Acoustic Wave Engineering Sensors October one, 2000 By: Bill Drafts,
Windows 7 Starter Key, Microsensor Techniques Inc.
Acoustic wave products have been in business use for a lot more than sixty decades. The telecommunications industry could be the greatest customer, accounting for ~3 billion acoustic wave filters yearly, mainly in mobile cell phones and base stations. They're typically surface area acoustic wave (Observed) products, and act as bandpass filters in equally the radio frequency and intermediate frequency sections of the transceiver electronics. Many of the rising purposes for acoustic wave products as sensors may possibly ultimately equivalent the demand with the telecommunications market. These incorporate automotive apps (torque and tire pressure sensors), health-related programs (chemical sensors), and industrial and business programs (vapor, humidity,
Microsoft Office 2010, temperature, and mass sensors). Acoustic wave sensors are competitively priced, inherently rugged, quite sensitive, and intrinsically dependable. Some are also able of currently being passively and wirelessly interrogated (no sensor energy resource required).
Acoustic Wave Technological innovation Overview
Acoustic wave sensors are so named due to the fact their detection mechanism can be a mechanical, or acoustic, wave. As the acoustic wave propagates by way of or within the surface from the material, any adjustments towards the traits of your propagation path affect the velocity and/or amplitude of the wave. Modifications in velocity can be monitored by measuring the frequency or stage attributes with the sensor and may then be correlated for the corresponding bodily amount being measured.
Virtually all acoustic wave devices and sensors use a piezoelectric content to generate the acoustic wave. Piezoelectricity was found by brothers Pierre and Paul-Jacques Curie in 1880, received its name in 1881 from Wilhelm Hankel, and remained mostly a curiosity right up until 1921, when Walter Cady learned the quartz resonator for stabilizing digital oscillators [1]. Piezoelectricity refers towards the production of electrical costs through the imposition of mechanical anxiety. The phenomenon is reciprocal. Applying an proper electrical subject to a piezoelectric content results in a mechanical anxiety. Piezoelectric acoustic wave sensors utilize an oscillating electrical subject to produce a mechanical wave, which propagates with the substrate and is then converted back again to an electric subject for measurement.
Piezoelectric Substrate Materials
Figure one. Acoustic wave units are produced by way of a photolithographic procedure related to that employed to create ICs. The only big difference is the fact that no junction exists in acoustic wave sensors.
Among the piezoelectic substrate supplies that may be employed for acoustic wave sensors and products, the commonest are quartz (SiO2), lithium tantalate (LiTaO3), and, to a lesser diploma, lithium niobate (LiNbO3). Each has specific positive aspects and drawbacks, which consist of price, temperature dependence, attenuation, and propagation velocity. Table one lists some related specs for every material, including the most well-liked cuts and orientations [2]. An interesting house of quartz is that it can be achievable to pick the temperature dependence with the material by the lower angle and the wave propagation route. With suitable choice, the first buy temperature result could be minimized. An acoustic wave temperature sensor might be developed by maximizing this impact. This is not genuine of lithium niobate or lithium tantalate, exactly where a linear temperature dependence often exists for all substance cuts and propagation instructions. Other components with industrial prospective contain gallium arsenide (GaAs), silicon carbide (SiC), langasite (LGS), zinc oxide (ZnO), aluminum nitride (AlN), lead zirconium titanate (PZT), and polyvinylidene fluoride (PVdF).
Fabrication of Acoustic Wave Devices
Figure two. A standard acoustic wave device includes two sets of interdigital transducers. 1 transducer converts electric subject power into mechanical wave energy; another converts the mechanical energy back again into an electrical area.
The sensors are made by a photolithographic process (see Figure 1). Manufac turing begins by meticulously polishing and cleaning the piezoelectric substrate. Metal, typically aluminum, is then deposited uniformly onto the substrate. The device is spin-coated which has a photoresist and baked to harden it. It is then exposed to UV light by way of a mask with opaque places corresponding for the places to get metalized within the final device. The uncovered areas endure a chemical adjust that enables them to become taken out having a developing solution. Last but not least, the remaining photoresist is taken out. The pattern of steel remaining on the gadget is known as an interdigital transducer, or IDT. By changing the duration, width, position, and thickness of the IDT, the overall performance with the sensor could be maximized.
