Research Article A Wireless Pressure Microsensor ...downloads.hindawi.com/journals/ijdsn/2015/974742.pdf · Research Article A Wireless Pressure Microsensor Fabricated in HTCC Technology

Research ArticleA Wireless Pressure Microsensor Fabricated inHTCC Technology for Dynamic Pressure Monitoring inHarsh Environments

Ronghui Gao,1,2 Yingping Hong,1,2 Huixin Zhang,1,2 Wenyi Liu,1,2 Ting Liang,1,2

Wendong Zhang,1,2 and Jijun Xiong1,2

1Key Laboratory of Instrumentation Science & Dynamic Measurement, Ministry of Education, North University of China,Tai Yuan 030051, China2Science and Technology on Electronic Test & Measurement Laboratory, North University of China, Tai Yuan 030051, China

Correspondence should be addressed to Jijun Xiong; [email protected]

Received 25 August 2014; Revised 7 December 2014; Accepted 17 December 2014

Academic Editor: Gour C. Karmakar

Copyright © 2015 Ronghui Gao et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The partially stabilized zirconia (PSZ) ceramic has wide applications due to its excellent mechanical toughness and chemicallyinert and electrical properties for fabricating various devices. In this paper, a novel high temperature pressure sensor with thePSZ was designed and fabricated. The sensor was designed based on the small deflection theory, which enables its theoreticpressure-capacitance capability up to 60 bar. HTCC process technology was used to fabricate the sensor, which would realize acompletely passive LC resonant circuit integrated on the ceramic substrate. According to the coupling principle, noncontact testingis achieved using the designed readout system, with average sensitivity up to 38 kHz Bar−1 presented. Compared to the fabricationandmeasurement of traditional sensors, excellent packaging process is demonstrated, and the sensor can be completely tested from0 to 60 bar.

1. Introduction

Instantaneous precise measurement pushes the limit of thedynamic testing technology continuously. Until now, thetest technology referring to the key parameters under harshenvironment is still unreachable [1]; the wireless pressuremeasurement especially under high temperatures has becomeincreasingly critical in automotive, aerospace, and industrialapplications [2–4]. The conventional pressure sensors basedon silicon material have been used widely in different occa-sions. However, they would face great challenges in hightemperatures for the intrinsic limits of the material. Othermaterials, such as silicon nitride and silicon carbide, despitehaving excellent robustness, are rarely used for standardfabrication because their process is not so mature comparedto silicon. Seen as a new-typematerial, the partially stabilizedzirconia ceramic (PSZ) has very high fracture toughness andhas one of the most highest maximum service temperatures(0–1850∘C) among all of the ceramics. It would also keep

mechanical properties when the temperature is close to itsmelting point (2500∘C), which makes great sense for fabri-cation of pressure-sensitive devices under high temperature.

Noncontact wireless passive telemetric sensing is one ofthe methods using frequency for continuous and reliablepressure measurements. The sensing methodology requiresan external reader to interrogate environment pressure vari-ations electrically registered by an implanted sensor througha wireless inductive coupling link. The concept, shown inFigure 1, was first proved in 1967 using a sensor with resonantcircuitry implanted to the anterior chamber of the eye [5],which then was further researched in the development ofvarious pressure microsensors [6–8]. The model enablesstraightforward pressure sensing by utilizing an implantedsensor that records pressure variations in high-temperatureenvironments, so that the pressure can be directly measuredby using an external reader wirelessly interrogating theimplant. In the last decades, due to the development ofmicromachining technology,microelectromechanical system

Hindawi Publishing CorporationInternational Journal of Distributed Sensor NetworksVolume 2015, Article ID 974742, 11 pageshttp://dx.doi.org/10.1155/2015/974742

2 International Journal of Distributed Sensor Networks

Impedance analysis

Sensor (inside the engine)

External reader antenna(outside the engine)

Inductive coupling

−−

Zeq

V1LsLr

RsC0

++

Cs

Lm

Figure 1: Conceptual schematic of the passive wireless pressure sensing.

