In this paper, we investigate the frequency band-gap features of micromachined air/silicon phononic band structures using layered slanted finger interdigital transducers (SFIT). In order to achieve the applications of phononic crystals on the microelectromechanical system related components, the frequency band-gap widths of surface waves are studied both theoretically and experimentally in micrometer scale phononic crystals. For further integration with the complementary metal-oxide semiconductor processing techniques, silicon is chosen as the base material of the two-dimensional phononic crystals in this study. To cover the frequency band-gap width of the phononic crystal, the wideband SFIT- and the SFIT∕ZnO∕Si-layered structures in the measurement are analyzed and discussed. For layered structures, the dispersive relation is calculated by the effective permittivity approach, and the frequency response of the layered SFIT is then simulated by the coupling-of-modes model. The frequency band-gap width and ...

Frequency band-gap measurement of two-dimensional air/silicon phononic crystals using layered slanted finger interdigital transducers

Acoustic devices have found wide applications in chemical and biosensing fields owing to their high sensitivity, ruggedness, miniaturized design and integration ability with on-field electronic systems. One of the potential advantages of using these devices are their label-free detection mechanism since mass is the fundamental property of any target analyte which is monitored by these devices. Herein, we provide a concise overview of high frequency acoustic transducers such as quartz crystal microbalance (QCM), surface acoustic wave (SAW) and film bulk acoustic resonators (FBARs) to compare their working principles, resonance frequencies, selection of piezoelectric materials for their fabrication, temperature-frequency dependency and operation in the liquid phase. The selected sensor applications of these high frequency acoustic transducers are discussed primarily focusing on the two main sensing domains, i.e., biosensing for working in liquids and gas/vapor phase sensing. Furthermore, the sensor performance of high frequency acoustic transducers in selected cases is compared with well-established analytical tools such as liquid chromatography mass spectrometry (LC-MS), gas chromatographic (GC) analysis and enzyme-linked immunosorbent assay (ELISA) methods. Finally, a general comparison of these acoustic devices is conducted to discuss their strengths, limitations, and commercial adaptability thus, to select the most suitable transducer for a particular chemical/biochemical sensing domain.

https://www.mdpi.com/1424-8220/19/20/4395/pdf

An Overview of High Frequency Acoustic Sensors-QCMs, SAWs and FBARs-Chemical and Biochemical Applications.

The tapered transducer was first demonstrated over 25 years ago but until recently has seen little commercial use. This paper reviews the theory that describes how the wideband response of a tapered transducer is achieved by the summation of narrow-band channels. In addition, it discusses weighting techniques appropriate to tapered transducers including block weighting, and how the use of SPUDT electrode structures allows the same reduction in IL and triple-transit over a wide bandwidth normally achieved only with relatively narrow-band filters. Experimental results are presented along with a discussion of applications that benefit from the unique properties of tapered transducers such as a single-bounce dispersive delay line, a multi-tap delay line with low spurious reflections, a channelizer or filter bank, an AO Bragg cell, and a radar pulse compression system without range/velocity ambiguity in the presence of Doppler.

Tapered transducers-design and applications

A frequency selective acoustic transducer (FSAT) is proposed for directional sensing of guided waves. The considered FSAT design is characterized by a spiral configuration in wavenumber domain, which leads to a spatial arrangement of the sensing material producing output signals whose dominant frequency component is uniquely associated with the direction of incoming waves. The resulting spiral FSAT can be employed both for directional sensing and generation of guided waves, without relying on phasing and control of a large number of channels. The analytical expression of the shape of the spiral FSAT is obtained through the theoretical formulation for continuously distributed active material as part of a shaped piezoelectric device. Testing is performed by forming a discrete array through the points of the measurement grid of a scanning laser Doppler vibrometer. The discrete array approximates the continuous spiral FSAT geometry, and provides the flexibility to test several configurations. The experimental results demonstrate the strong frequency dependent directionality of the spiral FSAT and suggest its application for frequency selective acoustic sensors, to be employed for the localization of broadband acoustic events, or for the directional generation of Lamb waves for active interrogation of structural health.

A frequency selective acoustic transducer for directional Lamb wave sensing.

This paper describes a useful design technique to achieve a nonlinear phase SAW filter using slanted finger interdigital transducers (SFITs) or tapered interdigital transducers which are suitable for wide-band filters in intermediate frequency stages. A required nonlinear phase response in the passband can be obtained by changing center-to-center distances between input and output SFITs along an axis perpendicular to the SAW propagation axis. The design is based on a building-block approach in the frequency domain. A nonlinear phase SAW filter with a center frequency of 70 MHz and a fractional bandwidth of about 10% is demonstrated on x-cut 112.2/spl deg/ y-propagating LiTaO/sub 3/. Because the substrate has a power flow angle of 1.55 degrees, the SFIT pattern is tilted along that angle. Good agreement between theoretical and experimental results is obtained.

Design technique for nonlinear phase SAW filters using slanted finger interdigital transducers

This paper describes useful design techniques for wide-band linear phase SAW filters using slanted finger interdigital transducers (SFITs): to increase stop band attenuation, to flatten the pass band response, and to draw SFIT patterns for photo-masks. In order to increase stop band attenuation, a withdrawal-weighted SFIT is employed. A combination of a withdrawal-weighted SFIT and an unweighted SFIT provides good stop band responses, even though the number of finger pairs is small. In order to achieve a desired amplitude response in the pass band, an aperture-weighting technique for divided channels of SFITs is presented. A design method for drawing SFIT patterns for photo-masks is also described. A linear-phase SAW filter using SFITs with a 50% fractional bandwidth is designed and fabricated on 128/spl deg/ y-x LiNbO/sub 3/. A sidelobe suppression of 40 dB and a 3 dB to 40 dB shape factor of 1.12 were realized. Good agreement between theoretical and experimental results is obtained.

H. Yatsuda

Papers

Design techniques for SAW filters using slanted finger interdigital transducers