a film capacitive temperature using barium strontium … · 2019. 8. 1. · possible andliquidphase...

Electrocomponent Science and Technology. 1979, Vol. 6, pp. 13-180305-3091/79/0601-0013 $04.50/0

(C) 1979 Gordon and Breach Science Publishers, Inc.Printed in Great Britain

A THICK FILM CAPACITIVE TEMPERATURE SENSOR USINGBARIUM STRONTIUM TITANATE GLASS FORMULATIONS

S. LEPPVUORI, T. HANNULA and A. UUSIM.KIDepartment ofElectrical Engineering, University of Oulu, Oulu, Finland

This paper describes a novel use of thick film techniques to produce a temperature sensor. Ferroelectric materialsabove their Curie temperature exhibit a dielectric constant which is inversely dependent upon temperature. Themeasuring range of the sensor can be altered by varying the ratio of the ferroelectric components used (BaTiOa andSrTiOa ). By using this ceramic together with a glass frit to form a paste, it is possible to employ standard thick filmtechniques to produce the sensors. Sensors with a composition (Ba0.5 Sr0.5)TiO were subjected to various temper-ature and ambient conditions to investigate their temperature performance and stability. The sensors were funda-mentally stable and exhibited a capacitance change as large as 65% of their initial value over a temperature rangeof 100 C and yet the dependence was linear to within 1.5 C.

1. INTRODUCTION

The dielectric constant of ferroelectric materials suchas barium titanate is strongly dependent upon tem-perature and increases up to the Curie temperature(Tc), where the material changes from the ferro-electric state to the paraelectric state. Above Tc, thedielectric constant is inversely dependent upon tem-perature. This temperature dependence is a disadvan-tage when a ferroelectric material is used as a capaci-tor dielectric. However, it has in the past been usedto advantage in temperature compensation capaci-tors.

Another possible use of these dielectrics in theparaelectric state is for temperature sensing),a Thereactance of a capacitor using such materials isstrongly dependent upon temperature and therelationship is virtually linear. Temperature sensorsbased upon this principle may be manufacturedusing thick f’tim techniques. By varying the com-position of the ceramic component of the thick filmpaste the properties of the resulting device can becontrolled.

This is a new application of thick f’tim materialsand temperature sensors of this type offer highlinearity and sensitivity, compatibility with hybridmicroelectronic production and a competitive pricefor the device.

2. PROPERTIES OF BARIUM STRONTIUMTITANATE GLASS FORMULATIONS

2.1 Properties of the CeramicBarium titanate (BaTiOa) and strontium titanate(SrTiO$) are ferroelectric materials which have a

perovskite crystal structure. In the ferroelectric statebelow the Curie temperature they are tetragonal butin the paraelectric state above Tc they are cubic.BaTiO and SrTiO have a complete solid solutionand the melting point of this solution increasesapproximately linearly from that of BaTiO(1618 C) to that of SrTiOa (2080 C).4 CeramicBaTiOa, SrTiOa and solid solutions of the two areproduced by solid-state sintering. The dielectricproperties of ceramic BaTiOa are almost indepen-dent of the grain size in the paraelectric state, where-in the dielectric constant follows the Curie-Weisslaw e C/T- To, where C is the Curie constant andTo is the Curie-Weiss temperature, which is withinabout 10C of the Curie temperature, s The depen-

oKX:)

-2(X)

SrTiO3 20 0 60 80 BTiO3WEIGHT PERCENT

FIGURE 1 The Curie-Weiss temperature TO of (Ba,Sr)-TiO as a function of weight percent of BaTiO.

13

14 S. LEPP)i.VUORI, T. HANNULA AND A. UUSIMKI

dence of the Curie-Weiss temperature on theBaTiO3/SrTiO3 ratio is quite linear in the (Ba,Sr)-TiOa-solution (Figure 1).

