Office Professional 2007 Acoustic Wave Technology

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Acoustic Wave Technological innovation Sensors October one, 2000 By: Bill Drafts, Microsensor Methods Inc.

Acoustic wave gadgets are in industrial use for over 60 many years. The telecommunications business is the greatest client, accounting for ~3 billion acoustic wave filters each year, mainly in cellular cell phones and base stations. These are generally surface area acoustic wave (Observed) gadgets, and act as bandpass filters in both the radio frequency and intermediate frequency sections from the transceiver electronics. Several of the rising programs for acoustic wave products as sensors may possibly ultimately equivalent the demand from the telecommunications industry. These contain automotive applications (torque and tire strain sensors), healthcare programs (chemical sensors), and industrial and industrial purposes (vapor, humidity, temperature, and mass sensors). Acoustic wave sensors are competitively priced, inherently rugged, quite sensitive, and intrinsically trustworthy. Some will also be capable of becoming passively and wirelessly interrogated (no sensor power supply essential).
Acoustic Wave Technological innovation Overview

Acoustic wave sensors are so named since their detection mechanism can be a mechanical, or acoustic, wave. As the acoustic wave propagates by way of or on the floor of the content, any changes to the attributes with the propagation path have an effect on the velocity and/or amplitude with the wave. Adjustments in velocity might be monitored by measuring the frequency or stage qualities of the sensor and may then be correlated for the corresponding physical quantity becoming measured.
Virtually all acoustic wave products and sensors use a piezoelectric content to make the acoustic wave. Piezoelectricity was discovered by brothers Pierre and Paul-Jacques Curie in 1880, received its title in 1881 from Wilhelm Hankel, and remained largely a curiosity right up until 1921, when Walter Cady discovered the quartz resonator for stabilizing electronic oscillators [1]. Piezoelectricity refers towards the creation of electrical charges from the imposition of mechanical tension. The phenomenon is reciprocal. Applying an acceptable electrical subject to a piezoelectric substance results in a mechanical anxiety. Piezoelectric acoustic wave sensors utilize an oscillating electrical field to build a mechanical wave, which propagates with the substrate and it is then converted back again to an electric subject for measurement.
Piezoelectric Substrate Materials

Figure 1. Acoustic wave units are manufactured by way of a photolithographic procedure related to that employed to produce ICs. The one variation is the fact that no junction exists in acoustic wave sensors.
Among the piezoelectic substrate supplies which can be used for acoustic wave sensors and gadgets, the most common are quartz (SiO2), lithium tantalate (LiTaO3), and, to a lesser degree, lithium niobate (LiNbO3). Every single has certain strengths and drawbacks, which consist of cost, temperature dependence, attenuation, and propagation velocity. Table one lists some pertinent specifications for each substance, like the most popular cuts and orientations [2]. An exciting residence of quartz is always that it really is possible to pick the temperature dependence of the material by the reduce angle and the wave propagation route. With correct choice, the primary order temperature effect may be minimized. An acoustic wave temperature sensor could be created by maximizing this influence. This isn't true of lithium niobate or lithium tantalate, exactly where a linear temperature dependence constantly exists for all content cuts and propagation instructions. Other components with industrial possible incorporate 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 2. A common acoustic wave device includes two sets of interdigital transducers. One particular transducer converts electrical subject energy into mechanical wave power; the opposite converts the mechanical vitality back into an electric subject.
The sensors are created by a photolithographic process (see Figure 1). Manufac turing commences by very carefully polishing and cleansing the piezoelectric substrate. Metal, generally aluminum, is then deposited uniformly onto the substrate. The gadget is spin-coated using a photoresist and baked to harden it. It can be then subjected to UV light through a mask with opaque places corresponding towards the areas to become metalized about the ultimate device. The exposed regions endure a chemical adjust that allows them to become eliminated having a establishing answer. Lastly, the remaining photoresist is removed. The pattern of metal remaining on the system is called an interdigital transducer, or IDT. By modifying the duration, width, position, and thickness with the IDT, the overall performance of the sensor may be maximized.
Acoustic Wave Propagation Modes
Acoustic wave gadgets are explained from the mode of wave propagation by means of or on the piezoelectric substrate. Acoustic waves are distinguished mainly by their velocities and displacement directions; many combos are achievable, relying around the materials and boundary situations. The IDT of every sensor offers the electric subject essential to displace the substrate and therefore sort an acoustic wave. The wave propagates with the substrate, exactly where it's transformed back again to an electric field at the IDT on the other aspect. Figure 2 reveals the configuration of a normal acoustic wave system. Transverse, or shear, waves have particle displacements which might be standard for the path of wave propagation and which may be polarized so that the particle displacements are both parallel to or typical for the sensing surface. Shear horizontal wave movement signifies transverse displacements polarized parallel for the sensing surface area; shear vertical movement indicates transverse displacements typical to the surface area.

