IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING

J. Micromech. Microeng. 17 (2007) 515–523 doi:10.1088/0960-1317/17/3/014

A novel 440 MHz wireless SAWmicrosensor integrated withpressure–temperature sensors and ID tagKeekeun Lee, Wen Wang, Taehyun Kim and Sangsik Yang

Department of Electronics Engineering, Ajou University, Suwon, 443-749, Korea

E-mail: keekeun@ajou.ac.kr

Received 24 August 2006, in final form 14 November 2006Published 9 February 2007Online at stacks.iop.org/JMM/17/515

AbstractThis paper presents the development of a 440 MHz range surface acousticwave (SAW)-based microsensor integrated with pressure–temperaturesensors and ID tag. Two piezoelectric substrates were bonded, in which a∼150 µm air gap was structured by metal poles. The pressure sensor wasplaced on the top substrate, whereas the ID tag and temperature sensor werelocated on the bottom substrate. Coupling of modes (COM) modeling wasused to find optimal design parameters. Using the extracted optimal designparameters, the SAW device was fabricated. In wireless device testing usinga network analyzer, sharp reflection peaks with high S/N ratio, small signalattenuation and small spurious peaks were observed in the time domain. Allthe reflection peaks were well matched with the predicted values from thesimulation. With 10 mW RF power from the network analyzer, a ∼1 mreadout distance was observed. Depending on applied external pressure, thephase shifts of the reflection peaks were linearly varied. The evaluatedsensitivity was about ∼2.9◦ kPa−1. Eight sharp ON reflection peaks wereobserved for the ID tag. The temperature sensor was characterized from20 ◦C to 200 ◦C. A large phase shift per unit temperature change wasobserved.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

In recent years, interest in surface acoustic wave (SAW)-based microsensors has greatly increased for application to tirepressure monitoring systems (TPMSs), temperature sensors,biosensors and environmental gas sensors [1–4]. SAW-based microsensors present many advantages over existingsemiconductor-based sensors: (1) they do not require a batteryor any power supply to operate, (2) they can be applied evenunder extremely harsh environment conditions, (3) they can beaccessed wirelessly at particularly inaccessible locations suchas hazardous and high voltage areas. Several groups havereported SAW-based RFID tags and pressure–temperaturesensors with different designs and operating principles [5–8].For TPMS application, Schimetta et al reported a wirelesslyrequestable passive pressure–temperature sensor based onthe combination of SAW transponder technology with a

high quality (Q) capacitive pressure sensor and demonstratedtemperature-corrected pressure measurement [5]. For SAWRFID tags, Hartmann reported 2.4 GHz range RFID tagson 128◦ LiNbO3 substrate with 64 bit data capacity usingsimultaneous time position and phase shifting [7]. However,despite some reported success stories, present SAW-basedmicrosensors suffer from large signal attenuation, broadreflection peaks, high spurious peaks and signal evaluationerrors.

For human recognition in intelligent buildings, a 440 MHzrange wireless SAW microsensor integrated with pressure–temperature sensors and ID tag was fabricated for the first timeand then wirelessly characterized using a network analyzer.Figure 1 shows schematic diagrams of the integrated SAWsensors. Two piezoelectric substrates were bonded with aconductive silver paste, in which a ∼150 µm air gap wasstructured. The pressure sensor was placed on the top

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temperature sensorReflectors for ID tagMetal pole

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temperature sensor

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pressure sensorIDT

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Figure 1. Schematic views of the integrated microsensor.(a) Overall cross-sectional view, (b) top view of the ID tag andtemperature sensor on the bottom substrate, (c) flip-over view of thetop substrate for the pressure sensor and (d) predicted reflectionpeaks in the time domain.

substrate, whereas the ID tag and temperature sensor werelocated on the bottom substrate. The interdigital transducers(IDTs) were placed on both top and bottom substrates andwere electrically connected to each other with ∼140 µmthick metal poles. Metal poles were deposited by a nickelelectroplating technique. A RF pulse is transmitted from thenetwork analyzer to the integrated SAW microsensor throughantennas. The IDT converts electromagnetic (EM) signalsinto mechanical acoustic waves. The SAW propagates onboth piezoelectric substrates and is partially reflected by thereflectors. The reflected waves are reconverted into an EMwave by the IDT and are transmitted back to the networkanalyzer. In the network analyzer, the reflected peaks arearranged in a time domain depending on the distance from theIDT. By appropriate arrangement of the reflector positions, wecan extract RFID, temperature and pressure information.

