Diode-based microfabricated hot-plate sensor

In recent years, different types of microfabricated silicon 'hot plates' have been used as substrates for a variety of gas sensors and in some cases as gas thermal conductivity probes. The temperature-sensing elements in all reported devices have been resistors with varying types of thermal coefficients of resistance. The major difficulty with this approach is the reproducibility of the temperature coefficient of resistance from batch to batch. It is well known that the forward-biased voltage of a constant-current diode is directly proportional to the absolute temperature. While the constant of proportionality depends on the fabrication process, this parameter tends to be reproducible and easily controlled. Utilizing the characteristics of a forward-biased diode also eliminates the need to use materials such as platinum, which is both expensive and catalytic, as the resistive element. In this paper we report on the design and construction of a microfabricated silicon 'hot plate' that employs a diode-based temperature-measurement system. The noise level of the measurement corresponds to a temperature difference in the 100 mu K range in the vicinity of room temperature. The simplicity and utility of the structure described herein makes it an attractive element for various types of thermal measurements, particularly of the thermal conductivity of pure and mixed gases. (C) 1998 Elsevier Science S.A.