Design and packaging of an iron-gallium (Galfenol) nanowire acoustic sensor for underwater applications

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A novel acoustic sensor incorporating cilia-like nanowires made of magnetostrictive iron-gallium (Galfenol) alloy has been designed and fabricated using micromachining techniques. The sensor and its package design are analogous to the structural design and the transduction process of a human-ear cochlea. The nanowires are sandwiched between a flexible membrane and a fixed membrane similar to the cilia between basilar and tectorial membranes in the cochlea. The stress induced in the nanowires due to the motion of the flexible membrane in response to acoustic waves results in a change in the magnetic flux in the nanowires. These changes in the magnetic flux are converted into electrical voltage changes by a GMR (giant magnetoresistive) sensor. As the acoustic sensor is designed for underwater applications, packaging is a key issue for the effective working of this sensor. A good package should provide a suitably protective environment to the sensor, while allowing sound waves to reach the sensing element with a minimal attenuation. In this thesis, design efforts aimed at producing this MEMS bio-inspired acoustic transducer have been detailed along with the process sequence for its fabrication. Package materials including encapsulants and filler fluids have been identified based on their acoustic performance in water by conducting several experiments to compare their impedance and attenuation characteristics and moisture absorption properties. Preliminary test results of the sensor without nanowires demonstrate the process is practical for constructing a nanowire based acoustic sensor, yielding potential benefits for SONAR applications and hearing implants.