Investigation of the hermeticity of BCB-sealed cavities for housing (RF-)MEMS devices

This paper reports on the hermeticity testing of MEMS cavities using BCB as the sealing and bonding material. Hermeticity has been tested according to the MIL-STD-883D. Gross leak testing based on liquid fluorocarbons revealed that BCB sealed cavities are leak tight, which means that the MEMS devices are well protected during handling and back-end processing (e.g., wafer dicing). Further, it is shown that the He fine leak testing of the MIL-STD is not fully applicable to small volumes (<1000 nl), typically encountered for MEMS. The problem is that the undefined regime normally existing in the MIL-STD is largely extended for small cavity volumes. Microbolometers have been used as test vehicles to confirm this. Large (>10,000 nl) cavities are needed to cover the entire leakage spectrum.