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ACOUSTIC WAVE SENSORS

I. Types and Principles of Operation

II. Markets and Applications

III. Technical Examples

This presentation is divided into three sections. The first section describes theconstruction and operation of the various types of acoustic wave sensing elements.The second section explains some of the sensor markets and applications. The finalsection is the bulk of the presentation and presents four sensor examples in detail.


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ACOUSTIC WAVE SENSORSPart I

Types and Principles of Operation

This first section presents the basic operation of BAW and SAW sensors in detailand briefly describes surface transverse wave, love mode, and acoustic plate modesensors.


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Acoustic Wave Sensors

Definition of a Sensor:

A device that responds to a physical, chemical,biological, or electrical stimulus by producing anelectrical output signal that is a function of the inputstimulus

For acoustic wave devices, we monitor the change inoscillation frequency when the device responds to theinput stimulus

ACOUSTIC WAVE SENSOR

INPUT STIMULUS

PHYSICAL,

CHEMICAL,

BIOLOGICAL,

ELECTRICAL

OUTPUT SIGNAL

CHANGE

INRESONANT

FREQUENCY

TRANSDUCTION OF THE

INPUT STIMULUS TO

THE ACOUSTIC WAVE

DEVICE

Definition of a Sensor:

A device that responds to a physical, chemical, biological, or electrical stimulus byproducing an electrical output signal that is a function of the input stimulus.

For acoustic wave devices, we monitor the change in oscillation frequency when thedevice responds to the input stimulus.

The input stimulus could be physical such as a changing pressure, temperature, orstress. It could be chemical such as a particular gas concentration or simply thepresence of a chemical agent. It could also be biological. Examples of which couldbe the concentration of particular type of bacteria, the presence of a biologicalagent, or even the concentration of antibodies for a particular type of disease.Finally, it could also be electrical such as the strength of a electric or magnetic fieldor a changing conductivity.

The transduction of the input stimulus to the acoustic wave device allows theacoustic wave device to respond by changing its resonant frequency.


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AT-CUT BAW

Thickness Shear Mode(TSM) Device

Fine Tune Spotf(ppm)

Film Thickness (Angstroms)0

We all know from our own experiences with manufacturing acoustic wave basedproducts that they can be very sensitive to a variety of environmental factors such asvariations in temperature, pressure, and packaging stress.

We also know that the amount of metallization on the surface of a Quartz BulkAcoustic Wave (BAW) device can dramatically shift the resonant frequency. Weactually utilize this property to fine tune the resonant frequency to within somewindow of tolerance. This process is called mass loading. For a Surface AcousticWave (SAW) delay line, metal deposited in the delay path of the device will resultin an increase in delay. When combined with a feedback amplifier to create anoscillator, the result is a decrease in the oscillation frequency.

Knowing how sensitive these devices are to changes occurring on their surfaces,how can we utilize acoustic wave devices to sense or detect something of interest.


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Maximum shear displacementoccurs on the crystal faces inthe plane of the crystal plate

Can be used for both gas andliquid sensing applications

Sensitive to mechanical and

electrical perturbations massloading, visco-elasticproperties, and electro-acousticinteractions


Vertically polarized displacementoccurs within one wavelength ofthe surface

Can only be used for gassensing applications

Sensitive to mechanical andelectrical perturbations massloading, visco-elastic properties,and electro-acoustic interactions

To answer this, we should take a look at the basic operation of these devices.Starting with the surface acoustic wave (SAW) delay line, we can see thatpropagating wave is confined to the top surface of the substrate. Because of this,the SAW is a very sensitive probe for measuring mechanical and electricalproperties on its surface. We also note that since there is a vertically polarized

displacement, the SAW can only be used for gas sensing applications. Putting theSAW in an aqueous environment will result in the SAW being completely dampedout.

For the BAW device, or more specifically an AT cut Thickness Shear Mode (TSM)device, we can see that the thickness shear vibration is in the plane of the crystalplate with the maximum displacements occurring on the crystal faces. This is idealfor sensing application and because there is no vertical component of displacement,the BAW device can be used for liquid-based applications without experiencingexcessive damping.

Both the SAW and BAW devices are sensitive to mechanical and electricalproperties occurring on their surfaces. For mechanical properties, they are sensitiveto mass loading and visco-elastic changes like stiffening and softening. Forelectrical properties, the devices can be sensitive to any property that interacts withthe electrical field that is coupled to the propagating acoustic wave. This effect hasbeen given the term electro-acoustic interactions.


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Application as a gas sensor is achieved by placing a gas specific sensingfilm on the surface of the device

When the sensing film gets exposed to a gas, mechanical and electricalperturbations in the sensing film will cause a corresponding change inthe oscillation frequency of the acoustic wave device oscillator.