(MEMS) sensors made of silicon are the major devicesused in detecting pressure [9]. However, silicon sensorswith PN-junctions exhibit a restriction; that is, they cannotbe used above 150∘C, since the leakage current across thejunctions drastically increases above 150∘C [10]. In addi-tion, the mechanical properties of silicon will deteriorateas the material becomes easily deformable when pressure isapplied above 500∘C [11]. Using SOI material can increasethe operation temperature of sensor, but sensor becomesinvalid because the silicon material will lose elasticity at500∘C [12]. Sensors demonstrated in the literature havecertain deficiencies: the Georgia Institute of Technology hasdesigned a wireless high temperature pressure sensor usinglow temperature cofired ceramic (LTCC) material. However,the sensor is only tested up to 450∘C [13–16]. Anotherteam in Novi Sad (Serbia) demonstrated a better structure,but worse performance [17–20]. Recently, Professor Xiong’steam has demonstrated a greater sensitivity and a betterperformance, but the measuring range was not up to 50 barand the operating temperature was only up to 500∘C [21,22]. However, the coupling distance and performance of thesensors designed before do not operate so efficiently and thepressure tests for these elements have not been taken undercomposite high-temperature environment.

These facts seriously hinder its application especiallyunder extraordinaryhigh-temperature environments (>800∘C).Thus, we need to further improve the full-scale range and theoperating temperature of the sensor.

In this paper, we developed a microfabricated wirelesspassive pressure sensor with HTCC technology [13] to realizea better applicability and feasibility of the sensor and morestable pressure data in real time under harsh environmentssuch as high-temperature environment. The working princi-ple is described as follows: the external pressure would causevariation of capacitor, so we can obtain the pressure valuefrom the frequency as the LC resonant frequently changes. Areadout system is designed to extract the pressure informa-tion contained in the sensor’s resonant frequency. Differentfrom the previous sensor, a new material is applied for theproposed sensor, and an embedded structure design andfabrication process are studied. Measurement and character-ization analysis in high-pressure environment (0–60 bar) are

achieved. Featuring zirconia ceramic as the high temperatureresistant material, the pressure sensor is designed to be com-pletely implantable in the high-temperature environment sothat long-term pressure monitoring inside the engine can befulfilled.

2. Design

2.1. Principle of Measurement. The concept of the wirelesspressure sensing system in the passive electrical sensingscheme is shown in Figure 1; the implanted sensor can recordreliable pressure variations using corresponding electricalcharacteristic changes, which are measured from the externalreader through a wireless inductive coupling link. The asso-ciated pressure monitoring method as shown in Figure 2 isproposed for continuous monitoring in harsh environments.

The sensor implant is designed to have an electrical LCresonant circuit with a corresponding resonant frequencyrepresented as

𝑓𝑠

=

1

2𝜋

√1

𝐿𝑠

𝐶𝑠

−

𝑅2

𝑠

𝐿2

𝑠

≅

1

2𝜋√𝐿𝑠

𝐶𝑠

(1)

if

𝑅2

≪

𝐿

𝐶

, (2)

where 𝐿𝑠

, 𝐶𝑠

, and 𝑅𝑠

are the inductance, capacitance, andresistance of the sensor, respectively. By using an externalreader antenna to build an inductive coupling link with theimplanted sensor, the equivalent input impedance viewedfrom the reader antenna side can be derived using circuitanalysis as

𝑍eq = 𝑅𝑟 + 𝑗2𝜋𝑓𝐿𝑟

× (1 − (

𝑓𝑟

𝑓

)

2

+

𝑘2

(𝑓/𝑓𝑠

)2

1 + (𝑗/𝑄𝑠

) (𝑓/𝑓𝑠

) − (𝑓/𝑓𝑠

)2

) ,

(3)

where 𝑅𝑟

and 𝑓𝑟

are the resistance and the self-resonancefrequency of the reader antenna, respectively, 𝑓 is the excita-tion frequency, 𝑄

