Encapsulated Sub-Millimeter Piezoresistive Accelerometers for Biomedical Applications

Abstract

Micromachined accelerometers have been introduced in the late 1970s and have been used in various applications. The applications range from inertial navigation and data logging in wells to body activity monitoring for pacemakers. Although the size of the accelerometers was sufficient for their applications, there were not many efforts in pushing the limits of accelerometer miniaturization. In this study, we utilized film deposition packaging technology and other modern microfabrication techniques to miniaturize the size and mass of the packaged accelerometers, two orders of magnitude smaller than any accelerometers ever reported. We used these ultra miniature accelerometers to offer sensing capabilities for biomedical applications which was not possible with any other means. A novel design of the accelerometer and packaging has been developed for miniaturization. The accelerometer consists of a proof mass suspended by a single high¬aspect-ratio beam attached to the substrate. Piezoresistors are implanted on the sidewall of the beam to sense the maximum stress applied on the beam. A thick layer of epitaxial silicon is deposited on the accelerometer to form a mechanically robust yet compact package. The new packaging method enables reduction in die area up to 70% compared to conventional wafer bonded package. A new polyimide flexible circuit is also developed to route the signals from the ultra-miniature accelerometers to a conventional package. The new technology is used in experimental biomedical applications. The accelerometer is evaluated as an implantable sound sensor for cochlear implants which can possibly replace the externally worn microphones. It is also used as an electrical stethoscope to measure respiratory and heart signal of neonatal mice. There are many other possible applications in the biomedical field such as imaging artifact reduction for live animal microendoscopy. This technology has the potential to open up new realms of motion sensing in the biomedical science and engineering.