Microelectronic Engineering 88 (2011) 82–86 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee Accelerated Publication Study on I–V characteristics of lead free NTC thick film thermistor for self heating application Shweta Jagtap a, Sunit Rane a,⇑, Suresh Gosavi b, Dinesh Amalnerkar a a b Thick Film Materials Laboratory, Centre for Materials for Electronics Technology (C-MET), Panchawati, Off. Dr. Bhabha Road, Pune 411008, India Department of Physics, University of Pune, Ganeshkhind, Pune 411007, India a r t i c l e i n f o Article history: Received 26 April 2010 Received in revised form 2 July 2010 Accepted 26 August 2010 Available online 6 September 2010 Keywords: NTC thermistor Lead free Thick film Heater Response and recovery time a b s t r a c t The resistivity of several materials varies predictably with temperature; this makes them suitable for the use as temperature sensors. If these material gets heated due to the electric current passing through it and if it preserves a uniform temperature distribution during heating, then its total resistance would accurately reflect its temperature, allowing it to simultaneously act as both a temperature sensor and a self heater. This paper describes the results of indigenously formulated lead free thick film NTC thermistor paste composition with sheet resistance of 1 kO/h used for heater application. The current–voltage, temperature–current characteristics, dissipation constant, response and recovery time of the heater are reported. The maximum current handling capacity of the prepared thick film thermistor was observed at 300 mA and the temperature achieved to 340 °C. Therefore, the heater was tested at a constant current of 300 mA for 24 h, which did not show any extreme change in behaviour and the temperature of the thermistor/ heater remained constant to 340 ± 5 °C. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction The resistivity of several materials varies predictably with temperature; this makes them suitable for the use as temperature sensors. If these material gets heated due to the electric current passing through it and if it preserves a uniform temperature distribution during heating, then its total resistance would accurately reflect its temperature, allowing it to simultaneously act as both a temperature sensor and a heater. Such heater/sensor would eliminate the need for two metal films (heater and sensor) on a chip, reducing the real-estate usage of electronics and rendering the chip more readily adaptable for higher levels of integration [1]. The name thermistors derived from thermally sensitive resistors was coined to described a form of resistive device that posses a large temperature co-efficient of resistance [2–4]. Thick film negative temperature co-efficient (NTC) thermistors are made up of ceramic materials usually based on Mn, Co, Ni and Fe oxides mixtures, which crystallize in the spinel structure [5,6]. Thermistors prepared using thick film technology has already gained good rank in the families of advanced solid sensor technology. Three important characteristic of thermistors make them extremely useful in measurement and control applications: (a) the resistance–temperature characteristic, (b) the voltage–current characteristic and (c) the current–time characteristic. All of these applications are based on ⇑ Corresponding author. Tel.: +91 20 2589 9273; fax: +91 20 2589 8180. E-mail address: sunitrane@yahoo.com (S. Rane). 0167-9317/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.08.025 the resistance temperature characteristics of the thermistor which depends on the semiconducting material. In the voltage–current characteristic of a thermistor, if a large amount of current flows through a thermistor, it will generate heat which will raise the temperature of the thermistor above that of its environment and its resistance decreases further. This characteristic of self heat provides an entirely new field of applications for the thermistor. In the self heat state the thermistor is sensitive to anything that changes the rate at which heat is conducted away from it. It can so be used to measure flow, pressure, liquid level, composition of gases etc. If, however, the rate of heat removal is fixed, the thermistor is sensitive to power input and can be used for voltage or power-level control. It may be noted here that even though the thick film thermistors are widely used for temperature sensing since past 4–5 decades, however, the extensive literature survey on thick film thermistors revealed that no adequate research available for thick film thermistor in particular to self heater application. Therefore, the present work is focused on the study of the indigenously formulated lead free thick film NTC thermistor for the self heater applications. 