𝑠

= 𝑅−1

𝑠

(𝐿𝑠

𝐶𝑠

− 1)1/2 is the quality factor of

International Journal of Distributed Sensor Networks 3

Insert

ing

PCB test antenna

Sensor

Waterproof device

Signal collecting

Signal output

unit

Computer

3 cm

Readout

Figure 2: Integrated reader antenna and the sensor on the waterproof device in harsh environments.

the sensor at resonance, 𝑘 represents the coupling coefficientof the inductive link (totally dependent on the physicalgeometries planar size of the sensor and the reader antennacoils and the separation distance between the inductancecoils) [16, 17]. Therefore, from (3), an input impedance-risetechnique can be applied to wirelessly detect the resonantfrequency of the sensor. When the sensor is excited atresonance, the real part of the equivalent input impedance𝑍eq becomes

max𝑓

Re {𝑍eq} ≈ Re {𝑍eq}𝑓=𝑓

𝑠

= 𝑅𝑟

+ 2𝜋𝑓𝑠

𝐿𝑟

𝑘2

𝑄𝑠

. (4)

The real part-rise magnitude determines the signalstrength in the wireless sensing system and is dependenton 𝑘 and 𝑄

𝑠

. Because of such relation between the inputimpedance’s real part and the resonance frequency of thesensor, the latter can be identified if one can detect areal part-rise in the frequency scan of the equivalent inputimpedance. As long as the real part-rise is detectable inthe frequency scan, the resonant frequency of the sensorcan be accurately characterized. As a result, if the sensorhas pressure-sensitive electrical components, its resonantfrequency will be changed because of external pressure vari-ation; thus the environmental pressure would be recorded.This change can be interrogated using the external readerantenna so that continuous wireless pressure monitoring canbe accomplished. As a result, by finding the updated real partof 𝑍eq change, the frequency shift can be obtained, and thecorresponding pressure change can be analyzed.The involvedelectrical characteristic change can be interrogated usingthe external reader antenna coil to accomplish continuouswireless pressure monitoring.

In case of high-temperature environment, it is demon-strated that the sensor’s 𝑄 value which related to the changesof temperature limits the operating temperature at 400∘C[13, 14]. The value of mutation point of impedance’s real partat peaks changes less obviously as the𝑄 value varies; even theresonant characteristics for extracting the resonant frequency

20.8 20.85 20.9 20.95 21 21.05 21.1 21.15 21.2 21.25 21.31

2

3

4

5

6

7

8

Frequency (MHz)

Q = 270.17

Q = 135.08

Q = 90.06

Q = 67.54

Q = 54.03

Q = 45.03

Q = 38.09

Q = 33.77

Q = 30.02Q = 27.02

Q = 24.56Q = 22.51Q = 20.78Q = 19.30

Q = 18.01

|Z𝜃(f

)|(kΩ

)

Figure 3: The real part-rise magnitude of antenna coil from inputimpedance for the quality factor variation.

disappear. The real part-rise magnitude of antenna coil frominput impedance for the quality factor variation is shownin Figure 3. The main reason is that the relative dielectricconstant and loss of inductance coil from ceramic substrateof sensing elements become different as temperature changes.The characteristics disappear when the value of 𝑄 reduces,in other word, the coupling energy cost totally at the activepower rather than reactive power which beneficial for detect-ing.


0

10

20

30

40

50

60

Frequency (MHz)

|Z𝜃(f

)|(kΩ

)

20.5 20.6 20.7 20.8 20.9 21 21.1 21.2 21.3 21.4 21.5

k1 = 0.1k1 = 0.2k1 = 0.3

k1 = 0.4

k1 = 0.5

k1 = 0.6

k1 = 0.7

k1 = 0.8

k1 = 0.9

Figure 4:The impedancemagnitude of antenna coil for the couplingcoefficients variation.