2.2 Properties of Ceramic Glass FormulationsBecause of the low sintering temperature of the thickfilm process solid state sintering of the ceramic is notpossible and liquid phase sintering of the glass com-ponent of the paste must be used. The thick filmdielectric layer has a larger porosity than the ceramic,the volume fraction of which depends mainly uponthe volume fraction of the glass component. If theglass frit content is greater than 25 vol. % then mostof the porosity is eliminated.6 However, the glasscomponent, together with the porosity, result in thedielectric properties of the mixture being differentfrom those of the pure ceramic. The total value ofthe dielectric constant (eT) can be evaluated fromthe logarithmic rule, log er Y,ivi log ei, where vi isthe volume fraction and ei the dielectric constant ofeach component. 7

In order to examine the effect of the glass com-ponent on the dielectric constant both pressed discsand thick films were used. Figure 2 shows thedielectric constant for various glass frit contents ofpressed discs. The plotted point with 0% glass fritcontent refers to a disc made by solid state sintering.The theoretical curve shown was calculated from thelogarithmic rule assuming no porosity. All otherplotted points refer to liquid state sintered sampleswhich have experienced the same temperature profile

TABLEConstants C, To, emax and TC to (Bao. Sro. )TiO with

different glass frit contents

Glass fritcontent C To emax(TC) TC(%) (xl0*) (C) (C)

0 8.7 -70 12100 -653 3.8 -75 850 -606 4.2 -77 1060 -6012 4.2 -88 920 -5824 2.6 -140 450 -56

as the thick films. The differences between thedielectric values of the pressed discs (glass content3 and 6 vol. %) and the theoretical curve are causedby porosity. The volume fraction of porosity is about8% with a 3% glass frit content calculated from thelogarithmic rule.

Table shows the constants C and To correspon-ding to the constants of the Curie-Weiss law andapproximated from the measured temperaturedependence curves of the reciprocal dielectric con-stant, the measured maximum dielectric constante(Tc) and the Curie temperature of (Bao.s Sro.s)-TiOa with different glass frit contents (again, thepoint at 0% glass frit content refers to a solid statesintered sample). The results in Table and inFigure 2 show that for the maximum dielectricconstant, a glass frit content of between 6 vol. %and 12 vol. %, is optimum.

Figure 3 shows the reciprocal dielectric constants

1000

800

600

/00

200

CERAMIC

0 0" 6 12 18GLASS FRIT CONTENT (VOL. %)

FIGURE 2 The dielectric constant for various glass fritcontents of (Ba0. St0. )TiO pressed discs taken at 23 C.Also shown is the theoretical curve calculated by the log-arithmic rule assuming no porosity.

1,0

08

0,6

0,2

FIGURE 3

THICK FILMCERAMIC 6 % GLASSCERAMIC

/

-60 0 60 120TEMPERATURE (C)

The reciprocal dielectric constant (proportionalto reactance) normalized to 120C as a function of tem-perature.

A THICK FILM TEMPERATURE SENSOR 15

normalized to a temperature of 120C as a functionof temperature for (Bao .s Sro .s)TiO3 with a glassfrit content of 0% (curve 3) and 6 vol. % (curve 2).Curve refers to a sample made by thick film tech-niques. The most important effect is the reductionin the temperature sensitivity of curves 2 andcompared with the ceramic curve 3. Also, it can beseen that curves 2 and 3 have generally a slightlyreduced linearity compared with curve 3 and inparticular that the thick film material cannot betaken close to the Curie-Weiss temperature of theceramic without serious non-linearity occurring.Thus it is necessary for good linearity to restrict theoperating range of the device to temperatures above(To + 70C) i.e. in the case of (Ba,Sr)TiO3 sensors,the minimum working temperature is between180 C to +170 C (Figure 1).

3. PREPARATION OF THE CERAMIC AND THESENSOR PASTE

The starting points for the preparation of the ceramicfor the temperature sensitive sensor paste are com-mercial grade barium titanate and strontium titanatepowders. After mixing, the powder is homogenisedby milling with alcohol or distilled water, pressedinto discs and sintered. The material is then milleduntil the mean particle size is of the order of a fewmicrons. X-ray diffraction analysis indicates that asecond sintering process is necessary in order toachieve a satisfactory crystal structure. After thesecond sintering the material is milled ready for thepreparation of the thick film paste. At this stage themean particle size is still in the order of a few microns.Particles smaller than 1/am are removed by a sedi-mentation process in an ultrasonic bath.

