Film Bulk Acoustic Resonators for Parallel Mass Sensing
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Film Bulk Acoustic Resonators for Parallel Mass Sensing Shih-Jui Chen1, Anderson Lin1, Lukas Baumgartel2, and Eun Sok Kim1 1 Department of Electrical Engineering, University of Southern California, CA 90089, USA. 2 Department of Physics, University of Southern California, CA 90089, USA.
ABSTRACT This paper describes the principle of array sensing with film bulk acoustic resonators (FBARs) for combinatory mass sensing and the use of the arrayed FBAR resonant mass sensor for parallel detection of protein-ligand reactions in liquid environment. Various ligands were immobilized on the gold layer on the FBAR’s sensing surface for selective protein detection. The FBARs of the arrayed FBAR were fabricated to have different resonant frequencies from one another by adding slightly varied amount of mass loading on the resonators. Results showed that the arrayed FBAR could detect specific bindings on its surfaces as the concentration of the target ligand was varied.
INTRODUCTION Protein chip [1] potentially offers low cost and simplicity for combinatory detection of proteins. Current detection method for proteins uses fluorescent or radioactive tags on the target proteins, which when bound to probe proteins are detected by an optical detector. Since such a labeled optical detection is expensive, a label free detection of proteins is highly desirable. For example, various resonant mass sensing technologies have been studied over past ten years in either dry or wet environment. As the mass on the resonator increases, the resonant frequency of a resonator decreases. Thus, no tags would be needed for the detection of protein. A microcantilever based resonant mass sensor can detect the mass accumulated on its surface with high sensitivity. However, the quality factor (Q) of the micro-cantilever drops too much when it is exposed to liquid, making the micro-cantilever-based mass sensor impractical in liquid environment or incapable as a real time sensor. Surface plasmon resonance (SPR) [2] is another label-free approach for protein detection, as it detects mass adsorption based on changes in refractive index. But SPR requires a relatively large device size as well as optical source and detector that make SPR not a single chip solution. In this paper, we describe an arrayed film bulk acoustic resonator (FBAR) for parallel detection of chembio interactions without the complexity of optical measurement and with reasonably high Q.
THEORY AND DESIGN Film bulk acoustic resonator (FBAR) is composed of a thin film of piezoelectric material sandwiched by two layers of metals [3]. The piezoelectric material converts an applied electrical field into mechanical strain, which in turn affects the electrical field. When an ac electrical signal
is applied to the resonator in the thickness direction, a longitudinal acoustic wave propagating along thickness direction would be excited. Due to the acoustic impedance mismatch on the interface between the electrode and air, the acoustic wave reflects from the interface and forms a standing wave at th
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