Acoustic Wave Propagation Modes
Acoustic wave devices are explained through the mode of wave propagation by means of or on the piezoelectric substrate. Acoustic waves are distinguished primarily by their velocities and displacement directions; several combinations are feasible, relying within the materials and boundary problems. The IDT of every sensor supplies the electric subject necessary to displace the substrate and as a result kind an acoustic wave. The wave propagates with the substrate, in which it really is transformed back to an electrical subject on the IDT on the other aspect. Figure two reveals the configuration of the common acoustic wave gadget. Transverse, or shear, waves have particle displacements which might be standard to the route of wave propagation and which may be polarized so that the particle displacements are either parallel to or normal to your sensing surface area. Shear horizontal wave movement signifies transverse displacements polarized parallel to your sensing surface; shear vertical movement signifies transverse displacements standard to your surface area.
Figure three. Though it's the oldest acoustic wave gadget, the thickness shear mode resonator is still used for measuring steel deposition prices.
A wave propagating with the substrate is termed a bulk wave. One of the most generally used bulk acoustic wave (BAW) units will be the thickness shear mode (TSM) resonator and also the shear-horizontal acoustic plate mode (SH-APM) sensor.
If the wave propagates around the surface area with the substrate, it's identified as a surface area wave. Probably the most widely utilized surface area wave units will be the floor acoustic wave sensor and also the shear-horizontal surface area acoustic wave (SH-SAW) sensor, also known as the floor transverse wave (STW) sensor.
All acoustic wave units are sensors in they are sensitive to perturbations of many diverse bodily parameters. Any adjust inside the qualities with the route over which the acoustic wave propagates will outcome in a change in output. All of the sensors will operate in gaseous or vacuum environments, but only a subset of them will run effectively once they are in speak to with liquids. The TSM, SH-APM, and SH-SAW all produce waves that propagate mostly from the shear horizontal motion. The shear horizontal wave doesn't radiate appreciable electricity into liquids, allowing liquid operation without extreme damping. Conversely, the Observed sensor has a considerable surface-normal displacement that radiates compression waves into the liquid, hence causing excessive damping. An exception to this rule takes place for units making use of waves that propagate at a velocity lower compared to sound velocity from the liquid. Regardless of the displacement elements, this kind of modes do not radiate coherently and therefore are hence reasonably undamped by liquids.
Other acoustic waves which might be promising for sensors contain the flexural plate wave (FPW), Love wave, surface-skimming bulk wave (SSBW), and Lamb wave. Prior to turning to application examples, it really is valuable to briefly critique every sensor sort.
Figure 4. Inside the shear-horizontal acoustic plate mode (SH-APM) sensor, the waves journey among the best and bottom surfaces with the plate, enabling sensing on either facet.
Bulk Wave Sensors
Thickness Shear Mode Resonator. The TSM, extensively known as a quartz crystal microbalance (QCM), will be the best-known, oldest, and simplest acoustic wave system. As proven in Figure 3, the TSM typically consists of a skinny disk of AT-cut quartz with parallel circular electrodes patterned on equally sides. The software of a voltage among these electrodes brings about a shear deformation of the crystal.
This device is known as a resonator because the crystal resonates as electromechanical standing waves are produced. The displacement is maximized with the crystal faces, producing the system delicate to surface interactions. The TSM resonator was initially employed to meas ure steel deposition costs in vacuum methods where it was frequently used in an oscillator circuit [3]. The oscillation frequency tracks the crystal resonance and indicates mass accumulation within the device surface. Inside the late 1960s, the TSM resonator was demonstrated to operate being a vapor sensor.
The TSM features simplicity of manufacture, capability to withstand harsh environments, temperature balance, and excellent sensitivity to added mass deposited about the crystal surface [4]. On account of its shear wave propagation part, the TSM resonator is also capable of detecting and measuring liquids, creating it a good candidate for any biosensor. Unfortunately, these gadgets have the lowest mass sensitivity with the sensors examined right here. Common TSM resonators run between 5 and thirty MHz. Making quite skinny devices that run at higher frequencies can increase the mass sensitivity, but thinning the sensors beyond the typical array leads to fragile gadgets which can be hard to manufacture and deal with. Current work has long been completed to type high-frequency TSM resonators utilizing piezoelectric films and bulk silicon micromachining tactics [5].