In the sensor paste a leadborosilicate glass is usedfor the glass frit and the vehicle consists of ethylcellulose and butyl carbitol acetate or terpineol. The

O Osoftemng point can be shxfted from 600 C to 500 Cand the linear thermal expansion coefficient (TCE)from 4.5 x 10-6/C to 9.5 x 10 -6/C by varying thecomposition of the glass (PbO 60... 80%, SiO215 5%, B2 O3 10... 25%). It is therefore possibleto adjust the TCE of the glass so that stresses betweenthe active ceramic powder (TCE 10 x 10-6 [OC) andthe alumina substrate (TCE 6.5 x 10-6 [oC) areminimized.

The glass frit is made by mixing the componentoxides and firing in a crucible. The liquid glass isquenched in de-ionised water and the lumps areground until the particle size is comparable with that

ACTIVE COMPONENT GLASS FRIT

MIXING

1,

IMILLING TO FINAL|[PARTICLE SIZEJ

PbO B203 SiO

MIXING

IGLASS FOR’IN

MILLING TO FINAL!PARTICLE SIZE

REGULATIONOF VISCOSITY

,I"FINISHEDPASTE,,

VEHICLE

BUTYLETHYL CARBIT.CELLULOSE ACETATE

[,MIXING]

FIGURE 4 Preparation of the sensor paste.

of the final barium strontium titanate powder. Theceramic powder is mixed with the glass frit and theorganic vehicle is added. The paste is then thoroughlymixed and the correct viscosity is obtained by vary-ing the amount of organic vehicle. Figure 4 summarisesthe processes involved in the paste preparation.

Pressed discs were prepared for the examinationof the properties of ceramics and the effect of theglass frit. Pure ceramic disc capacitors were takenfrom the standard process after the second sintering.Discs with some glass frit were pressed and thensintered in the same way as the thick film dielectricmaterials. Electrodes for the discs were made usingPd/Ag thick film conductor paste or by vacuumevaporation.

4. THICK FILM TEMPERATURE SENSOR

4.1 Manufacture and EncapsulationThe sensors were printed onto Kyocera 96% AIO3substrates (1 in x in x 0.025 in) using Du PontPd-Ag 8228 terminations. The required dielectricthickness of about 40 vm was achieved with the useof a 60 mesh screen. The dielectric was then fired at

16 S. LEPPJ(VUORI, T. HANNULA AND A. UUSIMJ(KI

,-,,......./q.d,j)IELECTRIC LAYER_’2"):-);%’):79

OTTOM ELECTROO(

ii ilIIII IIII iiII II

LL:: j:-:LJ 3S0

CONSTRUCTION CONSTRUCTION 2

FIGURE 5 Schematic diagrams of two versions of thethick film temperature sensor.

a peak temperature of 900C for about 10 minutes.The terminations were fired separately.

The sensors were manufactured in two differentdesigns (Figure 5). The first structure was encap-sulated in a protective coating, ESL 240-SB, whichhad been double printed. In the second type, wherethe top electrode covered nearly all the dielectric,encapsulation was achieved by solder-dipping thecomplete sensor to ensure that the electrode wasnon-porous. The uncovered part of the dielectric wascovered with ESL 240-SB.A sensor designed for the temperature range 0C

to +100C was selected for the tests to evaluate theproperties of the devices. The glass content of thepaste was 6 vol. %, and the active component(Bao .s Sro .s)TiO3, the Curie-Weiss temperature ofwhich is about -70C. These selections give a sensorwith good sensitivity and linearity (Section 2.2).

4.2 Properties of the SensorThe temperature response of the sensor wasmeasured in a standard environmental chamber at afrequency of 1 kHz (Figure 6). The change in reac-tance over the temperature range 0C to +100Cwas 65% and the deviation from the best possiblestraight line relationship between reactance andtemperature represents a temperature change of1.5C. Figure 7 shows a block circuit diagram of aCMOS-oscillator using a thick film capacitor sensor.Figure 8 shows the output frequency of the oscillatoras a function of temperature.

The thermal time constants of the sensors weredetermined by measuring the response to a steptemperature change from +23C to +100C. Thevelocity of the air circulation was 2 m/s. The response

400

3OO

250

2O0

150-60 0 .60 o120

TEMPERATURE (C)

FIGURE 6 The temperature response of a sensor for useover the range 0C to +100 C. The measurement frequencyis 1 kHz and the area of the capacitor is 10 mm2.

was exponential and the results indicated 95% of thetotal change took place in less than minute.

