wireless and passive sensors for high temperature measurements

Wireless and Passive Sensors for High Temperature Measurements

Bruno Francois∗, Denny Richter†, Holger Fritze†, Zacharis J. Davis‡, Christophe Droit§,Blandine Guichardaz∗, Valerie Petrini∗, Gilles Martin∗, Jean-Michel Friedt§ and Sylvain Ballandras∗

∗FEMTO-ST, time and frequency department, Besancon, FranceEmail: [email protected]

†TU Clausthal, Institut fur Energieforschung und Physikalische Technologien, Clausthal, GermanyEmail: [email protected]

‡Danish Technological Institute , Taastrup, DenmarkEmail: [email protected]

§SENSeOR SAS, Besancon, FranceEmail: [email protected]

Abstract—Surface acoustic waves have been studied for morethan 50 years and are mainly used as passive components(resonators, frequency filters, sensors, etc.) for signal pro-cessing, and more specifically. As demonstrated by Bao etal, surface acoustic wave devices can operate without on-board power supply using a dedicated interrogation unit. Usingappropriate design considerations, these devices are sensitive toexternal conditions including temperature, pressure, strain orchemical/biological mass loading. Therefore, the unique charac-teristics of such devices allows for an effective implementationproviding new opportunities for remote control of physical andchemical parameters.

Keywords-Surface acoustic wave; high temperature; sensor;langasite; quartz; aging; radiofrequency; reader; passive.

I. INTRODUCTION

The work presented in this paper has been achievedwithin the SAWHOT European project frame [1], devoted tothe development of sensor and interrogation system for anunprecedented temperature range (from cryogenic conditionsto temperature in excess of 650oC). In the following, thedevelopment of temperature sensors operating from roomto high temperature is described, the application concern-ing high temperature furnace monitoring, turbine operationcontrol. Two different types of substrate have been used forthe fabrication of SAW (Surface Acoustic Wave) sensors toassess first the possibility for accurate measurements using adifferential structure, second to reach temperatures in excessof 700oC using a single resonator sensor to demonstratethe capability of these devices to withstand such operationconditions. Quartz [2] and Langasite-based [3] solutionshave been respectively implemented in that purpose [4].

The first part of the paper briefly recalls the principleof wireless SAW sensors. Design rules are subsequentlyreported considering the specific problem of temperaturerobustness. Particularly, the specific problem of packagingand antenna assembly is discussed. The final sections aredevoted to effective temperature measurements in the above-mentioned ranges. As a conclusion, the capability of LGS-

based sensors to reach temperature in excess of 700oC isdiscussed.

II. WIRELESS SAW SENSOR INTERROGATION PRINCIPLE

SAW sensors are based either on delay lines on LithiumNiobate operating in the Industrial-Scientific-Medical (ISM)2.45 GHz band or on resonators working at lower frequen-cies, i.e. ISM bands centered in 434, 868 or 915 MHz. Forthe later case, the basic principle of the interrogation strategyis a combination of a frequency-sweep network analyzerused to identify the resonance frequencies of the device, anda monostatic pulse-mode RADAR strategy [5] for improvingthe isolation between emission and reception stages. Thewhole interrogation system is usually called reader as itactually reads the resonance frequencies. A radio-frequencypulse gates a carrier at variable frequencies within the ISMband. If the emitted pulse spectrum overlaps the bandpass ofone resonator, the acoustic device stores the energy. Once theantenna of the interrogation unit switched from the emissionto the reception stages of the reader, the resonator restoresthe loaded energy at its own resonance frequency, shiftingwith the variation of the physical parameter measured bythe resonator [6]. This pulse is received by the interrogationunit using a wideband power detector. The returned powervs frequency curves allows for the identification of theresonance frequency [7].

