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.
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 OPERATIONOF 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).
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 yearApplications: Cellular phones and TV (largest market) Military (Radar, filters, advanced systems Currently emerging – sensors, RFID 7
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.
• In resonators, the reflections of the wave from either metal stripes or grooves of small depths are used. Fig-2 One-port SAW resonator
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 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.
SAW Materials to Meet Sensor Needs15 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
RFID Sensor16Two 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
Diversity for Identification17 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
Brief Introduction toWireless 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
Reflective Delay Line Sensor19 “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
SAW Chirp Sensor20 “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
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
SAW RFID Practical Approaches22 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
SAW Resonator experimental23 -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
SAW CDMA Delay Line CDMA TagCDMA 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
SAW OFC Delay Line25 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 2.4OFC 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)
Resonator/CDMA/OFC26 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
OFC Historical Development27 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
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
Schematic of OFC SAW ID Tag 29 f1 f4 f2 f6 f0 f5 f3 Piezoelectric Substrate 1Sensor bandwidth is 0.8Time domain chipsdependent on 0.5realized of chips andnumber in Bragg Magnitude (Linear)reflectors havingsum of chip 0.6differing carrierbandwidths. 0frequenciesdomainFrequency and 0.4frequencies are non-of Bragg reflectors:sequential whichcontiguous in 0.5provides codingfrequency but 0.2shuffled 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
Example 915 MHz SAW OFC30 Sensor US Quarter SAW Sensor f4 f3 f1 f5 f2 SAW OFC Reflector Chip Code FFT
Synchronous Correlator31 Transceiver
Temperature ExtractionUsing Adaptive Corelator32 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 -20Stationary plots represent idealized received SAW sensor -25RFID signal at ADC. Adaptive filter matches sensor RFID -30 -0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2temperature at the point when maximum correlation Time ( s)occurs.
Synchronous Correlator33 ReceiverBlock diagram of a correlator receiver using ADCOFC 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)
250 MHz Wireless Pulsed RF OFC SAW System - 2nd Pass34 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.
RF Transceiver: Sensor Overview35 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
SAW 915 MHz OFC Sensor36 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
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
RF Chirp Transceiver38 Parameters Power to antenna = 30dBm Pulse-length = 700ns, 20Vpp Antenna Gain = 9dB Bandwidth = 74MHz Receiver Gain = 45dB NF = 15dB PGC= 49 = 17 dB
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
UCF OFC Sensor Successful Demonstrations40 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
Differential SAW OFC Thin Film41 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
Temperature Sensor using Differential Delay Correlator Embodiment f3 f5 f0 f6 f2 f4 f1 f1 f4 f2 f6 f0 f5 f3Temperature Sensor Example Piezoelectric Substrate250 MHz LiNbO3, 7 chipreflector, OFC SAW sensortested using temperaturecontrolled RF probe station 42
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
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 temperaturesensor results and Measurementcomparison with system withthermocouple measurements liquid nitrogenat cryogenic temperatures. Dewar andTemperature scale is between vacuum+50 to -200 oC and horizontal chamber forscale is relative time in DUTminutes. 44
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 changein differential delay senses the gas (τ1 = τ2) (in progress) 45
Hydrogen Gas sensor Palladium Background Information46 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
Pd Films on SAW Devices Schematic of Test Conditions47 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)
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 72Loss (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
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
Current to Future50
Vision for Future51 • 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