Donald F. McCann

A lateral-field-excited LiTaO 3 high-frequency bulk acoustic wave sensor

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM), which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM, the exciting electric field is primarily perpendicular to the crystal surface, resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation (LFE) in which electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that can support temperature-compensated TSMs and operate efficiently at high frequencies resulting in increased sensitivity. In this work, theoretical search and experimental measurements are performed to identify the existence of high-frequency temperature-compensated TSMs in LiTaO3. Prototype LFE LiTaO3 sensors were fabricated and found to operate at frequencies in excess of 1 GHz and sensitively detect viscosity, conductivity, and dielectric constant changes in liquids.

Recent advances in harsh environment acoustic wave sensors for contemporary applications

There is a significant need for wireless sensor systems capable of operation up to 1100°C and beyond, in abrasive or corrosive harsh environments, in particular for the energy, steel, aerospace, oil and gas exploration industries. These environments and applications preclude the use of batteries and normally require wireless and multiple sensor interrogation. The University of Maine and Environetix Technologies have successfully responded to these needs by researching and developing surface acoustic wave (SAW) sensors based on the langasite family of crystals and co-deposited Pt/Rh/ZrO2 thin-film electrode technology. This paper reports on the recent achievements, which include: long term operation in furnace and technology validation in jet-engine static and rotating parts up to 53,000 gs; stable and repetitive wired and wireless responses of temperature sensors; multiple wireless sensor interrogation; and associated packaging (tests run in the 200°C to 1000°C range).

Wireless acoustic wave sensors and systems for harsh environment applications

This paper reviews current progress in the area of wireless microwave acoustic sensor technology, and discusses advances in wireless interrogation systems that can operate in harsh environments. The use of wireless, battery-free, low maintenance surface acoustic wave (SAW) sensors has been successfully demonstrated in applications including high temperature turbine engines and inflatable aerospace structures. Wireless interrogation of multiple sensors up to 910°C has been established and sensor tests in gas turbine engine are reported. This paper elaborates on several aspects of the technology, including: high-temperature thin-film electrode and sensor development, temperature cycling, thermal-shock behavior, testing in turbine engine environments, sensor packaging and attachment, wireless operation, and adaptation to energy and industrial applications.

Recent advances in lateral field excited and monolithic spiral coil acoustic transduction bulk acoustic wave sensor platforms

The quartz crystal microbalance (QCM) has been used extensively as a bulk acoustic wave (BAW) platform for applications such as chemical and biological sensors and rate monitors in thin film deposition systems. Although the QCM is capable of measuring mechanical property changes critical in many thin film deposition systems, it cannot measure electrical property changes that can occur in many sensor applications. In this paper we review the recent developments of two novel transducer configurations for BAW sensors. In the first sensor, called the lateral field excited (LFE) sensor, the transverse shear mode (TSM) in AT-cut quartz is excited by two electrodes on the reference surface, resulting in a bare sensing surface which allows both electrical and mechanical properties of target analytes to be measured. In the second sensor, called the monolithic spiral coil acoustic transduction (MSCAT) sensor, the TSM is excited by a photolithographically deposited spiral antenna on the reference surface which can excite high-order harmonics in the substrate, and potentially lead to increased sensitivity. The responses of both the LFE and MSCAT sensors to electrical and mechanical property changes of liquids have been examined and compared to the response of the standard QCM. In addition, results relating to the detection of chemical and biological target analytes using the LFE and MSCAT sensor platforms are presented.

Acoustic mode behavior in Lateral Field Excited sensors

Lateral Field Excited (LFE) sensors have been shown to have several very attractive features [1], the most important of which is a bare sensing surface. In contrast to the standard Quartz Crystal Monitor (QCM), the LFE sensor surface allows one to sensitively monitor both mechanical and electrical property changes in adjacent media or analyte selective biological or chemical films. Although LFE sensors have been shown to function very efficiently in liquid media, LFE sensor operation in gaseous environments has been plagued by spurious mode responses. It is the purpose of this work to study electrode dimensions, sensor curvature, and surface etching in order to minimize the spurious mode behavior found in LFE Quartz and Lithium Tantalate (LTO) gas sensors.