Figure three. Although it's the oldest acoustic wave device, the thickness shear mode resonator continues to be used for measuring metal deposition rates.
A wave propagating with the substrate is called a bulk wave. The most generally utilized bulk acoustic wave (BAW) gadgets would be the thickness shear mode (TSM) resonator and the shear-horizontal acoustic plate mode (SH-APM) sensor.
If the wave propagates around the surface of the substrate, it can be identified as being a surface wave. One of the most widely used floor wave devices would be the surface acoustic wave sensor and also the shear-horizontal surface area acoustic wave (SH-SAW) sensor, also called the floor transverse wave (STW) sensor.
All acoustic wave products are sensors in that they are delicate to perturbations of several diverse physical parameters. Any adjust inside the characteristics of your route more than which the acoustic wave propagates will end result inside a adjust in output. Every one of the sensors will operate in gaseous or vacuum environments, but only a subset of them will run efficiently when they are in get in touch with with liquids. The TSM, SH-APM, and SH-SAW all create waves that propagate primarily in the shear horizontal motion. The shear horizontal wave isn't going to radiate appreciable power into liquids, allowing liquid operation without having excessive damping. Conversely, the Observed sensor incorporates a considerable surface-normal displacement that radiates compression waves into the liquid, hence causing extreme damping. An exception to this rule occurs for gadgets employing waves that propagate at a velocity reduced than the sound velocity inside the liquid. Regardless of the displacement parts, this kind of modes tend not to radiate coherently and therefore are thus reasonably undamped by liquids.
Other acoustic waves which are promising for sensors include the flexural plate wave (FPW), Adore wave, surface-skimming bulk wave (SSBW), and Lamb wave. Prior to turning to software examples, it is valuable to briefly critique each sensor sort.
Figure 4. Within the shear-horizontal acoustic plate mode (SH-APM) sensor, the waves travel between the top and bottom surfaces from the plate, enabling sensing on either side.
Bulk Wave Sensors
Thickness Shear Mode Resonator. The TSM, broadly referred to as a quartz crystal microbalance (QCM), is the best-known, oldest, and easiest acoustic wave system. As demonstrated in Figure 3, the TSM generally includes a thin disk of AT-cut quartz with parallel circular electrodes patterned on equally sides. The software of a voltage among these electrodes ends in a shear deformation of the crystal.
This device is called a resonator because the crystal resonates as electromechanical standing waves are created. The displacement is maximized at the crystal faces, producing the system delicate to surface interactions. The TSM resonator was originally used to meas ure metal deposition costs in vacuum techniques wherever it had been generally utilised in an oscillator circuit [3]. The oscillation frequency tracks the crystal resonance and indicates mass accumulation on the gadget floor. From the late 1960s, the TSM resonator was demonstrated to operate as a vapor sensor.
The TSM attributes simplicity of manufacture, capability to withstand harsh environments, temperature stability, and great sensitivity to added mass deposited on the crystal surface area [4]. Due to its shear wave propagation part, the TSM resonator can be capable of detecting and measuring liquids, generating it a good candidate for the biosensor. Regrettably, these gadgets have the lowest mass sensitivity from the sensors examined here. Standard TSM resonators operate between 5 and 30 MHz. Creating extremely thin units that operate at increased frequencies can increase the mass sensitivity, but thinning the sensors outside of the standard assortment brings about fragile units which are difficult to manufacture and handle. Recent function continues to be completed to type high-frequency TSM resonators using piezoelectric films and bulk silicon micromachining methods [5].
Figure 5. Rayleigh waves move vertically in a course typical towards the surface area plane of the floor acoustic wave (Found) sensor. Found waves are incredibly sensitive to floor alterations, but do not work effectively for the majority of liquid sensing programs.
Shear-Horizontal Acoustic Plate Mode Sensors. These products use a skinny piezoelectric substrate, or plate, operating as an acoustic waveguide that confines the energy between the upper and reduced surfaces of your plate (see Figure four). Because of this, equally surfaces endure displacement, so detection can arise on either side. This is an important benefit, as 1 side consists of the interdigital transducers that have to be isolated from conducting fluids or gases, although one other facet may be used because the sensor.
As with all the TSM resonator, the relative absence of a surface-normal component of wave displacement makes it possible for the sensor to arrive into make contact with with liquid for biosensor apps. SH-APM sensors are successfully employed to detect microgram-per-liter levels of mercury, which is sufficient for Secure Consuming Water Act compliance testing [6]. Though much more delicate to mass loading than the TSM resonator, SH-APM sensors are less delicate than surface wave sensors. There are two causes: The initial is the sensitivity to mass loading along with other perturbations is dependent about the thickness of your substrate,Office Pro 2007 Key, with sensitivity rising because the device is thinned. The minimum thickness is constrained by manufacturing processes. Second, the power of your wave is just not maximized in the surface area, which decreases sensitivity.
Surface Wave Sensors

