Microactuation using Wafer-level Integrated SMA Wires
Micro-aandrijving door middel van vormgeheugendraden geïntegreerd op wafer-niveau
Clausi, Donato; S0197560
The work density of shape memory a lloy (SMA) exceeds that of other actuation mechani sms available at the microscale by at least one or der of magnitude and remains constant upon miniatu rization, thus making SMA actuators attractive for applications requiring high forces and large displacements. Despite its favorable charact eristics, full exploitation of SMA in Micro E lectro-Mechanical Systems (MEMS) has been hindered , partially because of a lack of cost-efficient an d robust integration methods. This thesis addresse s this issue and presents novel manufacturing sche mes for fabricating MEMS actuators using wafer-lev el integration of SMA wires to silicon struct ures. The SMA wires are integrated to a rrays of micromachined single crystalline sil icon cantilevers, which provide the cold state res et mechanism. The wires are eccentrically placed o nto the cantilever beams, thus providing ecce ntric loading of the cantilevers and resulting int o out-of-plane deflection. Tensioning of NiT i wires, alignment and placement to the wafer ¨is performed using specially built wire fram e tools and a coarse adhesive bonding approach.Anchoring of the wires to the individual devices is demonstrated using two different m ethods: the first method is based on adhesive bond ing of the SMA wires to the array of micromac hined cantilevers using a negative photoresist (SU -8) and the actuators are operated using an extern al energy source. The second method uses elec troplating to form nickel anchors that both provid e a strong mechanical coupling between the SMA wir es and the silicon microstructures and that serve¨ as electrical contact pads. The fabricated actuato rs are operated using Joule heating of the SM A wires and exhibit both a high reliability d uring long term cycling and displacements that are ¨among the highest yet reported. High-flow g as valves serve as a case study to illus trate the benefits of the developed actu ators with respect to the current state-of-the-art . A new design for knife gate microvalves is presented and the first batch-manufactured si licon valves using SMA wires for actuation ar e fabricated. These microvalves allow controlling¨ high gas flows at a low voltage actuation and at l ow power consumption.