SURFACE ACOUSTIC WAVE (SAW) BASED SENSOR

Shreesh Mohan Verma

Tanuj Agarwal

Surface Acoustic Wave

A surface acoustic wave (SAW) is an acoustic wave traveling along the surface of a material exhibiting elasticity, with amplitude that typically decays exponentially with depth into the substrate.

Surface acoustic waves were discovered in 1885 by Lord Rayleigh, and are often named after him: Rayleigh waves. A surface acoustic wave is a type of mechanical wave motion which travels along the surface of a solid material.

The velocity of acoustic waves is typically 3000 m/s, which is much lower than the velocity of the electromagnetic waves.

Waves

Longitudinal Wave

Transverse Wave

Rayleigh surface wave

Surface Acoustic Wave Sensors

Surface acoustic wave sensors are a class of microelectromechanical systems (MEMS) which rely on the modulation of surface acoustic waves to sense a physical phenomenon.

The sensor transduces an input electrical signal into a mechanical wave which, unlike an electrical signal, can be easily influenced by physical phenomena.

The device then transduces this wave back into an electrical signal. Changes in amplitude, phase, frequency, or time-delay between the input and output electrical signals can be used to measure the presence of the desired phenomenon.

Conventional fields of application – communications and signal processing Other application - as identification tags, chemical and biosensors, and as sensors of different physical quantities.

The SAW sensors are passive elements (they do not need power supply) and can be accessed wirelessly, enabling remote monitoring in harsh environment. They work in the frequency range of 10 MHz to several GHz.

They have the rugged compact structure, outstanding stability, high sensitivity, low cost, fast real time response, extremely small size (lightweight).

BASIC PRINCIPLE OF OPERATION OF SAW DEVICES

The operation of the SAW device is based on acoustic wave propagation near the surface of a piezoelectric solid. This implies that the wave can be trapped or otherwise modified while propagating.

The displacements decay exponentially away from the surface, so that the most of the wave energy (usually more than 95 %) is confined within a depth equal to one wavelength.

The surface wave can be excited electrically by means of an interdigital transducer (IDT).

7

What is a typical SAW Device? A solid state device

Converts electrical energy into a mechanical wave on a single crystal substrate

Provides very complex signal processing in a very small volume

Approximately 4-5 billion SAW devices are produced each year

Applications:Cellular phones and TV (largest market)

Military (Radar, filters, advanced systems

Currently emerging – sensors, RFID

PRINCIPLE

A basic SAW device consists of two IDTs on a piezoelectric substrate such as quartz. The input IDT launches and the output IDT receives the waves.

The basic structure of a SAW device

PRINCIPLE

The interdigital transducer consists of a series of interleaved electrodes made of a metal film deposited on a piezoelectric substrate as shown above.

The width of the electrodes usually equals the width of the inter-electrode gaps giving the maximal conversion of electrical to mechanical signal, and vice versa.

The minimal electrode width which is obtained in industry is around 0.3 μm, which determines the highest frequency of around 3 GHz.

PRINCIPLE

The commonly used substrate crystals are: quartz, lithium niobate, lithium tantalate, zinc oxide and bismuth germanium oxide. They have different piezoelectric coupling coefficients and temperature sensitivities. The ST quartz is used for the most temperature stable devices.

The wave velocity is a function of the substrate material and is in the range of 1500 m/s to 4800 m/s, which is 105 times lower than the electromagnetic wave velocity. This enables the construction of a small size delay line of a considerable delay.

The input and output transducers may be equal or different. It depends upon the function which the SAW device has to perform. Usually, they differ in electrode’s overlaps, number and sometimes positioning.

If the electrodes are uniformly spaced, the phase characteristic is a linear function of frequency, e.g., the phase delay is constant in the appropriate frequency range. This type of the SAW device is than called delay line.

In the second type of SAW devices – SAW resonators , IDTs are only used as converters of electrical to mechanical signals, and vice versa, but the amplitude and phase characteristics are obtained in different ways.

Fig-2One-port SAW resonator

• In resonators, the reflections of the wave from either metal stripes or grooves of small depths are used.

In the one-port SAW resonator only one IDT, placed in the center of the substrate, is used for both, input and output, transductions.

