Surface acoustic wave sensors rely on modulating and transducing surface acoustic waves to sense physical phenomena. They have advantages including compact size, high sensitivity, fast response, and ability to operate wirelessly in harsh environments. A basic SAW sensor consists of a piezoelectric substrate with input and output interdigital transducers to launch and receive surface acoustic waves. The transducers convert between electrical and mechanical surface waves, allowing remote sensing by analyzing signal changes induced by external factors interacting with the waves.

1. SURFACE ACOUSTIC WAVE
(SAW) BASED SENSOR
Shreesh Mohan Verma
Tanuj Agarwal

2. 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.

3. Waves
Longitudinal Wave
Rayleigh surface wave
Transverse Wave

4. 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.

5.  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).

6. 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 7

8. 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

9. 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.

10. 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.

11.  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.

12. • In resonators, the reflections of the wave from either metal
stripes or grooves of small depths are used.
Fig-2
One-port SAW
resonator

13.  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.

14. 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 Diagra
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.

15. SAW Materials to Meet Sensor Needs
15
Coupling Temperatur SAW
Material Crystal cut Max Temp
coefficient e coefficient Velocity
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 wave
SAW wavelength @ 1 GHz ~ 3 um

16. RFID Sensor
16
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.
 RFID Acquisition  Measurand Extraction
 Priority for system  RFID is acquired
 Coding approach
 S/N ratio
 Demodulation approach
 Accuracy
 System Parameters
 Acquisition rate

17. Diversity for Identification
17
 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

18. Brief Introduction to
Wireless SAW Sensors
 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
18

19. Reflective Delay Line Sensor
19
“Wireless Interrogator System for SAW-Identification-Marks and SAW-Sensor Components”,
F. Schmidt, et al, 1996 IEEE International Frequency Control Symposium
 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

20. SAW Chirp Sensor
20
“Spread Spectrum Techniques for Wirelessly Interrogable Passive SAW Sensors”,
A. Pohl, et al, 1996 IEEE Symposium on Spread Spectrum Techniques and Applications
 Increased sensitivity when compared with simple
reflective delay line sensor
 Multi-sensor operation not possible due to lack of coding

21. Impedance SAW Sensors
“State of the Art in Wireless Sensing with Surface Acoustic Waves”,
W. Bulst, et al, IEEE UFFC Transactions, April 2001
 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
21

22. SAW RFID Practical Approaches
22
 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

23. SAW Resonator
experimental
23 -2 predicted
-4
S11 magnitude (dB)
-6
-8
Grating IDT Grating
-10
Q~10,000
-12
D D
-14
354.6 354.8 355 355.2 355.4 355.6 355.8 356 356.2 356.4
Frequency, MHz
• Resonant cavity
• Frequency with maximum returned
power yields sensor temperature
• High Q, long time response
• Coding via frequency domain by
“Remote Sensor System Using Passive SAW
separating into bands
Sensors”,
W. Buff, et al, 1994 IEEE International
Ultrasonics Symposium

24. SAW CDMA Delay Line
CDMA Tag
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
24

25. SAW OFC Delay Line
25
OFC Tag
f1 f4 f2 f6 f0 f5 f3
20
Piezoelectric Substrate
30
Magnitude (dB)
40
50
Experimental Micrograph of device
COM Simulated under test (DUT)
60
0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
OFC Tag Time (us)
•Multi-frequency (7 chip example)
•Long chips, high reflectivity Bragg reflector gratings at
differing frequencies
•Orthogonal frequency reflectors –low loss (6-10 dB) - RF probe
DUT
connected to
transducer
•Example time response (non-uniformity due to transducer)

26. Resonator/CDMA/OFC
26
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 CDMA OFC
•Minimal delay •Delay as reqd. ~ •Delay as reqd. ~
•Narrowband PG~1 1usec 1usec
•Fading •Spread Spectrum •Spread Spectrum
•Frequency domain Fading immunity Fading immunity
coding Wideband Ultra Wide Band
•High Q – long PG >1  PG >>1
impulse response •Time domain coding •Time & frequency
•Low loss sensor •Large number of domain coding
codes using PPM •Large number of
codes using PPM and
diverse chip
frequencies

