Design, fabrication and characterization of thin film resistances for heat flux sensing application
Introduction
Knowledge of the heat flux appears as a very important element in many areas of engineering. It enables to optimize lots of thermal systems, with applications in building heat transfer, aeronautics, aerospace and process industry … Several methods are used to measure heat flux depending on the nature of heat transfer: conduction, convection or radiation. For heat flux sensors measuring the conductive heat flux through a surface, three different methods may be considered: spatial temperature gradient, temperature variation with time and surface heating ones [1], [2]. For gradient heat flux sensors, a temperature gradient is measured on each side of an element of known features (thermal properties like conductivity, specific heat capacity, density, geometry…). The heat flux is then determined by applying Fourier’s law. With such a method, the gradient can be either transverse or tangential to the surface of the element. Concerning heat flux sensors based on temperature variations, a thermal sensor is attached to a substrate surface and the heat flux is determined on the other substrate’s face by assuming one-dimensional transient heat conduction in semi-infinite body [3]. Most of the commercial sensors have a thermopile as temperature-sensitive element, and many references on this subject are given in Diller’s work [1]. However, using Resistance Temperature Detectors (RTD), instead of thermopile, has several practical advantages. RTDs provide indeed highly accurate absolute values of temperature, instead of differential temperature measurements (such as those given by thermopiles). They also enable to improve resistance to electromagnetic disturbances and measurement noise, which is also an advantage. Moreover, during the manufacturing process, the use of one metallic material (instead of several in the case of thermocouples) is another advantage. Therefore, RTDs are frequently used in heat flux sensors based on temperature variation. The sensor response time decreases with its thickness, therefore applications requiring very short response time use thin-film technologies, such as in aeronautics for measuring unsteady heat flux in supersonic flow. Miniaturization of the sensing device is also beneficial for limiting heat transfer distortion by the sensor. For example, Maulard and Jourdin [4] developed heat flux sensors based on temperature variation with time for the study of compressor noise. The resistor was a sputtered platinum film of 100 nm thickness. Different kinds of substrate can be used: for instance, Fralick et al. [5] deposited platinum resistors on a ceramic substrate whereas Guo et al. [6] used a polyimide substrate, and Kumar et al. [7] deposited a platinum ink on Pyrex glass. Cornelis et al. [8] studied issues of using this type of heat flux sensor in a hydrogen engine. In addition, RTDs are also used for heat flux sensor based on gradient temperature. Andretta et al. [9] achieved a heat flux sensor based on resistive sensors to estimate the effects of solar performance on natural cooling devices. The heat flux sensor consisted of two coils of copper wire placed on each side of a Plexiglas disc. Klems and DiBartolomeo [10] developed a heat flux sensor that measures low non-uniform heat flux to study the performance of buildings insulation. The surface sensor (0.09 m2) was made of a nickel wire supported on a fibreglass substrate, and the insulation was achieved with a phenolic resin honeycomb. Another solution has been proposed by Hayashi et al. [11]: two nickel resistors were sputtered on each side of a silicon monoxide substrate. Epstein et al. [12] developed a heat flux sensor for measuring a dynamic heat flux up to 100 kHz. This heat flux sensor, a gradient type one, was made of nickel resistors of 10 nm thickness deposited on a 25 microns polyimide substrate. The sensor was bonded to the piece to be instrumented. Mockitat and Herwig [13] have developed a heat flux sensor made of nickel film resistors on a polyimide substrate. Hamadi et al. [14] and Aamar et al. [15] used gold resistors of 85 nm thickness on a substrate of borosilicate glass. Their goal was to achieve heat flux measurements in microchannels. Azerou et al. [16] used for their sensors a 28 μm deposit of copper resistors on a plate of epoxy of 1.6 mm thick. Recent works are related to the utilization of new substrate materials. For example, Seo et al. [17] developed a sensor on a polymer derived ceramic. Follador et al. [18] worked on the protective deposition of diamond on the platinum (of the sensor) for high temperatures applications. Finally, D’Aleo and Prasser [19] used a platinum deposit on a borosilicate glass substrate for temperature and heat flux measurement in tubes subjected to thermal fatigue in the nuclear industry.
This work focuses on the development and characterization of platinum thin film RTDs for a heat flux sensor based on spatial temperature gradient. This sensor is designed to be used in a Micro Electro Mechanical System (MEMS)and at operating temperatures from ambient to 250 °C. The design choices are presented in the first part. Then, the microfabrication techniques are exposed, followed by the characterization of deposits. In the third part, temperature calibration, self-heating and uncertainties of the temperature measurement are presented. The influence of annealing, of 2-wire or 4-wire connections and of self-heating on reproducibility and uncertainty are also quantified. The last part presents the heat flux test bench and the heat flux sensor results.
Section snippets
Design choices
One objective of the sensor is to measure heat flux in MEMS, made of glass and silicon, for operating temperatures from ambient one to 250 °C. Due to materials, size and maximal operating temperature of the MEMS engine in which the sensor is planned to be implemented, no commercial sensors could have been used. We thus had to develop a specific sensor for our application.
