Silicon flow sensors

Abstract

A short review is given of the principles of thermal flow sensors in silicon. Sensor structures based on the thermal anemometer principle are discussed, as are the integrated flow sensors developed at Delft University of Technology which use the detection of an onchip temperature difference. Recent research on one-and twodimensional sensors for the measurement of both flow magnitude and direction are presented, including experimental results.

Constant power operation of a two-dimensional flow sensor

“…1 variety is given in [1]. There it is shown that such sensors obey the following relationship, known as King's law [11] (1) where is the total power dissipated in the heaters, is the overheat with respect to ambient temperature, and is the flow velocity.…”

“…The layout of a typical integrated 2-D thermal flow sensor is shown in Fig. 1 [1]- [4]. It consists of a square silicon chip (or membrane) in which a diode, four thermopiles, and four resistive heaters are integrated.…”

“…F LOW speed and direction may be determined by the use of two-dimensional (2-D) thermal flow sensors [1]- [4]. These devices utilize the fact that airflow over a heated plate will cool it asymmetrically, with the upstream portion of the plate being cooled more than the downstream portion.…”

“…This may be conveniently done using thermopiles [1]- [4], which, being self-generating transducers, are intrinsically offset free. Their sensitivity, however, is process and temperature dependent, and will therefore contribute to the overall process and temperature dependence of the flow sensor's characteristics [6].…”

“…To compensate for ambient-temperature variations, such sensors are usually operated in a constant temperature difference (CTD) mode [1]- [3]. Here, a control loop maintains the sensor at a constant temperature (known as the overheat) above ambient temperature.…”