Flexible MEMS Shear Stress Sensor with Improved Performance for Wind Tunnel Measurements

Abstract

This paper describes a flexible micro-electro-mechanical system (MEMS) shear stress sensor, fabricated on a polyimide substrate using sputtering and a lift-off technique. The width of the nickel thermistor is minimized to several microns by an optimized manufacture process. The performance of the nickel thermistor is notably enhanced by heat-treatment during the fabrication. The sensor's characteristics are tested, including the temperature coefficient of resistance (TCR), time constant (cutoff frequency), and sensitivity. As a result of the optimized fabrication and the appropriate annealing heat-treatment, the TCR of the nickel thermistor is high, which has a considerable effect on the sensitivity; these techniques and results are an improvement on results in literature. The cutoff frequency test and sensitivity calibrations of the sensor were performed in a wind tunnel. The results show that the improved sensor with higher TCR and smaller size can be used in wind tunnel measurements effectively.

Recrystallization‐Induced Laser Lift‐Off Strategy for Flexible Thermal Sensors with Near‐Limit Sensitivity

“…The TCR of bulk nickel [ 28 ] is ≈6.4–6.9‰°C −1 , while the TCR of the thin film is generally lower than that of the block due to the electron scattering effect. In this work, the TCR of the thermal sensor can reach 6.2‰°C −1 , which is closer to the limit of bulk nickel and higher than other works [ 17–19,21,22,28,37–60 ] as shown in Figure 1e.…”

“…Traditional thermal sensors exhibit flexibility but low sensitivity due to the temperature limitations of the flexible substrate. For example, when the annealing temperature of flexible thermal sensors [28,36,37] is increased to 400 °C (the upper temperature limit of polyimide), the sensor sensitivity is improved to ≈5.1‰°C −1 , which cannot be further improved by increasing the annealing temperature. The recrystallization-induced laser lift-off strategy allows the nickel film to be transferred from a rigid substrate to a flexible one after high-temperature annealing (≈800 °C), enabling the sensor to achieve both flexibility and high sensitivity.…”

“…Although some inorganic materials [ 27 ] can withstand temperatures as high as 1200 °C, such elevated temperatures may lead to carbonization and decomposition in thin, flexible organic materials. In previous studies, [ 28 ] the temperature‐sensitive metal nickel, with the highest TCR among pure metals, is deposited on a polyimide substrate and annealed at 400 °C (the glass transition temperature of polyimide) to obtain a high sensitivity of 5.0‰°C −1 at a thickness of ≈1 µm. For flexible metal‐based thermal sensors aiming to further improve their performance, a natural contradiction arises: high sensitivity requires high‐temperature annealing (e.g., 800 °C), but organic materials cannot withstand such high temperatures.…”

Design of readout circuit for high sampling miniaturized MEMS wall shear force sensor

“…The measurement of high-precision wall shear stress has a wide range of applications in aerodynamic research [1], industrial process control, biomedicine [2], and other fields [3]. At present, there are three kinds of wall shear stress measurement technologies: oil film interference technology [4], liquid crystal coating technology, and sensor technology based on micromachining (MEMS) [5]. The first two are indirect measurement techniques, which need to be combined with other directly measured physical parameters to obtain the wall shear stress to be measured through indirect calculation.…”