Another key parameter which affects the extraction ofresonance frequency feature is the coupling factor 𝐾, whichis related to geometry size, coupling distance, and magneticpermeability of inductance coil of antenna and passive sensorranging from0 to 1.The impedancemagnitude of antenna coilfor the coupling coefficients variation is shown in Figure 4.In the actual detection system, the coupling factor𝐾 reducesunder 0.3 when coupling distance is far away or the sizeof the inductance coil is very small; the mutual inductancecoupling characteristics of the remote sensor cannot bedetected at reading antenna (even including the high preci-sion impedance analysis instrument).Therefore, the couplingfactor 𝐾 of readout testing system proposed in this work ischosen at 0.3∼1; however the ideal case 𝐾 = 1 cannot berealized since there always exists a certain coupling distancebetween the sensor and readout antenna inductance coil.

2.2. Sensor Circuit Structure Parameters. Figure 5 shows thedesign of the microsensor comprising a pressure-sensitiveparallel-plate variable capacitor embedded in a deformablediaphragm chamber, a spiral metal wire serving as planarinductance, and a flexible ceramic sensitive membrane withelectrodes on as the capacitor.

The design particularly features a larger square substrateto incorporate a larger planar spiral inductance. From theengineering aspect, the electrical characteristics of the sensorcan be determined by using the established models [22],where the electrical inductance of such a circular spiral coilcan be calculated as

𝐿𝑠 = 2.34𝑢0

𝑛2

𝑑avg

1 + 2.75𝐹

, (5)

Spiral inductor

Pressure-sensitive variable capacitor

Via

Capacitance plate

Ceramicsubstrate

Sealed cavityd0

Figure 5: Pressure sensor designing schematics with zirconiaceramic substrate.

where 𝑛 is the number of turns of the inductor coil, 𝑑avgindicates the averaged diameter of the coil windings, (𝑑avg =((𝑑in + 𝑑out)/2)), and 𝐹 is the fill ratio defined as 𝐹 = (𝑑out −𝑑in)/(𝑑out + 𝑑in), where 𝑑in and 𝑑out are the inner and outerdiameters, respectively. 𝑢

0

is permeability of vacuum. Theelectrical resistance is mainly contributed by the inductorwire which imperatively has a series resistance calculated,with consideration of the high-frequency skin effect, as

𝑅𝑠 =

𝜌𝑙

𝑤√(𝜌/𝜋𝑓𝑢) (1 − 𝑒−𝑘⋅(𝜋𝑓𝑢/𝜌)

)

, (6)

where 𝜌 is the electrical resistivity of the metal, 𝑢 is themagnetic permeability of the metal, 𝑤 is the metal linewidth, and ℎ indicates the metal line height. The electricalcapacitance of the sensor can be expressed as

𝐶𝑠 =

𝜀0

𝑎2

𝑡𝑔

+ (𝑡𝑚

/ (2 ∗ 𝜀𝑟

))

⋅

tanh−1 (√𝑑0

/ (𝑡𝑔

+ (𝑡𝑚

/ (2 ∗ 𝜀𝑟

))))

√𝑑0

/ (𝑡𝑔

+ (𝑡𝑚

/ (2 ∗ 𝜀𝑟

)))

;

(7)

𝑑0

is the center defection of the membrane assumes bothbending and stretching of a uniformly loaded circular plateand is given by [10]

𝑑0

=

3𝑃𝑎4

(1 − V2)

16𝐸 (𝑡𝑚

)3

⋅

1

1 + 0.448 (𝑑0

/𝑡𝑚

)2

, (8)

where 𝑎 is the length of the square electrode, 𝑃 is theatmospheric pressure outside the sensor, 𝑡

𝑔

represents thedepth of the cavity and 𝑡

𝑚

is the thickness of the membrane,and 𝜀

0

and 𝜀𝑟

are the free space permittivity and relativedielectric constant, respectively.


Table 1: Parameters of the designed sensor.