27. OFC Historical Development
27
 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

28. Why OFC SAW Sensors?
 A game-changing  Radiation hard
approach
 All advatageous of SAW  Wide operational
technology temperature range
 Wireless, passive and
multi-coded sensors
 Frequency & time offer
greatest coding diversity
 Single communication
platform for diverse
sensor embodiments
28

29. Schematic of OFC SAW ID Tag
29
f1 f4 f2 f6 f0 f5 f3
Piezoelectric Substrate
1
Sensor bandwidth is 0.8
Time domain chips
dependent on 0.5
realized of chips and
number in Bragg
Magnitude (Linear)
reflectors having
sum of chip 0.6
differing carrier
bandwidths. 0
frequenciesdomain
Frequency and 0.4
frequencies are non-
of Bragg reflectors:
sequential which
contiguous in 0.5
provides coding
frequency but 0.2
shuffled in time
10
00 0.2 1 0.4 2
0.6 0.8 3 1 41.2 1.4 5 1.6 6
1.8 7
Normalized Time (Chip Lengths)
Normalized Frequency

30. Example 915 MHz SAW OFC
30
Sensor
US Quarter
SAW Sensor
f4 f3 f1 f5 f2
SAW OFC Reflector Chip Code
FFT

31. Synchronous Correlator
31
Transceiver

32. Temperature Extraction
Using Adaptive Corelator
32
Comparison of ideal and measured
matched filter of two different SAW
sensors : 5-chip frequency(below)
Normalized amplitude (dB) versus time
0
Experimental
-5
NS401
Amplitude (Normalized)
Ideal
-10
-15
-20
-25
-30
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
Time ( s)
0
NS403 Experimental
Amplitude (Normalized)
-5
Ideal
-10
-15
-20
Stationary plots represent idealized received SAW sensor -25
RFID signal at ADC. Adaptive filter matches sensor RFID -30
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
temperature at the point when maximum correlation Time ( s)
occurs.

33. Synchronous Correlator
33
Receiver
Block diagram of a correlator receiver using ADC
OFC Single Sensor Signal
Correlation
Output
0
Experimental
-5
Temperature
Amplitude (Normalized)
Ideal
-10
Extraction -15
-20
-25
-30
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2
Time ( s)

34. 250 MHz Wireless Pulsed RF OFC SAW System - 2nd
Pass
34
50 cm 50 cm
An OFC SAW temperature sensor data
run on a free running hotplate from an
30 cm 30 cm
Interrogator
(Transmitter)
Receiver
improved 250 MHz transceiver system.
SAW
78°C
Sensor/Tag
Thermal
The system used 5 chips and a
Couple
Thermal
Controller
Hot Plate
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.

35. RF Transceiver: Sensor Overview
35
 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

36. SAW 915 MHz OFC Sensor
36
 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

37. Parameter Definitions
(extensive list of variables)
 ADC= ideal analog-to-digital  PG= signal processing gain
converter of the system (= τ·B)
 MDS= minimum detectable  PL= path loss
signal at ADC
 NF= receiver noise figure
 S= signal power measured at
ADC  Next= external noise source
 N= noise power measured at referenced to antenna
ADC output
 kT= thermal noise energy  NADC= ADC equivalent noise
 EIRP= equivalent radiated  Nsum= number of
power synchronous integrations in
 GRFIDS= RFIDS gain (less than ADC
unity for passive device)  PGC = pulse compression
 GRx-ant= gain of the receiver gain from chirp
antenna interrgogation
 GRx= receiver gain from
antenna output to ADC 37

38. RF Chirp Transceiver
38
Parameters
 Power to antenna = 30dBm
 Pulse-length = 700ns, 20Vpp
 Antenna Gain = 9dB
 Bandwidth = 74MHz
 Receiver Gain = 45dB
 NF = 15dB
 PGC= 49 = 17 dB

39. UCF Sensor Development
 The following are a few of
 There is an extensive body
the successful UCF sensor
of knowledge on sensing
projects
 Wired SAW sensing has
 The aim is to enable
quite an extensive body of
wireless acquisition of the
knowledge and continues
sensors data
 Wireless SAW sensing has
 The further goal is to
been most successfully
develop a multi-sensor
demonstrated for single, or
system for aerospace
very few devices and in
applications
limited environments
 Successful wireless sensing
has been demonstrated for
temperature, liquid, closure,
and range 39