Figure five. Rayleigh waves transfer vertically within a path regular to your floor aircraft of the surface acoustic wave (Found) sensor. Noticed waves are very delicate to surface alterations, but don't operate properly for many liquid sensing programs.
Shear-Horizontal Acoustic Plate Mode Sensors. These products use a thin piezoelectric substrate, or plate, working as an acoustic waveguide that confines the energy between the upper and reduced surfaces of the plate (see Figure 4). Consequently, equally surfaces endure displacement, so detection can happen on possibly aspect. That is a vital benefit, as 1 aspect consists of the interdigital transducers that have to be isolated from conducting fluids or gases, whilst the other side can be used since the sensor.
As together with the TSM resonator, the relative absence of a surface-normal part of wave displacement permits the sensor to come into get in touch with with liquid for biosensor programs. SH-APM sensors have already been efficiently used to detect microgram-per-liter levels of mercury, which is adequate for Safe Drinking Water Act compliance testing [6]. Although a lot more sensitive to mass loading than the TSM resonator, SH-APM sensors are significantly less sensitive than surface wave sensors. You can find two reasons: The first is always that the sensitivity to mass loading as well as other perturbations is dependent within the thickness of your substrate, with sensitivity increasing as the system is thinned. The minimal thickness is constrained by manufacturing processes. 2nd, the vitality from the wave isn't maximized at the surface area, which minimizes sensitivity.
Surface Wave Sensors
Figure 6. The wave electricity is confined to within one wavelength in the surface of a Observed sensor. This attribute yields a sensor that is certainly very sensitive to interactions together with the surface.
Surface Acoustic Wave Sensors. In 1887, Lord Rayleigh learned the surface acoustic wave mode of propagation [7] and in his traditional paper predicted the properties of those waves. Named for their discoverer,
Office 2010 Discount, Rayleigh waves have a longitudinal plus a vertical shear part which will couple with a medium in contact together with the device’s surface area (see Figure 5,71). This sort of coupling strongly affects the amplitude and velocity from the wave. This feature enables Noticed sensors to directly sense mass and mechanical properties. The surface movement also enables the devices to get used as microactuators. The wave features a velocity that's ~5 orders of magnitude much less than the corresponding electromagnetic wave, generating Rayleigh surface area waves amongst the slowest to propagate in solids. The wave amplitudes are usually ~10 Å and also the wavelengths variety from one to 100 microns [8].
Figure six details the deformation field caused by a Saw propagating along the Z-axis and the associated distribution of potential electricity. Because Rayleigh waves have virtually all their acoustic electricity confined inside of a single wavelength of your surface, Found sensors have the highest sensitivity of the acoustic sensors reviewed.
Typical Noticed sensors run from 25 to 500 MHz. One particular disadvantage of these devices is Rayleigh waves are surface-normal waves, making them poorly suited for liquid sensing. When a Saw sensor is contacted by a liquid, the resulting compressional waves cause an excessive attenuation with the surface wave.
Photo 1. Acoustic wave sensors are commercially available in many form factors. Most sensors begin as processed wafers which might be then tested, diced, and mounted into packages.
Shear-Horizontal Surface Acoustic Wave Sensors. In the event the minimize of your piezoelectric crystal materials is rotated appropriately, the wave propagation mode alterations from a vertical shear Found sensor to a shear-horizontal Saw sensor. This dramatically minimizes loss when liquids come into contact with all the propagating medium, allowing the SH-SAW sensor to operate as a biosensor (see Figure 7).
Comparison of Acoustic Wave Sensors
In general, the sensitivity from the sensor is proportional to the amount of power from the propagation path getting perturbed. Bulk acoustic wave sensors usually disperse the electricity in the surface area with the bulk content to your other floor. This distribution of electricity minimizes the power density around the floor, that is exactly where the sensing is accomplished. Noticed sensors, conversely, focus their energy about the surface area, tending to make them far more sensitive [9,10] (see Table two). Other design considerations when selecting acoustic wave sensors contain oscillator stability and noise level.
