(680h) Dual Delay Line SAW Sensor Utilizing Multistrip Coupler for Liquid Biosensing Applications

Mandek Richardson¹, Changbao Wen^2,3, and Venkat R. Bhethanabotla¹

¹Sensors Research Laboratory, Department of Chemical & Biomedical Engineering, University of South Florida, Tampa, Florida 33620, USA

²Institute of Micro-nanoelectronics, School of Electronics and Control Engineering, Chang’an University, Xi’an 710064, People’s Republic of China

³Institute of Vacuum Microelectronics, School of Electronics and Information Engineering, Xi’an 710064, People’s Republic of China

Surface Acoustic Wave (SAW) devices can be used as direct, label free biosensors that monitor the interaction between a receptor and its target in real time, through changes in the properties of the traveling wave (i.e., frequency, phase, and amplitude)  [1].   A typical SAW device consists of two metal interdigital transducers (IDTs) separated from each other where one transducer is used to generate acoustic waves and the other receives acoustic waves and converts them into electrical signals. SAW IDTs not only generate surface waves but also bulk waves that have deleterious effects on sensor performance.  For example, the generation of bulk waves reduces SAW power, which increases insertion loss, and interference between SAW and bulk waves causes amplitude and phase distortions that increase ripples in the pass band, which affect linear sensor calibration [2, 3].  A technique used to address problems stemming from bulk wave generation was to implement a multistrip coupler into SAW filter designs [2].   An MSC consists of an array of identical electrodes oriented parallel to the surface-wave wavefront; in which, the electrodes are not electrically connected.  The purpose of the MSC is to divert the surface wave to an output IDT that is offset from the input IDT; this decouples SAWs and bulk waves allowing just the SAW to arrive at the output IDT.  For practicality, SAW substrates with large electromechanical coefficients (K²) must be used [2].  

In this work we present a dual delay-line SAW liquid sensor with weighted input/output IDTs and two MSCs fabricated on a 41° Y-X LiNbO₃ piezoelectric substrate.  We choose 41° Y-X LiNbO₃ because of its high acoustic velocity (4792 m/s), large electromechanical coupling coefficient, K² (17.3%), and leaky wave propagation [4].  Substrates that support leaky wave modes can be used as liquid sensors because particle displacement is parallel to the surface, which limits the dissipation of energy into the adjacent liquid.  Incorporation of the dual-delay line structure coupled with MSCs eliminates spurious responses caused by the external environment and bulk wave generation [5].  Furthermore, the presence of MSCs improve sensor calibration by reducing the amount of phase distortion in the pass band through the suppression of bulk waves, and they increase sensor sensitivity by trapping the leaky wave at the surface.  In addition, weighted IDTs were used as the input and output transducers to improve side lobe rejection levels [5].  Devices based on the design were fabricated and compared to a standard SAW dual delay-line to show its feasibility as a biosensor.  

References

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[2]           C. Campbell, Surface acoustic wave devices for mobile and wireless communications / C.K. Campbell: San Diego : Academic Press, c1998., 1998.

[3]           D. W. Branch and T. L. Edwards, "Love Wave Acoustic Array Biosensor Platform for Autonomous Detection " IEEE Ultrasonics Symposium Proceedings, pp. 260-263, 2007.

[4]           K. Lee, W. Wang, T. Kim, and S. Yang, "A novel 440 MHz wireless SAW microsensor integrated with pressure-temperature sensors and ID tag," Journal of Micromechanics and Microengineering, vol. 17, pp. 515-523, Mar 2007.

[5]           C. Wen, C. Zhu, Y. Ju, H. Xu, and Y. Qiu, "A Novel Dual Track SAW Gas Sensor Using Three-IDT and Two-MSC," IEEE Sensors Journal vol. 9, pp. 2010-2015, 2009.