2. Experimental 2.1. Preparation of test heater patterns Thick film NTC thermistor paste composition with sheet resistance as 1 kO/h value was used for the fabrication of planar 83 S. Jagtap et al. / Microelectronic Engineering 88 (2011) 82–86 heaters. The thermistor composition is based on spinels of oxides of Mn, Co and Ni as a functional material with thermistor system as (Mn1.85 Co0.8 Ni0.35) O4, RuO2 as a conducting phase and lead free glass frit as a permanent binder. It should be noted here that ruthenium dioxide based thick film NTC thermistors has been found a most suitable choice since it showed linear current–voltage (I–V) characteristics, significant decrease in the resistance value, moderate thermistor constant and good stability [7,8]. Considering the above phenomenon, in the present study RuO2 was selected as a conducting phase. These ingredients were then blended with the organic vehicle to form the viscous paste. The inorganic to organic ratio was kept as 70:30 while the NTC spinel powder material and glass frit was kept at 90:10. The detailed thermistor compositions and paste formulations process were already explained in our earlier study [9–15]. Planar thick film thermistors/heater patterns of 1  1 mm size was screen printed on the pre-fired lead free silver electrodes (ESL 9912-K) onto the 100  100 alumina substrate (96%, Kyocera). The thickness of the alumina substrate was 635 lm. The printed heaters were then dried under IR lamp for 10– 15 min in order to evaporate the volatile organic solvents and then fired at 850 °C for 10 min in the BTU furnace. Thickness of fired films was measured using Talysurf thickness profiler and was in the range 25 ± 5 lm. 2.2. Experimental set-up for heater characteristics The factors which govern the rate at which an NTC thermistor will self heat are the power which is applied to the device and its thermal mass, construction of the device, the nature and temperature of its surrounding ambient and its means of connection in the circuit also affect the heat flow, either through the device itself or alternatively to its surroundings. Therefore, in order to maintain the operational environment and avoid the surrounding affects while measuring the electrical parameters, the sample was mounted on a fixed support in a closed metallic chamber to maintain the constant surrounding ambient. The temperature and electrical characteristics of the fired thick film thermistor was measured using the constant current source (Keithley model-220, max current = 100 mA), Regulated power supply (Aplab 7222S, max current = 2.5 Amp), Data acquisition system (DAQ) (Agilent, Model 34970A) with K type thermocouple (Agilent make) with accuracy of 1 °C. Different values of currents were passed through the films and voltage across the film along with the corresponding change in temperature of the film was acquired using the data acquisition system. Initially, the input current (up to 100 mA) was supplied though the constant current source (Keithley) which was later replaced by regulated power supply source (Aplab make). The schematic diagram of the measurement set-up is shown in Fig. 1. 3. Result and discussion Fig. 2. Backscattered SEM image of fired thick film NTC thermistor. film revealed the sintered microstructure with connected grain boundaries. The bright particles seen in the matrix are nothing but the conducting RuO2 grains. The cavities/small pinholes occurred due to the evolution of gases produces due the burning of organic solvents during firing of the thick film. 3.2. XRD analysis Fig. 3 shows the X-ray diffractorgram of the thermistor film sample. The XRD confirms the formation of spinel phases such as NiMn2O4, CoMn2O4, NiCo2O4 and conducting phase RuO2 is also detected in film sample. However, the spinel phase NiMn2O4 is dominant with the average peak intensity of 50% while existence of RuO2 phase with the average peak intensity of 43% is observed. Also, feeble presence of other spinel phases such as CoMn2O4 and NiCo2O4 with the average intensities of 5% and 8%, respectively has been noted. 3.3. R–T characteristics The change in resistance with respect to temperature was measured for the sample. Fig. 4 shows a plot of resistance (R) as a function of temperature (T) for the fired thermistor films. It can be seen from the graph that, the resistance of the thermistor decreases almost linearly (i.e. quasi linear behaviour) with the increase of temperature. This change in R–T characteristics of thermistor from exponential to quasi linear is mainly due to the contribution of RuO2 in the composition. As stated earlier, RuO2 has relatively low specific resistivity with high positive, linear metallic like dependence of resistivity with respect to temperature. These 3.1. Microstructure analysis Agilent Constant current Temperature 34970A Thermistor source Voltage DAQ RS 232 Computer control system Intensity (Arb units) Fig. 2 shows the backscattered SEM image obtained from the surface of fired thick film thermistor. The SEM of the thermistor NiMn2O4 CoMn2O4 NiCo2O4 RuO2 20 30 40 50 60 70 80 2θ Fig. 1. Schematic diagram of the measurement set-up used. Fig. 3. X-ray diffractogram of the fired thick film thermistor sample. 