Parameters ValueOverall dimension 25mm × 25mm × 0.9mmCapacitance plate radius 4mmHeight of the embedded cavity 100 umHeight of the sensitive membrane 400 umDiameter of inner inductor 22.5mmDiameter of outer inductor 12mmNumber of coil turns 9.5Width of coil turns 0.4mmDistance between neighbor coils 0.4mmVia dimension 0.2mm × 0.2mmThickness of the printed pattern 20 um

Readoutcircuit

Reader antenna Pressure sensor

Figure 6: Major components of the designed readout system.

For the case when the deflection is small compared to theplate thickness (𝑑 ≪ 𝑡

𝑚

), (8) above simplifies to

𝑑0

=

3𝑃𝑎4

(1 − V2)

16𝐸 (𝑡𝑚

)3

(9)

which is exactly the maximum center deflection for purebending of a circular plate with clamped edges.

Deduced from the aforementioned description, parame-ters of the sensor including capacitance plate radius, height ofthe embedded cavity, and height of the sensitive membranewere designed and the specific parameters of the sensordesigned were shown in Table 1.

2.3. Readout System. In this paper, a readout system isdesigned tomeasure the pressure sensor’s resonant frequency.Figure 6 demonstrates the major components of the read-out system, which include the reader antenna fabricatedon printed circuit boards, the pressure sensor inductivelycoupled to the reader antenna, the readout circuit, and themeasurement cable (regular 50Ω coaxial). The sweep signalis generated with a direct digital synthesizer (DDS). Thistype of oscillator has a wide output frequency range and

Table 2: Parameters of the ESL 5570.

Parameters ValueScreen mesh/emulsion ∼325mesh/25 ± 5 𝜇mResistivity (17.5) ∼30 ± 10Ω/squareFiring range 1500∘C ± 10∘CTime at peak temperature ∼120minViscosity 70 ± 20 Pa⋅sSodium concentration ≤50 ppm

Table 3: Parameters of the PSZ tape 42020.

Parameters ValueYoung’s modulus ∼220GpaPoisson’s ratio ∼0.32Stabilizing agent Y2O3Unfired thickness ∼125 um ± 10%𝑋, 𝑌 shrinkage ∼16.5 ± 1.0%𝑍 shrinkage ∼18.0 ± 1.0%

Fired density (1450∘C for 1.5 hours) ≥95% of theoreticalcalculating value

very fine frequency resolution and the output frequencychanges almost instantly. The sweep signal and the voltagesignal across a reference resistance aremultiplied through theGilbert cell-basedmixer (multiplier), and a low-pass filter cir-cuit is used to filter the mixer’s output signal into a dc outputvoltage while the dc output voltage is processed and digitizedin a digital system by a fast 16-bit ADC (AD7667). Thereadout circuit also contains a microcontroller unit (MCU)for communicating with the PC via a USB interface andperforming the requested frequency sweeps at the specifiedspeed and with the specified starting and ending frequencies.The PC postprocessing software analyzes the digital data andcalculates and obtains the sensor’s resonant frequency.

The readout system is able to measure frequenciesbetween 1MHz and 100MHz. The method used to extractthe sensor’s resonant frequency is based on the changes in theshape of themeasured dc output voltage curve.Themeasureddc output voltage is related to the real part of reader antennaimpedance: when the sensor is excited at resonance, the realpart of reader antenna impedance changes greatly; then, thereadout system can extract the sensor’s resonant frequencyfrom the dc output voltage information.

2.4. Sensor Structure Scheme. Similar to devices designedwith LTCC (low-temperature cofiring ceramic) material,devices with HTCC (high-temperature cofiring ceramic)material mainly take three essential parts into consideration:formation of a flexible membrane, a sealed cavity, and theintegration of an LC resonant circuit. One of the majordifferences between the previous sensor and the sensor wedesigned is the introduction of ESL 5570 and PSZ tape 42020,whose parameters are shown in Tables 2 and 3, respectively.


Capacitor electrode

Via

Diaphragm

Inductor coils

Sealed cavity

P

P

Figure 7: Schematic cross-sectional view of the sensor.