The sensing films can be metal, metal oxide, metal nitride, polymer, orbiological material (antigens, bacterial biofilms, or cell cultures)


TopElectrode

BottomElectrode

Sensing Film

ATQuartz

Sensing Film

ST Quartz SAW Delay Line

Input IDT Output IDT

BAW (TSM) and SAW Gas-Based Sensors:

Application of the SAW or BAW device is achieved by placing a gas specificsensing film on the surface of the device. When the sensing film gets exposedto the target gas, mechanical and electrical perturbations in the sensing film willcause a corresponding change in the oscillation frequency of the acoustic wavedevice oscillator.

Mechanical examples:

1. Mass loading as a concentration of gas adsorbs (sticks onto) onto the surface ofthe sensing film will result in a decrease in oscillation frequency.

2. Changes in a sensing film as a concentration of gas diffuses into the bulk of thesensing film can result in elastic stiffening or softening. Elastic stiffening willresult in an increase in the oscillation frequency, while elastic softening orswelling of the sensing film will result in a decrease in the oscillation frequency.

Electrical examples:

1. Conductivity changes in the sensing film as it gets exposed to a concentration ofgas can result in either an increase or decrease in oscillation frequencydepending on whether the gas causes the conductivity of the sensing film toincrease or decrease.

Note that in some cases where the sensing film is metal, the device electrodesthemselves can become the sensing material. Also note that when you have ametal sensing film, you will not be able to observe any electrical propertiesbecause the metal film will short out any electric field that is coupled to thepropagating acoustic wave.


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Application as a liquid sensor can be achieved with a sensing filmor by direct contact of the liquid onto the surface of the BAWdevice

Sensing film case - Attachment of the chemical or biological

stimulus results in mechanical perturbations causing acorresponding change in resonant frequency Direct contacting case - Mechanical properties of the fluid causes

perturbations on the surface of the BAW (TSM) device resulting inresonant frequency changes

BAW (TSM) Liquid-Based Sensors:

o-ring

Sensing Film

Liquid

AT Quartzo-ring

Fluid Cell

o-ring

Liquidundertest

AT Quartzo-ring

Fluid Cell

Application as a liquid sensor can be achieved with a sensing film or by directcontact of the liquid onto the surface of the BAW device.

For the sensing film case, attachment of the chemical or biological stimulus resultsin mechanical perturbations causing a corresponding change in the resonant

frequency.Mechanical examples:

1. Mass loading as a concentration of chemical adsorbs onto the surface of thesensing film or when a bio-molecule attaches to some selective surfacechemistry. Mass loading results in a decrease in resonant frequency.

2. Visco-elastic changes of the sensing film as a concentration of chemical diffusesinto the bulk of the sensing film. Elastic stiffening of a sensing film will result inan increase in the resonant frequency, while elastic softening or swelling of thesensing film will result in a decrease in the resonant frequency.

For the directly contacting case, the mechanical properties of the fluid causesperturbations on the surface of the BAW device resulting in resonant frequency

changes.Mechanical examples:

1. Viscous loading of the BAW: The density and viscosity of the fluid will stronglyaffect the BAW equivalent circuit parameters and resonant frequency.

Electrical Properties: Note that for a sensing film placed on top of a metalelectrode, the BAW device will not detect any electrical properties because themetal film will short out the electric field that is coupled to the propagatingacoustic wave.


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Surface Transverse Wave (STW):


InputIDT

OutputIDTSurface

DisplacementMetal

TrappingGrating

Similar to the SAW, the Surface Transverse Wave (STM) device uses input andoutput transducers to launch and receive the propagating acoustic surface wave.However, the differences are two-fold. First, the STW device uses a metal trappinggrating structure to trap the propagating wave to the surface of the substrate.Without the grating, the wave propagates at a slight angle into the bulk of thesubstrate resulting in attenuation of the wave. Secondly, the displacement of thepropagating STW is in the plane of the substrate without any vertical component ofdisplacement.

Since there is no vertical component of displacement, the STW could theoreticallybe used for liquid-based applications. In a practical sense, use as a liquid-basedsensor is unachievable. Placing water on the surface of the device will short out theIDT electrodes preventing the excitation of the STW before it even has a chance topropagate. Isolation of the IDTs from the liquid has been tried with various types ofpackaging, but this usually results in significant losses in the device characteristics.

For this reason, the STW is typically only used for gas sensing.-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

R.L. Baer, C.A. Flory, M. Tom-Moy, and D.S. Solomon, STW Chemical Sensors,Proceedings IEEE Ultrasonics Symposium, pp. 293-298, 1992.


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Love Mode:


InputIDT

OutputIDTSurfaceDisplacement

ThinFilm

TrappingLayer

Similar to the SAW and STW, the Love Mode device uses input and outputtransducers to launch and receive the propagating acoustic surface wave. Alsosimilar to the STW, the displacement of the propagating love mode is in the plane ofthe substrate without any vertical component of displacement. The love modedevice, however, uses a thin trapping layer to trap the propagating wave to thesurface of the substrate instead of a metal grating.

For the love mode device, the same argument regarding liquid-sensing applicationapplies.

