Sensors based on surface-launched acoustic wave devices have been developed since the 1980's for application to physical measurements (temperature, pressure, torque, strain, etc.) and to a wide range of chemical and biological detection problems. These widely varying devices have utilized several operating modes and corresponding wave propagation modes, including the traditional Rayleigh wave (or Surface Acoustic Wave—SAW), the surface transverse wave (STW) or the shear horizontal SAW (SH-SAW), the surface skimming bulk wave (SSBW), the SH-SAW or SSBW that has been guided to the surface and into a surface layer of slower acoustic wave velocity, known as the Love wave, the shear-horizontally polarized acoustic plate mode (SH-APM), the flexural plate wave (FPW) or Lamb wave, the layer guided acoustic plate mode (LG-APM), and the thickness shear mode (TSM) bulk wave (as used in the quartz crystal microbalance—QCM). A number of different device types have been recognized using these diverse wave modes, including resonators, delay lines, differential delay lines, and reflective delay lines (tag or ID devices). These devices have been operated within a wide range of wired and wireless interrogation system architectures, which have generally been designed specifically to operate with the selected sensor(s). In most cases, wireless interrogation has been applied to physical sensors, and not to biological or chemical sensors. These system architectures include pulsed radar-like delay measurement systems, Fourier transform measurement systems, and delay line and resonator-based oscillator systems. The system architecture has usually been selected based on specific device characteristics and application requirements, and generally involves absolute or differential measurements of sensor frequency, phase, delay, amplitude, or power spectral density, and changes in these quantities with exposure, to provide the output sensor measurement.
The relative advantages of each wave mode and device type make them suitable for different applications. Rayleigh wave sensors, for instance, involve particle displacements that include a component normal to the substrate surface. When used in a liquid, this component generates a compressional wave in the liquid, causing wave energy to leak into the liquid. This energy leakage results in large attenuation of the Rayleigh wave, often referred to as “damping”. This effect makes Rayleigh waves useful only for gas phase sensing, and not applicable to sensing in the liquid phase. This energy leakage occurs whenever the wave motion in the substrate involves a component of displacement normal to the substrate surface, and the speed of the sound wave in the device is greater than the speed of sound in the liquid (or in the layer coating the device). Certain wave modes, such as flexural plate waves (FPWs), do involve a normal component of displacement, but have wave velocities lower than the speed of sound in the liquid. Leakage therefore does not occur, and FPW devices can operate successfully in liquid environments. Other wave modes that do not involve components of displacement normal to the substrate surface are also operable in both gas and liquid phase. These include Love waves, STW or SH-SAW, SH-APM, and LG-APM.
Rayleigh waves coated with polymers have been used extensively for chemical vapor detection. QCM devices have also been applied to characterization of interfacial chemistry in both vapor and liquid environments. In recent years, there has been significant research into the application of STW or SH-SAW, APM, FPW, and Love waves to liquid based biosensing. Michael Thompson and David Stone provide an overview of surface launched acoustic wave sensors in their book, Surface-Launched Acoustic Wave Sensors—Chemical Sensing and Thin-Film Characterization [ISBN 0-471-12794-9, 1997 John Wiley & Sons Inc, NY]. In journal literature, Love waves are often cited as having the highest possible mass sensitivity for example by Electra Gizeli et al., in “A novel Love-plate acoustic sensor utilizing polymer overlayers”, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 39, No. 5, September 1992, pp. 657-659. STW devices have the practical drawback of difficulty of creating a physical interface between the fluid (liquid or gas) sample chamber and the active device surface. In STW devices, the fluid must be constrained to interact with the surface of the device on which the wave is generated and propagates. This involves making a liquid or gas tight contact on the substrate surface without interfering with the generation and propagation of the acoustic wave. FPW, APM, and LG-APM devices, by comparison, have an advantage in that a gas or liquid sample can interact with the back side of the device, leaving the wave generation process (on the front side of the device) unaffected.