ESL 42020 high-temperature PSZ (partially stabilized zirco-nia tape) tape is a flexible cast film of partially stabilizedzirconia (PSZ) powder dispersed in an organic matrix. Thismaterial is designed to be sintered in the temperature rangeof 1450∘C–1550∘C to yield a dense white-colored ceramic.ESL 42020 tape is provided on a silicone-coated polyesterfilm to protect the tape from mechanical damage and aidin handling. Pt inks (ESL 5570 Series) are used for printing,which can be cofired for use applications such as planarsensors.

ESL 49000, a flexible cast film of fugitive powder dis-persed in an organic matrix, is firstly introduced to generatethe sealed cavity.Thismaterial is designed to be burned out inthe temperature range of 600∘C–800∘C to yield a void wherethe tape was placed. ESL 49000 tape is provided on a silicone-coated polyester film to protect the tape from mechanicaldamage and aid in handling.

In addition, according to the small deflection theory,increase of the thickness of the sensitive membrane anddecrease of the sensitive membrane area would both con-tribute to improvement of the pressure range. Embeddedcapacitance electrode inside the sensor is designed to increasethe capacitance as the distance between capacitor plates isshortened, as shown in Figure 7. Capacitance increases willlead to the decrease of the frequency, to some extent; thisis advantageous to signal collection in low frequency range.By the simulation using the ANSYS software, the deflection,stress, and strain of the zirconia ceramic membrane under60 bar and at 800∘C, respectively, are shown in Figure 8.

3. Fabrication

The first step is to cut the green tape using punchingmachine. The punch file is used to cut accurate cavity, via,and alignment holes, as illustrated in Figure 9, step 1. Detailedschematic layout and geometrical values used for cutting thisdesign are described in Table 1.

To achieve embedded passives series resonance circuitwithin the substrate, ESL 5570 platinum conductor was

screen-printed while the ceramic tape was in a green state,illustrated in Figure 9, step 1. The top planar spiral and elec-trodes are screen-printed on the first layer, shown in Figure 9,step 1. The wet ink is allowed to dry in an oven at 150∘C for 15minutes prior to lamination. The fifth layer is defined as thecavity. Capacitance electrode plates are printed in the fourthand the seventh layers, respectively. All sheets, except for thelast three sheets, have vias on, through which the inductorand capacitor would be connected to form a series resonantcircuit; the last two layers are used merely for increase ofthe sensitive membrane thickness. All layers are assembledto form a ceramic body. ESL 5575 platinum conductor, usedto fill the via hole, is then allowed to dry in oven at 170∘C for5 minutes. Assembly of the device begins with laminating thenine layers separately illustrated in Figure 9, step 2, in vacuumcondition. The top section is assembled over the bottom andmiddle sections to form the final stack and then is laminatedunder pressure of 21MPa at 80∘C, which ensures that areasover the cavity are well laminated before final assembly. Thetop, middle, and bottom sections are then assembled andlaminated, illustrated in Figure 9, step 2. Contact betweenthe top metal spiral and via during lamination is sufficientto ensure the metal melts and create a contact during sinter-ing.

As shown in Figure 9, step 3, the laminated stack issintered in a box furnace in air for 85 minutes from 430∘Cto 600∘C (2∘C⋅min−1 ramp rate) to bake off the organics; slowheating rate from 600∘C to 800∘C (3∘Cmin−1 ramp rate) isused to make the fugitive tape volatile and then 120 minutesat 1500∘C to form a dense zirconia; then the structure iscooled at 5∘Cmin−1 ramp rate or slower; the specific sinteringcurve is shown in Figure 10. The microfabricated sensorswith sealed cavity and image of inductor coils observedby scanning electron microscope (SEM) are illustrated inFigure 11.

4. Results and Discussion

The sensor testing was conducted using the PCB readerantenna connected to the readout circuit to serve as theexternal reader for electrical measurements. The microfab-ricated devices were tested on-bench to characterize theirelectrical, physical behaviors. Electrical parameters of thefabricated microsensor were firstly obtained by analyzingthe measurement data from both the actual device withthe external wireless readout method and several test struc-tures with on-chip probing. Table 4 lists the experimentalresults which were in good agreement with device designestimates.

