(593e) Hexagonal Saw Devices for Enhanced Sensing

We present the design, fabrication and testing of a novel, hexagonal array based on 36^o YX lithium tantalate for non destructive evaluation of thin inorganic and biological films. Propagation along the x-axis generates the leaky shear horizontal (SH) mode where off axis propagation excites Rayleigh type surface acoustic waves (SAW). Our approach permits rapid and simultaneous extraction of multiple film parameters. Given that SAW devices are in widespread use for chemical and biological sensing; a need exists to rapidly interrogate the interface for several parameters. This is especially relevant in biological applications when the sample quantity can be very limited. Our design allows for the simultaneous extraction of multiple properties (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 determination of the concentration. We present this device as an alternative to a SAW sensor array configuration that does not allow for simultaneous probing and which would require larger devices and packaging. These cost advantages are significant when working with novel piezoelectric materials. The device is based on a double split finger delay-line with widths of 4 µm and a delay path of 197λ. The individual delay path of each hexagonal device intersects 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 3452 m/s, 3161 m/s, and 3331 m/s, which correspond to the theoretical range of all acoustic waves in the crystal of velocities 3290-4160 m/s. This variation is sufficient to allow for the simultaneous solution of the same wave parameter dependent equation that will allow for the multiple properties of the sensing film to be extracted.