The love mode device is typically used for gas sensing.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

J.C. Andle and J.F. Vetelino, Acoustic Wave Biosensors,Proceedings IEEEUltrasonics Symposium, pp. 451-460, 1995.


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Shear Horizontal Acoustic Plate Mode (SHAPM) :


The shear horizontal acoustic plate mode (SH-APM) device combines the bestproperties of both the BAW and SAW devices. It employs separate input and outputtransducers in order to allow differential signal measurements like the SAWstructures but also allows the sensor crystal to be employed as a physical barrierbetween the electronics and the sensing medium.

The wave is a waveguide mode with energy throughout the bulk of the crystal and isdependent on the thickness of the substrate. Like all the previous surface launchedacoustic wave devices, the SH-APM device uses input and output IDTs to launchand receive the acoustic wave. Similar to the BAW thickness shear mode device,the maximum displacements occur on the top and bottom surfaces of the plate.Similar to the STW and Love Mode devices, the surface displacement is shear and inthe plane of the plate so it can be used for liquid-based applications.

The waveguide modes have energy distributed between the two surfaces as astanding wave as in the BAW sensor but traveling along the surface as in a SAW.The continuous exchange of energy between the two surfaces allows the signalbetween the IDTs to be influenced by changes on the opposite surface..

Since the wave interacts with both surfaces of the plate, either surface can be used asthe sensing surface. For liquid sensing applications and for corrosive or explosivegases, this is a great advantage over the STW and Love Mode device because youcan isolate the sensing medium from the electrodes by making the bottom surface ofthe SH-APM device the sensing surface.------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

S.J. Martin, A.J. Ricco, and G.C. Frye, Sensing in Liquids with SH Plate ModeDevices,Proceedings IEEE Ultrasonics Symposium, pp. 607-611, 1988.


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ACOUSTIC WAVE SENSORSPart II

Markets and Applications

This section discusses some of the markets and applications for sensors in general.Most of these applications could be achieved by using acoustic wave sensors.


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Potential Markets and Applications

Major Markets Military Automotive

Industrial andEnvironmental

Food Industry

Medical

Types of Sensors Chemical Biological

Physical

The major markets are military, automotive, industrial and environmental, foodindustry, and medical. The sensors are based on chemical, biological, and physicalsensing properties.


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Military Applications Chemical Agent Detectors

Blister - Mustard, Lewisite, etc.

Blood - Hydrogen Cyanide, Cyanogen Chloride, etc.

Choking/Lung - Chlorine, Cyanide, etc.

Incapacitating Canniboids, Phenothiazines, etc.

Nerve Sarin, Soman, etc.

Bio-Agent Detectors Anthrax, Botulism, Plague, Smallpox, Q-fever, etc.

Funding Agencies Army Research Laboratory (ARL), Office of Naval Research

(ONR), Air Force Research Laboratory (AFRL), Department ofDefense (DOD), and Defense Advanced Research Projects Agency(DARPA)


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Automotive Combustion Control

Hydrocarbons (HC) Carbon monoxide (CO) Nitric Oxides (NO and NO2) Oxygen (O2) Particulates

Engine Performance Control Air/Fuel ratio Oil quality/viscosity

Other Physical Sensors Tire pressure, torque, andacceleration

Funding Agencies Department of Energy (DOE), Department of Transportation

(DOT), and the National Science Foundation (NSF)


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Industrial and Environmental

Combustion Emissions Monitoring NOx, SOx, CO, CFCs, Lead, and Mercury Electrical Power Generation (Coal-Fired Combustion) Municipal, Medical, and Low Level Nuclear Waste

Incineration

Water Quality Aquaculture, Coastal, Inland, Drinking Water Reserves, and

Ground Water Oxygen, Conductivity/Salt/pH, Chlorophyll

Work Place Monitoring Process or byproduct gases Chemical fumes

Funding Agencies Department of Energy (DOE), Environmental Protection

Agency (EPA), and the National Science Foundation (NSF)


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Food Industry Process Control

Moisture Content

Viscosity and Texture

pH and Conductivity (acidity and Salt Content)

Sugar Content (Glucose and Sucrose)

Food Freshness Microbial Detection (E. Coli, Salmonella, etc.)

Microbial Toxin Detection (liquid and gas)

Ingredient Freshness (Milk, Meat, etc.)

Frying Oil (Viscosity and Chemical)

Food Quality : Taste (Electronic Nose)


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Medical Point-of-care Diagnostics

Emergency Room Diagnostics Bio-chemical screening Detection and identification of bacterial infection

Neonatal and Critical Care Monitoring

Nitric Oxide Treatment with Nitric Oxide Oxygen Carbon Dioxide

Drug Discovery and Development Detection of biomolecular interactions

Attachment of candidate drug molecules with proteins Identifying specificity of proteins with other drugs

Funding Agencies National Institutes of Health (NIH), the National ScienceFoundation (NSF), and the U.S. Army Medical Research andMateriel Command (USAMRMC)


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ACOUSTIC WAVE SENSORSPart III

Technical Examples

The following section will give four sensor examples in a fair amount of detail.

