Bio-inspired MEMS Aquatic Flow Sensor Arrays

This thesis describes the attempt to design and fabricate flow sensors based on the function of sensory organs of the lateral line of fish. The driving force is the sensitivity and robustness of these sensory organs and the wide range of engineering appli¬cations that can benefit from direct velocity measurement with such accuracy. Three different schemes are proposed and the designs and fabrication procedures which pro¬vide the platform for realisation of the functional sensors are described. Chapter two shortly introduces the morphology and the importance of the lateral line of fish. The physical nature and range of the relevant stimuli and then the concept of Biomimetics is reviewed. Some examples of the use of Micro-Electro-Mechanical Systems (MEMS) technology in biomimetic practice are briefly presented. This is followed by the presentation of the state of the art engineered flow sensors inspired by nature or based on drag induced torque. The description and justification of the approach that is taken concludes the chapter. Chapter three introduces different designs of aquatic capacitive flow sensors. Detailed analysis is done to extract the design parameters which influence the characteristics of the sensor and can therefore be used to tune the performance. A qualitative comparison between different designs is provided at the end which highlights the mer¬its and drawbacks of each scheme. Chapter four includes the fabrication procedures of the proposed capacitive flow sensors. Each fabrication step has been characterised and detailed to result in an optimised procedure. The main objective is to devise a robust and high yield process. The general characteristics, drawbacks and complexity of the steps are compared to give an evaluation of the process integrity. In the next chapter details of SU-8 processing are presented. SU-8 plays an important role in all the schemes and its process requires careful optimisation. This is followed by the fabrication of passive canal embedded hair-like structures from SU-8 on glass wafers. These structures were intended to be used for Particle Image Velocimetry (PIV) measurements. The differences between structural and thermal properties of glass and silicon results in considerable variation of SU-8 process parameters. These differences were investigated and used to adapt the fabrication process and the design. Chapter six presents the results from the measurements carried out in air. These results help to understand the mechanical and electrical behaviour of the sensor before its immersion in water. Mechanical characterisation using laser vibrometer as well as electrical measurement are presented. At the time of writing the characterisation of the sensors in water has not been started. Chapter seven includes the conclusion and outlook. The long mathematical calculations are presented as appendices at the end of the thesis together with detailed fabrication process documents.