External pressure induces the bending of the diaphragm,which leads to time and phase angle shifts of reflection peaks.

By analyzing the phase shifts, we can extract the pressurevalues. For the ID tag, each bar-code-like reflector creates animpulse peak in the measurement unit. The reflection peaksrepresent the tag’s ID and data. Using peak position and phaseangle, the unique ID number can be extracted. Temperaturevariation induces position shifts of the reflection peaks in thetime domain because a change in temperature results in avariation of the acoustic path length and a variation of theSAW phase velocity. The resulting change of propagationtime leads to phase shifts.

To find optimal design parameters, coupling of modes(COM) modeling was used. According to the extracted designparameters, the device was fabricated and then characterizedusing an RF network analyzer. In this paper, we describethe process used to fabricate the integrated SAW sensor,the electrical and mechanical device performance, and acomparison between simulated and measured results.

2. Design consideration

The primary goals of the SAW-integrated sensors are high S/Nratio, sharp reflection peaks, small spurious peaks, long rangewireless reading distance and high sensitivity to temperatureand pressure changes. The materials, structure parameters,processing and testing methods were varied to find the optimaldevice performance.

2.1. Overall device structure

The main design parameters regarding the overall devicestructure are the air gap height, the thickness of the toppiezoelectric substrate and the metal pole material. A∼150 µm air gap was designed to minimize interferencebetween the top and bottom SAW energies. ∼350 µmdiaphragm thickness was employed. A thin diaphragmprovides better sensitivity than a thick one, but the maximummeasurable pressure range can be decreased due to a limitedcavity depth and weak diaphragm properties. Metal poles usedto connect two IDTs and for the supporting poles with uniformthickness can induce impedance mismatching due to additionalinduction and resistance values during RF signal transmission.High quality nickel poles of ∼140 µm thickness were carefullyelectroplated.

2.2. Piezoelectric substrate

A 41◦ YX LiNbO3 piezoelectric substrate was used forboth the pressure sensor and ID tag/temperature sensorbecause it has high SAW propagation velocity (4792 m s−1)and large electromechanical coupling factor K2 (17.2%)[9, 10]. High SAW velocity provides easy device patterningin fabrication. A large value of K2 allows high reflectivityfrom the reflectors and low insertion loss. 41◦ YX LiNbO3

has a leaky SAW propagation mode. The leaky SAW devicespossess many attractive features over its SAW counterpart,Rayleigh: (1) leaky SAW devices can be less sensitive tosurface contamination and environmental conditions becauseleaky SAW propagation occurs beneath the piezoelectricsurface. However, the leaky SAW is still susceptible tosurface conditions, so several gas sensors (or environmentalsensors) are made on the leaky SAW substrate. (2) LeakySAW devices are capable of handling high RF power because

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the leaky SAW penetrates deeper into the substrate than theRayleigh wave. Quartz has almost zero temperature sensitivityat room temperature, so it can offer good stability in a harshenvironment. However, 41◦ LiNbO3 provides larger K2 todecrease the insertion loss. So in this proto-type device, wechose the 41◦ YX LiNbO3 as the piezoelectric substrate for thepressure sensor. A clean surface without any damage was usedto lower resistive and scattering losses and thus to minimizeinsertion loss during SAW propagation.

2.3. IDT

Two IDTs were placed on the top and bottom substrates andwere connected to each other by two metal poles. An IDTwith uniform finger spacing was designed. To obtain highS/N ratio and sharp reflection peaks from the reflectors, thenumber of finger pairs was set to 10. According to theIDT design rule (λsaw = vsaw/f), the width was ∼2.4 µm.The metallization layer (aluminum) should be kept thin tominimize self-reflection from the IDT itself. However, thebonding pads require a thicker metallization layer to ensurefaultless bonding. To prevent any damages (e.g., large holes,scratch, contamination and peeling) of small IDT patternsduring UV lithography and lift-off processes, we chose a onestep metallization process for the IDT and bonding pads. Thethickness was targeted at ∼1500 A. The SAW experiencessome angular spreading due to the finite width of the IDTaperture. This results in increased insertion loss. The angularspreading can be decreased by employing a wider acousticaperture. A large aperture of ∼796 µm (80 λ) was used.