Some general references on acoustic wave sensors are:---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

D.S. Ballantine, R.M. White, S.J. Martin, A.J. Ricco, E.T. Zellers, G.C. Frye, and H.Wohltjen, Acoustic Wave Sensors : Theory, Design, and Pysico-chemicalApplications, Academic Press, 1997.

J.C. Andle and J.F. Vetelino, Acoustic Wave Biosensors,Proceedings IEEEUltrasonics Symposium, pp. 451-460, 1995.

E. P. EerNisse, R. W. Ward, and R. B. Wiggins, Survey of Quartz Bulk ResonatorSensor Technologies,IEEE Transactions on Ultrasonics, Ferroelectrics, andFrequency Control, vol. 35, no. 3, pp. 323-330, May 1998.

E. Benes, M. Groschl, F. Seifert, A. Pohl, Comparison Between SAW and BAWSensor Principles,Proceedings IEEE International Frequency Control Symposium,pp. 5-20, 1997.


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Examples:

SAW GAS SENSOR Mass Loading of a SAW Mercury Sensor

Electrical Changes of a Metal Oxide Coated NO Monitor

BAW (TSM) LIQUID SENSOR

Viscous Liquid Loading of a BAW (TSM) Viscometer

Liquid Conductivity Measurements using a Modified BAW(TSM) Device

There are four examples covered in this next section. The first exampledemonstrates the SAW devices ability to detect changes in mass of a sensing filmplaced on the surface of a delay line. The second example demonstrates the SAWdevices ability to detect changes in conductivity of a sensing film placed on thesurface of a delay line. The third example demonstrates how the BAW device can

be used to detect changes in viscosity of a liquid. The fourth example demonstrateshow one can modify the electrode structure of a BAW device to enhance itssensitivity to electrical effects of a solution or sensing film.


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Dual Delay Line SAW Mercury Sensor:

QuartzSubstrate

SensorDelayLine

Reference

DelayLine

TemperatureSensors

GlassPassivation

LayerGold

SensingFilm

PassivatedReference

GoldFilm


The figure above demonstrates the construction of a dual delay line SAW Mercurysensor. One delay line is used as the sensor, while the other delay line is used as areference to cancel out environmental fluctuations such as variations in temperatureand pressure. To achieve matched temperature-frequency characteristics for bothdelay lines, the same material and thickness film is used. The reference film isplaced underneath a passivation layer (glass) preventing it from reacting to mercury,while the sensing film is placed on top of the passivation layer allowing it to respondto the gaseous mercury. The difference between the two delay lines is the mercuryresponse independent of temperature and pressure fluctuations.

The sensors ability to detect mercury is due to the strong interaction between goldand mercury, known as amalgamation. Upon sorption of mercury onto the surfaceof the gold, changes in film mass induce corresponding changes in the propagationof the SAW which results in changes in the oscillation frequency. The rate offrequency change is a function of the mercury concentration.-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

R. Haskell, J. Caron, M. Duplisea, J. Ouellette and J. Vetelino, Effects of FilmThickness on Sensitivity of SAW Mercury Sensors, Proceedings IEEE UltrasonicsSymposium, pp. 429-434, 1999.

R.B. Haskell, J.J. Caron, Temperature Compensated Surface-Launched AcousticWave Sensor, U.S. Patent 6,378,370, April, 2002.


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Perturbation Theory For Mechanical Effects

( )

++++=

2

420

321v

kkkfhf

f

- fractional coverage of the gold film over the IDT center-to-center distance - nominal operating frequencyh - film thicknessKi - normalized particle velocities in the xi direction film density nominal saw velocity and Lam constants of the film


Predicted Mass Loading Due to a Monolayer of Hg

For a 261 MHz ST-quartz SAW delay line with a sensing film withdimensions of 1mm x 1.25mm, the expected shift in frequency due toone monolayer of mercury is 8600 Hz

The fractional change in oscillation frequency due to mass and elasticityperturbations can be expressed using the Tiersten formula shown above. Examiningthe equation closely, one can see that the mass-induced frequency variations areembodied in the terms, while the elasticity-induced changes are embodied in theterms containing and . It is apparent from the equation above that increases in

film mass result in a decrease in oscillation frequency, while increases in mechanicalstiffness result in increases in oscillation frequency. In the case of mercuryamalgamation to a gold film, both of these situations arise. By choosing a gold filmthat is just continuous (75 to 100) mass loading is optimized as described in thereferenced works below.---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

H.F. Tiersten, B.K. Sinha, Journal of Applied Physics, 49(1), pp. 87-95 (1978).

R. Haskell, J. Caron, M. Duplisea, J. Ouellette and J. Vetelino, Effects of FilmThickness on Sensitivity of SAW Mercury Sensors, Proceedings IEEE UltrasonicsSymposium, pp. 429-434, 1999.

