Chip scale studies of BCB based polymer bonding for MEMS packaging

This paper presents chip scale investigation of polymer based bonding processes for producing microcavities for encapsulation of MEMS and other microscale devices. The polymer sealing rings were fabricated on the glass covers using the photosensitive BCB polymer and photolithography. They were bonded onto a variety of substrates including silicon substrates, aluminum coated substrates and glass substrates with feedthrough structures to investigate the properties of polymer bonding under different conditions. A high-yield BCB bonding procedure was established. For hotplate based bonding, a pre-bonding step is essential to ensure defect-free polymer seal. The optimized bonding process requires pre-bonding at 150degC for 5 minutes followed by final bonding at 250degC for 15 minutes. The capability of sealing feedthrough structures on a substrate of the BCB polymer was demonstrated using 0.5 mum thick titanium line structures. Fast bonding using laser assisted curing was achieved in less than 10 seconds, representing a factor of ~100 of improvement over the hotplate based approach. The quality of the polymer seal was evaluated using optical inspection and leak and shear tests.