AeroMEMS Wall Hot-Wire Anemometer on Polyimide Substrate Featuring Top Side or Bottom Side Bondpads

Abstract

Design, manufacturing, calibration, and basic characterization of a microelectromechanical systems (MEMS) wall hotwire sensor on a flexible polyimide substrate are presented. A configuration exhibiting bondpads on the top side of the foil, as well as an improved setup featuring a through-foil metallization and bottom side bondpads were established. Both sensor designs make use of a highly sensitive nickel thin-film resistor spanning a reactive ion etched cavity in a polyimide substrate. The polyimide base material enables the sensor to be adapted to curved aerodynamic surfaces, e.g., airfoils and turbine blades. A mismatch of curvature of aerodynamic surface and silicon sensor surface, as observed with previously presented MEMS hot-wire anemometers is avoided. The combination of polyimide's low thermal conductivity and a cavity featuring FEM-optimized dimensions accounts for a very low-power consumption (<25 mW). Fluctuations in wall shear stress up to 85 kHz can be resolved in constant-temperature mode. An average sensitivity of 0.166 V/(N/m 2 ) is achieved in a wall shear stress range from 0 to 0.72 N/m 2 . The specifically designed through-foil metallization process allows for electrical contacts to be positioned on the backside of the substrate, thus effectively minimizing aerodynamic disturbances.Index Terms-Microelectromechanical systems (MEMS), polyimide, via, wall hot-wire.

On Improving the Performance of a Triaxis Vortex Convective Gyroscope Through Suspended Silicon Thermistors

“…A larger difference in the thermistor temperature compared to the airflow temperature will yield Conduction losses into the substrate between the two types of thermistors under different power consumption. Based on (5), the sensitivity of the gyroscope can be written as follows: (11) where S gyro is the sensitivity; and T is the difference of the mean temperatures of the thermistors due to the application of the angular rate ω. According to [10] and [13], the flow velocity change V vort ex at the location of the thermistors has a linear relation with the applied angular rate ω, as shown in the equation as follows:…”

“…This type of clampedclamped thermistor will cause a large thermal stress when the working temperature changes. Furthermore, more heat will dissipate into the substrate because the thermistors are not suspended [11]. The thermal stress and heat loss will significantly deteriorate the performance of the sensor.…”

Membrane-based thermal flow sensors on flexible substrates

“…First approaches of thermal flow sensors on flexible substrates have been done by Buder et al [7,8], Tan et al [9,10] and Liu et al [11]. Buder et al present a hot-wire anemometer being realized by placing either one or two freestanding nickel wires on a 125 m thick polyimide foil.…”

“…The measurement principle used here is anemometric: the nickel wires are heated up to a defined temperature and the current is measured. With increasing flow the convective heat loss rises leading to a higher induced current which can be interpreted as flow rate [7,8]. A polyimide foil is also used as substrate material by Tan et al [9,10].…”

“…This approach is, however, expensive and might impact the flow channel's structural health. For those issues a wide setup of thermal flow sensor approaches on flexible substrates can be found in literature [7][8][9][10][11]. These approaches have in common that the sensors' characteristics are reduced due to their substrate modification which is often coming along with a sensor design differing from the ideal one.…”