3I-4 A Monolithic Spiral Coil Acoustic Transduction Sensor

A monolithic spiral coil acoustic transduction (MSCAT) sensor that combines and improves upon the positive features of other acoustic wave sensors has been developed. The MSCAT sensor consists of an AT-cut quartz substrate that has a bare sensing surface and a photolithographically deposited multi-turn gold spiral coil on the opposite surface. The spiral coil acts as an antenna that radiates a time varying electric field that penetrates into the AT-quartz wafer. As a result of the piezoelectric effect, the time varying electric field sets up a time varying stress in the wafer. The MSCAT sensor can operate at high frequencies by efficiently exciting high harmonics with the application of a high frequency RF signal to the spiral coil. It is shown that resonant acoustic waves up to the 63rd order harmonic can be efficiently excited. The MSCAT sensor was used to measure the viscosity of corn syrup. When compared to the performance of the standard quartz crystal monitor (QCM), the MSCAT sensor was found to be over three times more sensitive to viscosity changes. Finally, the MSCAT sensor was shown to be capable of detecting conductivity changes in liquids