The following tests were performed to examinethe stability of the sensors

Dry heat (IEC 68-2-2), at +100C for 96 hours.Damp heat, steady state (IEC 68-2-3), at +40Cand 93% RH for 96 hours.Change of temperature (IEC 68-2-14), where eachcycle lasted 10 hours of which 4 hours was at 0Cand 4 hours at +100C. The number of cycles was5 and the rate of temperature change, 1.65C/min.

The measurements before and after these tests weremade at a constant temperature (23C) and at afrequency of 1 kHz.

The maximum deviation from the starting valueswhich occurred in these tests represents 1.1C to1.8C uncertainty of measurement which can beimproved by improving the encapsulation.

Table II summarizes the properties of the thickf’flm sensor.

i/z+ CO 4001 lit, co 4oo /z, CO 400

RTcCTC

.’---1

FIGURE 7 The block diagram of a CMOS-oscillator usinga thick film capacitive sensor (CTc).

A THICK FILM TEMPERATURE SENSOR 17

4.5

3,5

,,3.0

0 20 40 60 80 100TEMPERATURE (C)

FIGURE 8 The output frequency of the oscillator (Fig-ure 7) as a function of temperature.

The stability tests indicated that the device wasstable providing good encapsulation was used.Because they were used at temperatures in excessof the Curie temperature there were no problemsencountered with the long term instability normallyassociated with ferroelectric materials.

This new type of sensor which has been manu-factured using standard thick film techniques can beused either as a discrete sensor or alternatively inte-grated with an oscillator to make a hybrid trans-ducer, the output frequency of which is proportionalto temperature. Calibration and instrumentation ofthis kind of transudcer is simple and because themeasurements are made using a.c. there are noerrors due to contact resistance or potentials.

ACKNOWLEDGEMENTS

The authors wish to thank the Tauno Terming Foundationand the Foundation of Technology in Finland for theirfinancial suppport.

5. CONCLUSIONS

The thick film capacitive temperature sensor des-cribed in this paper can be used over a temperaturerange of 100C. The maximum change in the reac-tance of the sensor is 65%, the uncertainty ofmeasurement due to non-linearity is 1.5C and theinstability is less than 2C. The minimum temper-ature of the sensor can be varied between-180Cand +170C by adjusting the composition of thesensing material.

REFERENCES

1. R. A. Delaney and H. D. Kaiser, "Multiple-Curie-PointCapacitor Dielectrics." IBMdournal, 11, 501 (1967).

2. S. Leppiivuori, P. Niemela and A. Uusimiki, "A capaci-tive thermal sensor using hig K dielectric pastes withnegative temperature coefficient." Prec. INTERNEPCON,101 (1977), Brighton.

3. S. Leppiivuori and P. Niemela, "A thick film capacitivetemperature sensor." Prec. Electron. Components Conf.IEEE, 47 (1978).

4. E. M. Levin et al. Phase Diagrams for Ceramists (TheAmerican Ceramic Society, Columbus, 1964) p. 195.

TABLE IIProperties of the sensor

Construction 1 Construction 2

Protective coatingChange in reactance

(0 C to +100 C)Non-linearity(0C to +100C)

Instability(Max. changes in environ, tests)

Thermal time constant(10 ram2 capacitor)

Capacitance per unit area(1 kHz, +23 C)

Dissipation factor(1 kHz, +23 C)

ESL 240-SB65%

+1.5%

-0.4... +1,1%

19s

65 pF/mm

0.6%

Solder (2% Ag)65%

+/-1.5%

-0.6%... 1.8%

14.5

65 pF/mm

0.6%

18 S. LEPP,VUORI, T. HANNULA AND A. UUSIMKI

5. K. Kinoshita and A. Yamaji, "Grain-size effects ondielectric properties in barium titanate." Z App. Phys.,47, 371 (1976).

6. H. Nester and D. Mason, "Thick Film Capacitors."Proc. Electron. Components Conf. IEEE, 233 (1968).

7. W.D. Kingery, Introduction to Ceramics (John Wiley &Sons, New York, 1960) Chap. 20, p. 720.

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a film capacitive temperature using barium strontium … · 2019. 8. 1. · possible andliquidphase...

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