Sensor Applications
All acoustic wave sensors are delicate, to varying degrees, to perturbations from several distinct physical parameters. Some commercially available acoustic wave sensors are shown in Photo 1. Like a matter of fact, all acoustic wave devices created for the telecommunications industry has to be hermetically sealed to prevent any disturbances since they will be sensed through the system and cause an unwanted alter in output.
Figure seven. By correctly selecting the orientation of content cut, shear-horizontal surface area acoustic waves (SH-SAW) will dominate. These waves have a very displacement that is parallel for the device’s surface.
The range of phenomena that may be detected by acoustic wave gadgets may be greatly expanded by coating the devices with materials that undergo alterations in their mass, elasticity, or conductivity upon exposure to some bodily or chemical stimulus. These sensors become stress, torque, shock, and force detectors under an applied stress that modifications the dynamics from the propagating medium. They become mass, or gravimetric, sensors when particles are allowed to contact the propagation medium, modifying the anxiety on it. They become vapor sensors when a coating is applied that absorbs only distinct chemical vapors. These units perform by effectively measuring the mass of the absorbed vapor. If your coating absorbs specific biological chemicals in liquids, the detector becomes a biosensor. As previously noted, a wireless temperature sensor may be created by selecting the correct orientation of propagation. The propagating medium modifications with temperature, affecting the output. Detailed below are some of the a lot more frequent applications of acoustic wave sensors.
Temperature Sensor. Surface area wave velocities are temperature dependent and therefore are determined through the orientation and sort of crystalline materials employed to fabricate the sensor. Temperature sensors based on Found delay line oscillators have millidegree resolution, great linearity, and low hysteresis [11]. They may be, however, really delicate to mass loading and so have to be sealed in a hermetic package. A 124 MHz ST-cut quartz, surface-skimming bulk wave temperature sensor was recently reported to possess a temperature coefficient of 32 ppm/ÞC plus a resolution of 0.22ÞC [12]. It also exhibited three orders of magnitude much less sensitivity to mass loading than do Saw sensors. The response time was found for being 0.three s, 10 three faster than BAW sensors. These temperature sensors possess the additional advantage of requiring no energy and of getting wireless, producing them nicely suited for use in remote locations.
Pressure Sensor. In 1975, the very first reported use of Observed engineering to get a sensor application was in the sort of a stress sensor [13]. Saw velocities are strongly affected by stresses applied for the piezoelectric substrate on which the wave is propagating. A Found pressure sensor is therefore designed by generating the Found system into a diaphragm (see Figure 8,).
Figure 8. The frequency of your Found modifications with stress. Because the diaphragm flexes due to pressure, the Saw sensor adjustments its output. Sadly, changes in temperature also cause a change in output.
The uncompensated temperature drifts that tend to interfere with Found strain sensors can be minimized by placing a reference Observed system close to the measuring Noticed within the same substrate and mixing the two signals [14]. A single sensor acts as being a temperature detector, whose proximity to the pressure sensor ensures that equally are uncovered to the same temperature. However, the temperature sensor Observed must be isolated from the stresses the pressure Noticed experiences (see Figure 9).
SAW stress sensors are passive (no energy needed), wireless, low price, rugged, and extremely small and lightweight, creating them nicely suited for measuring stress in moving objects (e.g., car and truck tires). These traits offer benefits over technologies this kind of as capacitive and piezoresistive sensors, which require operating power and are not wireless. A Noticed strain sensor weighing <one g, with a resolution of 0.73 psi, was recently integrated into a car tire with excellent outcomes [15]. These a system allows the operator to view the strain in each tire from the comfort of the cabin. Correctly inflated tires lead to improved safety, greater fuel efficiency, and longer tire life. This technological innovation is particularly interesting for the new run-flat (also known as zero strain or extended mobility) tire marketplace.