R. B. Haskell, A Surface Acoustic Wave Mercury Vapor Sensor,Masters Thesis,Univ. of Maine, 2002.

J.J. Caron, R.B. Haskell, C.J. Freeman, J.F. Vetelino, Surface Acoustic WaveMercury Vapor Sensors, U.S. Patent 5,992,215, November, 1999.


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75 Gold Film Hg SAW Sensor Response10 ppb Hg @ room temperature

-75

-50

-25

0

25

60 90 120 150 180 210 240

time (min)

f(ppm)

-0.3

-0.2

-0.1

0.0

0.1

Freq.

Slope(kHz/min)

Slope

Frequency

Hg Off

Hg On

Hg Off

The response of the SAW mercury sensor to a 10 ppb mercury exposure is shownabove. When operated at room temperature, the amalgamation process is essentiallyirreversible. Almost all of the mercury that comes into contact with the film sticksto it indefinitely until it is removed by heating to 150C. The rate of adsorption ontothe surface of the gold film is a function of the mercury concentration.


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75 Gold Film Hg SAW Sensor Response20, 40, 60, 80, and 100ppb Hg @ 50C

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

55 60 65 70 75

time (min)

f(ppm)

Hg Off Hg On

20

40

60

80

100

The figure above shows the response magnitudes for concentrations of 20, 40, 60,80, and 100 ppb mercury at an operating temperature of 50C. You can see that theinitial slope of the response changes significantly as a function of mercuryconcentration.


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75 Gold Film Hg SAW Sensor ResponseFrequency Slope vs. Mercury Concentration

y = -0.0083x - 0.1108

R2= 0.9693

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0 20 40 60 80 100 120

Concentration (ppb Hg)

FrequencySlope(kHz/min)

By taking the derivative (calculating slope) of the initial part of the response of theprevious set of plots, you can plot the frequency slope as a function of the mercuryconcentration as shown above. The plot shows that the mercury sensor is extremelylinear with respect to mercury concentration. The equation above can be stored in amicroprocessor and used to calculate mercury concentration when it encounters ameasured frequency slope.


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Dual Delay Line SAW NO Sensor:

QuartzSubstrate

SensorDelay Line

ReferenceDelay Line

AluminumReferenceMatching

Film


WO3:RuSMO

SensingFilm

The figure above demonstrates the construction of a dual delay line SAW NitricOxide (NO) sensor. The sensor delay line uses an RF sputtered ruthenium-dopedtungsten trioxide (WO3:Ru) film as the sensing element, while the reference delayline is used to cancel out environmental fluctuations such as variations in temperatureand pressure. The reference delay line uses an aluminum film to match thetemperature-frequency characteristics of the sensing delay line.

The sensor utilizes the electroacoustic effect to detect changes in the conductivity ofthe WO3:Ru film caused by interactions with NO. In the electroacoustic effect, theelectric fields associated with the SAW penetrate into the adjacent semi-conductingmetal oxide (SMO) film. Because the electric fields interact with the charge carriersin the film, they are effected by changes in the film conductivity. The net effect ofthis interaction is an alteration of the SAW properties (i.e. phase velocity).-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

J.J. Caron, T.D. Kenny, L.J. LeGore, D.G. Libby, C.J. Freeman, and J.F. Vetelino, ASurface Acoustic Wave Nitric Oxide Sensor,Proceedings IEEE InternationalFrequency Control Symposium,pp. 156-162, 1997.

J.J. Caron, R.B. Haskell, J.C. Andle, J.F. Vetelino, Temperature Stable PiezoelectricSubstrates for SAW Gas Sensors, Sensors and Actuators, B:Chemical v B35 n 1-3 pt1, pp. 141-145 (1996).


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Perturbation Theory for Electroacoustic Interactions:


( )

++

=

f

ssc

f

f

v

k

01

1

2 22

- fractional coverage of the film over the IDT center-to-center distancek2 - piezoelectric coupling coefficient for the substrate

- short circuit SAW velocity

0 - dielectric constant of free spaces - dielectric constant of the substratef sheet conductivity of the sensing film

A closed-form expression can be derived for the relationship between SAWoscillation frequency changes and film conductivity changes using simplifiedperturbation theory. Ignoring mechanical perturbations to the film (which have aminor effect compared with electrical perturbations for this particular gas-filmsystem), the relationship is shown above.

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

B.A. Auld, Acoustic Fields and Waves in Solids, Vol II, Wiley IntersciencePublication, 1973.

R. Lec, R.S. Falconer, Z. Xu and J.F. Vetelino, Macroscopic Theory of SurfaceAcoustic Wave Gas Microsensors,Proceedings IEEE Ultrasonics Symposium, pp.585-589, 1988.

J.D. Galipeau, An Experimental Study of a Surface Acoustic Wave Hydrogen

Sulfide Microsensor, Masters Thesis, Univ. of Maine, 1996.


