Thin Film Bulk Acoustic Wave Resonators for Continuous Monitoring in the Physical, Chemical and Biological Realms
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Thin Film Bulk Acoustic Wave Resonators for Continuous Monitoring in the Physical, Chemical and Biological Realms. Greg Ashley.1 Jack Luo 1 Paul Kirby.2 Tim Butler.3 and Dave Cullen2 1 Centre for Materials Research and Innovation. University of Bolton. BL3 5AB 2 Cranfield Health/Microsytems and Nanotechnology Centre. University of Cranfield. MK43 0AL 3 Centre for Advanced Photonics and Electronics. University of Cambridge. CB3 0FA
Abstract: A transducer that can act as a highly sensitive and reliable universal sensor capable of detecting and continuously monitoring changes in the physical, chemical and biological domains is a potentially useful scientific tool. The Thin Film Bulk Acoustic Wave Resonator (FBAR) is a microwave device that is becoming increasingly recognised as a universal transduction platform with the added advantage of potential integration into CMOS architecture and array-like formats. This work shows preliminary results on FBAR where a continuous monitoring arrangement demonstrated the capability of FBAR to respond to changes in physical parameters such as temperature and light levels, the work goes on further to show the ability of FBAR to respond to changes in humidity in a gas flow and can have sensitivity increased with the addition of hygroscopic polymers on its surface and finally how FBAR can be adapted to act as a biosensor in the form of an immunosensor with sensitivity some orders of magnitude greater than traditional lower frequency bulk acoustic wave platforms.
Introduction The FBAR device is a resonating transducer so can also be exploited as a mass-loading or socalled gravimetric sensor in a manner similar to the quartz crystal microbalance (QCM) but with orders of magnitudes more sensitivity due to its low base mass and high natural resonant frequency of operation. Progress has already been made with FBAR biosensors1,2and in modification of the control of the motional direction of the device.3 The FBAR can also measure force in the physical domain.4. FBARs offer great potential as mass loading sensors because their resonant frequency can be controlled by the thickness of the deposited thin-film piezoelectric membrane. Additional fine-frequency tuning of FBAR by ion milling has also been reported allowing precise selection of sensor frequency range.5 In this work it is confirmed that the FBAR is extremely sensitive to changes in physical parameters namely temperature and light, as well as micro-mass loadings in chemical and biological sensing. The work is unambiguous in showing that the FBAR immunosensor platform is much more responsive to immunoaccumulation in terms frequency shift than the present state of the art quartz crystal microbalances. The mass loading sensitivity of a resonating transducer increases with the square of its fundamental operating frequency (f0), meaning that there will be an increase in the absolute negative frequency change (-∆f) per unit of mass adsorbed on the surface of a higher frequency device. This is expressed in the Sauerbrey equation
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