III. SAW SENSOR DESIGN FOR HIGH TEMPERATUREMEASUREMENTS

An important knowledge has been developed for 20 yearsbased on work devoted to the simulation and computation ofsurface acoustic wave devices (SAW), [8]. The two mainlyexploited methods are FEM (Finite Element Method) [9]and BEM (Boundary Element method) [10], both combinedto accurately represent effective SAW boundary conditions.FEM is used to account for the inhomogeneous part ofthe transducer whereas of BEM is used to represent thecontribution of the substrate. This later function is achieved

46Copyright (c) IARIA, 2012. ISBN: 978-1-61208-208-0

SENSORDEVICES 2012 : The Third International Conference on Sensor Device Technologies and Applications

using Green’s function of any layered substrates (Fig. 1)assuming flat interface between each layer of the stack.These computations are used to derive mixed-matrix orCoupling-Of-Mode (COM) parameters necessary to simulatethe electrical response of actual devices.

Meshed part : Finite Element Analysis

Radiation : Green function

Junction by BEM

p

a

h

Period

MirrorsMirrors Transducers

Aperture

Figure 1. FEM/BEM synoptic.

Two high temperature ranges are targeted, below and over500oC. For “low temperature”, sensors based on Quartz areused with aluminum-copper electrodes allowing for highquality factor resonances (≥ 10000) [11]. Regarding theCurie temperature of Quartz, this material can not be usedat temperature higher than 573oC (limited to 540oC inpractice); therefore Langasite (lanthanum gallium silicate)substrate [12] has been chosen for temperature measurementover 500oC [13], [14].

Quartz sensors used for first tests are TSEAS10 (5x5mm2, Fig. 2) from SENSeOR (Mougins, France), this deviceuses two resonators for differential measurements reducingthe influence of correlated noise and aging effect on the re-sponse. SAW devices based on Quartz with Al-Cu electrodesprovide high Q factor as required for wireless interrogationconsidering the capability of the sensor to store energy fromthe RF wave, and to restore it at its own frequency dependingof environmental physical parameters [15].

Fig. 2 shows a quasi-perfect match between the simulationand the experiment for this type of sensor (association ofquartz substrate with aluminum for interdigital transducers,IDT).

IV. PACKAGING OF SAW DEVICES FOR HIGHTEMPERATURE

Packaging is one key-point of the project. The device mustbe mounted on a carrier to avoid applying any stress on thedevice. Therefore, package and substrate materials requirecoefficients of thermal expansion as close one another aspossible. The whole package is composed of a standardSMD ceramic case with tungsten/molybdenum pads (Ky-

Figure 2. Comparison between computation and measurement of theTSEAS10 double resonators sensor.

ocera 7.1*9.1 mm2 A440), connected to device by gold wire-bonding (Fig. 3).

Figure 3. The different stages of the packaging process.

At the moment, devices are sealed using another upside-down package as lid. The case is connected to an aluminaplate with patterned connection, this footprint is realized inAgPb alloy to avoid the migration of the solder, stainlesssteel antennas and case are connected to the pattern thanksto tin solder.

An additional conditioning is required to preserve thedevice integrity when exposed to high temperature. Thecorresponding process is currently under patent applicationprocess and will be described when presented at the confer-ence.

V. TEMPERATURE MEASUREMENTS WITH SAW SENSORSBASED ON QUARTZ

The first experiment helps us to understand what happensto Quartz-based sensor between 25 oC and 250 oC.

The interrogator includes adapted SAW filters yieldingoperating in the 434MHz-centered ISM frequency band(434 MHz±0.85MHz). The algorithm used by the readeris based on the separation of the above 1.7MHz band intwo equilibrated frequency sub-bands. Each resonance ofthe differential sensor is located in one of the band forthe whole temperature excursion. The problem in our caseis that the sensor is used out of its specifications (roomtemperature to 250oC instead of -15oC to 165oC); therefore



4.33e+08

4.335e+08

4.34e+08

4.345e+08

4.35e+08

0 2000 4000 6000 8000 10000 12000 0

50

100

150

200

250

Fre

quency (

Hz)

Tem

pera

ture

(degC

)

Time (s)

Frequency 1 (Hz)Frequency 2 (Hz)

Temperature (degC)

Figure 4. Behavior of resonator at intermediate temperatures, exhibitingthe crossing of the higher frequency resonance (starting as the green curve)into the low-frequency band between dates 500 and 600, for temperaturesabove 180oC.

the resonance frequencies are overlapping over 180oC. Withthis algorithm, the first band begins at 433.05MHz andfinishes at 433.89MHz, while the second band starts at433.89MHz and stops at 434.83MHz. Each band is used tomeasure only one frequency. if two resonance frequenciesarise in the same band, the measured signal is the onewith the largest amplitude. On Fig. 4, the first frequency(green curve) is leaving the upper band to penetrates thesecond one (>434 MHz), the power received by the readercorresponding to this resonance is larger than for the otherone (red curve).

