Fully Micromachined Lifesize Cochlear Model

Abstract

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/Si 3 N 4 /SiO 2 /Si 3 N 4 ), 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 Si 3 N 4 . 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. LDV results show traveling waves with similarity to those seen in the insensitive cochlea. The device responds at high frequencies (10-70 kHz) due to the low compliance of the BM (residual stress is 500MPa). The magnitude of the observed displacements (0.2-0.5 nm/Pa) is smaller than those observed physiologically, due to the lack of a middle ear, lower BM compliance, and lack of any OHC-like structures. A complete description of a similar device including mathematical models can be found in a recent work by the authors [4].

Toward a neuromorphic microphone

“…Pressure waves are applied hydro-mechanically, and the movement is read using a laser vibrometer. White and Grosh ( 2005 ) test a silicon nitride (Si 3 N 4 ) membrane, using vapor deposition: in 2006 (White and Grosh, 2006 ) built a BM from a stacked thin film structure using Si 3 N 4 /SiO 2 /Si 3 N 4 with additional metallization (Au and Cr) to enable optical detection of movement. In 2008, White et al ( 2008 ) report using the composite membranes with a fluid-filled chamber.…”

Microfabrication of coupled fluid–structure systems with applications in acoustic sensing

“…Development of the fabrication process was motivated by the authors' ongoing efforts to produce lifesize micromachined cochlear models and biomimetic cochlear-like sensors [1][2][3]. In order to accomplish this task, a fabrication process was required to produce a deep (wafer thick), fluid-filled chamber coupled to an exponentially tapered membrane.…”

“…In addition, multiple sensing structures needed to be integrated into the structure to measure vibration of the membrane in response to traveling fluid-structure waves. For additional details on hydromechanical cochlear models, see work by the authors [1][2][3] and other researchers [4][5][6][7].…”