Until recently, most reported FPW devices have been fabricated using silicon substrates with deposited surface layers with desired properties. The silicon substrates are then etched away in the region beneath the sensor active region, leaving a membrane consisting of the surface layer only. These layers may be composed of various films, and the backside of the device may be used to allow exposure of the device to liquid samples, while keeping the electrical connections of the device separated from the sample. Typical films consist of a structural component such as silicon nitride (Si3N4), combined with a ground electrode layer (often aluminum), followed by a piezoelectric film layer such as zinc oxide (ZnO), and surface fabricated electrodes. In the literature, a good example of this type of device is provided by B. J. Costello et. al., in “Acoustic plate-wave biosensing”, Proceedings of the IEEE Engineering in Medicine and Biology Society 11th Annual International Conference, 1989. Composite layer thicknesses typically range from around 3 microns to around 6 microns in thickness, and the resulting devices have operating frequencies in the low MHz range. APM devices, by comparison, have generally been fabricated from plates of piezoelectric materials, often using the thickness of standard wafers. Typical devices may utilize substrates with thickness of 0.5 mm (20 mils). APM devices have been demonstrated on quartz and on high coupling substrates such as lithium niobate. Typical APM devices operate in the low hundred MHz range. In the literature, R. Dahint et. al. discuss APM immunosensors in “Operation of acoustic plate mode immunosensors in complex biological media”, Anal. Chem. Vol. 71, 1999, pp. 3150-3156. Also in the literature, C. Zimmermann et.al. compare the performance of APM devices with other acoustic sensors in “Evaluation of Love waves chemical sensors to detect organophosphorous compounds: comparison to SAW and SH-APM devices”, Proceedings of the 2000IEEE International Frequency Control Symposium, pp. 47-51. In published U.S. Patent Application No. US 2007/0000327 A1, filed Jan. 6, 2006, an acoustic wave sensing device with integrated micro-channels is presented that utilizes flexural plate waves (FPWs) on thinned single crystal piezoelectrics. Both single and double sided processes are discussed for substrate thinning, and protective electrode layers are included to prevent deterioration of the electrodes due to exposure in the sensor. An array of two or more acoustic wave sensing devices are taught, and microfluidic channels connecting the sensing elements are integrated with the FPW devices.
Love wave devices consist of a substrate that is thick relative to an acoustic wavelength (and thus can be viewed as semi-infinite) and a top layer that acts as a guiding layer for the acoustic wave. Generally, the substrate is piezoelectric, such as quartz, and the guiding layer is made of a material with a sound speed lower than the wave speed in the substrate. This structure produces a shear wave with amplitude that decays with depth in the substrate, and with varying amounts of wave energy penetrating the guiding layer (depending on specific device materials and operating parameters). On quartz, amorphous SiO2 and various polymers (PMMA, etc.) are often utilized as the guiding layer. Standard thickness piezoelectric substrates are generally used, with varying thicknesses of guiding layers based on device design. Fundamental and harmonic device operation have been evaluated, resulting in operation frequencies ranging from roughly 100 MHz to over 300 MHz.
Finally, layer-guided SH-APMs (LG-SH-APMs) have been identified as shear horizontally polarized waves that occur in a system that consists of a finite substrate covered by a finite guiding layer of slower shear acoustic speed, and are analogous to either Love waves or to SH-APMs, depending on the precise structure of the device under consideration. It has been suggested that these devices will be capable of higher mass sensitivity than other previously identified device structures. In the literature, Glen McHale et. al. provide a comprehensive discussion of the relationship between Love waves and layer guided SH-APMs in “Layer guided shear horizontal acoustic plate mode sensors”, Proceedings of the 2002 IEEE International Frequency Control Symposium and PDA Exhibition; and in “Layer guided shear horizontally polarized acoustic plate modes”, Journal of Applied Physics, Vol. 91, No. 9, pp. 5735-5744. In these publications, the authors clearly explain the close connection between love wave modes, which are shear horizontally polarized surface acoustic waves localized to the surface of a semi-infinite half space by a guiding layer that has a shear acoustic speed less than that of the half space material, and LG-SH-APMs. SH-APMs are resonant shear waves that occur in substrates of finite thickness. These waves do not have wave energy that decays with depth into the substrate as Love waves do, but rather have displacement that is resonant throughout the thickness of the plate, with displacements on both the upper and lower surfaces. Because the wave energy penetrates the entire plate, it is possible (similar to a FPW) to excite an APM with transducers on one side of the substrate and use motion on the other side of the substrate to effect sensing. Placing a layer with slower shear acoustic speed on one side of a substrate capable of supporting SH-APM propagation will result in a LG-SH-APM or a Love wave, depending on the relative thicknesses and shear acoustic speeds of the substrate and film, as well as device operating frequency. These references also make it clear that the high mass sensitivity of Love wave sensors and LG-SH-APM sensors is not due simply to localization of the acoustic wave energy at the surface as widely accepted, rather the high mass sensitivity is primarily due to the combination of high frequency operation and operation in a combined film thickness and frequency regime wherein any change in mass due to surface deposition of an analyte to be measured results in a large change in the phase speed of the wave mode being used. The same can be said for surface changes resulting in changes in physical film properties such as stiffness, elasticity, etc., and for conductivity changes. The large increase in sensitivity possible using LG-SH-APM, combined with the feasibility of exciting these waves using transducers on the substrate face opposite that where sensing takes place makes devices using this construction highly desirable for chemical and biological sensing applications.