2.4. Reflectors

∼1500 A thick aluminum was used as the reflector. Themain insertion loss mechanisms of the propagation SAW areconsidered as the distribution of the energy to the individualreflectors and the resistive and scattering losses of the SAWon its propagation path. Among several different typesof SAW reflectors (e.g., open-circuited metal stripes, short-circuited metal stripes, a IDT-type reflector and a single-bar-type reflector), short-circuited metal stripes (figure 2(a)) wereused for the reflectors in order to obtain higher reflectivityfrom reflectors and lower insertion loss due to almost zero self-reflection and strong reflectivity [11]. On the bottom substratefor the ID tag and temperature sensor, two acoustic trackswere used to prevent undesirable spurious peaks which comefrom multiple reflections between closely spaced reflectors.The multi reflections induce a spurious reflection peak wherean off peak is intended. The distance between the IDTand the first reflector of the ID tag was set to 2.19 mm toseparate environmental echoes from the response signal. Exactpositioning of reflectors (n · λ: where n is an integer) alongSAW propagation paths was designed for precise impedancematching.

2.5. Long range reading distance

Several parameters affect the maximum reading distance r.Using the radar equation, we can infer r [12] as

r = 1

4π· 4

√P0 · G2

i · G2e · λ4

kT0 · B · F · S/N · D. (1)

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Figure 2. (a) Schematic views of the integrated microsensor dividedinto two individual SAW sensors for COM modeling. (b) Thesimulated reflection coefficient S11 in the time domain withoutdiaphragm bending.

Here, λ is the electromagnetic wavelength, Po is the RF power,Gi and Ge are the gains of the antennas, k is Boltzmann’sconstant, To is the absolute temperature, B is the systembandwidth, F is the noise figure, S/N is the minimum signal-to-noise ratio required to safely detect the received signaland D is the insertion loss. Among many parameters, theantenna gain is the largest impact parameter to obtain themaximum reading distance because the operating wavelengthis already determined in our desired application. Based on ourfabricated devices, typical values for all the parameters arePo = 10 dBm, Gi = 12 dBi, Ge = 6 dBi, To = 300 ◦K, B =50 MHz, F = 5 dB and S/N = 10 dB. Therefore, the calculatedmaximum reading distance was ∼2 m.

2.6. Temperature sensor

A 41◦ YX LiNbO3 (temperature coefficient of delay:∼69 ppm ◦C−1) substrate was used for the temperature sensorbecause it has a large sensitivity to temperature variation [13].As a function of temperature T, the phase difference betweenthe 1st and 2nd reflection peaks can be described by

�2−1(T ) = �2−1(Tref) · [1 + T CD × (T − Tref)], (2)

where Tref is the reference temperature (or room temperature)and TCD stands for the temperature coefficient of delay.However, the evaluation of the phase differences induces anambiguity when the phase shift exceeds 360◦. Therefore,

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three reflectors were placed on the bottom substrate for thetemperature sensor to eliminate this ambiguity. Using thetime delay, τ 31, between the 3rd and 1st reflectors and the timedelay, τ 21, between the 2nd and 1st reflectors, the ambiguityof the phase evaluation can be overcome, and the sensitivitycan be evaluated from [14]

�T = k × 2πf × (�τ31 − 2�τ21), (3)

where k means the ratio of the third-to-first reflector distanceto the second-to-first reflector distance. The distance betweenthe 1st and 2nd reflectors was 0.415 mm, and the distancebetween the 2nd and 3rd reflectors was 1.09 mm.

2.7. Pressure sensor

External pressure induces the bending of the diaphragm,resulting in a variation of the acoustic path length, L, anda variation of the SAW phase velocity, vs . The variations inL and vs lead to time and phase angle shifts of the reflectionpeaks. By evaluating the phase shifts of the reflection peaks,we can determine the external pressure values. There are tworegions along the bent diaphragm: stretched and compressedregions. The stretched section is observed near the centerof the diaphragm, whereas the compressed strain section isobserved near the edge of the diaphragm. It has been knownthat the SAW propagation velocity is lower in the stretchedsection, whereas it is higher in the compressed section. Thepressure sensor requires at least three reflectors to minimizethe temperature dependence effect. The first two reflectorswere placed at a stretched region and the third reflector wasplaced at a compressed region. The phase shift of each reflector(��i) includes pressure information (��ip) and temperatureinformation (��it). The phase shift ��i for each reflectionpeak is described as

