A THICK FILM CAPACITIVE TEMPERATURE SENSOR USING BARIUM STRONTIUM TITANATE GLASS FORMULATIONS

This paper describes a novel use of thick film techniques to produce a temperature sensor. Ferroelectric materials above their Curie temperature exhibit a dielectric constant which is inversely dependent upon temperature. The measuring range of the sensor can be altered by varying the ratio of the ferroelectric components used (BaTiOa and SrTiOa ). By using this ceramic together with a glass frit to form a paste, it is possible to employ standard thick film techniques to produce the sensors. Sensors with a composition (Ba0.5 Sr0.5)TiO were subjected to various temperature 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 range of 100 C and yet the dependence was linear to within 1.5 C.


INTRODUCTION
The dielectric constant of ferroelectric materials such as barium titanate is strongly dependent upon temperature and increases up to the Curie temperature (Tc), where the material changes from the ferroelectric state to the paraelectric state. Above Tc, the dielectric constant is inversely dependent upon temperature. This temperature dependence is a disadvantage when a ferroelectric material is used as a capacitor dielectric. However, it has in the past been used to advantage in temperature compensation capacitors.
Another possible use of these dielectrics in the paraelectric state is for temperature sensing),a The reactance of a capacitor using such materials is strongly dependent upon temperature and the relationship is virtually linear. Temperature sensors based upon this principle may be manufactured using thick f'tim techniques. By varying the composition of the ceramic component of the thick film paste the properties of the resulting device can be controlled. This is a new application of thick f'tim materials and temperature sensors of this type offer high linearity and sensitivity, compatibility with hybrid microelectronic production and a competitive price for the device.  dence of the Curie-Weiss temperature on the BaTiO3/SrTiO3 ratio is quite linear in the (Ba,Sr)-TiOa-solution ( Figure 1).

Properties of Ceramic Glass Formulations
Because of the low sintering temperature of the thick film process solid state sintering of the ceramic is not possible and liquid phase sintering of the glass component of the paste must be used. The thick film dielectric layer has a larger porosity than the ceramic, the volume fraction of which depends mainly upon the volume fraction of the glass component. If the glass frit content is greater than 25 vol. % then most of the porosity is eliminated. 6 However, the glass component, together with the porosity, result in the dielectric properties of the mixture being different from those of the pure ceramic. The total value of the dielectric constant (eT) can be evaluated from the logarithmic rule, log er Y, ivi log ei, where vi is the volume fraction and ei the dielectric constant of each component. 7 In order to examine the effect of the glass component on the dielectric constant both pressed discs and thick films were used. Figure 2 shows the dielectric constant for various glass frit contents of pressed discs. The plotted point with 0% glass frit content refers to a disc made by solid state sintering. The theoretical curve shown was calculated from the logarithmic rule assuming no porosity. All other plotted points refer to liquid state sintered samples which have experienced the same temperature profile as the thick films. The differences between the dielectric values of the pressed discs (glass content 3 and 6 vol. %) and the theoretical curve are caused by porosity. The volume fraction of porosity is about 8% with a 3% glass frit content calculated from the logarithmic rule.   normalized to a temperature of 120C as a function of temperature for (Bao .s Sro .s)TiO3 with a glass frit content of 0% (curve 3) and 6 vol. % (curve 2).
Curve refers to a sample made by thick film techniques. The most important effect is the reduction in the temperature sensitivity of curves 2 and compared with the ceramic curve 3. Also, it can be seen that curves 2 and 3 have generally a slightly reduced linearity compared with curve 3 and in particular that the thick film material cannot be taken close to the Curie-Weiss temperature of the ceramic without serious non-linearity occurring. Thus it is necessary for good linearity to restrict the operating range of the device to temperatures above (To + 70C) i.e. in the case of (Ba,Sr)TiO3 sensors, the minimum working temperature is between 180 C to +170 C (Figure 1).

PREPARATION OF THE CERAMIC AND THE SENSOR PASTE
The starting points for the preparation of the ceramic for the temperature sensitive sensor paste are commercial grade barium titanate and strontium titanate powders. After mixing, the powder is homogenised by milling with alcohol or distilled water, pressed into discs and sintered. The material is then milled until the mean particle size is of the order of a few microns. X-ray diffraction analysis indicates that a second sintering process is necessary in order to achieve a satisfactory crystal structure. After the second sintering the material is milled ready for the preparation of the thick film paste. At this stage the mean particle size is still in the order of a few microns.
Particles smaller than 1/am are removed by a sedimentation process in an ultrasonic bath. In the sensor paste a leadborosilicate glass is used for the glass frit and the vehicle consists of ethyl cellulose and butyl carbitol acetate or terpineol. The The glass frit is made by mixing the component oxides and firing in a crucible. The liquid glass is quenched in de-ionised water and the lumps are ground until the particle size is comparable with that  of the final barium strontium titanate powder. The ceramic powder is mixed with the glass frit and the organic vehicle is added. The paste is then thoroughly mixed and the correct viscosity is obtained by varying the amount of organic vehicle. Figure 4 summarises the processes involved in the paste preparation.
Pressed discs were prepared for the examination of the properties of ceramics and the effect of the glass frit. Pure ceramic disc capacitors were taken from the standard process after the second sintering. Discs with some glass frit were pressed and then sintered in the same way as the thick film dielectric materials. Electrodes for the discs were made using Pd/Ag thick film conductor paste or by vacuum evaporation.

Manufacture and Encapsulation
The sensors were printed onto Kyocera 96% AI O3 The terminations were fired separately.
The sensors were manufactured in two different designs ( Figure 5). The first structure was encapsulated in a protective coating, ESL 240-SB, which had been double printed. In the second type, where the top electrode covered nearly all the dielectric, encapsulation was achieved by solder-dipping the complete sensor to ensure that the electrode was non-porous. The uncovered part of the dielectric was covered with ESL 240-SB.
A sensor designed for the temperature range 0C to +100C was selected for the tests to evaluate the properties of the devices. The glass content of the paste was 6 vol. %, and the active component (Bao .s Sro .s)TiO3, the Curie-Weiss temperature of which is about -70C. These selections give a sensor with good sensitivity and linearity (Section 2.2).

Properties of the Sensor
The temperature response of the sensor was measured in a standard environmental chamber at a frequency of 1 kHz ( Figure 6). The change in reactance over the temperature range 0C to +100C was 65% and the deviation from the best possible straight line relationship between reactance and temperature represents a temperature change of 1.5C. Figure 7 shows a block circuit diagram of a CMOS-oscillator using a thick film capacitor sensor. Figure 8 shows the output frequency of the oscillator as a function of temperature.
The thermal time constants of the sensors were determined by measuring the response to a step temperature change from +23C to +100C. The velocity 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 use over the range 0C to +100 C. The measurement frequency is 1 kHz and the area of the capacitor is 10 mm2.
was exponential and the results indicated 95% of the total change took place in less than minute.
The following tests were performed to examine the stability of the sensors  The stability tests indicated that the device was stable providing good encapsulation was used. Because they were used at temperatures in excess of the Curie temperature there were no problems encountered with the long term instability normally associated with ferroelectric materials.
This new type of sensor which has been manufactured using standard thick film techniques can be used either as a discrete sensor or alternatively integrated with an oscillator to make a hybrid transducer, the output frequency of which is proportional to temperature. Calibration and instrumentation of this kind of transudcer is simple and because the measurements are made using a.c. there are no errors due to contact resistance or potentials.