A Lateral Field Excited Acoustic Wave Sensor

A lateral field excited (LFE) acoustic wave sensor with a bare sensing surface was developed. In addition to sensing mechanical and electrical changes in liquids, film-coated LFE devices were used to sense mechanical and electrical film property changes caused by chemical and biological analytes in solution. The sensor was tested as a chemical and biological sensor, detecting phosmet and Escherichia coli, respectively. A method to acoustically isolate multiple LFE sensors on a single substrate is also described. The results show that the LFE sensor can be used to sense analytes critical to homeland security, the military, agriculture, and health.

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

The electromagnetic fields radiated from a monolithic spiral coil acoustic transduction sensor

The monolithic spiral coil acoustic transduction (MSCAT) sensor platform is a novel bulk acoustic wave (BAW) device which is excited by a gold spiral coil antenna photolithographically deposited on one side of an AT-quartz wafer. The MSCAT platform can operate at very high frequencies by efficiently exciting high harmonic transverse shear modes (TSMs) with the application of a high frequency RF signal to the spiral coil. Since one surface of the MSCAT device is bare, this device can be used as a sensing platform upon which one deposits analyte selective chemical or biological films. The bare surface allows the detection of analyte induced mechanical (mass and viscoelasticity) and electrical (conductivity and dielectric constant) property changes in the film. Although previous work has shown that the MSCAT sensor is a highly sensitive sensor platform for chemical and biological analyte detection, the form of the electric field in the AT-quartz substrate is not well understood. In this work finite element analysis (FEA) was used to obtain the electromagnetic fields radiated by the spiral coil antenna deposited on AT-cut quartz. The theoretical results were compared to experimental measurements for specific spiral coil geometries. The spiral coil antenna was shown to produce electric fields that have components in both the lateral and thickness directions. These fields were responsible for exciting the TSM in the MSCAT sensor platform. Further, it was also shown that liquid properties on the sensor surface significantly alter the form of the radiated electric field. Also, the relative magnitude of the electric field in the AT-cut quartz vary significantly dependent on operating frequency and the spiral coil geometry.

The detection of chemical and biological analytes using a monolithic spiral coil acoustic transduction sensor

The monolithic spiral coil acoustic transduction (MSCAT) sensor platform is a novel bulk acoustic wave (BAW) device which is excited by a gold spiral coil antenna photolithographically deposited on one side of an AT-quartz wafer. The MSCAT platform can operate at very high frequencies by efficiently exciting high harmonic transverse shear modes with the application of a high frequency RF signal to the spiral coil. Since one surface of the MSCAT device is bare, this device can be used as a sensing platform upon which one deposits analyte selective chemical or biological films. The bare surface allows the detection of analyte induced mechanical (mass and viscoelasticity) and electrical (conductivity and dielectric constant) property changes in the film. In order to demonstrate the applicability of a MSCAT device as a sensor, the MSCAT platform is coated with biological and chemical films selective to Escherichia coli (E. coli) O157:H7, the E. coli strain most often responsible for serious illnesses in humans, and saxitoxin (STX), the most dangerous neurotoxin associated with shellfish poisoning stemming from red tide, respectively.

Novel transducer configurations for bulk acoustic wave sensors

Two novel transducer configurations for bulk acoustic wave (BAW) sensors have been developed. In the first sensor, called the lateral field excited (LFE) sensor, the transverse shear mode (TSM) in AT-cut quartz is excited by two electrodes on the reference surface resulting in a bare sensing surface which allows both electrical and mechanical properties of target analytes to be measured. In the second sensor, called the monolithic spiral coil acoustic transduction (MSCAT) sensor, the TSM is excited by a photolithographically deposited spiral antenna on the reference surface which can excite high order harmonics in the substrate, which can lead to increased sensitivity. In order to demonstrate the applicability and advantages of the LFE and MSCAT sensor platforms, the sensorspsila responses to chemical and biological target analytes were examined and compared to the responses of a standard quartz crystal microbalance (QCM).