��i = (��ip + ��it ) = 2πf0 × �τi, (4)

where f0 is the center frequency and �τ i is the time delayof each reflection peak. Based on equation (2), there is arelationship between ��32t and ��21t,

��32t = L3/L2 × ��21t (5)

where ��32t and ��21t are the phase shifts due to thetemperature effect, L2 is the distance between the 1st and2nd reflectors, and L3 is the distance between the 2nd and 3rdreflectors. The total combined phase shifts (��s) of the bentdiaphragm are obtained by

��s = (��21p + ��21t ) − w × (��32p + ��32t ), (6)

where w is the ratio of the first-to-second reflector distanceto the second-to-third reflector distance. By insertingequation (5) into equation (6), equation (6) providesonly pressure information, so the temperature effect iscompensated. As w is increased, the above temperaturecompensation method can be more effective way [15–17].

2.8. ID tag

The primary concerns of the RFID system are anti-collision,low insertion loss, high tag accuracy and high tag density[1, 7]. The RFID reader requires the ability to read multiple IDtags at one time. When an RFID reader receives multiple returnsignals from multiple ID tags simultaneously, the reader must

accurately determine the unique ID of the individual ID tag.Because the SAW ID tags are passive components without anyactive logic on the chip, they cannot be addressed individually.To access more than one ID tag, frequency division multipleaccess (FDMA), time division multiple access (TDMA) orcode division multiple access (CDMA) must be incorporated.A high K2 piezoelectric substrate, shorted-grating reflector,no damaged surface and optimal device dimension were usedfor low insertion loss. For high tag accuracy, two acoustictracks were employed on the bottom substrate to minimizeundesirable spurious peaks from multiple reflections. Anadequate distance between the IDT and the first reflector(∼2.19 mm) was set to separate environmental echoes fromthe response signal. Precise positioning and weighting ofthe reflectors were used on its acoustic path. Close spacingbetween bar-code-type reflectors was structured to obtain ahigh number of data bits within a given limited region. Eightbar-code-like reflectors were arranged in a row to realize anID tag. Each peak represents one data bit. By using eightreflectors, 28 objects can be identified.

3. Device simulation

The SAW reflective delay line with various configurationscan be effectively simulated using the COM modeling[18, 19]. To obtain the reflection coefficient S11 using theCOM theory, we divided the integrated microsensor into twoSAW devices as shown in figure 2(a). One device has an IDTand three shorted-grating reflectors for the pressure sensor andthe other has two acoustic tracks, eight reflectors for the IDtag and three reflectors for the temperature sensor. Two SAWdevices were electrically connected through conductive metalpoles, which can be considered to be additional resistances(figure 2). Using the admittance matrix solution, S11 for eachdevice can be described as

S11top = (YG−y11top)×(YG+y22top)+y12top×y21top

(YG+y11top)×(YG+y22top)−y12top×y21top,

(7)S11bott = (YG+YM−y11bott)×(YG+YM+y22bott)+y12bott×y21bott

(YG+YM+y11bott)×(YG+YM+y22bott)−y12bott×y21bott.

Here, S11top is S11 for the top substrate device, S11bott is S11 forthe bottom substrate device, ytop is the calculated admittancematrix for the top device, ybott is the admittance matrix for thebottom device, YM is the input admittance for the metal polesand YG is the resource and load inductance. The combinedoverall S11 for the two SAW devices can be determined by

S11 = S11top + S11bott. (8)

Using the parameters listed in table 1 and the FFT program,the integrated SAW device was simulated to obtain S11 in thetime domain.

As shown in figure 2(a), the top device has one IDTand three reflectors in an acoustic track, whereas the bottomdevice has one IDT and two acoustic tracks. Both IDT have thesame dimension and structure. However, the aperture of thereflectors located on the top device is two times larger than thatof the bottom reflectors. Sharp reflection peaks, few spuriousnoises and nearly equal intensities between the peaks wereobserved in COM modeling (figure 2(b)). From the simulatedresults, we found that (1) among three simulated deviceswith different IDT finger pairs (10, 50 and 100 IDT finger

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Al LiNbO3

PR

Ni

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Figure 3. Fabrication procedure for the integrated sensor.(a) Aluminum patterning on LiNbO3 for the IDT and reflector,(b) PR patterning for electroplating, (c) electroplating of nickel,(d) PR removal, (e) aluminum deposition on LiNbO3 for the ID tagand temperature sensor, ( f ) aluminum patterning for the IDT andreflectors and (g) wafer bonding with a conductive silver paste.