The input electrical signal connected to IDT, via antenna or directly, forms a mechanical wave in the piezoelectric substrate which travels along the surface on both sides from the transducer.

The wave reflects from the reflective array and travels back to the transducer, which transforms it back to the electrical signal. The attenuation of the signal is minimal if the frequency of the input signal matches the resonant frequency of the device.

Device Layout

The basic surface acoustic wave device consists of a piezoelectric substrate, an input interdigitated transducer (IDT) on one side of the surface of the substrate, and a second, output interdigitated transducer on the other side of the substrate.

Surface Acoustic Wave Sensor Interdigitated Transducer Diagram

The space between the IDTs, across which the surface acoustic wave will propagate, is known as the delay-line. This region is called the delay line because the signal, which is a mechanical wave at this point, moves much slower than its electromagnetic form, thus causing an appreciable delay.

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SAW Materials to Meet Sensor Needs

Material Crystal cut Coupling coefficient

Temperature coefficient

SAW Velocity Max Temp

LiNbO3 Y,Z 4.6% 94 ppm/ºC 3488 m/s ~500 ºC

128ºY,X 5.6% 72 ppm/ºC 3992 m/s ~500 ºC

LiTaO3 Y,Z 0.74% 35 ppm/ºC 3230 m/s ~500 ºC

Quartz ST 0.16% 0 ppm/ºC 3157 m/s 550 ºC

Langasite Y,X 0.37% 38 ppm/ºC 2330 m/s >1000 ºC

138ºY,26ºX 0.34% ~0 ppm/ºC 2743 m/s >1000 ºC

SNGS Y,X 0.63% 99 ppm/ºC 2836 m/s >1000 ºC

SAW travels ~ 105 slower than EM waveSAW wavelength @ 1 GHz ~ 3 um

RFID Sensor

RFID Acquisition Priority for system Coding approach Demodulation

approach System Parameters

Measurand Extraction RFID is acquired S/N ratio Accuracy Acquisition rate

Two primary system functions: RFID and extraction of the measurand. The RFID must first be acquired and then the measurand extracted. The presentation will address these issues for a temperature sensor system.

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Diversity for Identification

Frequency Spectrum Diversity per Device Coding Divide into frequency bands

Time Delay per Device Different offset delays per device Pulse position modulation Time allocations minimize code collisions

Spatial Diversity – device placement Sensor & Tx-Rx Antenna Polarization Use combinations of all to optimize system

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18

One port devices return the altered interrogation signal

Range depends on embodiment Range increased using coherent

integration of multiple responses Interrogator used to excite devices Several embodiments are shown next

Brief Introduction to Wireless SAW Sensors

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Reflective Delay Line Sensor

First two reflectors define operating temperature range of the sensor

Time difference between first and last echoes used to increase resolution of sensor

No coding as shown

“Wireless Interrogator System for SAW-Identification-Marks and SAW-Sensor Components”,

F. Schmidt, et al, 1996 IEEE International Frequency Control Symposium

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SAW Chirp Sensor

Increased sensitivity when compared with simple reflective delay line sensor

Multi-sensor operation not possible due to lack of coding

“Spread Spectrum Techniques for Wirelessly Interrogable Passive SAW Sensors”,

A. Pohl, et al, 1996 IEEE Symposium on Spread Spectrum Techniques and Applications

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Impedance SAW Sensors

External classical sensor or switch connected to second IDT which operates as variable reflector

Load impedance causes SAW reflection variations in magnitude and phase

No discrimination between multiple sensors as shown

“State of the Art in Wireless Sensing with Surface Acoustic Waves”,

W. Bulst, et al, IEEE UFFC Transactions, April 2001

SAW RFID Practical Approaches

Resonator Fabry-Perot Cavity Frequency selective, SAW device Q~10,000

Code Division Multiple Access (CDMA) Delay line – single frequency Bragg reflectors Pulse position encoding

Orthogonal Frequency Coding (OFC) Delay line, multi-frequency Bragg reflectors Pulse position encoding Frequency coupled with time diversity

