On the characterization of an ElectroHydroDynamic instability patterning (EHDIP) fabrication process for polymer based MEMS
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This thesis is concerned with the patterning of polymers by ElectroHydroDynamic instabilities (EHDIP). This process represents contactless, photolithography free micromachining ability. A patterned electrode is held above a thermosetting polymer heated above its glass transition temperature. Capillary waves set up by the electrostatic pressure at the air/polymer interface disrupt the polymer surface so that it flows into the same shape as the patterned electrode. With relatively quick process times and low process temperatures, EHDIP has the potential to become an important process in the MEMS industry with a range of applications thanks to its ability to create unique topographies in the polymers, including continuous surfaces. The outcome of the work in this thesis was achieving the characterisation of important aspects of the EHDIP process, the influence of various applied forces (Electrostatic, Hydromechanical and acoustic) and physical parameters such as applied voltage, E-field shape, temperature and the filling gap between air/polymer. Further, demonstration was made of a prototype small batch processing setup, highlighting the importance of precision control of small gap heights to the outcome of the process in terms of both accuracy and repeatability of this process. In this thesis it was concluded that there are several important factors at work in refining a useable EHDIP process. It was shown that the capacitor/air interface filling ratio is important to maximising the uniformity of the patterned polymer surface topography, and that the applied voltage is important in defining the speed at which structures are grown out of the polymer disruption. Further it was demonstrated that EHDIP has a narrow process window and that great care must be taken in establishing both a uniform micron scale air gap and minimising other disruptive interfacial forces, such as convective thermal currents, and charged surface particles. Demonstration of a method for modelling the complex electrostatic field analytically via Schwarz Christoffel mapping in order to further understanding of the underlying mechanics of the process was also achieved. It will show that it is possible to record the growth rate of the polymer structures in realtime via capacitive sensing. Finally a proof of concept of the potential for directly injected mechanical waves via Surface Acoustic Wave (SAW) technology in increasing the uniformity of the polymer structures was shown to influence the growth dynamics of the polymer.