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Frequency Shift as a Function of Film Sheet Conductivity

27 RYC Quartz : =0.4

-250

-200

-150

-100

-50

0

50

1E-9 1E-8 1E-7 1E-6 1E-5Sheet Conductivity (Seimens)

f(ppm)

The perturbation equation for electroacoustic interactions is plotted above for a 27rotated y-cut quartz substrate.

In order to optimize the electroacoustic interaction for sensing applications, theSMO film conductivity must fall within the range where the curve of the above

figure has a significant slope. The film conductivity is tailored to fall within thisrange by precisely controlling parameters such as film thickness, operatingtemperature, and deposition and annealing procedures.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

J.J. Caron, T.D. Kenny, L.J. LeGore, D.G. Libby, C.J. Freeman, and J.F. Vetelino,A Surface Acoustic Wave Nitric Oxide Sensor,Proceedings IEEE InternationalFrequency Control Symposium,pp. 156-162, 1997.


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400 WO3:Ru Coated SAW Sensor Response3ppm Nitric Oxide @ 250C

0

20

40

60

80

100

120

140

160

180

200

0 2 4 6 8 10

Time (min)

f(ppm) Response

Magnitude

This plot demonstrates the NO sensors response to 3ppm Nitric Oxide. The sensingfilm was a 400 WO3 film doped with Ru and the operating temperature was 250C.

Since the SAW nitric oxide sensor is operated at an elevated temperature (250C),the conductivity induced SAW response is driven by the diffusion of nitric oxide

into the bulk of the WO3 film instead of adsorption of nitric oxide onto the surface ofthe WO3 film. Because of this, the response shape is very different than the SAWmercury sensor response. The amount of response magnitude, instead of the initialresponse slope, is determined by the nitric oxide concetration.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



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400 WO3:Ru Coated SAW Sensor ResponseVaried Concentrations of Nitric Oxide @ 250C

0

50

100

150

200

250

0 20 40 60 80 100 120

Time (min)

Df(ppm)

400 ppb500 ppb

600 ppb

700 ppb

800 ppb

900 ppb

1 ppm

2 ppm

3 ppm 4 ppm 5 ppm

The figure above shows the sensors response to exposures of NO ranging from 400ppb to 5 ppm. The data shows that the sensor responds in a very linear fashion up toabout 3 ppm. Concentrations above 3ppm NO cannot be measured due to the factthat high concentrations of NO reduce the film conductivity to the point that it is nolonger in the range for which the SAW device is sensitive.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



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400 WO3:Ru Coated SAW Sensor ResponseResponse Magnitude vs. NO Concentration

y = 67.767x - 0.520

R2= 0.996

0

50

100

150

200

250

0 0.5 1 1.5 2 2.5 3NO Concentration (ppm)

Df(ppm)

Taking the response magnitudes of the previous plot and plotting them as a functionof NO concentration results in the figure above. It is obvious that the sensor behavesin a very linear fashion, and is ultimately capable of measuring extremely lowconcentrations of NO.

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------



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BAW (TSM) Liquid Viscometer:


AT Quartz

q q,

Viscously-EntrainedLiquid

Top Electrode

Displacement

(y)ux

Liquid

,

x

y

When a thickness shear mode (TSM) resonator is placed in contact with a liquid, theresonant frequency and series resistance is dependent on the density and viscosity ofthe contacting liquid.

The illustration above shows the cross-sectional displacement profile for a TSM AT-

cut resonator contacted by a viscous liquid. As shown, the oscillating surfacegenerates plane-parallel laminar flow in the contacting liquid. The viscously-entrained liquid undergoes a phase lag that increases from the distance from thesurface of the TSM device.

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

S.J. Martin, K.O. Wessendorf, C.T. Gebert, G.C. Frye, R.W. Cernosek, L. Casaus,and M.A. Mitchell, Measuring Liquid Properties with Smooth and TexturedSurface Resonators,Proceedings IEEE International Frequency ControlSymposium,pp. 603-608, 1993.

K.K. Kanazawa and C.E. Reed, Study of Liquids in Shear Using a QuartzResonator,Proceedings IEEE International Frequency Control Symposium,pp.350, 1987.


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Viscous Liquid Loading for BAW (TSM) Resonators:


qqss

ff

2/3

=

density of the liquid under test

viscosity of the liquid under test

q quartz shear stiffness

q quartz mass density

The change in resonant frequency is proportional to the square-root of thedensity/viscosity product as shown above.


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5MHz Fundamental AT Quartz Impedance Magnitude%wt Solutions of Glycerol

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

1600.00

1800.00

2000.00

4985000 4990000 4995000 5000000 5005000 5010000

Frequency (Hz)

ImpedanceMagnitude(W)

DI water M

5% M

10% M

20% M

40% M

60% M

80% M

Air M

The plot above demonstrates the series impedance dependence on the viscosity ofvarious glycerol solutions.