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SAW Resonator

D D

Grating GratingIDT

354.6 354.8 355 355.2 355.4 355.6 355.8 356 356.2 356.4-14

-12

-10

-8

-6

-4

-2

Frequency, MHz

S11

mag

nitu

de (

dB)

experimentalpredicted

“Remote Sensor System Using Passive SAW Sensors”,

W. Buff, et al, 1994 IEEE International Ultrasonics Symposium

Q~10,000

• Resonant cavity• Frequency with maximum returned

power yields sensor temperature• High Q, long time response• Coding via frequency domain by

separating into bands

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SAW CDMA Delay Line

CDMA Tag Concept•Single frequency Bragg reflectors

•Coding via pulse position modulation

•Large number of possible codes

•Short chips, low reflectivity - (typically 40-60 dB IL)

•Early development by Univ. of Vienna, Siemens, and others

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CDMA Tag

SAW OFC Delay Line

Piezoelectric Substrate

f1 f4 f6 f0f2 f5 f3

0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.460

50

40

30

20

ExperimentalCOM Simulated

Time (us)

Mag

nitu

de (

dB)

OFC Tag

•Multi-frequency (7 chip example)

•Long chips, high reflectivity

•Orthogonal frequency reflectors –low loss (6-10 dB)

•Example time response (non-uniformity due to transducer)

OFC Tag

DUT - RF probe connected to transducer

Bragg reflector gratings at differing frequencies

Micrograph of device under test (DUT)

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Resonator/CDMA/OFC

Resonator, CDMA, and OFC embodiments have all been successfully demonstrated and applied to various applications. Devices and systems have been built in the 400 MHz, 900 MHz and 2.4 GHz bands by differing groups.

Resonator•Minimal delay•Narrowband PG~1•Fading•Frequency domain coding•High Q – long impulse response•Low loss sensor

CDMA•Delay as reqd. ~ 1usec•Spread Spectrum

Fading immunity Wideband PG >1

•Time domain coding•Large number of codes using PPM

OFC•Delay as reqd. ~ 1usec•Spread Spectrum

Fading immunity Ultra Wide Band PG >>1

•Time & frequency domain coding•Large number of codes using PPM and diverse chip frequencies

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OFC Historical Development

Several different OFC sensors demonstrated Chose 1st devices at 250 MHz for feasibility Demonstrated harmonic operated devices at 456,

915 MHz and 1.6 GHz Fundamental device operation at 915 MHz Devices in the +1 GHz range in 2010 First OFC system at 250 MHz Current OFC system at 915 MHz First 4 device wireless operation in 2009 Mnemonics demonstrates first chirp OFC corelator

receiver in 2010

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Why OFC SAW Sensors?

A game-changing approach

All advatageous of SAW technology

Wireless, passive and multi-coded sensors

Frequency & time offer greatest coding diversity

Single communication platform for diverse sensor embodiments

Radiation hard Wide operational

temperature range

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Piezoelectric Substrate

f1 f4 f6 f0f2 f5 f3

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

0.2

0.4

0.6

0.8

Normalized Frequency

Mag

nitu

de (

Lin

ear)

Schematic of OFC SAW ID Tag

0 1 2 3 4 5 6 71

0.5

0

0.5

1

Normalized Time (Chip Lengths)

Time domain chips realized in Bragg reflectors having differing carrier frequencies and frequencies are non-sequential which provides coding

Sensor bandwidth is dependent on number of chips and sum of chip bandwidths. Frequency domain of Bragg reflectors: contiguous in frequency but shuffled in time

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Example 915 MHz SAW OFC Sensor

FFT

US QuarterSAW Sensor

SAW OFC Reflector Chip Code

f4 f3 f1 f5 f2

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Synchronous Correlator Transceiver

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Temperature ExtractionUsing Adaptive Corelator

Comparison of ideal and measured matched filter of two different SAW sensors : 5-chip frequency(below)

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2-30

-25

-20

-15

-10

-5

0

Time (s)

Am

plitu

de (

Nor

mal

ized

)

Experimental

Ideal

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2-30

-25

-20

-15

-10

-5

0

Time (s)

Am

plitu

de (

Nor

mal

ized

)

Experimental

IdealNS403

NS401

Normalized amplitude (dB) versus time

Stationary plots represent idealized received SAW sensor RFID signal at ADC. Adaptive filter matches sensor RFID temperature at the point when maximum correlation occurs.