The second experiment aims at using Quartz sensor attemperature up to 450oC without frequency band programof the reader (the contrary of the last experiment), providingthe flexibility needed to measure SAW sensors out of theirspecifications.

4.3e+08

4.31e+08

4.32e+08

4.33e+08

4.34e+08

4.35e+08

0 5000 10000 15000 20000 25000 30000 0

50

100

150

200

250

300

350

400

450

Fre

quency (

Hz)

Tem

pera

ture

(´°

C)

Time (s)

Temperature (degC)Frequency 1Frequency 2

Figure 5. Behavior of resonator at intermediate temperature (around450oC).

The sensor is based on differential measurements, butthe two corresponding resonators do not exhibit the samebehavior versus temperature. The 434 MHz frequency (bluebetween 0 and 5000s) is the reference mode exhibitingless frequency variation than the sensor mode (433 MHz)corresponding to the sensor mode. The turnover of thereference mode is situated in the temperature range from

room temperature to 200oC, then, the frequency is dra-maticaly decreasing as shown from 5000s to 10000s. Theturnover of the other mode is situated at higher temperature,therefore the frequency shift of this mode versus temperatureis less important than for the reference mode. The higherfrequency is crossing the other resonance between 5000sand 7500s, then both frequencies have changed their placesin the frequency band when the oven stoped to heat upat 22500s. With this later algorithm, the reader is ableto measure frequencies, which are separated from at least200 kHz (depending of the averages number performed andthe frequency band measured,considering 150 steps in a 4MHz wideband, each “valid” measure separated by 7 steps).More experiments have been led on Quartz-based sensors toevaluate if a solution with this substrate can be an alternativeto high temperature substrate at intermediate temperatures(room temperature to 500oC). As shown on Fig. 6, thisresonator suffered 50h at 450oC without visible damages.

Figure 6. Picture of the IDT of TSEAS10 sensor used during 50 hours at450oC while measured by wireless interrogator. IDT are polluted after theopening of the ceramic package using milling tools.

VI. AGING AND TEMPERATURE MEASUREMENTS WITHSAW SENSORS BASED ON LANGASITE

The first steps of the experiments are focusing on theevaluation of the efficiency of the wireless sensor at inter-mediate (Fig. 8), and then at high temperature (Fig. 13).Moreover, the frequency measurements of the devices willhelp understanding and evaluating the frequency variationsof the resonators. The necessity of a “preaging” process forLangasite sensors then will be emphasized.

For the first experiment, the resonator was placed in aceramic oven (able to warm up up to 1200oC, Fig7).

The resonator has been submitted at a temperature of500oC during 50h. Meanwhile, its resonance frequency hasbeen wirelessly monitored with a refreashing rate of aboutone measurement per second.

As expected, the temperature cycle applied to the res-onator did operate as an aging process and the frequency ofthe resonator has evolved. As shown on Fig. 8, a differenceof 300 kHz is observed at room temperature before and afterthe 50h-500oC cycle (at dates 50000s and 320000s). Thefrequency is shifting along time (during the 500oC heatingprocess), showing a difference of 50 kHz between the dates



Figure 7. High temperature bench : ceramic oven with non-metallic backfor RF wireless measurement associated with oscilloscope for debuggingsignal issue from reader unit, another temperature probe is used to measuretemperature as close as possible of the device.

4.3e+08

4.32e+08

4.34e+08

4.36e+08

4.38e+08

4.4e+08

0 50000 100000 150000 200000 250000 300000 350000

Fre

quency (

Hz)

Time (s)

Resonance frequency of LGS sensor

Figure 8. Wireless measurement of the sensor at 500oC during 50h.Between 0s and 50000s, the sensor is packaged (Fig3) and placed in theoven for a first preliminary cycle (100oC for 2h, 270oC for 2h and 360oCfor 2h). Between 80000s and 280000s, the sensor withstands a temperatureof 500oC.