Torque Sensor. If a Noticed system is rigidly mounted to a flat spot on a shaft, and also the shaft experiences a torque, this torque will anxiety the sensor and turn it into a wireless, passive, lightweight torque detector. Because the shaft is rotated a single way, the Found torque sensor is placed in tension; rotated the other way, it really is placed in compression. For practical programs, two Found torque sensors are employed this kind of that their centerlines are at right angles (see Figure 10) [16]. As a result, when a single sensor is in compression, one other is in tension. Since equally sensors are subjected to the same temperature, the sum with the two signals minimizes any temperature drift effects.
Figure 9. Adding a 2nd, strategically placed Found effectively minimizes the temperature drift of the Saw strain sensor.
In comparison to other torque sensors, which includes resistive strain gauges, optical transducers, and torsion bars, Saw torque sensors offer reduced expense, larger reliability, and wireless operation. Monitoring torque on trucks and cars will significantly improve handling and braking since torque meas ures wheel traction much better compared to rpm sensors in current use.
Mass Sensor. Of all of the products evaluated here, Noticed sensors are the most delicate to mass loads. This opens up several applications including particulate sensors and film thickness sensors. If the sensor is coated with an adhesive substance, it becomes a particulate sensor; any particle landing about the floor will remain there and perturb the wave propagation. A mass resolution of three pg to get a 200 MHz ST-cut quartz Found continues to be reported, which was 1000 3 the sensitivity from the 10 MHz TSM resonator tested [17]. Particulate sensors are employed in cleanrooms, air quality monitors, and atmos pheric monitors.
Thickness sensors run on basically the same principle as particulate sensors, except that they are not coated. The measured frequency shift is proportional towards the mass with the deposited film, so the sensor offers thickness data by measuring the film density and acoustic impedance. This method is accurate, provided the film is skinny (ideally no greater than a few percent of the acoustic wavelength) [18]. Most commercially available thickness sensors are based on TSM resonators. Even though not so delicate as Saw sensors, these gadgets offer ease of use and ample sensitivity.
Dew Point/Humidity Sensor. If a Found sensor is temperature controlled and subjected to the ambient atmosphere, drinking water will condense on it in the dew point temperature, producing it an effective dew point sensor. Current business instruments for high-precision dew point measurements are based on optical methods, which have expense, contamination, accuracy, sensitivity, and long-term stability issues. A 50 MHz YZ-cut lithium niobate Saw dew point sensor continues to be developed that is immune to typical contaminants, features a resolution of ±0.025°C (vs. ±0.2°C for an optical sensor), is low price, and it is significantly far more stable [19].
Figure 10. The anxiety from the shaft is transferred to your Found sensor, which adjustments its output frequency with pressure and, therefore, torque. The addition of another Observed minimizes temperature effects.
Acoustic wave sensors with an elastic hygroscopic polymer coating make excellent humidity detectors. Three operational mechanisms contribute to the sensors’ response: mass loading, acoustoelectric effects, and viscoelastic effects, each and every of which might be effectively controlled to yield an accurate, low-cost, humidity sensor. A 50 MHz YZ-cut lithium niobate Found sensor coated with polyXIO has long been demonstrated as a humidity sensor with a variety of 0%–100% RH as well as a hysteresis on the purchase of 5% [20]. In addition, a 767 MHz AT-cut quartz SH-SAW sensor coated which has a plasma-modified hexamethyldisiloxane (HMDSO) has recently been demonstrated as a humidity sensor, which has a sensitivity of 1.4 ppm/% RH plus a 5% hysteresis. This was found for being 4–10 3 a lot more delicate than a 14 MHz TSM resonator coated with the same polymer [21].
In the same category, a 434 MHz YZ-cut lithium niobate Observed sensor has long been utilised as a remote drinking water sensor [22], and an 86 MHz XY-cut quartz Really like wave sensor has become demonstrated as an ice sensor [23].
Vapor Chemical Sensor—Coated and Uncoated. Chemical vapor sensors based on Found products were initial reported in 1979 [24]. Most of them rely about the mass sensitivity of the detector, in conjunction which has a chemically selective coating that absorbs the vapors of interest and ends in an increased mass loading from the system (see Figure 11). As using the temperature-compensated strain sensors, 1 Observed is used as a reference, effectively minimizing the effects of temperature variations.