40. UCF OFC Sensor
Successful Demonstrations
40
 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

41. Differential SAW OFC Thin Film
41
Gas Sensor Embodiment
f3 f5 f0 f6 f2 f4 f1 f1 f4 f2 f6 f0 f5 f3
Piezoelectric Substrate
6.75 mm
1.25 mm 1.38 mm 2.94 mm 1.19 mm
f3 f5 f0 f6 f2 f4 f1
2.00 mm

42. Temperature Sensor using Differential Delay
Correlator Embodiment
f3 f5 f0 f6 f2 f4 f1 f1 f4 f2 f6 f0 f5 f3
Temperature Sensor Example
Piezoelectric Substrate
250 MHz LiNbO3, 7 chip
reflector, OFC SAW sensor
tested using temperature
controlled RF probe station
42

43. Temperature Sensor Results
Temperature Sensor Results
200
180
160
140
Temperature (C)
120
100
80
60
40
LiNbO3 SAW Sensor
20
Thermocouple
0
0 20 40 60 80 100 120 140 160 180 200
Time (min)
 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
43

44. OFC Cryogenic Sensor Results
50
Thermocouple
LiNbO3 SAW Sensor
0
Temperature (C)
Scale -50
Vertical: +50 to -200 oC -100
Horizontal: Relative time (min) -150
-200
0 5 10 15 20 25
Time (min)
OFC SAW temperature
sensor results and
Measurement
comparison with system with
thermocouple measurements liquid nitrogen
at cryogenic temperatures. Dewar and
Temperature scale is between vacuum
+50 to -200 oC and horizontal chamber for
scale is relative time in DUT
minutes.
44

45. Schematic and Actual OFC Gas Sensor
Differential mode OFC Sensor Schematic
f3 f0 f2 f1 f1 f2 f0 f3
 
Piezoelectric Substrate
Actual device with RF
probe
•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)
45

46. Hydrogen Gas sensor
Palladium Background Information
46
 The bulk of PD research has
Without H2
been performed for Pd in the
CONTACT
CONTACT
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 – With H2
called Hydrogen Induced Lattice
CONTACT
CONTACT
Expansion (HILE) – Resistivity
reduces
 Pd absorbs H2 gas which causes
palladium hydride formation –
Resistivity increases
 Examine these effects for ultra- HILE - Each small circle
thin films (<5nm) on SAW represents a nano-sized
cluster of Pd atoms
devices

47. Pd Films on SAW Devices
Schematic of Test Conditions
47
 Control: SAW delay line on YZ
LiNbO3 wafers w/ 2
transducers and reflector w/o
Pd film 1.27 mm
 Center frequency 123 MHz
Pd Film
 (A) SAW delay line w/ Pd in (A)
propagation path between
transducer and reflector
 (B) SAW delay line w/ Pd on Pd Film
reflector only (B)

48. Nano-Pd Film – 25 Ang.
20
Hydrogen Gas Sensor 24
28
Normalized Magnitude (dB)
32
Results: 2% H2 gas 36
40
44
48
52
56
Theory (lines) versus measurement data
Pd Film
60
Propagation Loss (dB/cm) and Velocity(m/s) vs. Film Resistivity 64
240 3500 68
SAW Velocity (m/s)
200 3485 72
Loss (dB/cm)
160 3470 76
120 3455 80
1.7 1.8 1.9 2 2.1 2.2
80 3440
•The change in IL
40 3425 Time (micindicates a <20 dB
ro-seconds)
0 3410
100 1 10
3
1 10
4
1 10
5
Delay Line w/o Pd sensitivity range and
Resistivity (ohm-cm) After Pd Film further tests were <
Loss/cm @ 123 MHz During 1st H2 Exposure
50 dB!
Pd Film
Loss/cm due to Pd Film
After 1st H2 Exposure
Loss/cm due to Pd Film After Final H2 Gas Exposure
Loss/cm due to succ essive H2 exposure During 2nd H2 Exposure •Sensitive hydrogen
SAW Velocity After 2nd H2 Exposure sensor is possible.
SAW Velocity due to Pd Film
SAW Velocity due to Pd Film After Final H2 Gas Exposure During 3rd H2 Exposure
SAW Velocity due to successive H2 exposure After 3rd H2 Exposure
During 4th H2 Exposure
After 4th H2 Exposure 48

49. 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
49

50. Current to Future
50

51. Vision for Future
51
• 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