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FULLY MICROMACHINED LIFESIZE COCHLEAR MODEL

A life-size hydrodynamical cochlear model is demonstrated. The structure is fully micromachined and suitable for batch fabrication. Laser Doppler Velocimetry (LDV) measurements show cochlear-like traveling fluid structure waves with a phase lag of approximately 3 cycles (6π radians) and displacement magnitude of 0.2-0.5 nm/Pa at the location of maximum response. The device responds in the 10-70 kHz band. To date, three groups have reported the construction of life-sized physical cochlear models [1,2,3]. These systems use a polymer membrane on a micromachined support as the “basilar membrane” (BM), and use casting, milling, or small-scale electro-discharge machining (EDM) to create two fluid ducts on either side of the membrane. The device described in this paper is fully micromachined, has fluid on only one side of the BM, and is driven by airborne sound. The BM is 3 cm long, made of a stacked thin film structure (Au/Cr/Si3N4/SiO2/Si3N4), and tapers exponentially in width from 170 μm to 1.9 mm. The three-layer dielectric is used to partially compensate the high tensile residual stress in the Si3N4. The metals are for optical reflectance. The fluid duct is 6.25 mm wide, 0.5 mm high, and filled with 200 cSt silicone oil. The high viscosity fluid was needed to suppress the formation of standing waves. Figure 1 shows a drawing of the device and the fabrication process.