Measurements for the sensor were taken using pressurecylinder to simulate the high-pressure environments. Thepressure can be controlled from atmospheric pressure upto 20MPa using air pump as a source of stress and apressure control instrument. A customized pressure controlconfiguration, as shown in Figure 12, was utilized for wireless


(a) (b) (c)

(d) (e) (f)

Figure 8: Ansys simulation of the membrane deflection (a), stress (b), and strain (c) load under 60 bar. ANSYS simulation of membranedeflection (d), stress (e), and strain (f) at 800∘C.

Table 4: Electrical parameters of the sensors.

Parameters ValueInductance ∼3.6 𝜇HResistance ∼5.3ΩCapacitance ∼3.6 pFResonant frequency ∼43MHZQuality factor at resonance ∼186Sensing distance ∼3 cmPressure sensitivity ∼38 kHz/bar

pressure sensing demonstration. Accurate environmentalpressure variations could thus be created for the device withthis pressure control setup. The device was placed insidethe cylinder connected to the designed readout system tocomplete the high-pressure testing.

Although the pressure sensitivitywas expected to be smallfrom the device design, the sensing was compensated by thehigh sensor resonant frequency to reach reasonable pressureresponsibility for detection of the readout circuit’s dc outputvoltage shift with respect to environmental pressure varia-tions. Figure 13 shows the measured dc output voltage curvesfor the sensor by varying the pressure from atmosphericpressure to 60 bar.

For the sensors in the variable capacitor design, thenormalized shifted resonant frequency can be written as

𝑓max (Δ𝑃)

𝑓max (Δ𝑃 = 0)=

1/2𝜋√𝐿𝑠 (𝐶𝑠 + Δ𝐶𝑠)

1/2𝜋√𝐿𝑠𝐶𝑠

≅ (1 − 𝛼Δ𝑃)1/2

,

(10)

where Δ𝐶𝑠 is the changed capacitance due to diaphragmdeflection and 𝛼 is the fitting parameter incorporating themechanical behavior of the diaphragm. Accurate environ-mental pressure variations could thus be created for thedevice (Δ𝑃 = 𝑃outside sensor − 𝑃inside sensor) with this pressurecontrol setup. The wireless pressure sensing behavior ofthe device was characterized with the measured dc outputvoltage curves as shown in Figure 14 with approximately38 kHz/bar.

In the previous literatures of our research team, somestatic performances of sensor at high temperature are testedin the closed furnace. In this work, pressure tests are designedunder 650∘C high-temperature environment on the high-temperature platform as shown in Figure 17.The experimentsunder high-temperature environment consist of two parts intotal: one is 50∘C∼650∘C temperature changes experimentat 100 kpa and the result is shown in Figure 15; the other ispressure testing experiments with a heat preservation after 90min under 650∘C and the result is shown in Figure 16. As theplatform takes water cooling way to reduce the temperatureof readout antenna, the tests should be taken after the systemreaches its thermal equilibrium; the pressure range is from70 kpa to 190 kpa and step value is 20 kpa; the test results are


Bottom section

Top section

Center section

1

987

654

2 3

ESL Pt inks 5570

ESL fugitive film 4900

ESL PSZ tape 42020

(1) Laser-cut samples of green tape.Then screen-print top and bottom coils

(2) Laminate all threesections separately

(3) Laminate sections together

(4) Sinter laminated sections togetherFigure 9: Fabrication process for pressure sensor.

0 200 400 600 800 1000 1200

0

200

400

600

800

1000

1200

1400

1600

Sintering time (min)

Sint

erin

g te

mpe

ratu

re (∘

C)

(610, 1500) (735, 1500)

(552, 800)

(485, 600)

(400, 430) (850, 445)

(1050, 25)

Figure 10: Temperature process control curve.

shown in Figures 13 and 14. The resonant frequency range ofsensor is 84 kHz with a linearity of 1.17% and repeatabilityof 7.1%, and the hysteresis is 1.94% and the sensitivity is51 kHz/bar under 650∘C as well.