100000s and 280000s. The observed variance at short termof the measured frequency is 10 kHz.

The resonator then is used at 700oC to push ahead theexperiment (Fig. 9) and to continue to estimate the observedshift of the frequency.

4.3e+08

4.32e+08

4.34e+08

4.36e+08

4.38e+08

4.4e+08

4.42e+08

0 10000 20000 30000 40000 50000 60000 70000 80000 90000

Fre

quency (

Hz)

Time (s)

LGS sensor 700 degC test after 50h at 500 degC

Figure 9. Wireless measurement of the sensor at 700oC. Between 0s and18000s, the oven is warming up. The sensor is then operating at 700oCduring 36000s (from 18000s to 54000s), then the sensor does not operateanymore.

After 10h of measurements, the interrogator does not

measure any response. In order to determine the origin of thefailure, the package has been opened and it was identifieda break of the wire-bonding induced by the deformation ofthe metallization used for the connection pad of the case.Concerning data acquired during these 10h of operation, thesignal is too noisy to determine the frequency shift. As aconclusion, this resonator was able to operate during 50h at500oC and then 10h more at 700oC. The noise measuredduring both experiments (Fig. 8 and Fig. 9) is related tothe laboratory activities. Note that the resonator was stilloperating when probed with tips.

In parallel, another resonator was used for stabilizing atemperature controlled oscillator based on a Collpits-likestructure. The device was placed in the oscillation looponce dicing process performed without any preaging. Thisresonator (Fig. 10) showed a Q factor of 5000 with acoupling factor of 0.04%. The temperature of the systemwas locked at 50oC. This test was performed to estimateaging effects of LGS resonators used at room temperatureto corroborate the need for pre-aging process application ingeneral.

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

425 426 427 428 429 430 0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0.022

0.024

0.026

Adm

itta

nce (

S)

Conducta

nce (

S)

Frequency (MHz)

Y11 realY11 imag

Figure 10. Reponse of the resonatorused in the oscillator loop.

Figure 11. Variation of frequencyof the oscillator over time (1.09−5

for LGS versus 1−7)

Regarding the frequency variation curve (Fig. 11) andthe frequency variation at intermediate temperature (Fig. 8),the langasite based devices would need the developmentof a “preaging” process to reduce as much as possible thefrequency shift of the sensor in harsh environments.

For the second experiment, a different resonator is usedfor life time tests, regarding the low (10-20 cm) rangeinduced by its “low” coupling coefficient (0.19%, low com-pared to the other devices issued of the same wafer) and itslow Q factor (<500), it is not possible to use this sensorfor wireless measurement. The resonator is placed in theoven with another case up and down as hood, this packagewill protect the resonator against a direct contact with hightemperature environment (as it will be in packaged devicefor wireless measurements).

The sensor is measured using tip-prober between eachsuccessive cycle to evaluate the impact of the temperatureon the response of the device. The temperature cycle usedin this experiment is composed of a 5h warming from roomtemperature to 700oC, and then keeping the temperatureconstant during 10h.



0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

4.3e+08 4.32e+08 4.34e+08 4.36e+08 4.38e+08 4.4e+08 4.42e+08 4.44e+08 4.46e+08

Adm

itta

nce (

S)

Frequency (Hz)

Langasite sensor - life time (30 h at 700 degC)

Before 1st cycleBefore 2nd cycleBefore 3rd cycleBefore 4th cycle

Death of the sensor after the 4th cycle

Figure 12. Life time test: temperature cycles are performed from roomtemperature to 700oC during 10h, showing a life time of 30 hours.

As shown on Fig. 12, the dynamics of the resonatoradmittance is changing along cycles . The coupling coeffi-cient of the device remains constant despite successive cycleapplication. However, after 4 cycles, no more response ismeasured.

The third experiment aims at using langasite sensors inhigh temperature environment (>500 oC). The goal of thisexperiment is to observe the evolution of the resonancefrequency at 700oC (Fig13) and to evaluate the effect ofoperating in harsh environment on the response of thedevice.

