Multiple Lateral Field Excited Sensors on a Single Substrate (2011)

A sensor array consists of multiple sensing devices working together in tandem to measure or sense multiple measurands simultaneously. An ideal arrangement would be to have a sensor array implemented on a single small substrate chip. Such a device would be small, compact and portable. It could result in cost-savings by replacing many separate, bulky pieces of sensing equipment in laboratory or medical settings with a single low cost device. Such technology could theoretically be used to probe a liquid sample for several measurands simultaneously. This could save human lives in poor and remote areas of the world if it were used to quickly and inexpensively test drinking supplies for contaminants. In recent years, prototype sensor arrays based on bulk acoustic wave (BAW) devices have been reported in the scientific literature. Specifically, these consist of multiple quartz crystal microbalances (QCM) that are monolithically integrated onto a single quartz crystal substrate. However, these arrays are limited by the fundamental nature of the QCM, namely the requirement of having electrodes on both side of the substrate, which precludes sensing electrical property changes caused by the target measurand. This electrode configuration also results in a cumbersome arrangement of wiring interconnects running across the surface of the array substrate. The lateral field excited (LFE) sensor is a novel BAW device that was developed at the University of Maine. It uses the same BAW mode as the QCM. Unlike the QCM, however, the LFE uses a pair of lateral electrodes that are located on the backside of the crystal substrate. This is a simplified electrode geometry that leaves the sensing surface of the LFE bare, allowing the LFE to sense both electrical and mechanical property changes in the measurand. These unique characteristics make the LFE a better choice for implementing a sensor array. In this thesis, several approaches to the implementation of a monolithic LFE sensor array are explored. These methods include selective X-axis inversion of specific regions of the quartz substrate, the machining of well-shaped structures into the surface of the substrate, and the machining of trench-shaped structures. Several dual LFE sensor arrays were fabricated and tested with impedance measurements. The devices were exposed to varying concentrations of saline and glycerol-water solutions, to test the sensor response to mechanical and electrical liquid property changes. Experimental results indicated successful lateral field excitation of well structures in quartz. X-axis inversion was successfully achieved and results indicated that a dual LFE array based on inversion was plausible. The best results were achieved using trench structures. Several dual LFE arrays based on these structures were shown to respond to mechanical and electrical property changes in liquid. The individual LFE elements were shown to operate as independent sensors with minimal cross-talk.A sensor array consists of multiple sensing devices working together in tandem to measure or sense multiple measurands simultaneously. An ideal arrangement would be to have a sensor array implemented on a single small substrate chip. Such a device would be small, compact and portable. It could result in cost-savings by replacing many separate, bulky pieces of sensing equipment in laboratory or medical settings with a single low cost device. Such technology could theoretically be used to probe a liquid sample for several measurands simultaneously. This could save human lives in poor and remote areas of the world if it were used to quickly and inexpensively test drinking supplies for contaminants. In recent years, prototype sensor arrays based on bulk acoustic wave (BAW) devices have been reported in the scientific literature. Specifically, these consist of multiple quartz crystal microbalances (QCM) that are monolithically integrated onto a single quartz crystal substrate. However, these arrays are limited by the fundamental nature of the QCM, namely the requirement of having electrodes on both side of the substrate, which precludes sensing electrical property changes caused by the target measurand. This electrode configuration also results in a cumbersome arrangement of wiring interconnects running across the surface of the array substrate. The lateral field excited (LFE) sensor is a novel BAW device that was developed at the University of Maine. It uses the same BAW mode as the QCM. Unlike the QCM, however, the LFE uses a pair of lateral electrodes that are located on the backside of the crystal substrate. This is a simplified electrode geometry that leaves the sensing surface of the LFE bare, allowing the LFE to sense both electrical and mechanical property changes in the measurand. These unique characteristics make the LFE a better choice for implementing a sensor array. In this thesis, several approaches to the implementation of a monolithic LFE sensor array are explored. These methods include selective X-axis inversion of specific regions of the quartz substrate, the machining of well-shaped structures into the surface of the substrate, and the machining of trench-shaped structures. Several dual LFE sensor arrays were fabricated and tested with impedance measurements. The devices were exposed to varying concentrations of saline and glycerol-water solutions, to test the sensor response to mechanical and electrical liquid property changes. Experimental results indicated successful lateral field excitation of well structures in quartz. X-axis inversion was successfully achieved and results indicated that a dual LFE array based on inversion was plausible. The best results were achieved using trench structures. Several dual LFE arrays based on these structures were shown to respond to mechanical and electrical property changes in liquid. The individual LFE elements were shown to operate as independent sensors with minimal cross-talk.