Sensor design
To measure heat flux through a medium (for instance, a wall), the sensor must be located inside this medium (embedded sensor). The heat flux measurement for this type of sensor requires to minimize thermal disruption and to maximize the temperature gradient across its two faces. On way to minimize thermal disruption is to use a very thin and highly conductive substrate for the sensor, but this leads to very low temperature gradients. Therefore, in order to avoid thermal disruption, we chose to
Microfabrication steps
The RTDs for the heat flux sensor were manufactured in the MIMENTO cleanroom at FEMTO-ST institute. The methodology used to manufacture our prototype is derived from microelectronics techniques that enable to achieve complex planar structures and multiple layers deposits. In our case, the choice of methods of photolithography and thin film deposition by sputtering offers the possibility of obtaining a hundred sensors on a four inch diameter wafer. Cleaning and degreasing were performed on the
Temperature calibration of the RTDs, uncertainties
The calibration bench was composed of a portable calibration oven (550 Gemini LRI), a highly accurate Pt100 platinum reference probe (0.005 °C) and a reference thermometer (PHP 601) (cf. Fig. 9). This calibration bench was certified by the company AOIP. The RTDs were placed into the oven. A removable thermal mass of aluminium was inserted into the oven to ensure a uniform temperature between the sensors to be calibrated and the reference probe. The RTDs were measured with two precision 61/2
Test bench
The test bench was made of a polyamide cylinder heated on one side and in contact with ambient air on the other side (Fig. 19). It was insulated with cork. The heating element consisted of two electric heating pads sandwiched between two copper disks to ensure thermal homogenization on the surfaces. One side (on the left side on Fig. 19) of the heating parts was insulated with fiberglass, the other side was in contact with the polyamide cylinder. The sensor was in contact with air and polyamide
Conclusion
This article presents the design, fabrication and characterization of two thin-film platinum RTDs for a heat flux sensor based on spatial temperature gradient method. The RTDs were deposed by PVD on a Borofloat glass substrate (with a chromium adhesion layer). The analysis and experimental results showed an influence of the annealing on structures and measurements repeatability. After three thermal cycles in a calibration oven, best results were achieved for sensors which were subjected for 6 h
Acknowledgements
The authors gratefully acknowledge The French National Research Agency and the Mistic project for funding.
Aymen ZRIBI PhD student at the University of Bourgogne Franche-Comté. Since October 2015, he is temporarily attached to education and research in the same university. He had his diploma of electromechanical engineer in 2012 from the National Engineering School of Sfax Tunisia. His current research area in the energy department of FEMTO-ST Institute is heat flow measurement.
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Aymen ZRIBI PhD student at the University of Bourgogne Franche-Comté. Since October 2015, he is temporarily attached to education and research in the same university. He had his diploma of electromechanical engineer in 2012 from the National Engineering School of Sfax Tunisia. His current research area in the energy department of FEMTO-ST Institute is heat flow measurement.
Magali Barthès received her PhD degree in Mechanics and Energetic at the University of Aix-Marseille in 2005. She is an Associate Professor since 2006 at the University of Franche-Comté, in the Femto-ST institute. Her scientific skills are in phase change phenomena (boiling), thermocapillary convection and also in development and characterization of thermal (probes) and optical (PIV, PTV...) metrology methods.
Sylvie Begot associate professor at Femto-st/Université de Franche-Comté. Engineer at Ecole Centrale de Lille 1990—PhD at Université de Franche-Comté in 2001. She worked for Alcatel as a test engineer for 5 years, then for Alstom as a research engineer for 3 years, and since 2001 in FEMTO-ST as research engineer then associate professor. She is in charge of “Energy in buildings” and “Renewable power”, “Forced convection” teaching courses. Her current research areas are Stirling engines, and heat flux measurement.
François Lanzetta received the Ph.D. degree in Science for Engineer and Microtechnics (Energy) in 1997 from Université de Franche-Comté, France. He is currently Full Professor of Energy with the Université Bourgogne Franche-Comté and head of the Energy research department, Institut FEMTO-ST. His research concerns heat transfer, thermodynamics and power generation. Currently, its research interest focuses on micro sensors (temperature, flow, heat flux) and their application to fluid mechanics and heat transfer in microsystems and heat engines.
Jean-Yves Rauch was born in 1969, he recieved his PhD degree in chemical-physics of the University of Franche comte in 2000. He is currently working as Research Engineer in the same University, in the Femto-ST Institute. His scientific fields are every thin coatings layers by PVD, CVD and ICPECVD, vacuum and plasma technologies, and several surface analysis, MEB, FIB, XPS, XRD and mass spectroscopy.
Virginie Moutarlier obtained her PhD in 2003 in the Laboratory of Material Chemistry and Interface (Université de Franche-Comté). The following year she worked as an engineer in the Surface Treatment Institute of Franche-Comté until 2007. Since then she has been working in the Institute UTINAM as a CNRS engineer. She is now a technical manager of the analysis device platform for surface characterization and Member of Management Council and Laboratory management committee. Her research activity is mainly focused in developing high-end analyzing methods based on XRD, GDOES, PM-IRRAS, AFM and SEM.