All-silicon micromachined acoustic ejector arrays for micro propulsion and flow control.

Abstract

This research reports a high-density all-silicon micromachined acoustic ejector (MACE) array utilizing forced Helmholtz resonators. The devices are coupled with an acoustic ejector for the generation of high-speed micro-jets needed in applications such as micro propulsion, jet cooling, and pumping. The ultimate goal in developing such an acoustic ejector is in generating vertical thrust sufficient for lifting and hovering miniature platforms. To realize the proposed MACE structure, a novel three-dimensional MEMS fabrication technology is developed. The MACE array, consisting of four Helmholtz resonators surrounding an ejector hole, is formed by bonding two pre-processed micromachined silicon wafers, one supporting a diaphragm and the other a drive electrode. Fabrication is performed using a low-temperature localized wafer bonding with BCB (Benzocyclobutene) in combination with DRIE (deep reactive ion etch) and 3D micromachining. Actuation of the diaphragm is achieved using electrostatic drive utilizing thick perforated electrode under the diaphragm. At acoustic resonance, the device is capable of producing air jets with a velocity of at least a few meters per second. A reduced-order numerical model for the forced Helmholtz resonator is used as a guideline for the design optimization. Theoretical and finite-element analysis techniques are also used to understand and optimize the device structural behavior. Perforation on the drive electrode has been carefully designed based on theoretical analysis to minimize all damping effects. A high-density (23 units/cm<super>2</super>) all-silicon MACE array has been designed and fabricated with a yield as high as 90%. Detailed testing and characterization through interferometry, hot-wire anemometry, and flow visualization have been performed to verify the theory and design of the micromachined forced Helmholtz resonator. MACE (&sim;1.6mm x 1.6mm x 1 mm) with air jet velocity >1 m/sec has been tested using hot-wire anemometry at a distance of 560mum from the resonator throat at an actuation frequency of &sim;70kHz. Flow entrainment and visualization have demonstrated a jet column longer than 10cm. The fabricated MACE has demonstrated a thrust of 1.35muN per ejector/resonator. It also has demonstrated a gas pumping rate of &sim;54 ml/min per ejector. A MACE chip with 20 ejectors has been tested as an active heat sink and shows a cooling flux of >600W/m<super>2 </super> at a distance of 1 cm from a heated chip (&sim;100&deg;C). The device has consistently operated for more than 6 billion cycles without failures. To further improve MACE performance, a planar fabrication technology for constructing out-of-plane curved surfaces in silicon with an arbitrary profile using a single masking and etching step has also been developed. This technology has been designed and used to fabricate an out-of-plane curved drive electrode for MACE actuation. The new design has demonstrated that the jet velocity is increased by a factor of 2 without collapsing the diaphragm. The equivalent thrust output has been increased by 4 times. By incorporating an ejector shroud into MACE, it is believed that the MACE chip could generate sufficient thrust for levitation.