Hexagonal Surface Acoustic Wave Devices for Enhanced Sensing and Materials Characterization

摘要

We present the design, fabrication and testing of a hexagonal surface acoustic wave (SAW) array device fabricated in YZ lithium niobate for non-destructive evaluation of thin organic, inorganic and biological films. Propagation along the Y-axis generates a Rayleigh mode wave where off-axis propagation excites a mixture of other SAWs. Our approach permits rapid and simultaneous extraction of multiple film parameters (film material density or thickness, Lamé and shear moduli, sheet conductivity) of a thin film material to achieve a more complete characterization than when a single SAW device is utilized. In sensor applications, this capability translates to better discrimination of the analyte and possibly more accurate quantification. The device is based on a double split finger delay-line design with a line width of 4 mum and a delay path of 197 lambda. The individual delay paths of each hexagonal device intersect in the center of the die producing a single region for sensor analysis. Additionally, the central region where the acoustic waves intersect is shorted to reduce the number of modes of waves traversing the surface. Initial testing has shown the pass band frequency of the individual delay paths to be centered around 97 MHz. The acoustic velocities of the rotated device have been measured to be 3593 m/s, 3721 m/s, and 3620 m/s, which correspond to the theoretical values of 3542 m/s, 3646 m/s, and 3622 m/s, respectively. Vapor sensing tests were conducted by exposing a poly(isobutylene)-coated device to various concentrations of benzene, chloroform, and n-hexane in the range of 0.8 to 16.6 volume percent. Measured attenuation and phase angle shifts at a fixed, near-center frequency revealed significant, signature-type differences for the three delay-paths at each exposure concentration. These responses can be exploited in constructing better sensors and sensor arrays utilizing these hexagonal SAW devices.