Mechanical characterization of thermal flow sensors membranes

The aim of this paper is to determine the mechanical properties of the films that typically compose a membrane based thermal flow sensor. Departing from a sample composed of a single layer of Si3N4 deposited by low pressure chemical vapour deposition, used as a basic mechanical support of a great variety of micromechanical sensors, the residual stress introduced by two different passivating layers deposited by plasma enhanced chemical vapour deposition has been determined. The characterization has been performed by means of the bulge test. This technique allows to obtain not only the residual stress of the samples under test, but also the Young modulus of the stacked layers. In addition it has to be emphasized that up to now, this test has been done only in bare membranes and at ambient temperature; in this work, bulge test has been carried out directly on the thermal flow sensors. As these devices operate in the range of 100-200 °C, three different temperatures within this range have been tested, providing a direct measurement of the evolution of the average residual stress onto biased sensors membranes.