Materials selection for microsystems actuators

Abstract

The recent developments of novel thin film materials and their associated processes have marked the onset of change in the design philosophy of Micro-Electro- Mechanical Systems (MEMS) from process centric to performance centric. This presents an opportunity to improve the performance of MEMS devices by materials selection beyond the commonly preferred candidates compatible with the Complimentary-Metal-Oxide-Semiconductor (CMOS) processes which forms the principal motivation of the present research. This thesis focuses on the selection of suitable materials choices for realising high performance MEMS actuators. The thermomechanical design for a generic cantilever bi-layered structure was evolved using a temperature dependent quasi-static analysis. The closed form solutions for mechanical performances (displacement/tip slope, blocked force/moment, work per unit volume) were obtained applying Timoshenko bimetallic theory for an Euler- Bernoulli beam subjected to electrothermal, piezoelectric and shape memory actuations. The thermal performance (actuation frequency) was evaluated using lumped heat capacity models. Contours of equal performance were plotted in the domain of governing material properties on Ashby’s selection maps to identify and rank promising candidates for further analysis. A few novel material combinations such as Zn and Ni on Si and Diamond-like-carbon (DLC) substrates perform better than conventional material combination (Al on Si) for high work per volume bimaterial electrothermal actuators. Actuation frequencies of the order of ~10 kHz can be achieved electrothermally at scales less than 100 ?m using engineering alloys. Although engineering polymers on Si are promising for high displacement electrothermal actuators, their low elastic moduli have to be compensated by a large thickness for an optimal performance. Pb based piezoceramics on Si/DLC substrates are promising for high force piezoelectric MEMS actuators. Piezoelectric actuators operate at mechanical resonance (> 100 kHz) and hence the achievable frequencies are greater than that of the electrothermal actuators. However, the work per unit volume delivered is lower than that of the electrothermal actuators. Nitinol (NiTi shape memory alloy) on Si/DLC are promising material combination for high work per volume actuation at a few hundred Hz. Actuation achieved by electrothermal buckling of a fixed-fixed structure was found to be superior to the bimaterial flexural ii actuators in delivering a large work per volume. A detailed comparison of the maximum achievable performance for different actuation schemes was made to facilitate the selection of actuators and the associated material choices for any application. The suitability of Al-Si bimaterial electrothermal actuators for low speed distributed flow control applications was assessed by comparing their performance with the more obviously suitable Si-PZT bimaterial piezoelectric actuators. A detailed processing route for microfabricating Al-Si3N4 bimaterial electrothermal actuators was developed and the associated micromachining issues were discussed. The experimental evaluation of the mechanical and thermal performance metrics of the microfabricated structures is expected to be accomplished in the future, for comparison with the analytical estimates and for subsequent validation by finite element analysis. The general framework of the materials selection strategy and the ranking of the potential candidates presented here will form a basis for the rational design of the MEMS actuators with an improved performance. The outcomes of this thesis also have set up an agenda for long term research goals which include exploration of novel actuator shapes/schemes, understanding of the process-property relations to tailor thin film properties and a comprehensive assessment of other novel substrate materials and their processes for MEMS actuator structures.

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ORCID for S.M. Spearing: orcid.org/0000-0002-3059-2014

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