Measurement technique for the thermal properties of thin-film diaphragms embedded in calorimetric flow sensors

Abstract

In diaphragm-based micromachined calorimetric flow sensors, convective heat transfer through the test fluid competes with the spurious heat shunt induced by the thin-film diaphragm where heating and temperature sensing elements are embedded. Consequently, accurate knowledge of thermal conductivity, thermal diffusivity, and emissivity of the diaphragm is mandatory for design, simulation, optimization, and characterization of such devices. However, these parameters can differ considerably from those stated for bulk material and they typically depend on the production process. We developed a novel technique to extract the thermal thin-film properties directly from measurements carried out on calorimetric flow sensors. Here, the heat transfer frequency response from the heater to the spatially separated temperature sensors is measured and compared to a theoretically obtained relationship arising from an extensive two-dimensional analytical model. The model covers the heat generation by the resistive heater, the heat conduction within the diaphragm, the radiation loss at the diaphragm's surface, and the heat sink caused by the supporting silicon frame. This contribution summarizes the analytical heat transfer analysis in the microstructure and its verification by a computer numerical model, the measurement setup, and the associated thermal parameter extraction procedure. Furthermore, we report on measurement results for the thermal conductivity, thermal diffusivity, and effective emissivity obtained from calorimetric flow sensor specimens featuring dielectric thin-film diaphragms made of plasma enhanced chemical vapor deposition silicon nitride.

Implantable Multi-Modal Sensor to Improve Outcomes in Hydrocephalus Management

“…This geometry allows localised heating of an 'active area', rather than heating of the whole silicon die and serves to markedly reduce the power consumption of the device [33]- [35]. Device sensitivity is also increased as heat loss via thermal conduction to the substrate is minimized allowing the flow dependent thermal convection to the bulk fluid flow to become dominant [36], [37].…”

Simulationsgestützte Designoptimierung zur Reduktion des Winkelfehlers bei miniaturisierten thermischen Windsensoren

“…Parallel dazu wurde ein Niedertemperatur-Prozess für die plasmaunterstützte chemische Gasphasenabscheidung (Plasma Enhanced Chemical Vapor Deposition, PECVD) entwickelt, mit dem mechanisch spannungsarme Dünnschichten aus Siliziumnitrid (SiNx) gefertigt werden können. Dieses PECVD-SiNx wird einerseits als Passivierungsschicht für das aGe eingesetzt und erlaubt andererseits die Herstellung von mechanisch robusten, (sub-)mikrometerdicken Dünnschichtmembranen mit geringer thermischer Leitfähigkeit (unter 1 W/(Km), [5]). Bedingt durch die geringe thermische Masse dieser Membranstrukturen ergibt sich eine exzellente thermische Entkopplung zwischen den Temperatursensoren auf der Siliziumnitrid-Membran und dem die Membran tragendenden Sensorchip aus Silizium.…”

Frequency Domain Based Measurement Method for the Thermal Parameters of a Thin-film Diaphragm Embedded in a MEMS Multi-parameter Wind Sensor

“…Common techniques for their determination require fabrication of custom test structures (e.g., thin-film cantilevers [3]). In [4], we derived the thermal thin-film parameters of rectangular diaphragms from measurements directly carried out on calorimetric flow sensor devices. In this contribution, we apply and significantly extend this method to multi-parameter wind sensors.…”