84 S. Jagtap et al. / Microelectronic Engineering 88 (2011) 82–86 Vmax 50 Voltage(volts) 45 40 35 30 25 20 15 10 0 50 100 150 200 250 300 Current (mA) Fig. 5. Current–voltage (I–V) characteristics of an NTC thick film thermistor as a function of ambient temperature. Fig. 4. Resistance–temperature characteristics of the thick film NTC thermistor as self heater. characteristics of RuO2 in conjunction with spinel phases are responsible for the quasi linear nature which is an important behaviour of thermistors [15]. The data on electrical properties pertaining to the prepared lead free thermistor paste composition along with the data available on ‘lead’ based commercial thermistor paste compositions reported by some researchers [16] is given in Table 1. It is seen from the table that the electrical properties of the prepared thermistor paste composition is compatible with the commercially available lead based NTC thick film compositions. The R–T characteristics of the commercially available thermistor compositions are exponential in nature which may be due to the absence of conductor phase (RuO2) in the reported commercial samples. 3.4. Current–voltage characteristics One of the most interesting and useful property of an NTC thermistor is the behaviour of the voltage drop ‘V’ measured across the device as the DC current ‘I’ through the device is increased. Fig. 5 illustrates the typical I–V curve of the thick film NTC thermistor at an ambient temperature of 30 °C after sufficient time has been allowed for the thermistor to achieve a steady state. At very low currents, the power dissipated in the thermistor is too low (<1 mW) to heat the thermistor where Ohm’s law is obeyed. In this region, the current through the thermistor is not sufficient to raise the temperature of the device appreciably above the ambient and the variation in the resistance will depend only upon changes in the temperature of the surroundings. This linear region (Ohmic region) can be used for temperature measurement where as the overheating (near maximum voltage) is commonly used for flow sensors, vacuum monitoring and similar applications. An increase in current causes reduction in thermistor resistance. This decrease Table 1 Comparative data of thick film NTC thermistors prepared by indigenously formulated paste with the data available for the commercial paste samples. * Sample Sheet resistance (kX/h) Temperature range (°C) Thermistor constant (K) TA35C Remex NTC 4993* ESL NTC 2414* 1 1 10 25–300 25–125 25–125 1200 1200 1250 Commercial thick film NTC materials. in resistance is due to heat developed in the thermistor itself by the action of current through it (Joule heating). A peak value of voltage (Vmax = 47.56 volts) occurs at current of 60 mA where the thermistor body temperature must be above the ambient and further increase in the current (i.e. up to 300 mA) causes so much self heating that the resulting drop in resistance causes drop in voltage (Fig. 5). This region exhibits a negative resistance and a steady state is reached where thermistor dissipates as much power as is supplied to it. However, further increase in the current to 350 mA, thermistor was unable to withstand this high current which might cause the thermal mismatch between the thermistor and the substrate presumably damaged the alumina substrate as well as and the thermistor film. The portion of the breakage in the thermistor due to high current is also shown as circled in Fig. 6b. This effects was due to passing of high current through the thick film thermistor are shown in Fig. 6 (a, b). The silver electrode also turned to brownish as there is a great disparity between the thermistor and conductor resistivity, current crowding was occurred that turned the ageing of electrode which is shown in Fig. 6a and appearance of current crowding is shown in Fig. 6b. The current flows into the conductor at its leading edge, rather than being distributed over the overlap region which results in the current crowding causes ageing of the electrode [17]. The large voltage difference between the thermistor and silver electrode can also be the reason of turning the silver electrode to brownish [18]. From the above results it can noted that the developed thermistor can be used as a self heater up to the maximum temperature of 340 °C. 3.5. Current–temperature characteristics Fig. 7 illustrates the current–temperature characteristics of the thick film thermistor. It is observed here that at very low currents (1–9 mA); the power dissipated is too low to heat the thermistor and Ohm’s law is obeyed. However, at higher current value (P10 mA) the temperature of the thermistor raise above the ambient temperature due to Joule heating. It is seen from the figure that the maximum temperature achieved by the thermistor is 340 ± 5 °C. Further increase in current leads to the breakage in the heater. Based on the above conditions, the heater performance was tested for 24 h at a constant current of 300 mA. It was found that the heater withstands the current of 300 mA without the breakage or damage in the sample. Therefore, from the above results, we can say that the prepared lead free thermistor can be used as a self heater up to the temperature of 340 ± 5 °C. It assumed that the temperature distribution over the heater was uniform since the area of the heater is small (i.e. 1 mm2). 85 S. Jagtap et al. / Microelectronic Engineering 88 (2011) 82–86 Fig. 6. Effect of passing the high current on the silver electrode (a) and breakage in the alumina substrate as well as NTC film (b). 