Table 1. Simulated parameters of the integrated sensor in COMmodeling.

Top pressure Device on bottomsensor substrate

IDT finger pairs 10 10SAW acoustic tracks 1 2Reflector types Shorted circuit Shorted circuit

grating with grating withthree metal strips three metal strips

Acoustic aperture (λ) 80 80Substrate 41◦ YX-LiNbO3 41◦ YX-LiNbO3

IDT metal material Aluminum Aluminum

pairs), the device with the smallest number of IDT finger pairsprovided the highest S/N ratio of the reflection peaks in thetime domain, because a larger number of finger pairs inducesa stronger SAW radiation force, but at the same time the lossis also increased by the increase in SAW dampening due toexcessive mass loading, and the increase in self-reflection andstatic IDT capacitance. In addition, the larger IDT fingerpair number enlarges the bandwidth in the frequency domain,which results in the broader reflection peaks. Therefore, thesmaller IDT finger pair number can sharpen the reflected peakseffectively. (2) A shorted-grating reflector can reduce thespurious signals effectively due to almost zero self-reflectionand strong reflectivity, compared with IDT-type reflectors andbar-type reflectors. (3) Equal amplitude for all the reflectionpeaks can be obtained by appropriate control of the reflectoraperture and propagation path length. (4) Multiple acoustictracks can significantly reduce the multi-reflection between thereflectors.

4. Fabrication

Figure 3 shows schematic diagrams for the fabricationprocedure. For the pressure sensor, first 4′′ 41◦ YX LiNbO3

piezoelectric substrate with 350 µm thickness was cleaned inacetone and rinsed in de-ionized (DI) water. 1500 A thickaluminum was deposited using an electron beam evaporator.Then, 1 µm thick photoresist (PR) was spin-coated, exposed

and then patterned for the IDT and three reflectors. Ingeneral, SAW piezoelectric substrates are anisotropic. SAWpropagations are not constant in all directions. SAW velocity iseither a maximum or a minimum along a particular propagationdirection. Alignment of the IDT pattern with the required X-direction (X-axis wave propagation) was performed duringthe PR lithography process. Aluminum was wet-etchedin 4H3PO4:1HNO3:4CH3COOH:1H2O etchant (figure 3(a)).The PR was dissolved in acetone. Several rinses with DIwater were performed to remove any unwanted products.A ∼10 µm thick PR was spin-coated, exposed and thenpatterned for the seed layer in the following electroplatingprocess. A 2500 A thick Ti/Au layer was deposited using anelectron beam evaporator. The PR was dissolved in acetonefor 2 h for lift-off processing. A 20 µm thick PR was spin-coated, exposed and then patterned for the desired shape ofnickel poles (figure 3(b)). Ni electroplating was performed for20 h to form ∼150 µm thick poles (figure 3(c)). Processingparameters were carefully controlled to obtain high qualitynickel poles. After electroplating, the PR was dissolvedin acetone for 2 h (figure 3(d)). Lateral expansion of thenickel pole was observed because only a ∼20 µm thick PRstructure was patterned before electroplating. Further thickPR patterning can improve this lateral expansion problem, butit can also damage the IDT and reflector patterns. Next, thesubstrate was dicing-sawed for wafer bonding and package.

For the ID tag and temperature sensor, another 41◦ YXLiNbO3 piezoelectric substrate with 350 µm thickness wascleaned in acetone and rinsed in DI water. 1500 A thickaluminum was deposited using the electron beam evaporator(figure 3(e)). Then, a 1 µm thick PR was spin-coated, exposedand then patterned for IDT and reflectors. Aluminum waswet-etched. The PR was dissolved in acetone (figure 3( f )).The substrate was dicing-sawed. The two substrates werebonded with a conductive silver paste to complete the device(figure 3(g)). A two-dimensional planar antenna (45 mm ×45 mm) with 440 MHz central frequency and 21 MHzbandwidth was fabricated using 8 mil thick RO4003 substrate(dielectric constant k = 3.38). The soldering was performedfor electrical connection between the fabricated SAW deviceand antenna.

