Previous sections of this guide provided information on wall shear stress measurement techniques and transduction methods. This section, and the one preceding it, discuss shear stress sensor construction methods and how they impact and/or enable sensor capabilities.  In this final part before the full guide summary, micromachined MEMS wall shear stress floating element sensors are discussed.

MEMS Floating Element Sensors

In contrast to conventionally-built sensors, micromachined sensors use one or more microfabrication methods including thin-film deposition, oxidation, epitaxial growth, chemical and plasma etching, and polishing. Features are generally photolithographically defined (a method first championed for integrated circuit fabrication) to enable high precision of geometric features (down to the micron level).

The combination of thin-film materials and photolithography enable small, precise features with tight tolerances, leading to higher performance and lower uncertainty and variance between sensors. Additionally, small features enable miniaturization of the entire sensor, resulting in smaller sensors with higher spatial resolution and higher bandwidth.

Furthermore, micromachined sensors are generally batch-fabricated (i.e., hundreds to thousands of sensors are created on a single silicon wafer), enabling greater sensor-to-sensor consistency and phase matching.

On the downside, micromachined sensors can exhibit increased fragility, small gaps can trap debris, and temperature sensitivity is sometimes an issue. While these negatives are not exclusive to MEMS sensors, they can impose additional requirements to cleaning and measurement compensation procedures. 

The next and final section of this guide will provide a summary of the full guide.

Table of Contents

1. Overview
2. Comparing Techniques – Indirect Measurements
3. Comparing Techniques – Direct Measurements
4. Transduction Method – Piezoresistive
5. Transduction Method – Piezoelectric
6. Transduction Method – Capacitive
7. Transduction Method – Optical
8. Transduction Method – Summary and Guidelines
9. Sensor Construction – Conventional
10. Sensor Construction – MEMS
11. Summary and References

References

IC2’s DirectShear™ Sensing Systems