3.7. Response and recovery time 350 Response and recovery time of heater has practical importance in many applications since it gives a measure of the speed of response for the particular device to a temperature change. The response time of a heater can be obtained from the time required for the heater to change its output value from its initial value to 250 o Temperature ( C) 300 200 150 (a) 100 350 50 300 50 100 150 200 250 300 Current (mA) Fig. 7. Current–temperature-relationship of the thick film thermistor as heater. 3.6. Dissipation constant 250 O 0 Temperature ( C) 0 200 150 100 In Sections 3. 4 and 3.5, it is seen that as the power supplied to the NTC thermistor, Joule heating accompanying the change in resistance of the thermistor. Therefore, the dissipation constant ‘K’ can be defined as the power (in mW) required to raise the thermistor temperature by 1 °C above the ambient temperature. If the Newtonian cooling is assumed, the steady state relationship between the applied electrical power (P) and the thermal power, dissipated in the thermistor material as a heat loss is given by Macklen [1], 50 -50 0 50 100 150 200 250 300 350 Time (Sec) (b) 350 300 250 where K is independent of T and all parts of the thermistor are at the same temperature T. Accordingly, using the above equation the dissipation constant (K) of the lead free thick film NTC thermistor was 28 mW/°C (±2) which is quite reasonable as the dissipation constant range from 10 W/°C, 5–15 mW/°C and 100 mW/°C for bead type, disc as well as for rod devices and a disc mounted on a plate and held against a heat sink, respectively. It may be noted that the value of K depends greatly on the environment of the thermistor, its method of mounting, the thermal conductivity of its wires and envelope. In the present case, the measurements were carried out in the closed chamber having normal air ambient. The sample is in the planar form but without hermetically sealed which was mounted on a fixed support. 200 (T-Tamb) P ¼ V  I ¼ KðT  T amb Þ 150 100 50 0 0 50 100 150 200 250 300 Time (sec) Fig. 8. (a and b) Response and recovery time of the NTC thermistor as self heater. 86 S. Jagtap et al. / Microelectronic Engineering 88 (2011) 82–86 90% of its final settled value. The power applied to the device is the major factor which governs the rate at which the NTC thermistors will self heat [1]. To measure the response time of the thermistor used as a self heater application, 300 mA current was passed through the thermistor and the time required for the thermistor to reach at 340 °C was recorded using the data acquisition system (DAQ) with a scan rate of 500 ms/scan. The heater takes 2 min 35 s (±5 s) to reach the 340 °C which is the response time of a heater (Fig. 8a). Recovery time of the heater can be obtained as time required for the heater to reach 10% of its original temperature when the source was removed. To measure the recovery time of the thermistor used as a self heater, initially the thermistor was kept at its highest withstanding current i.e. 300 mA. At this stage, the temperature of the thermistor body was recorded as 340 °C. The current source was then removed and the change in temperature with respect to time was recorded using DAQ system at a scan rate of 100 ms/scan. The recovery time of the self heater was recorded to 45 s (±3) as shown in Fig. 8(b). The recovery time of thermistor as a heater is dependent on the type of thermistor fabrication and is 1 s for bead type thermistor, 15–30 s for disc type and small rod type thermistor while the maximum response time of 200 s was noted in case of disc encapsulated in a moulded resin case [1]. 4. Conclusion We have successfully demonstrated the indigenously formulated ‘lead free’ thick film NTC thermistor paste composition with sheet resistance of 1 kO/h used for self heater applications. The thermistor as a self heater showed the maximum voltage (Vmax) of 47.56 V at 60 mA current, then decreases even further increase in the current value. The developed thermistor can be used as self heater up to the maximum temperature of 340 °C with the current of 300 mA. However, further increase in the current to 350 mA, thermistor was unable to withstand this high current presumably damaged the alumina substrate as well as and the thermistor film. The response time of the thermistor is 2 min 35 s (±5 s) to reach at the maximum temperature of 340 °C with a recovery time of 45 s (±3) which is satisfactory and compatible to the available bulk devices. Acknowledgements The authors are grateful to Prof. R.C. Aiyer, Department of Physics, Pune University for the fruitful discussions and time to time help during this work. The work was supported through the sponsored project supported by Department of Information Technology, New Delhi. The authors are grateful to Department of Information Technology, Ministry of Communication and Information Technology, Government of India for the financial support and Dr. U.P. Phadke, Dr. Krishnakumar, Dr. S. Chatterjee, Department of Information Technology, Ministry of Communication and Information Technology, New Delhi for their active support related to the sponsored project. References [1] E.D. Macklen, Thermistors, Electrochemical Publications Ltd., Ayr, Scotland, 1979. [2] O. Shpotyuk, A. 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