Figure six. The wave electricity is confined to within 1 wavelength from the surface of a Saw sensor. This attribute yields a sensor that's very sensitive to interactions using the surface area.
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 his or her discoverer, Rayleigh waves have a very longitudinal and a vertical shear part which will several with a medium in speak to with all the device’s surface area (see Figure 5,71). These coupling strongly affects the amplitude and velocity from the wave. This feature enables Noticed sensors to directly sense mass and mechanical properties. The surface motion also permits the gadgets to get utilised as microactuators. The wave incorporates a velocity that is certainly ~5 orders of magnitude less than the corresponding electromagnetic wave, generating Rayleigh floor waves amid the slowest to propagate in solids. The wave amplitudes are generally ~10 Å along with the wavelengths range from one to 100 microns [8].
Figure 6 details the deformation area caused by a Found propagating along the Z-axis along with the associated distribution of prospective vitality. Simply because Rayleigh waves have practically all their acoustic energy confined inside of 1 wavelength with the surface area, Observed sensors have the highest sensitivity with the acoustic sensors reviewed.
Typical Saw sensors run from 25 to 500 MHz. A single disadvantage of these devices is that Rayleigh waves are surface-normal waves, producing them poorly suited for liquid sensing. When a Found sensor is contacted by a liquid, the resulting compressional waves cause an excessive attenuation with the floor wave.
Photo one. Acoustic wave sensors are commercially available in a number of sort factors. Most sensors begin as processed wafers that are then tested, diced, and mounted into packages.
Shear-Horizontal Surface Acoustic Wave Sensors. If the cut of the piezoelectric crystal materials is rotated appropriately,Office Professional 2007, the wave propagation mode changes from a vertical shear Found sensor to a shear-horizontal Observed sensor. This dramatically decreases loss when liquids arrive into make contact with together with the propagating medium, making it possible for the SH-SAW sensor to run as being a biosensor (see Figure seven).
Comparison of Acoustic Wave Sensors
In general, the sensitivity from the sensor is proportional to your amount of energy from the propagation path currently being perturbed. Bulk acoustic wave sensors usually disperse the energy from your floor through the bulk substance to the other floor. This distribution of electricity minimizes the vitality density within the surface, which is where the sensing is carried out. Observed sensors, conversely, focus their energy within the surface area, tending to produce them much more delicate [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 a lot of diverse physical parameters. Some commercially available acoustic wave sensors are shown in Photo one. As a matter of fact, all acoustic wave products made for the telecommunications business has to be hermetically sealed to prevent any disturbances simply because they will be sensed through the device and cause an unwanted modify in output.

Figure seven. By correctly selecting the orientation of content minimize, shear-horizontal surface acoustic waves (SH-SAW) will dominate. These waves have a very displacement that is certainly parallel towards the device’s floor.
The range of phenomena which can be detected by acoustic wave devices might be greatly expanded by coating the gadgets with supplies that endure modifications 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 tension that modifications the dynamics of your propagating medium. They become mass, or gravimetric, sensors when particles are allowed to get in touch with the propagation medium, changing the stress on it. They become vapor sensors when a coating is applied that absorbs only distinct chemical vapors. These gadgets function by effectively measuring the mass from the absorbed vapor. In the event the coating absorbs particular biological chemicals in liquids, the detector becomes a biosensor. As previously noted, a wireless temperature sensor can be developed by selecting the correct orientation of propagation. The propagating medium changes with temperature, affecting the output. Detailed below are some of the far more common programs of acoustic wave sensors.
Temperature Sensor. Surface wave velocities are temperature dependent and are determined by the orientation and sort of crystalline substance used to fabricate the sensor. Temperature sensors based on Observed delay line oscillators have millidegree resolution, very good linearity, and low hysteresis [11]. They're, however, very sensitive 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 have a very temperature coefficient of 32 ppm/ÞC plus a resolution of 0.22ÞC [12]. It also exhibited three orders of magnitude significantly less sensitivity to mass loading than do Noticed sensors. The response time was found for being 0.3 s, 10 3 faster than BAW sensors. These temperature sensors hold the further advantage of requiring no electrical power and of being wireless, producing them well suited for use in remote locations.
Pressure Sensor. In 1975, the first reported use of Saw engineering to get a sensor software was in the form of a pressure sensor [13]. Observed velocities are strongly affected by stresses applied for the piezoelectric substrate on which the wave is propagating. A Saw strain sensor is therefore designed by making the Noticed system into a diaphragm (see Figure 8,).

