radiation (5) which acts on the measuring cell (9) in a modulated manner. The modulation frequency of the radiation source (5) corresponds to the resonance frequency of the measuring cell (9), and the measuring cell and reference cells (9 to 10) are open on at least one side in relation to the gas and/or aerosol to be detected. The measuring arrangement is used as a smoke detector, gas detector, fire risk detector or a combined smoke and gas detector. The measuring cell (9) is subjected to radiation of a wavelength at which a relevant substance to be detected is absorbed and an optoacoustic effect is produced as a result.

Optoacoustic measuring arrangement and use thereof

In a photo-acoustic gas sensor having a measuring cell, a lamp, a photo-diode, a gas-permeable diaphragm and a microphone, a device is included for monitoring the gas-permeable diaphragm and the microphone. The device is operated in an explosion-proof manner. Intentionally, for diagnostic purposes, an artificial pressure modulation is generated in the measuring cell. The pressure modulation is converted by the microphone into electrical monitoring signals which are amplified by an amplifier circuit and assigned to preset ranges by a microprocessor, for determining the functional condition of the diaphragm and the microphone, and of microphone sensitivity. Depending on the range assigned to the monitoring signal, an output signal is generated indicating a fault in the microphone and/or the diaphragm. In the event of long-term drift in microphone sensitivity, a calibration is performed in which gas concentration values are re-calculated and stored in an EEPROM. A heater wire can be used, for modulated heating to generate the pressure modulation.

Martin Forster

Papers

Piezoelectric Pb(Zrx, Ti1−x)O3 thin film cantilever and bridge acoustic sensors for miniaturized photoacoustic gas detectors

Photo-acoustic gas sensor and its manufacture and use

Optoacoustic measuring arrangement and use thereof

Photo-acoustic gas sensor

Investigations for the environmentally friendly production of Na2CO3 and HCl from exhaust CO2, NaCl and H2O