Hermetic packaging and bonding technologies for implantable microsystems.

Abstract

The main objective of this research has been to develop a wafer-level hermetically-sealed package for a variety of MEMS devices, and especially for long-term use in bioimplantable microsystems. The research has: (1) developed two passive wireless humidity sensors; (2) produced long-term hermeticity and corrosion test results on glass-Si bonded micropackages; (3) demonstrated long-term hermeticity and biocompatibility tests in biological hosts, and (4) developed a novel low-temperature glass-to-Si wafer-level package technology. Micropackages formed by anodically bonding a glass cap to a silicon substrate for use in an implantable neuromuscular stimulator were soaked in phosphate buffered saline (PBS) at 85&deg;C and 95&deg;C, and have demonstrated an estimated lifetime >177 years at 37&deg;C. Control packages soaking in 25&deg;C PBS have maintained hermeticity for >10.8 years. This research also led to the development of two corrosion prevention techniques, Au galvanic biasing and degenerate Boron doping, to reduce the dissolution rate of silicon in PBS by more than two orders of magnitude thereby increasing package lifetime and reliability. Hybrid and integrated passive wireless humidity sensors have been developed for monitoring package hermeticity. These &sim;1.5x8mm<super>2</super> humidity sensors have a resolution of &sim;2.0%RH and a wireless range of &sim;2cm and provide a simple noninvasive wireless method for in-vitro and in-vivo package testing. General biocompatibility data from packages implanted in multiple locations of animal hosts for up to 2-years and explanted surrounding tissues demonstrate that these packages biocompatible and maintained hermeticity. Micropackages were also formed using a new and novel glass-to-solder-to-Si wafer bonding technique. It is found that by applying an anodic bias across a glass-to-Au-Si eutectic interface one can achieve robust and uniform bonds. This new technique makes a significant contribution to wafer bonding by providing a technique that is low temperature &sim;370&deg;C, provides hermetic sealing, and can planarize over wafer topology. The leak rate of air into these packages is 1.3x10<super>-14</super>[atm*cc/sec] which is well below that needed to qualify hermeticity. Test results from a packaged microcontroller chip and MEMS devices demonstrate IC and MEMS compatibility. This work makes significant advancement in the field of microsystems packaging and provides two packaging technologies that are ready for application trials.