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Change in Series Resonant Frequency5MHz Fundamental AT Cut Quartz

%wt Solutions of Glycerol in Water

-2000

-1500

-1000

-500

0

0 10 20 30 40 50 60 70 80

Glycerol (% Weight)

Df(ppm)

Plotting the series resonant frequency as a function of the percent weight of glycerolgenerates the plot above.


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Change in Series Resonant Frequency vs. (r*)1/25MHz AT Cut Quartz

%wt Solutions of Glycerol in Water

y = -224.73x

R2= 0.9984

-2000

-1800

-1600

-1400

-1200

-1000

-800

-600

-400

-200

0

0 1 2 3 4 5 6 7 8 9

(r*)1/2 (g cm-2 s-1/2)

Df(ppm)

And finally, plotting the shift in frequency as a function of the square-root of thedensity-viscosity product for the various concentrations of glycerol creates the plotshown above. You can see that the shift in frequency is directly proportional to thesquare-root of the density-viscosity product.


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BAW Sensors for Measuring Solution Conductivity:


AT Quartz

q q, C0

ConductiveIonic

Solution

,

FringingElectric

Field

Ground Electrode

For a commercial AT resonator, the C0 value is mostly a function of the quartz thickness andthe area of the electrodes and to a lesser extent the interaction of the fringing electric fields inthe quartz and the air surrounding the resonator. This C0 value determines a large part of theanti-resonant frequency of the device.

When one side of the resonator is placed in a conductive ionic solution, the part of C 0 caused

by the fringing fields will cause a corresponding change in the anti-resonant frequency andequivalent circuit parameters as a function of the solution concentration

A bulk acoustic wave TSM device can be used to measure the conductivity of non-viscous solutions. For a TSM device with equivalent electrodes (shown above), theinteraction with the conductive solution is not electro-acoustic in nature, but ismainly a function of the fringing electric fields.

Because the top electrode is metal, it will short out any electric field that is coupledto the propagating bulk acoustic wave. For this reason, the acoustic wave devicedoes not interact with the solution via the propagating BAW. Instead, the fringingfields interacting with the conductive solution directly influences the C0 of theresonator causing the anti-resonant frequency of the device to be dependent onconductivity of the solution.--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

M. Yang and M. Thompson, Perturbations of the Electrified Interface and theResponse of the Thickness Shear Mode Acoustic Wave Sensor Under ConductiveLiquid Loading,Anal. Chem., vol. 66, pp. 3591-3597, 1993.

F. Josse, Acoustic Wave Liquid-Phase-Based Microsensors, Sens. Actuators A, vol.

44, pp. 199-208, 1994.Z.A. Shana and F. Josse, Quartz Crystal Resonators as Sensors in Liquids Using theAcoustoelectric Effect, Anal. Chem., vol. 66, pp. 1955-1964, 1994.

F. Josse, Z.A. Shana, and H. Zong, Quartz Resonators as Effective Detectors forDilute Conductive Liquids,Proceedings IEEE Ultrasonics Symposium, pp. 425-430, 1993.


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5MHz AT Quartz TSM Sensor ResponseImpedance Magnitude for Varying NaCl Solutions

Equivalent Top and Bottom Electrodes (diam. 0.25")

0.00

2000.00

4000.00

6000.00

8000.00

10000.00

12000.00

4994000 4996000 4998000 5000000 5002000 5004000

Frequency (Hz)

Magnitude(Ohms)

DI water M

1x10-4 M

5x10-4 M

1x10-3 M

5x10-3 M

1x10-2 M

5x10-2 M

1x10-1 M

5x10-1 M

1mol M

SeriesResonantFrequency

ParallelAnti-resonant

Frequency

The plot above demonstrates how a 5MHz AT-cut (equivalent electrodes: 0.25)resonator responds to varying concentrations of conductive salt solution. Noticehow the series resonant frequency (minimum impedance) is unaffected by varyingconcentrations of salt, while the anti-resonant (maximum impedance) frequency isseverely affected by the changing salt concentration.


38/44


5 MHz AT Quartz TSM Sensor ResponseSeries and Parallel Resonant Frequency Change

vs. Molarity NaCl in Water

-350

-300

-250

-200

-150

-100

-50

0

0.00001 0.0001 0.001 0.01 0.1 1

NaCl Concentration (moles/L)

Df(ppm)

Series

Parallel

Plotting the frequency values for the minimum and maximum impedances for all thevarious salt concentrations results in the plot shown above. As can be seen, the anti-resonant frequency is very sensitive to conductive solutions.

The fact that the anti-resonant frequency changes considerably, while the series

resonant frequency is virtually unaffected eludes to a C0 change instead of anelectro-acoustic interaction. If the effect was electro-acoustic, then the seriesresonant frequency would be very sensitive to the conductivity of salt solution.


39/44


5 MHz AT Quartz TSM Sensor ResponseSeries and Parallel Resonant Impedances


-6000

-5000

-4000

-3000

-2000

-1000

0

0.00001 0.0001 0.001 0.01 0.1 1


DR()

Plotting the impedance values obtained at the series and parallel resonant frequencyresults in the plot shown above. This plot demonstrates how sensitive the anti-resonant frequency is to the conductivity of the bulk solution.

