Falk Herrmann

Properties of sensors based on shear-horizontal surface acoustic waves in LiTaO/sub 3//SiO/sub 2/ and quartz/SiO/sub 2/ structures

Acoustic wave devices based on waveguide modes with shear-horizontal polarization, i.e., Love modes, are very promising for sensor application, especially in liquid environments. They can be used for the determination of liquid density and viscosity as well as for chemical sensors. Up to now, several systems have been reported based on standard ST quartz. Those devices lack temperature stability, which is essential for field application. Thus, temperature-compensated systems based on different Y-rotated quartz and lithium tantalate (LiTaO/sub 3/) plates with SiO/sub 2/ guiding layers have been investigated. Temperature behavior as well as relevant acoustic properties were considered. Furthermore, experimentally determined data for device sensitivity are compared with theoretical predictions from numerical calculations.

Separate determination of liquid density and viscosity with sagittally corrugated Love-mode sensors

A novel microacoustic sensor is reported for separate determination of liquid density and viscosity. The device is based on a layered quartz/SiO2-system with sagittal corrugations, supporting acoustic waveguide Love-modes. The sensor combines the merits of Love-mode devices, e.g., robustness and high sensitivity, with liquid trapping as an effective approach to separate density and viscosity influences on acoustic shear modes. The outstanding features are extraordinary high and adjustable sensitivity to density changes with inherent large differences between density and viscosity responses. A model is discussed to describe the frequency response to liquid loading. Furthermore, studies of attenuation changes have shown that only little extra damping occurs due to the corrugations. Moreover, strategies are discussed for device optimisation in terms of temperature stability and sensitivity enhancement.

Properties of shear-horizontal surface acoustic waves in different layered quartz-SiO2 structures

Acoustic wave devices based on waveguide modes with shear-horizontal polarisation in Y-rotated quartz plates are very promising for sensor application. Up to now, several systems have been reported based on standard ST-quartz. Those devices lack temperature stability, which is essential for field application. However, appropriate combinations of crystal cut angle and SiO2 overlay thickness should provide temperature compensation. Thus, different systems based on Y-rotated quartz plates with cut angles between 30 and 42.75 degrees have been investigated. First- and second-order temperature coefficients of frequency change have been calculated and measured. Moreover, properties are compiled that are relevant for the device design, i.e. acoustoelectric coupling coefficients, effective dielectric constants, and phase as well as group velocities. In summary, device configurations could be identified that combine temperature stability comparable to that of surface skimming bulk waves in the AT-cut, a suitable coupling coefficient, and high gravimetric sensitivity.

Microacoustic Sensors for Liquid Monitoring

In recent years, much effort has been expended to developing miniaturized, reliable sensors for measuring physical and chemical liquid properties. Microacoustic devices may be employed to determinate physical quantities perturbing the propagation conditions of the acoustic wave, even in liquid environments. For liquid sensors, acoustic modes with shear polarization are often used in order to avoid radiation losses. Examples are shear bulk modes, surface skimming bulk waves, shear-horizontal acoustic plate modes, and Love modes. Furthermore, modes with sagittal polarization may be applied if their phase velocity is sufficiently lower than the sound velocity in the adjacent liquid. General interactions between acoustic waves and liquids are viscous coupling, acoustoelectric effects, and mass loading. The resulting changes in device frequency and attenuation may be utilized for sensing purposes. The general advantages of microacoustic sensors are high sensitivity, simple fabrication, and quasidigital frequency readout. Hence, a wide variety of devices based on different modes for very different applications has been discussed in the relevant literature. The aim of this review is to provide a systematic overview of microacoustic sensors particularlay suited for operation in liquids. First, we provide a brief introduction to surface acoustic wave devices and the underlying physics, basic microacoustic structures, and general measurement techniques. A description of the mechanisms of interaction between liquids and acoustic waves is then followed by a comprehensive survey covering the different types of microacoustic liquid sensors, with emphasis on the mode-specific device properties. Thereafter, aspects of material selection and device fabrication are summarized. Lastly, an overview is given indicating where microacoustic liquid sensors have already been put into practice and where their application may be expected in the near future.

The potential of microacoustic SAW- and BAW-based sensors for automotive applications - a review

In recent years, sensors and sensor systems have gained increasing importance for automotive electronics. In this paper, the suitability and the perspectives in the application of microacoustic surface- and bulk-acoustic-wave sensors in the fields of angular rate sensors, pressure sensors, wireless sensor readout, and liquid sensors are discussed.

Separation of density and viscosity influence on liquid-loaded surface acoustic wave devices

Love-mode sensors are reported for separate measurement of liquid density and viscosity. They combine the general merits of Love-mode devices, e.g., ease of sensitivity adjustment and robustness, with a highly effective procedure of separate determination of liquid density and viscosity. A model is proposed to describe the frequency response of the devices to liquid loading. Moreover, design rules are given for further optimization and sensitivity enhancement.

High-performance surface acoustic wave sensors for liquid environments

Acoustic wave devices based on waveguide modes with shear-horizontal polarisation, i. e. Love modes, are very promising for sensor application, especially in liquid environments. They can be used for determination of liquid density and viscosity as well as for chemical sensors. Up to now, several systems have been reported based on standard ST-quartz. Those devices lack temperature stability which is essential for field application. However, appropriate combinations of crystal cut angle and SiO2 overlay thickness should provide temperature compensation. Thus, different systems based on Y-rot quartz and lithium tantalate plates with SiO2 waveguiding films have been investigated. Temperature behavior as well as relevant acoustic properties are compiled. It has been proven that numerical calculations yield a very accurate description of the physical device properties. For both cases of material combinations it has been shown that systems are available combining temperature compensation with a high device sensitivity.