5. Results

5.1. Fabricated device

Figure 4 shows optical microscope and scanning electronmicroscopy (SEM) views of the fabricated devices. The IDTson the top and bottom substrates have the same dimensionand structure. The IDT finger pairs were 10, the width was∼2.4 µm, the thickness was 0.15 µm and overlapping aperturewas 796 µm. For the pressure sensor, three shorted-grating-type reflectors were arranged in a row on the top substrate.The distance between the IDT and the first reflector was7.11 mm, and the distance between the IDT and the thirdreflector was about 9.499 mm. The ratio of the first-to-secondreflector distance to the second-to-third reflector distance was5 to minimize the temperature dependence effect. The ID tagand temperature sensor were located on the bottom substrate,in which eight shorted-grating reflectors for the ID tag and

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(a)

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2.4 µm

ID tag reflectorsIDT Reflectors for

temperature sensor

Figure 4. Optical and SEM views of the fabricated devices. (a) Topview of the ID tag and temperature sensor, (b) flip-over view of thepressure sensor and (c) bonded devices.

three shorted-grating reflectors for the temperature sensor werestructured. Two substrates were bonded with a conductivesilver paste. The air gap between the two substrates was∼150 µm.

SAW sensor

Antenna

Figure 5. Measurement setup using the network analyzer.

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Figure 6. Comparison between experimentally measured reflectionpeaks and simulated peaks in the case of no compression force andat room temperature.

5.2. RF wireless testing

The reflection coefficient S11 was measured using an HP8510 network analyzer and a Cascade probe station. Twoantennas with ∼440 MHz central frequency were fabricated.One antenna was connected to the S11 port of the networkanalyzer and the other was connected to the integratedmicrosensor (figure 5). Under no compression force and atroom temperature, eight sharp reflection peaks from ID tagreflectors, three peaks from the temperature sensor and threepeaks from the pressure sensor were observed in a row inthe time domain (figure 6). The x-axis is the travel time ofthe impulse and the y-axis is the averaged reflection over thefrequency ranges. A large S/N ratio, sharp peaks and cleardistinction between peaks were observed. The first reflectionpeak occurred at 1 µs, and at that point S11 was ∼43 dB. All thereflected peaks were well matched with the predicted valuesfrom the simulation (figure 6). These results suggest that allthe device parameters had good impedance matching with thepropagating SAW.

To study the effects of resistance and inductance valuesfrom ∼150 µm Ni poles on device performance, the RF testingresults of the completed device were compared with the resultsobtained before wafer bonding. All the results were the same,

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Figure 7. (a) The reflection coefficient S11 of the ID tag andtemperature sensor measured before wafer bonding. (b) S11 of thepressure sensor measured before wafer bonding and (c) S11 of theintegrated sensor measured after wafer bonding under no diaphragmbending.

except for a little bit of decrease in the intensity of the reflectionpeaks (figure 7). For this result, we thought that an additionalexperimental resistance/inductance value from the metal polesinduces some signal losses of ∼10 dB, but it does not affect thetime and phase shifts of the reflected peaks we are interestedin at our aimed frequency region. The RF power from thenetwork analyzer was varied from −40 dBm to 10 dBm(∼10 mW) to find the maximum readout distance. As theapplied RF power is increased, the readout distance was alsoincreased. With 10 mW RF power from the network analyzer,a ∼1 m readout distance was observed. Further enlargement ofthe reading distance can be obtained by increasing the antennagain and applied RF power, and decreasing the insertion lossof the fabricated SAW device.

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Figure 8. Phase angle shifts in terms of temperature change.

5.2.1. ID tag. With 10 mW RF power from the networkanalyzer and a ∼30 cm request distance, eight sharp ONreflection peaks were observed (figure 6). Clear separationbetween the reflection peaks was observed. The time intervalbetween the IDT and the 1st reflector was 1 µs. This allowedadequate separation between environmental noise echoes andreflector peaks, because all the environment echoes fade awaywithin ∼1 µs. All the reflection peaks in the time domain werewell matched with the predicted values. Intensity was a littledecreased due to the attenuation of the SAW on its propagationpath. No clear multi-reflection and resultant spurious peakswere observed.