It should be noted, however, that this sensor configuration can only be sensitive to

the conductivity of the bulk solution due to the fringing electric fields at theelectrode edges. It will not be sensitive to electrical changes of a sensing filmplaced on top of the sensors electrode surface.


40/44

Lossy Electrode for electroacoustic interactions:


C0

RingElectrode

Lossy Electrode

ConductiveIonic

Solution

,

AT Quartz

q q,

+X

-X

ModeShape

The bulk wave sensor shown previously is useful for measuring liquid conductivity,but is difficult to implement. Ideally we would prefer to measure the series resonantfrequency using a standard oscillator circuit. The ring electrode structure shownabove was designed to enhance the electro-acoustic interaction (series resonantfrequency change) with a conductive solution or a sensing film placed on thesurface.

The plot shown above illustrates the strength of the acoustic mode as a function ofposition across the surface of the resonator. As can be seen, the acoustic activity inthe center of the resonator is not very strong, but is still somewhat present. Theacoustic wave in the center (no metal) does have an electric field that is coupled tothe propagating acoustic wave that can interact with a conductive solution or asensing film placed on the surface. Additionally, the liquid or sensing film in thecenter non-electrode region will behave as a lossy electrode coupling moreacoustic energy into the TSM device as a function of conductivity of the solution orsensing film on its surface. So as the conductivity increases, the series resonantfrequency, series resistance, and overall Q of the resonator will dramatically change

and can be easily measured using a standard oscillator circuit.---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

C. Zhang and J. Vetelino, Bulk Acoustic Wave Sensors for Sensing Measurand-Induced Electrical Property Changes in Solutions,IEEE Transactions onUltrasonics, Ferroelectrics, and Frequency Control, vol. 48, no. 3, pp. 773-778,May 2001.


41/44


5MHz AT Quartz TSM Sensor ResponseImpedance Magnitude for Varying NaCl Solutions

Ring Structure (o.d. 0.45", i.d. 0.30") Bottom Electrode (o.d. 0.25")

0.00

1000.00

2000.00

3000.00

4000.00

5000.00

6000.00

7000.00

8000.00

5011000 5013000 5015000 5017000 5019000 5021000

Frequency (Hz)

Magnitude(W)

DI water M

1x10-4 M

5x10-4 M

1x10-3 M

5x10-3 M

1x10-2 M

5x10-2 M

1x10-1 M

5x10-1 M

1mol M

The figure above shows the impedance plots for a 5MHz AT-Cut ring structure (o.d.0.45, i.d. 0.30, bottom o.d. 0.25) for various concentrations of salt solution. Ascan be seen, the series resonant frequency is very dependent on the conductivesolution concentration.


42/44


5MHz AT Quartz TSM Sensor ResponsesSeries Resonant Frequency Change


-400

-350

-300

-250

-200

-150

-100

-50

0

0.00001 0.0001 0.001 0.01 0.1 1


Df(ppm)

The figure above plots the series resonant frequencies as a function of the saltconcentration for a ring electrode and an identical structure with a solid electrode.The solid electrode structure has no electro-acoustic interaction with the conductivesolution resulting in no frequency shift. The ring electrode structure is very sensitiveto the concentration of salt in water. As the conductivity of the solution increases,the amount of acoustic coupling increases resulting in significant series resonantfrequency changes. Notice how the shift in frequency occurs over a certain range ofconductivity similar to the SAW device presented earlier.


43/44


5MHz AT Quartz TSM Sensor ResponsesSeries Impedance Change


-400

-300

-200

-100

0

100

200

300

400

0.00001 0.0001 0.001 0.01 0.1 1


DRs(W)

The figure above plots the series resonant resistance values as a function of saltconcentration for the ring electrode structure and an identical structure with a solidelectrode. There is no change observed for the solid electrode, while the ringelectrode resistance is very dependent on the salt concentration.


44/44


Summary Gas Sensing

SAW

BAW

STW and Love Mode

SH-APM

Liquid Sensing BAW

SH-APM

Mechanical and Electrical Sensitivity Mechanical Mass, Visco-elastic properties

Electrical - Conductivity

A wide range of acoustic wave devices can be used for gas and liquid based sensingapplications. For gas sensing, SAW, BAW, STW, Love Mode, and SH-APM makeexcellent platforms for a wide range of sensing applications. For liquid-basedsensing, BAW and SH-APM can be utilized.

All acoustic wave devices are sensitive to mechanical and electrical perturbations ontheir surfaces. For mechanic effects, they are sensitive to mass loading and visco-elastic changes. For electrical effects they are sensitive to any effect that interactswith the electric field that is coupled to the propagating acoustic wave.

In general, acoustic wave devices are very sensitive and make excellent platformsfor a variety of sensing applications.

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