The Fraunhofer Institute for Silicon Technology (ISIT) is engaged in the production of sensors and functional groups in the field of micro system technology. Complex micro mechanical devices need to be encapsulated with an assembly technology that features hermetic sealing and low mechanical stress. To encapsulate many identical chips within one process step, different waferbonding technologies can be applied depending on the actual system requirements. ISIT particularly bets on advanced metallic bonding processes, which work already at low bonding temperatures and are tolerant against wafer surface topography and scratches. The narrow sealing frames enable a high component density on the wafer, but exhibit a high strength despite. Additionally, the metallic wafer bonding forms an electrical contact to the cap, hence the electrical potential can be defined there without any supplementary wire bond in the following assembly process.
ISIT offers AuSn wafer bonding and eutectic AuSi- (or AuSiGe-) wafer bonding processes, both industrially qualified and documented via a production management system. These processes issue a sealing with a low and long-term stable stress level. Following a component calibration, sensors with extremely low drift error can therefore be manufactured. - In addition to eutectic bonding, ISIT provides anodic bonding with and without intermediate layer as a standard process. Classical wafer direct bonding with tempering is also available, but requires wafer surfaces of excellent quality.
The state-of-the-art infrastructure is configured for 200 mm wafers. Three eutectic waferbonders are available, two of them are also suited for anodic bonding. A special feature is the possibility of simultaneous anodic bonding of three wafers in the combination Glass-Si-Glass or Si-Glass-Si (in development).
The wafer bond processes are characterised by an excellent control of the atmosphere in the sensor cavity. Depending on the sensor specification, it is possible to define gas pressures in a range of approx. 10-4 mbar to 2000 mbar. The application of structured getter films stabilises the dampening atmosphere against outgassing effects for up to 20 years. The leak rate of resonant micro sensorsis verified by a Neon-ultra-fine-leak-test for each individual device. This test is patented by ISIT and will be incorporated into the new SEMI standard for hermeticity testing of micro components. By its application in a pilot production, a sealing yield of > 96% has been proven.
In future, not only passive caps will be manufactured, but also caps with sensory and actory elements. These elements have to be electrically connected using vertical micro contacts. First experiments at ISIT have shown the feasibility using the conventional eutectic bond process.