Thermal vertical bimorph actuators and their applications

Abstract

In this thesis, a novel concept for lateral actuators based on vertical bimorphs is presented. Vertical bimorphs consist of silicon beams side-coated with aluminium, which bend when heated due to the different thermal expansion coefficients of the two materials causing a displacement in the wafer plane. The heating of the actuator is provided by an electrical current through the silicon beam. The simplest implementation of a vertical bimorph actuator is a clamped-clamped beam. To, obtain higher deflections, a meander shaped actuator has been designed. By combining four meander actuators, a two-dimensional positioning stage has been realised. The meander actuator has also been applied for normally closed and normally open micro-relays.

Analytical calculations and ANSYS simulations have been carried out to predict the physical behaviour of the bimorph devices, including temperature distribution, static deflection, vertical stiffness, thermal time constant and lateral resonances. For both the clamped-clamped beam actuator and the one-dimensional meander actuator the above parameters have been measured on actual devices showing reasonable to good agreement with the theoretical results. The measurements have been carried out in an SEM, which provided a vacuum environment and enabled measurements using a cursor.

The fabrication required development of non-standard processes, including aluminium deposition onto the sidewalls of silicon beams by angled evaporation and wet-etching, chrome/gold sidewall deposition into contact gaps for micro-relays, photolithography on wafers with 30 μm deep trenches, and a release etch process based on dry-etching. The batch fabrication for the vertical bimorph actuators uses silicon on insulator substrates, deep reactive ion etching for the definition of the silicon beams, gold deposition for the contacts, the aluminium sidewall deposition process for the bimorphs and a release etch based on dry-etching of silicon.

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