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Synchronous Correlator Receiver

Block diagram of a correlator receiver using ADC

-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2-30

-25

-20

-15

-10

-5

0

Time (s)

Am

plitu

de (

Nor

mal

ized

)

Experimental

Ideal

OFC Single Sensor Signal

Correlation Output

Temperature Extraction

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250 MHz Wireless Pulsed RF OFC SAW System - 2nd Pass

An OFC SAW temperature sensor data run on a free running hotplate from an improved 250 MHz transceiver system. The system used 5 chips and a fractional bandwidth of approximately 19%. The dashed curve is a thermocouple reading and the solid curve is the SAW temperature extracted data. The SAW sensor is tracking the thermocouple very well; the slight offset is probably due to the position and conductivity of the thermocouple.

50 cm 50 cm

30 cm 30 cm

SAW Sensor/Tag

Interrogator(Transmitter)

Receiver

Hot Plate

78°CThermal

Controller

Thermal Couple

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RF Transceiver: Sensor Overview

OFC with single wideband transducer Center Frequency: 915 MHz Bandwidth: Chirp - ~78 MHz Number of Chips: 5 Chip length 54ns/each, total reflector

length 270ns Substrate: YZ LiNbO3

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SAW 915 MHz OFC Sensor

SAW sensor acts as RFID and sensor All antenna & transducer effects are doubled Antenna gain and bandwidth are dependent

on size scaled to frequency SAW propagation loss is frequency

dependent

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Parameter Definitions(extensive list of variables)

PG= signal processing gain of the system (= τ·B)

PL= path loss NF= receiver noise figure Next= external noise source

referenced to antenna output

NADC= ADC equivalent noise Nsum= number of

synchronous integrations in ADC

PGC = pulse compression gain from chirp interrgogation

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ADC= ideal analog-to-digital converter

MDS= minimum detectable signal at ADC

S= signal power measured at ADC

N= noise power measured at ADC

kT= thermal noise energy EIRP= equivalent radiated

power GRFIDS= RFIDS gain (less than

unity for passive device) GRx-ant= gain of the receiver

antenna GRx= receiver gain from

antenna output to ADC

RF Chirp Transceiver Parameters Power to antenna = 30dBm Pulse-length = 700ns, 20Vpp

Antenna Gain = 9dB Bandwidth = 74MHz Receiver Gain = 45dB NF = 15dB PGC= 49 = 17 dB

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UCF Sensor Development

The following are a few of the successful UCF sensor projects

The aim is to enable wireless acquisition of the sensors data

The further goal is to develop a multi-sensor system for aerospace applications

Successful wireless sensing has been demonstrated for temperature, liquid, closure, and range

There is an extensive body of knowledge on sensing

Wired SAW sensing has quite an extensive body of knowledge and continues

Wireless SAW sensing has been most successfully demonstrated for single, or very few devices and in limited environments

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UCF OFC Sensor Successful Demonstrations

Temperature sensing Cryogenic: liquid nitrogen Room temperature to 250oC Currently working on sensor for operation to

750oC Cryogenic liquid level sensor: liquid

nitrogen Pressure/Strain sensor Hydrogen gas sensor Closure sensor with temperature

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Differential SAW OFC Thin Film Gas Sensor Embodiment

2.00 mm

1.25 mm 1.38 mm 1.19 mm2.94 mm

6.75 mm

f3 f5 f0 f6 f2 f4 f1

Piezoelectric Substrate

f3 f5 f0 f6f2 f4 f1

f1 f4 f2 f6f0 f5 f3

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Temperature Sensor using Differential Delay Correlator Embodiment

Piezoelectric Substrate

f1 f4 f6 f0f2 f5 f3f1f4f6f0 f2f5f3

Temperature Sensor Example

250 MHz LiNbO3, 7 chip reflector, OFC SAW sensor tested using temperature controlled RF probe station

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Temperature Sensor Results

250 MHz LiNbO3, 7 chip reflector, OFC SAW sensor tested using temperature controlled RF probe station

Temp range: 25-200oC Results applied to simulated

transceiver and compared with thermocouple measurements

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

120

140

160

180

200Temperature Sensor Results

Time (min)

Te

mp

era

ture

( C)

LiNbO3 SAW Sensor

Thermocouple

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OFC Cryogenic Sensor Results

0 5 10 15 20 25-200

-150

-100

-50

0

50

Time (min)

Tem

pera

ture

( C

)

ThermocoupleLiNbO

3 SAW Sensor

Scale

Vertical: +50 to -200 oC

Horizontal: Relative time (min)

Measurement system with liquid nitrogen Dewar and vacuum chamber for DUT

OFC SAW temperature sensor results and comparison with thermocouple measurements at cryogenic temperatures. Temperature scale is between +50 to -200 oC and horizontal scale is relative time in minutes.

