Acoustic sensors Flashcards
What are acoustic sensors? (General intro)
Acoustic sensors are devices that employ elastic waves at frequencies in the megahertz to low gigahertz range to measure physical, chemical or biological quantities. They are competitively priced, rugged, very sensitive, intrinsically reliable and some are also capable of being passively and wirelessly interrogated.
They are so named because their detection mechanism is a mechanical, or acoustic wave. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity, frequency and/or amplitude of the wave. Changes can then be correlated to the corresponding physical quantity being measured.
Virtually all acoustic wave devices and sensors use a piezoelectric material to generate the acoustic wave. Piezoelectricity refers to the production of electrical charges by the imposition of mechanical stress and the phenomenon is reciprocal (generate electrical charge by forcing mechanical stress and by applying electric field generate mechanical stress). Piezoelectric acoustic wave sensors apply an oscillating electric field to create a mechanical wave, which propagates through the substrate and is then converted back to an electric field for measurement.
We may identify 4 main classes of acoustic sensors:
• Thickness shear mode (TSM) sensors, they employ vibrating piezoelectric crystal plates for
electronic oscillators.
• Surface Acoustic Wave sensors (SAW), exploit surface acoustic waves travelling on the
surface of the solid.
• Flexural plate waves (FPW), exploit flexural plane waves travelling in a very thin membrane.
• Acoustic Plate Mode (APM), in these devices waves bounce at an acute angle between
bounding planes of a plate.
Thickness shear mode sensors (TSM)
They employ vibrating piezolectric crystal plates for electronic oscillators.
It consists of a thin disk of quartz with parallel circular electrodes patterned on both sides: the application of a voltage between these electrodes results in a shear deformation of the crystal.
Some advantages are:
-they are very mature (50 yrs old);
-they are made from quartz, which is abundant in nature and inexpensive: low cost;
-quartz resonators have a very high quality factor so very sensitive measurements can be made;
-the AT-cut quartz crystals have a zero first-order temperature coefficient around room ambient;
-the frequency of operation is typically the low MHz range (typical operation between 5 and 30 MHz), so electronic measurements are straightforward.
Acoustic plate mode: APM
In these devices waves bounce at an acute angle between bounding planes of a plate.
They use a thin piezoelectric substrate or plate functioning as an acoustic waveguide that confines the energy between the upper and lower surfaces of the plate.
As a result, both surfaces undergo displacement, so detection can occur on either side. This is an important advantage, as one side contains the interdigital transducers that must be isolated from conducting fluids or gases, while the other
side can be used as the sensor.
The relative absence of a surface-normal component of wave displacement allows the sensor to come into contact with liquid for biosensor applications.
Though more sensitive to mass loading than the TSM, SH-APM (sh= shear horizontal) sensors are less sensitive than SAW sensors.
There are two reasons: the first is that the sensitivity to mass loading and other perturbations depends on the thickness of the substrate, with sensitivity increasing as the device is thinned (the minimum thickness is constrained by manufacturing processes), second, the energy of the wave is not maximized at the surface, which reduces sensitivity.
Surface acoustic wave: SAW
Surface acoustic (Rayleigh) waves have a longitudinal and a vertical shear component that can couple with a medium in contact with the device’s surface. Such coupling strongly affects the amplitude and velocity of the wave. This feature enables SAW sensors to directly sense mass and mechanical properties. The surface motion also allows the devices to be used as microactuators.
The wave amplitudes are typically ~10 Å and the wavelengths range from 1 to 100 microns.
Typical SAW sensors operate from 25 to 500 MHz.
These devices use a piezoelectric substrate with metal interdigital transducers/electrodes (IDTs or IDEs) deposited on one of the surfaces.
Application of an oscillatory voltage to the IDT generates a displacement of the surface. The displacement “wave” will propagate away from the IDT. If the wave propagates to a second IDT placed a distance away, we form a “delay line” device.
One disadvantage of these devices is that 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 of the surface wave.