Due to the sensitivity of surface-launched acoustic wave sensors to changes in many environmental parameters, it has been customary to utilize some sort of reference device in the sensors or in the sensor systems. This has been accomplished in various ways. For example, differential delay line devices have been used to eliminate variations in electronic signals common to both delay paths, resulting in sensors that are only sensitive to variations in the parameter being sensed. In the literature, the inventor and her research collaborators included differential delay lines in coded SAW sensors for temperature measurement, in “SAW Sensors Using Orthogonal Frequency Coding”, Proceedings of the 2004 IEEE International Ultrasonics, Ferroelectrics, and Frequency Control Symposium. Similarly, pressure sensors have been developed that utilize multiple transducer and/or reflector structures with wave propagation at different orientations on the substrate to provide information about temperature simultaneously with information about pressure, allowing for the unambiguous determination of both parameters using a single sensor device. U.S. Pat. Nos. 6,571,638 (Hines et. al.), 6,907,787 B2 (Cook et. al.), and U.S. Patent Application US2005/0056098 A1 (Solie) all relate to such sensors.
SAW-based chemical vapor sensor systems have historically utilized multiple polymer-coated SAW sensor devices in an array configuration. Polymers were selected for their chemical orthogonality, or their ability to selectively adsorb or absorb chemical vapors of interest. Patterns of vapor responses developed on the multi sensor arrays could then be characterized using pattern recognition techniques. Reference sensors that were hermetically sealed or otherwise protected from exposure to the vapors under test were generally included in the arrays in order to allow for accurate determination of the array response. These arrays were often temperature controlled, either through bulk temperature control of the sensor devices (using under package heating and cooling) or through on-chip heaters incorporated in the sensor devices. These temperature control elements (including on-chip heaters) could be used to thermally ramp sensors and observe the temperature (and thus time) dependent desorption of adsorbed of vapors, providing an additional metric useful for pattern recognition. Prior acoustic wave biosensor devices have generally been used individually or in pairs, where one device serves as a reference device for the pair. In all cases known to the inventor where arrays of sensors have been used in biological and/or chemical sensing (prior to published U.S. Patent Application No. US 2007/0000327 A1, filed Jan. 6, 2006) the array has been composed of multiple individual distinct sensor devices along with measurement electronics. Depending on the system configuration, the measurement electronics may be common (“shared” and used sequentially by all sensors in the array), or multi-channel electronics may be used, allowing the simultaneous (or near-simultaneous) measurement of all array elements. U.S. Patent Application No. US 2007/0000327 Al introduces an acoustic wave array sensing device with integrated micro-channels that utilizes flexural plate waves (FPWs) on thinned single crystal piezoelectrics. Integration of microfluidic channels connecting the sensing elements and utilization of a FPW acoustic mode are advantageous characteristics of that approach.
Prior SAW based RF ID tags and physical sensors have utilized various coding techniques to allow identification of individual sensors within multisensor networks. Such sensors have also been accessed primarily via wireless radio frequency (RF) communication techniques. To date, the ability to incorporate unique sensor identification and the potential wireless operation aspect of these sensors has not been exploited for chemical and biological sensing applications in vapors and liquids.
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