Figure 8. The frequency of your Saw alterations with pressure. As the diaphragm flexes due to stress, the Found sensor modifications its output. Unfortunately, changes in temperature also cause a change in output.
The uncompensated temperature drifts that tend to interfere with Noticed strain sensors might be minimized by placing a reference Saw device close towards the measuring Noticed around the same substrate and mixing the two signals [14]. One particular sensor acts as a temperature detector, whose proximity for the pressure sensor ensures that the two are exposed to your same temperature. However, the temperature sensor Found has to be isolated through the stresses the strain Noticed experiences (see Figure 9).
SAW stress sensors are passive (no power required), wireless, low value, rugged, and extremely small and lightweight, producing them nicely suited for measuring stress in moving objects (e.g., car and truck tires). These characteristics offer strengths about technologies this kind of as capacitive and piezoresistive sensors, which require operating electrical power and therefore are not wireless. A Found strain sensor weighing <1 g, with a resolution of 0.73 psi, was recently integrated into a car tire with excellent outcomes [15]. These a system enables the operator to view the pressure in every single tire from your comfort of the cabin. Correctly inflated tires lead to improved safety, greater fuel efficiency, and longer tire life. This technologies is particularly intriguing for the new run-flat (also called zero pressure or extended mobility) tire market place.
Torque Sensor. If a Found system is rigidly mounted to a flat spot on a shaft, as well as the shaft experiences a torque, this torque will tension the sensor and turn it into a wireless, passive, lightweight torque detector. Because the shaft is rotated one particular way, the Saw torque sensor is placed in tension; rotated one other way, it can be placed in compression. For practical applications, two Observed torque sensors are used such that their centerlines are at right angles (see Figure 10) [16]. Hence, when a single sensor is in compression, the other is in tension. Since the two sensors are uncovered towards the same temperature, the sum from the two signals minimizes any temperature drift effects.

Figure 9. Adding a second, strategically placed Found effectively minimizes the temperature drift with the Observed strain sensor.
In comparison to other torque sensors, such as resistive strain gauges, optical transducers, and torsion bars, Saw torque sensors offer reduce cost, increased reliability, and wireless operation. Monitoring torque on trucks and cars will significantly improve handling and braking since torque meas ures wheel traction much better than the rpm sensors in current use.
Mass Sensor. Of each of the devices evaluated right here, Noticed sensors are the most sensitive to mass loads. This opens up many purposes like particulate sensors and film thickness sensors. When the sensor is coated with an adhesive substance, it becomes a particulate sensor; any particle landing around the surface area will remain there and perturb the wave propagation. A mass resolution of three pg for the 200 MHz ST-cut quartz Noticed continues to be reported, which was 1000 3 the sensitivity with the 10 MHz TSM resonator tested [17]. Particulate sensors are utilised 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 for the mass of the deposited film, so the sensor provides thickness data by measuring the film density and acoustic impedance. This method is accurate, provided the film is skinny (ideally no a lot more than a few percent with the acoustic wavelength) [18]. Most commercially available thickness sensors are based on TSM resonators. Although not so sensitive as Found sensors, these units offer ease of use and satisfactory sensitivity.
Dew Point/Humidity Sensor. If a Observed sensor is temperature controlled and uncovered to the ambient atmosphere, water will condense on it in the dew point temperature, producing it an effective dew point sensor. Current industrial instruments for high-precision dew point measurements are based on optical techniques, which have price, contamination, accuracy, sensitivity, and long-term balance issues. A 50 MHz YZ-cut lithium niobate Found dew point sensor has become developed that is immune to widespread contaminants, has a resolution of &plusmn;0.025°C (vs. &plusmn;0.2°C for an optical sensor), is low cost, and is also significantly more stable [19].
Figure 10. The pressure inside the shaft is transferred towards the Found sensor, which alterations its output frequency with pressure and, therefore, torque. The addition of another Found minimizes temperature effects.
Acoustic wave sensors with an elastic hygroscopic polymer coating make excellent humidity detectors. Three operational mechanisms contribute towards the sensors’ response: mass loading, acoustoelectric effects, and viscoelastic effects, every of which may be effectively controlled to yield an accurate, low-cost, humidity sensor. A 50 MHz YZ-cut lithium niobate Found sensor coated with polyXIO has become demonstrated as being a humidity sensor having a array of 0%–100% RH as well as a hysteresis about the purchase of 5% [20]. In addition, a 767 MHz AT-cut quartz SH-SAW sensor coated having a plasma-modified hexamethyldisiloxane (HMDSO) has recently been demonstrated as a humidity sensor, using a sensitivity of 1.four ppm/% RH plus a 5% hysteresis. This was found to be 4–10 3 much more delicate than a 14 MHz TSM resonator coated using the same polymer [21].
In the same category, a 434 MHz YZ-cut lithium niobate Saw sensor has been utilized as being a remote drinking water sensor [22], and an 86 MHz XY-cut quartz Really like wave sensor has become demonstrated being an ice sensor [23].
Vapor Chemical Sensor—Coated and Uncoated. Chemical vapor sensors based on Saw devices were very first reported in 1979 [24]. Most of them rely within the mass sensitivity with the detector, in conjunction with a chemically selective coating that absorbs the vapors of interest and leads to an increased mass loading with the system (see Figure 11). As together with the temperature-compensated strain sensors, 1 Noticed is utilized being a reference, effectively minimizing the effects of temperature variations.

