Low temperature wafer scale MEMS packaging and intrapackage humidity monitoring

As Micro-Electro-Mechanical Systems technology is increasingly adopted into commercial products, packaging of MEMS is becoming more important for performance and cost reasons. To improve reliability and to reduce manufacturing costs, this research focused on novel low-temperature wafer-scale packaging and intrapackage humidity monitoring.; First, a hard-cured BCB bonding method was developed for low-temperature wafer-scale packaging compatible with bulk-micromachining techniques. The concept of our hard-cured BCB bonding is that a hard-cured BCB bonding layer is transferred onto a MEMS device wafer and the MEMS devices are then encapsulated at the wafer scale using the transferred BCB layer. The compatibility with common bulk-micromachining processes and the bondability of the hard-cured BCB bonding layer are major issues. For the compatibility issue, the BCB delamination and the undercut in TMAH wet etching decrease as the degree of BCB curing increases with a low-etch rate underlying layer. The BCB process is compatible with DRIE without any process alteration. For the bondability issue, our hard-cured BCB bonding showed no void formation at 2 bar in bulk bonding and no BCB overflow to microstructures in patterned bonding. In four-point bending tests and debonding tests, the measured critical adhesion energy was a little lower than that of normal BCB bonding and the bond between hard-cured BCB films was stronger than that of a BCB/AP/Si interface.; Second, a novel intrapackage humidity monitoring system and a mathematical model were developed for analyzing moisture diffusion into MEMS packages. The theoretical simulations and the experimental results for geometric parameters show that the seal width of packages is the dominant factor in moisture diffusion, regardless of the seal thickness and the volume of internal cavities. Moisture diffusion into MEMS packages can be mitigated by cavity formation inside the packages, depending on the volume. As a result, our intrapackage humidity monitoring method is a cost-effective solution for the analysis of moisture diffusion.