Pros/Cons:
-Since the acoustic energy is trapped near the surface, they are potentially much more sensitive than bulk wave devices like the TSM or APM resonator;
-SAW devices typically work at much higher frequencies (> 50 MHz) which make them more sensitive to perturbations;
-higher sensitivity because they can be made of piezoelectric materials that have a larger coupling coefficient;
-other piezoelectric materials do not have the zero temperature coefficient around room ambient
like quartz but the SAW devices can be fabricated in pairs so neighboring devices on the same substrate will act as sensor-reference combinations;
-Since detection takes place on a single surface (where the energy is trapped), the back side of the device can be bonded to a package without interference. This cannot be done with the TSM or APM resonators since bulk waves interact with both surfaces.
Shear-Horizontal SAW sensors
In these devices, needed for operating surface wave devices in liquids, the surface displacements are generated shear in direction, thus the wave displacement is perpendicular to the direction of wave propagation and in the plane of the crystal surface. The crystal cut of the piezoelectric substrate must be chosen so that application of the electric field by the IDTs produces a shear surface motion.
Flexural plate wave: FPW
These devices exploit flexural plane waves travelling in a very thin membrane.
The flexural plate wave device is typically made of silicon and silicon-based materials by micromachining processes. A piezoelectric film is located on one side of a thin supporting membrane and excited by interdigitated conducting electrodes, as in the SAW.
Because of its location to one side of the neutral plane of the composite membrane, the deformation of the piezoelectric excites a propagating wave that involves anti-symmetric flexure of the membrane.
The gravimetric response of the FPW device may be quite large because of the device very small volume-to-surface ratio. They can be used as actuators (micropumps !!!)
Acoustic sensors: construction
Sensors are usually made by a photolithographic process. Manufacturing begins by carefully polishing and cleaning the piezoelectric substrate. Metal, usually aluminum, is then deposited uniformly onto the substrate. The pattern of metal remaining on the device is called an interdigital transducer, or IDT. By changing the length, width, position, and thickness of the IDT, the performance of the sensor can be maximized.
For the design of acoustic sensors, the performance characteristics, reliability and acoustic parameters of the piezo films are considered essential device properties.
The major properties considered in design trade-offs for monolithic sensors are:
• value of electromechanical coupling (or piezoelectric coefficients)
• good adhesion to substrate
• resistance to environmental effects (e.g. humidity, temperature, …)
• VLSI process compatible (e.g. deposition methods and etching)
• temperature sensitivity (eventually)
• cost effectiveness
Materials for acoustic sensors
Among the piezoelectic substrate materials that can be used for acoustic wave sensors and devices, the most common are quartz (SiO2), zinc oxide (ZnO), aluminum nitride (AlN), lead zirconium titanate (PZT), lithium tantalate (LiTaO3), and, to a lesser degree, lithium niobate (LiNbO3).
- Quartz: it is possible to select the temperature dependence of the material by the cut angle and the wave propagation direction. With proper selection, the first order temperature effect can be minimized. An acoustic wave temperature sensor may be designed by maximizing this effect. This is not true of lithium niobate or lithium tantalate, where a linear temperature dependence always exists for all material cuts and propagation directions.
- ZnO: high piezoelectric coupling and stability. ZnO films of excellent quality have been grown with DC and RF magnetron sputtering or by laser-assisted evaporation using a CO2 laser and a ZnO powder source in a vacuum chamber.
- AIN: high acoustic velocity, endurance in humidity and high temperature, good coupling factor though lower than ZnO. It is usually depositedby RF magnetron sputtering.
- PZT: very high piezo coupling and dielectric constant, large pyroelectric response and spontaneous polarization.
Acoustic sensors- pressure sensors
SAW velocities are strongly affected by stresses applied to the piezoelectric substrate on which the wave is propagating. A SAW pressure sensor is therefore created by making the SAW device into a flexible diaphragm. The uncompensated temperature drifts that tend to interfere with SAW pressure sensing can be minimized by placing a reference SAW device close to the measuring SAW on the same substrate and mixing the two signals. One sensor acts as a temperature detector, whose proximity to the pressure sensor ensures that both are exposed to the same temperature. However, the temperature sensor SAW must be isolated from the stresses that the pressure SAW experiences.
SAW pressure sensors are passive (no power required), wireless, low cost, rugged, and extremely small and lightweight, making them well suited for measuring pressure in moving objects (e.g., car and truck tires). These characteristics offer advantages over technologies such as capacitive and piezoresistive sensors, which require operating power and are not intrinsically wireless.