Photo two. A commercially available handheld Saw chemical vapor analyzer incorporates an array of four Noticed sensors, every coated which has a various polymer.
Several design considerations have to be satisfied when selecting and applying the chemically sorptive coating. Ideally, the coating is completely reversible, meaning that it will absorb and then completely desorb the vapor when purged with clean air. The rate at which the coating absorbs and desorbs should be fairly quick, <1 s, for instance. The coating should be robust enough to withstand corrosive vapors. It should be selective, absorbing only extremely particular vapors whilst rejecting others. The coating must operate more than a realistic temperature variety. It should be stable, reproducible, and delicate. And finally, its thickness and uniformity are very important.
When many Found sensors, each and every using a unique chemically particular coating, are configured being an array, each will have a very various output when exposed to a given vapor. Pattern recognition software enables a diverse list of volatile organic compounds thus to be detected and identified, yielding a quite powerful chemical analyzer. A commercially available analyzer with an array of four Observed sensors is demonstrated in Photo 2.
TSM resonators have also efficiently been employed for chemical vapor sensing [25] but they can be significantly much less sensitive than their Observed counterparts. In addition, Saw chemical vapor sensors have been created without coatings. This method uses a gas chromatograph column to separate the chemical vapor elements, as well as a temperature-controlled Observed that condenses the vapor and measures the corresponding mass loading [26].
Figure 11. By coating a Saw system which has a chemically sorptive polymer, a chemical vapor sensor is manufactured. Adding another Noticed system minimizes the temperature drift and provides a manageable variation frequency.
Biosensor. Comparable to chemical vapor sensors, biosensors detect chemicals, but in liquids rather than vapors. As noted earlier, the Found device is really a poor choice for this software, because the vertical part from the propagating wave will be suppressed by the liquid. Biosensors happen to be fabricated making use of the TSM resonator, SH-APM, and SH-SAW sensors. Of all of the recognized acoustic sensors for liquid sensing, the Enjoy wave sensor, a special class of your shear-horizontal Observed, has the highest sensitivity [27]. To produce a Enjoy wave sensor, a waveguide coating is placed on the SH-SAW system this kind of that the energy of the shear horizontal waves is focused in that coating. A biorecognition coating is then placed within the waveguide coating, forming the complete biosensor. Successful detection of anti-goat IgG from the concentration assortment of 3 three 10–8–10–6 moles employing a 110 MHz YZ-cut SH-SAW having a polymer Enjoy wave guide coating has become achieved [28].
Guide to Abbreviations BAW
FPW
IDT
QCM
SAW
SH-APM
SH-SAW
SSBW
STW
TSM
Bulk acoustic wave
Flexural-plate wave
Interdigital trans-ducer
Quartz crystal- microbalance
Floor acoustic wave
Shear-horizontal acoustic plate mode
Shear-horizontal sur-face acoustic wave
Surface-skimming bulk wave
Surface transverse wave
Thickness shear mode
Conclusion
Acoustic wave sensors are extremely versatile products which are just beginning to realize their industrial possible. They are competitively priced, inherently rugged, really delicate,
Windows 7 Enterprise, and intrinsically trustworthy, and can be interrogated passively and wirelessly. Wireless sensors are beneficial when monitoring parameters on moving objects, this sort of as tire stress on cars or torque on shafts. Sensors that require no operating power are highly desirable for remote monitoring of chemical vapors, moisture, and temperature. Other applications consist of measuring force, acceleration, shock, angular rate, viscosity, displacement, and flow, in addition to film characterization. The sensors also have an acoustoelectric sensitivity, permitting the detection of pH levels, ionic contaminants, and electrical fields. Surface area acoustic wave sensors have proved to become the most sensitive in general consequently of their larger electricity density on the floor. For liquid sensing, a special class of shear-horizontal floor acoustic wave sensors known as Enjoy wave sensors proved to be essentially the most delicate. Much work is continuing in establishing these sensors for future purposes.
References
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Microsoft Office 2010 Acoustic Wave Technology Sen
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