Temperature control of micro heater using Pt thin film temperature sensor embedded in micro gas sensor
© The Author(s) 2017
Received: 28 April 2017
Accepted: 1 September 2017
Published: 12 September 2017
Pt thin film temperature sensors (Pt T sensors) are embedded in micro gas sensors to measure and control the working temperature. We characterized electrical resistances of Pt T sensors and micro heaters with temperature changing in the oven and by Joule heating. In order to enhance the accuracy of temperature measurement by the Pt T sensors, we investigated the correlation among the Pt T sensor, micro heater, and the working temperature, which was linear proportional relation expressed as the equation: T2 = 6.466R1–642.8, where T2 = temperature of the Pt micro heater and R1 = the electrical resistance of the Pt T sensor. As the error by physically separated gap between Pt T sensor and micro heater calibrated, measuring and controlling temperature of micro heater in micro gas sensors were possible through the Pt T sensors. For the practical use of Pt T sensor in micro gas sensor, the gas sensing properties of fabricated micro gas sensors to 25 ppm CO and 1 ppm HCHO gases were characterized.
KeywordsMicro gas sensor Pt thin film temperature sensor CO and HCHO gases
Micro gas sensors with micro-platforms, which consist of micro heaters and sensing electrodes on the membranes, have been actively researched, due to the possibility to miniaturize sensors and reduce power consumptions [1–4]. Micro heaters are necessary for elevated temperatures to operate micro gas sensors , because most of gas sensors need thermal energy to react gases [6, 7]. However, usually, it is hard to measure and control accurate temperatures of micro gas sensors with input powers to increase temperatures of micro heaters. In general, there are two kinds of measurement methods of temperature of micro heaters. One is contact type method such as thermocouples, negative temperature coefficient (NTC) thermistors and Pt resistance temperature detectors (RTDs), and so on. The other is non-contact type method such as IR cameras. Even though many researchers have measured the temperatures of micro gas sensors by IR cameras, there are some still problems in terms of measurement errors by setting the incorrect emissivity and not enough precision due to the resolution of the cameras . For instance, temperature measurement of the same device is remarkably changed with variations of emissivity. The measured temperature is highly dependent on emissivity of materials. The reason why it is difficult to determine the exact emissivity of IR camera is that other materials exist surrounding micro heater layer in the micro gas sensor as reported in [8, 9] and the emissivity is affected by not only the kinds of materials but also morphologies of surfaces and shapes of materials . Also, it is not possible to measure the operating temperature of packaged micro gas sensors using by IR cameras. In the case of the contact type of temperature sensors, it is also hard to measure the operating temperature of micro gas sensors with membranes due to the fracture problem from fragile structures of membranes and thermal conductivity problem between the micro heaters and contact type temperature sensors. So, special micro temperature sensors are needed.
We need to avoid some problems above such as emissivity error from IR cameras, fracture problem and thermal conductivity of contact type temperature sensors. Also, more exact temperature monitoring of micro heaters in MEMS gas sensors using by micro Pt thin film temperature sensors (briefly, Pt T sensors) in the micro-platform of micro gas sensors is needed. Temperature could be measured using by Pt T sensors no matter what materials surrounding micro heaters. So, Pt T sensors could avoid emissivity error problem from IR cameras and fracture problem and thermal conduction problem from contact type temperature sensors.
In this study, in order to enhance the accuracy of temperature measurement of micro heaters in micro-platform of micro gas sensors, the Pt T sensors were designed, fabricated, and characterized in micro-platform.
Measurement of the electrical resistance change of both Pt T sensors and micro heaters in the temperature oven (SU201, Espec, Japan) as a function of temperature from 25 to 150 °C.
Measurement of the electrical resistance of micro heaters and Pt T sensors by Joule heating with various input power of a DC power supply from 1 to 15 mW
Measurement of the temperature of the micro heaters by an IR camera during Joule heating with input power to compare with the results from (1) and (2) above regarding the micro heaters.
Calibration of the results from (1) and (2) above in order to measure and control the temperature of the micro heaters and compare with the result from (3).
Results and discussion
Pt T sensors
Pt micro heaters
The electrical resistance of the Pt micro heaters were 131 and 174 Ω at 2 and 15 mW, respectively.
R1 = the electrical resistance of Pt T sensor
R2 = the electrical resistance of Pt micro heater
P = the power consumptions.
Gas response results of SnO2 gas sensor with Pt T sensor to 25 ppm CO and 1 ppm HCHO gases
CO 25 ppm
HCHO 1 ppm
In this study, we fabricated the Pt T sensors integrated in micro gas sensors to measure and control the working temperature. There were heat losses between the Pt T sensor and the Pt micro heater due to 20 μm physical gap. In order to enhance the accuracy of temperature measurement by the Pt T sensors, we investigated the correlation between the Pt T sensor and the working temperature, which was linear proportional relation expressed as the equation: T2 = 6.466R1–642.8, where T2 = temperature of the Pt micro heater and R1 = the electrical resistance of the Pt T sensor. As the error by separation gap calibrated, measuring and controlling the temperature of micro gas sensors were possible through the Pt T sensors. In addition, we measured the gas response of the fabricated micro gas sensor to 25 ppm CO and 1 ppm HCHO gases and the working temperature with the integrated Pt T sensors. Further work is thermal analysis to compare with the above results and enhance the accuracy of temperature measurement.
JK has made substantial contributions to conception, acquisition of data and analysis. JSP and HJL have been involved in drafting the manuscript or revising it critically for important intellectual content. KBP, JS, EAL and SN have given approval of the version. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
This research was supported by the Project No. 10043800, of “S/W converged components technology development program” by KEIT and MOTIE in Korea. The authors appreciate for research funding. J.S.Park and J.G. Kang also would like to acknowledge thepartial support from the R&D Convergence Program of MSIP andNST of Republic of Korea (Grant CAP-13-1-KITECH).
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