5.2.2. Temperature sensor. The fabricated device was placedon a hot plate for testing, in which the temperature was changedfrom 20 ◦C to 200 ◦C. With 10 mW RF power from the networkanalyzer and at a request distance of 30 cm, three reflectionpeaks were recorded from three reflectors. Depending onchanges in temperature, the shifts (time position and phaseangle) of the reflection peaks were modulated. The phase angleshift was used to evaluate the sensitivity because it providesa much higher resolution than the time shift of the reflectionpeak. All three reflectors showed the same sign of the phaseshifts because the temperature effects are equal on all thereflectors. The high linearity of the phase shifts was observedup to 200 ◦C, as shown in figure 8. The evaluated sensitivitywas ∼10◦ ◦C−1. For these promising results, we thought that(1) LiNbO3 with large temperature coefficient of delay (TCD)of approximately −69 ppm ◦C−1 provides high sensitivityto temperature variations and (2) a shorted-grating reflectorallows small spurious peaks, high reflectivity and low insertionloss.

5.2.3. Pressure sensor. Under 10 mW RF power and ata request distance of 30 cm, external pressure was appliedto the diaphragm by placing an object onto the center ofthe diaphragm, and then the S11 parameter was measured toextract the time and phase deviations of the reflected peaks as afunction of the amount of applied mechanical force. Shifts ofthe reflected peaks were observed. The total combined phase

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25oC 45oC 65oC105oC

0 100 200 300 400 500 600

Figure 9. Phase angle shifts in terms of pressure change.

shift ��s from all three reflectors was obtained from equation(6). Depending on the amount of applied external pressure,the phase angle was modulated. For a small pressure range,linearity was observed, while nonlinearity was observed from∼350 kPa (figure 9). The sensitivity was evaluated to be about∼2.9◦ kPa−1. This value is better than other reported values.Scherr et al reported about 2.38◦ kPa−1 sensitivity and a linearregion up to 250 kPa [17, 20].

The long-term temperature dependence effect of thepressure sensor was tested by measuring the phase shiftsdepending on pressure under different temperature conditions(25 ◦C, 45 ◦C, 65 ◦C and 105 ◦C). Temperature insensitivitywas observed at the 25–65 ◦C temperature range, as shownin figure 9. However, the phase shifts at 105 ◦C deviateda little from the others. For this result, we thought thatfor a bent diagram under the same temperature conditionthe length change (�L) of the diaphragm due to thermalexpansion/contraction of the piezoelectric substrate is thesame at all three reflector positions, whereas the SAW velocityshifts (�v) are different at the respective reflector positions,resulting in nonlinearity of the phase shift at extremelyhigh temperature. To minimize this effect, a differentpiezoelectric substrate with very low TCD (e.g., ST Quartz)can be considered for the pressure sensor, while the IDtag/temperature sensors are made on the same 41◦ YX LiNbO3

substrate.An interference effect between the top and bottom SAW

energies was closely observed by comparing the reflectedpeaks of the ID tag and temperature sensor obtained in thecase of bending of the diaphragm with the reflection peaksobtained in the case of no diaphragm bending. Under thesame temperature condition, no noticeable differences of thereflection peaks (in S/N ratio, sharpness, and amplitude) wereobserved, suggesting that a ∼150 µm air gap is sufficientto avoid interference between two propagating SAWs at thetargeted pressure range.

6. Conclusion

We have presented a surface acoustic wave (SAW)-basedmicrosensor integrated with temperature–pressure sensors

and ID tag for human recognition on the floor of anintelligent building. COM modeling was used to predictdevice performance prior to fabrication. Optimal designparameters were determined from simulations. High S/Nratio, small signal attenuation and spurious peaks, and sharpreflected peaks were observed from wireless network analyzermeasurements. With 10 mW RF power from the networkanalyzer, a ∼1 m readout distance was observed. Eight sharpON peaks with close spacing were obtained in order to realizea high number of ID data bit within a given limited region. Thetime and phase shifts of the reflection peaks were modulateddepending on temperature. High linearity of the phase shiftswas observed up to 200 ◦C. The evaluated sensitivity was∼10◦ ◦C−1. When external pressure was applied to thediaphragm from the top, a time delay of the reflected peaksand a change in the phase angle were observed. The sensitivitywas approximately 2.9◦ kPa−1. From these results, we suggestthat this prototype SAW-integrated sensor is very promisingfor achieving wirelessly requestable and batteryless humanrecognition sensor applications on an intelligent building floor.

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