This paper reports the development of a batchfabrication technology for constructing out-of-plane curved surfaces in silicon using a single masking and etching step. With the well-characterized Deep Reactive Ion Etch (DRIE) lag effect in silicon, one can create an arbitrary out-of-plane curved profile by adjusting the trench etch openings. By using trenches ranging from 2pm to 50pm wide, a smooth curved surface (1200pmx1200pm) with maximum depth more than 15pm has been fabricated. The proposed technology is especially attractive for the fabrication of electrostatic MEMS actuators. A test curved drive electrode for vertical electrostatic actuators is also fabricated.

The developed bonding process utilizes AuSn solder and provides liquid-proof sealing and multiple reliable electrical connections between the bonded wafers. The bond can withstand 300ºC and features a thin bond line (2-3 µm), high bond strength, excellent bond gap control, and low stress due to small amount of bonding material. A Nb/Au seed layer was shown to be an optimal adhesion and barrier film.

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The performance of synthetic jets produced by an electrostatically driven acoustic resonator is discussed. A reduced order model of the coupled membrane motion and acoustic field in the resonator is used to determine thrust and power consumption. The coupled system presents unique features depending on the pressure coupling parameter defined as the ratio of the pressure force on the membrane to its inertia. Thrust output increases as the pressure coupling parameter decreases and for stiffer membranes. Theoretical thrust performance of the order of 50 µN per resonator, and power to thrust ratio in the range 20-60 m/s could be achieved. We report also progress in the development of a micro propulsion system based on synthetic jets. Measurements of the membrane deformation and flow at the exit of the synthetic jets are reported.

High speed micro-jets produced by acoustic streaming can be used for micro propulsion in miniature airborne vehicles. A wafer-level technology was developed to fabricate hundreds of resonators to form these jets on a 4-inch silicon wafer. In this paper, modeling and full characterization of these jets is presented. The performance of electrostatic resonators was tested by laser interferometry, video particle imaging and hotwire anemometry. The occurrence of non-linear streaming phenomenon and jet formation was verified by particle imaging. The effect of various design parameters such as throat size and perforation geometry on jet performance was investigated and an optimum experimental design was identifi'zd. Jet velocities as high as I d s were measured and by spatial investigation of the velocity field, the micro jet stream along and away from the centerline was measured and profiled. A coupled equivalent circuit that models the electrostatic drive and acoustic streaming is developed and shown to closely match experimental results.