Design, fabrication and characterization of thin film resistances for heat flux sensing application

Highlights

• Design of thin film resistances.

• Gradient heat flux sensor with 2 Resistance Temperature Detectors as sensing elements.

• Platinum thin film deposited by PVD on a chromium buffer layer and glass substrate: SEM, FIB and XRD analysis.

• Analysis of the impact of annealing, self heating, and measurement uncertainties.

• Heat flux measurements.

Abstract

The article presents the design, fabrication and characterization of platinum thin-film Resistance Temperature Detectors (RTDs) for heat flux sensing applications. The microfabrication process is described and the thin films are analysed by several methods including SEM, XRD and profilometry. Estimation of experimental uncertainties is performed. The influence of annealing, self-heating, 2-wire and 4-wire connections are studied. The analysis and experimental results show that the RTDs have a good repeatability when an annealing process is performed and a maximal injected current are used. A measurement uncertainty of 0.14 °C is achieved. The observed Temperature Coefficient Ratio is sufficient to ensure variation of the resistance in the temperature range. The heat flux measurements made with the sensor are validated in a dedicated test bench and present a good repeatability.

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.