24.5mm × 24.5mm(planar)

Figure 11: Device images: (left) microsensors; (right) micrograph ofthe planar spiral inductor in SEM; (underneath) micrograph of thesealed cavity.

5. Conclusions

In this paper, a design and fabrication method of a wirelesspassive microsensor fabricated in HTCC technology waspresented. Featuring zirconia ceramics (PSZ) and Pt inks,the sensor achieves a satisfactory performance suitable for


Airpump

Pressure controller

Impedance analysis

SensorAntenna

Pressure barrel

Figure 12: Schematic of the on-bench pressure testing setup forcharacterization of the sensor.

35 35.5 36 36.5 37 37.5 38 38.5 39 39.5 40

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

Frequency (MHz)

4bar8bar12bar16bar20bar24bar28bar32bar

36bar40bar44bar48bar52bar56bar60bar

Vou

t(V

)

Figure 13: Overlay plot of the measured dc output voltage curvesof the readout circuit by varying the pressure in on-bench wirelesspressure sensing test.

applications in high-pressure environments. A sealed cavity,flexible ceramicmembranes, and a fixed inductance𝐿 formedthe series resonance circuit of the sensor, which were inte-grated on amonolithic substrate without the need of complexprocess technologies. A readout system was designed to testthe sensor’s resonant frequency, and the performance ofthe sensor was demonstrated in high-pressure environment.From the test result, the pressure dependence of the sensorcan be tested up to 60 bar. The average sensitivity andaccuracy of the sensor are up to 38 kHz/bar. And the resultsprovide substantial evidence that the sensor has great poten-tial for fulfilling continuous dynamic pressure monitoring inharsh environments.

Freq

uenc

y ra

tiof

max/f

max

(ΔP=0

)

0.94

0ΔP (bar)Applied pressure difference

10 20 30 40 50 60

0.95

0.96

0.97

0.98

0.99

1.00 R = fmax /fmax|ΔP=0 ≅ (1 − 0.001761102ΔP)1/2

Responsivity ≈ 38kHz/bar

Curve fitMeasured data points

Sensitivity =ΔP=0

≈ 8850 ppm/bar⏐⏐⏐⏐⏐⏐⏐⏐⏐⏐⏐⏐

𝜕R

𝜕(ΔP)

Figure 14: Frequency versus pressure Δ𝑃 (Δ𝑃 = 𝑃outside sensor −𝑃inside sensor).

0 100 200 300 400 500 600 700

38.80

38.81

38.82

38.83

38.84

38.85

38.86

38.87

38.88Fr

eque

ncy

(MH

z)

Temperature (∘C)

Figure 15:The resonant frequency of the sensor versus temperature.

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

38.74

38.75

38.76

38.77

38.78

38.79

38.80

Freq

uenc

y (M

Hz)

Pressure (bar)

Pressure decline Pressure raise Pressure decline

Pressure raise Pressure decline Pressure raise

Figure 16: The resonant frequency of the sensor versus pressure at650∘C.


Readoutunit

Operating control centre

Pressurecontrol

Nitrogentank

Reading antenna Sensor

Temperaturecontrol

Figure 17: The customized temperature-pressure measurement system.

Conflict of Interests

The authors declare no conflict of interests.

Acknowledgments

Thiswork is supported by theNational Natural Science Foun-dation of China (61335008) and the State Key DevelopmentProgram of Basic Research of China (2010CB334703). Theauthors especially thank Dr. Xiong and Dr. Liang for theirvaluable comments on experimental procedures and WenyiLiu for his valuable discussion and assistance.

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Research Article A Wireless Pressure Microsensor ...downloads.hindawi.com/journals/ijdsn/2015/974742.pdf · Research Article A Wireless Pressure Microsensor Fabricated in HTCC Technology