430

432

434

436

438

440

442

444

0 10000 20000 30000 40000 50000 60000

freq (

MH

z)

time (s)

experiment 1: redexperiment 2 : green

experiment 3: blue

Figure 13. Behavior of a langasite sensor. The experiment cycle is dividedin 3 steps, 300 oC, 500 oC and 700 oC for 2h at each temperature.

Considering the used resonator, the frequency band of thereader had to be adapted and a wide band interrogator wasused (440 MHz ± 10 MHz instead of classical configuration434 MHz ± 0.85 MHz). As expected, the frequency bandneeded to measure the behavior of the Langasite sensor wasaround 10 MHz (441 MHz at room temperature versus 430.5MHz at 700oC).The frequency of the resonator decreasesby 300 kHz after each step at 700oC, as shown in Fig. 13,the frequency is only shifting at “low” temperature (room,300oC, 500oC), the evolution of the frequency at 700oCafter each process is less significant (50 kHz). After thislast experiment, the sensor was not able to operate anymore.The package has been opened to analyzed what happenedto the device (Fig. 14 and Fig. 15). It appears that the fail

is coming from the destruction of the IDT induced by a toolong exposure at high temperature [16], [17], [18].

Figure 14. State of langasite res-onator : picture of the IDT.

Figure 15. State of langasite res-onator : picture of the mirrors.

VII. CONCLUSION AND FUTURE WORK

In the frame of the SAWHOT project, SAW devices forwireless passive high temperature measurements have beendeveloped. Two main operating range have been identified,from room temperature to 500oC and beyond 500oC. Forboth ranges of temperature, sensors have been developed,starting from basic design opertaion (using FEM/BEM andmixed-matrix-based tools) to characterization in oven usingwireless measurements. Simulated results did match withdevices behavior when measured via tip-probing. Quartz-based sensors were found suitable for intermadiate temper-ature measurements (<500oC). This type of sensor showsthat this range of temperature can be measured using Al-Cu electrode on SAW Quartz resonators, withy numer-ous degree-of-freedom for optimizing specifications suchas sensitivity, frequency range and the number of res-onators (differential measurements or not). The algorithmfor resonance frequency tracking prevents the overlapping ofboth measured resonators. For high temperature (>500oC),Langasite-based sensors were found to operate up to 700oC.The firsts experiments have been done using a widebandreader, according to the frequency/temperature range. Theresults show that the sensors can be actually measured at700oC using SENSeOR’s interrogator with a 30 cm rangeinterrogation distance (across the oven back). The resonancefrequency of the sensor is evolving at high temperature,showing a decrease of 300 kHz of the frequency aftereach cycle at room temperature, 300oC, 500oC. At 700oC,the shift of frequency is reduced and is around 50 kHz.Moreover, the langasite device has been able to operate atthis temperature for 30h, during this period, the dynamicadmittance is increasing, while the coupling coefficientremains constant.

Further works are currently performed, focusing on theupgrading of the sensor parameters (Q factor, couplingcoefficient), the life time of sensor at high temperature andthe aging process for the devices.

ACKNOWLEDGMENT

This work was performed within the SAWHOT project[1], funded in the framework of the European CommunitysSeventh Framework Program ([FP7/2007-2013]) under grantagreement no[NMP4-SL-2009-247821].



The authors wish to thank the partners of the SAWHOTproject for fruitful discussions and technical exchanges.

REFERENCES

[1] “Sawhot project,” http://www.sawhot-project.com/, 2009.

[2] D. Ballantine, Acoustic wave sensors: theory, design, andphysico-chemical applications. Academic Pr, 1997.

[3] W. Buff, M. Binhack, S. Klett, M. Hamsch, R. Hoffmann,F. Krispel, and W. Wallnofer, “Saw resonators at high tem-peratures,” in Ultrasonics, 2003 IEEE Symposium on, vol. 1.IEEE, 2003, pp. 187–191.

[4] D. Damjanovic, “Materials for high temperature piezoelectrictransducers,” Current Opinion in Solid State and MaterialsScience, vol. 3, no. 5, pp. 469–473, 1998.