Photo two. A commercially available handheld Saw chemical vapor analyzer incorporates an array of four Saw sensors, each and every coated which has a diverse polymer.
Several design considerations must 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 resist corrosive vapors. It should be selective, absorbing only extremely specific vapors while rejecting others. The coating must run about a realistic temperature assortment. It should be stable, reproducible, and sensitive. And finally, its thickness and uniformity are very essential.
When a number of Found sensors, each and every which has a unique chemically particular coating, are configured as an array, every will have a very diverse output when uncovered to a given vapor. Pattern recognition software makes it possible for a diverse list of volatile organic compounds hence to be detected and identified, yielding a extremely powerful chemical analyzer. A commercially available analyzer with an array of four Observed sensors is proven in Photo two.
TSM resonators have also efficiently been used for chemical vapor sensing [25] but they can be significantly significantly less delicate than their Saw counterparts. In addition, Found chemical vapor sensors happen to be made with out coatings. This method uses a gas chromatograph column to separate the chemical vapor parts, as well as a temperature-controlled Found that condenses the vapor and measures the corresponding mass loading [26].
Figure 11. By coating a Saw device having a chemically sorptive polymer, a chemical vapor sensor is produced. Adding another Observed system minimizes the temperature drift and offers a manageable difference frequency.
Biosensor. Equivalent to chemical vapor sensors, biosensors detect chemicals, but in liquids rather than vapors. As noted earlier, the Saw gadget is actually a poor choice for this software, since the vertical part from the propagating wave will be suppressed from the liquid. Biosensors have already been fabricated using the TSM resonator, SH-APM, and SH-SAW sensors. Of all of the identified acoustic sensors for liquid sensing, the Love wave sensor, a special class of the shear-horizontal Saw, has the highest sensitivity [27]. To make a Adore wave sensor, a waveguide coating is placed on a SH-SAW device such that the power with the shear horizontal waves is focused in that coating. A biorecognition coating is then placed about the waveguide coating, forming the complete biosensor. Successful detection of anti-goat IgG from the concentration array of 3 three 10–8–10–six moles using a 110 MHz YZ-cut SH-SAW with a polymer Love wave guide coating continues to be 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
Floor transverse wave
Thickness shear mode

Conclusion
Acoustic wave sensors are extremely versatile gadgets which might be just beginning to realize their business potential. They are competitively priced, inherently rugged, very delicate, and intrinsically reputable, and may be interrogated passively and wirelessly. Wireless sensors are beneficial when monitoring parameters on moving objects, this sort of as tire strain 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 programs contain measuring force, acceleration,Office 2007 Serial, shock, angular rate, viscosity, displacement, and flow, in addition to film characterization. The sensors also have an acoustoelectric sensitivity, allowing the detection of pH ranges, ionic contaminants, and electric fields. Floor acoustic wave sensors have proved to be probably the most delicate in general therefore of their larger vitality density within the surface. For liquid sensing, a special class of shear-horizontal surface area acoustic wave sensors referred to as Love wave sensors proved to get the most sensitive. Much operate is continuing in creating these sensors for future programs.
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