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Schematic and Actual OFC Gas Sensor

Piezoelectric Substrate

f1 f0f2 f3f1f0 f2f3

•For palladium hydrogen gas sensor, Pd film is in only in one delay path, a change in differential delay senses the gas (τ1 = τ2) (in progress)

Differential mode OFC Sensor Schematic

Actual device with RF probe

Hydrogen Gas sensor Palladium Background Information

The bulk of PD research has been performed for Pd in the 100-10000 Angstrom thickness

Morphology of ultra-thin films of Pd are dependent on substrate conditions, deposition and many other parameters

Pd absorbs H2 gas which causes lattice expansion of the Pd film – called Hydrogen Induced Lattice Expansion (HILE) – Resistivity reduces

Pd absorbs H2 gas which causes palladium hydride formation – Resistivity increases

Examine these effects for ultra-thin films (<5nm) on SAW devices

HILE - Each small circle represents a nano-sized

cluster of Pd atoms

CO

NTA

CT

CO

NTA

CT

W ithout H2

CO

NTA

CT

CO

NTA

CT

With H2

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Pd Films on SAW DevicesSchematic of Test Conditions

Control: SAW delay line on YZ LiNbO3 wafers w/ 2 transducers and reflector w/o Pd film

Center frequency 123 MHz (A) SAW delay line w/ Pd in

propagation path between transducer and reflector

(B) SAW delay line w/ Pd on reflector only

Pd Film

(A )

(B )

Pd

Film

1.27 mm

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Hydrogen Gas Sensor Results: 2% H2 gas

48

1.7 1.8 1.9 2 2.1 2.280

76

72

68

64

60

56

52

48

44

40

36

32

28

24

20

Delay Line w/o PdAfter Pd FilmDuring 1st H2 ExposureAfter 1st H2 ExposureDuring 2nd H2 ExposureAfter 2nd H2 ExposureDuring 3rd H2 ExposureAfter 3rd H2 ExposureDuring 4th H2 ExposureAfter 4th H2 Exposure

Time (micro-seconds)N

orm

aliz

ed M

agni

tude

(dB

)

Pd

Film

100 1 103

1 104

1 105

0

40

80

120

160

200

240

3410

3425

3440

3455

3470

3485

3500

Loss/cm @ 123 MHzLoss/cm due to Pd FilmLoss/cm due to Pd Film After Final H2 Gas ExposureLoss/cm due to successive H2 exposureSAW VelocitySAW Velocity due to Pd FilmSAW Velocity due to Pd Film After Final H2 Gas ExposureSAW Velocity due to successive H2 exposure

Propagation Loss (dB/cm) and Velocity(m/s) vs. Film Resistivity

Resistivity (ohm-cm)

Los

s (d

B/c

m)

SA

W V

eloc

ity (

m/s

)

Pd

Film

Nano-Pd Film – 25 Ang.

•The change in IL indicates a <20 dB sensitivity range and further tests were < 50 dB!

•Sensitive hydrogen sensor is possible.

Theory (lines) versus measurement data

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Applications

Current efforts include OFC SAW liquid level, hydrogen gas, pressure and temperature sensors

Multi-sensor spread spectrum systems Cryogenic sensing High temperature sensing Space applications Turbine generators Harsh environments Ultra Wide band (UWB) Communication

UWB OFC transducers Potentially many others

Current to Future50

Vision for Future

• Multiple access, SAW RFID sensors• SAW RFID sensor loss approaching 6 dB

– Unidirectional transducers– Low loss reflectors

• New and novel coding• New and novel sensors• New materials for high temperature (1000oC) and

harsh environments• SAW sensors in test space flight and support

operations in 1 to 5 years

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