Research Summaries

Back Sputtering system for MEMS Directional Acoustic Sensor Microfabrication

Fiscal Year 2020
Division Graduate School of Engineering & Applied Science
Department Physics
Investigator(s) Alves, Fabio D.
Sponsor Office of Naval Research (Navy)
Summary Our group at the Naval Postgraduate School (NPS) has been conducting research on micro-electromechanical system (MEMS) directional sound sensors based on Ormia Ochracea fly's hearing system. It is a parasitic fly, evolved to accurately locate a particular cricket chirp, for the purpose of laying eggs in the cricket. In contrast, to determine sound direction, humans use the difference in arrival time of a sound wave and sound pressure field between the two ears. This is a successful strategy for an animal with ear spacing comparable to the incident sound wavelength. Unlike humans, the fly has two eardrums connected by a cartilaginous bridge that can be modeled as a coupled mechanical oscillator with two resonant modes. The cricket chirp sound wavelength is almost two orders of magnitude larger than the fly's eardrum separation. The biomimetic MEMS directional sound sensor consists of two wings, connected in the middle by a bridge. At the edge of each wing, interdigitated comb finger capacitors were incorporated to allow the transduction of the mechanical vibration of the wings under sound excitation into processable electronic signals. The achieved directional resolution approaches that of the fly. Potential applications of the MEMS directional sound sensors, include locating snipers, boats and UAVs as well weapons and vessels in undersea environment. These sensors have extremely low form factor, lower power consumption, higher sensitivity and lower cost than the available alternatives. Up to this point, design and characterization were performed at NPS facilities, while the microfabrication was outsourced to commercial multi-wafer/multi-university foundries. As the research expands, more sophisticated designs are required and the limitations imposed by the design rules of the commercial foundries are no longer acceptable. Furthermore, the downtime due to few fabrication runs offered throughout the year imposes severe delays. To expand the utility of the MEMS sensors and reach performances beyond the current state-of-the-art, novel structural and functional characteristics, not achievable using the accessible commercial foundries, are required. Custom microfabrication fabrication must be performed. In the past 15 years, NPS invested significantly to the microfabrication facility, which currently has a formidable capability in terms of substrate preparation and characterization, photolithography, metal and dielectric deposition, wet and dry etching and more. It is now possible to carry out full microfabrication, consisting of film deposition and surface micromachining to define MEMS structures and bulk micromachining, used to release the MEMS structures. Nevertheless, NPS facilities lack the capability to grow piezoelectric films, deposit heavy metals such as titanium and platinum, produce metallic alloys and metal/dielectric compounds. This capability, crucial for the advancement of our biomimetic MEMS directional acoustic sensor could be achieved with a single tool, a sputtering instrument, the subject of our proposal. With automated deposition capability, the proposed deposition tool uses RF, standard DC, and reactive pulsed-DC sputtering of metals and ceramics, including metal nitrides, to achieve large deposition versatility, allowing for extremely accurate thickness and uniformity control. The proposed system will greatly expand our capabilities and allow in-house fabrication of sensors with advanced topologies, material composition and transduction mechanisms, such as piezoelectric. In addition, the design-fabrication-characterization time will be greatly reduced. The proposed sputtering system will directly support the current research programs sponsored by ONR, NPS-ONR (NRP) and NPS-ONR (CRUSER). Furthermore, this instrument will be highly valuable for the two MEMS classes taught in the Graduate School of Engineering and Applied Sciences
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Data Publications, theses (not shown) and data repositories will be added to the portal record when information is available in FAIRS and brought back to the portal