Acoustic sensors- torque sensors
If a SAW device is rigidly mounted to a flat spot on a shaft, and the shaft experiences a torque, this torque will stress the sensor and turn it into a wireless, passive, lightweight torque detector.
As the shaft is rotated one way, the SAW torque sensor is placed in tension; rotated the other way, it is placed in compression.
For practical applications, two SAW torque sensors are used such that their centerlines are at right angles. Thus, when one sensor is in compression, the other is in tension. Since both sensors are exposed to the same temperature, the sum of the two signals minimizes any temperature drift effects.
In comparison to other torque sensors, including resistive strain gauges, optical transducers, and torsion bars, SAW torque sensors offer lower cost, higher reliability, and wireless operation.
Acoustic sensors: dew point and humidity sensors
If a SAW sensor is temperature controlled and exposed to the ambient atmosphere, water will condense on it at the dew point temperature, making it an effective dew point sensor.
Current commercial instruments for high-precision dew point measurements are based on optical techniques, which have cost, contamination, accuracy, sensitivity, and long-term stability issues.
Acoustic sensors- vapor(bio)chemical sensors
Most of them rely on the mass sensitivity of the detector, in conjunction with a chemically selective coating that absorbs the vapors of interest and results in an increased mass loading of the device.
As with the temperature-compensated pressure sensors, one SAW is used as a reference, effectively minimizing the effects of temperature variations.
Several design considerations must be satisfied when selecting and applying the chemically sorptive coating:
• Ideally, the coating should be 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 very specific vapors while rejecting others.
• The coating must operate over a realistic temperature range.
• It should be stable, reproducible, and sensitive.
Acoustic sensors- vapor chemical sensors
When several SAW sensors, each with a unique chemically specific coating, are configured as an array, each will have a different output when exposed to a given vapor.
Pattern recognition software allows a diverse list of volatile organic compounds thus to be detected and identified, yielding a very powerful chemical analyzer.
Acoustic sensors: measurement methods
Either a resonator or a delay line may be used.
With either a resonator or a delay line, we may make measurements on the device itself (passive approach) or incorporate it in a circuit involving electrical feedback to form an oscillator (active circuit approach).
For a passive delay line, the phase shift between input and output interdigital transducers, which are separated by a known distance, gives the velocity. Alternatively SAW delay lines can utilize the propagation time between IDTs.
For resonator or delay line oscillators, the oscillator frequency is usually desired; it can be measured easily and precisely with a digital counter.
To avoid errors due to cross sensitivity for temperature, etc., differential measurements are performed.
Therefore, two oscillator circuits controlled by SAW resonators are employed. One of the SAW resonators is effected by the measurand, the other is insulated from this. Both are exposed to temperature, etc. The output signals are multiplied (mixed). A low pass filter let pass the difference frequency,
carrying the information about the measurand compensated for
temperature.
Wireless acoustic sensor
We can also exploit these devices in a wireless manner, basically using them as reflective one port devices.
How are these systems made?
The IDT is connected to an antenna so that when a signal reaches the antenna it is translated into an electrical input for the IDT. Now the Delay Line system outlined earlier responds with a delayed signal. By accurately measuring the difference
between the signals at R1 and R2 (not the delay time) the measurand can be evaluated. Obviously this system is strongly influenced by the propagation
between original emitter and antenna—-> More than one reflectors are arranged so that differential measurement is employed. Moreover when multiple sensors are in place, the IDTs can be placed (or not) to create a different response for each sensor–>Digital Identification Code.
In recent times, impedance loaded SAW sensor devices have been proposed and verified. Basically one IDT is connected to the antenna again while the other one is loaded by an impedance. By tuning the load impedance the reflectivity of an IDT can be adjusted in magnitude and phase. In such a system the SAW isn’t the sensor anymore, it only acts as a transponder. The real measurement is performed by the external sensor which in our circuit is identified as the external impedance. Since the SAW
doesn’t measure anymore all cross sensitivities and external factors influencing the substrate don’t affect the measurement anymore.
By using a Magnetoresistor magnetic fields can be measured. The magnetic field changes the impedance of the magnetoresistor leading to a change in reflectivity.
The low propagation velocity of SAWs allows for a long delay time even in small devices.