[5] A. Pohl, F. Seifert, L. Reindl, G. Scholl, T. Ostertag, andW. Pietsch, “Radio signals for saw id tags and sensors instrong electromagnetic interference,” in Ultrasonics Sympo-sium, 1994. Proceedings., 1994 IEEE, vol. 1. IEEE, 1994,pp. 195–198.

[6] L. Reindl, G. Scholl, T. Ostertag, C. Ruppel, W. Bulst,and F. Seifert, “Saw devices as wireless passive sensors,”in Ultrasonics Symposium, 1996. Proceedings., 1996 IEEE,vol. 1. IEEE, 1996, pp. 363–367.

[7] J.-M. Friedt, C. Droit, G. Martin, and S. Ballandras, “Awireless interrogation system exploiting narrowband acousticresonator for remote physical quantity measurement,” Reviewof Scientific Instruments, vol. 81, no. 1, p. 014701, 2010.[Online]. Available: http://link.aip.org/link/?RSI/81/014701/1

[8] J. Garcia, T. Laroche, W. Daniau, E. Carry, G. Martin,S. Ballandras, and J. Friedt, “2-and-3d analysis of temperatureeffects on periodic transducers using a fea/bem approach,”in Frequency Control Symposium, 2009 Joint with the 22ndEuropean Frequency and Time forum. IEEE International.IEEE, 2009, pp. 424–429.

[9] W. Daniau, M. Lenczner, T. Laroche, J. Garcia, E. Carry,and S. Ballandras, “On the convergence of 2d and 3d fi-nite element/boundary element analysis for periodic acousticwaveguides,” in Frequency Control Symposium, 2009 Jointwith the 22nd European Frequency and Time forum. IEEEInternational. IEEE, 2009, pp. 430–434.

[10] S. Ballandras, R. Lardat, M. Wilm, T. Pastureaud, A. Rein-hardt, N. Champavert, W. Steichen, W. Daniau, V. Laude,R. Armati et al., “A mixed finite element/boundary elementapproach to simulate complex guided elastic wave periodictransducers,” Journal of Applied Physics, vol. 105, no. 1, pp.014 911–014 911, 2009.

[11] X. Bao, W. Burkhard, V. Varadan, and V. Varadan, “Sawtemperature sensor and remote reading system,” in IEEE 1987Ultrasonics Symposium. IEEE, 1987, pp. 583–586.

[12] E. Forsen, D. Nilsson, W. Daniau, B. Francois, and S. Ballan-dras, “Fabrication of saw resonators on quartz and langasiteusing nano-imprint lithography,” in Ultrasonics Symposium(IUS), 2010 IEEE. IEEE, 2010, pp. 2091–2094.

[13] J. Thiele and M. da Cunha, “High temperature surface acous-tic wave devices: fabrication and characterisation,” Electron-ics Letters, vol. 39, no. 10, pp. 818–819, 2003.

[14] J. Hornsteiner, E. Born, G. Fischerauer, and E. Riha, “Surfaceacoustic wave sensors for high-temperature applications,” inFrequency Control Symposium, 1998. Proceedings of the 1998IEEE International. IEEE, 1998, pp. 615–620.

[15] W. Bulst, G. Fischerauer, and L. Reindl, “State of the artin wireless sensing with surface acoustic waves,” IndustrialElectronics, IEEE Transactions on, vol. 48, no. 2, pp. 265–271, 2001.

[16] S. Firebaugh, K. Jensen, and M. Schmidt, “Investigation ofhigh-temperature degradation of platinum thin films with an insitu resistance measurement apparatus,” Microelectromechan-ical Systems, Journal of, vol. 7, no. 1, pp. 128–135, 1998.

[17] M. da Cunha, R. Lad, T. Moonlight, G. Bernhardt, andD. Frankel, “High temperature stability of langasite surfaceacoustic wave devices,” in Ultrasonics Symposium, 2008. IUS2008. IEEE. IEEE, 2008, pp. 205–208.

[18] D. Richter, S. Sakharov, E. Forsen, E. Mayer, L. Reindl, andH. Fritze, “Thin film electrodes for high temperature surfaceacoustic wave devices,” Procedia Engineering, vol. 25, pp.168–171, 2011.



wireless and passive sensors for high temperature measurements

Documents