Mechanical and electrical characterization of BCB as a bond and seal material for cavities housing (RF-)MEMS devices

Abstract

This paper reports on the mechanical and electrical characterization of benzo-cyclo-butene (BCB) as a bonding and sealing material for 0-level packages (cavities) housing (RF-)MEMS devices. Shear strength and hermeticity of BCB-sealed cavities are experimentally investigated as functions of the geometrical parameters of the BCB sealing ring and the bonding conditions. The leak rate of BCB-sealed cavities strongly depends on the BCB width, and leak rates as low as 10−11 mbar l s−1 are measured for large BCB widths (>800 µm), dropping to 10−8 mbar l s−1 for BCB widths of around 100 µm. Depending on the bonding conditions, shear strengths as high as 150 MPa are achieved. BCB is also used in 0-level packaging of RF-MEMS devices, such as RF-switches and coplanar waveguides (CPWs). The electrical influence of the 0-level package is studied for different capping materials. It is experimentally shown that a 0-level package using capping chips made of low-loss high-resistivity materials (AF45 glass and high-resistivity silicon) and having a cavity height larger than about 45 µm above RF-MEMS devices, has a negligible impact on the microwave characteristics of an RF-MEMS device. Finally, some reliability testing is performed on BCB-sealed 0-level packages in order to study the influence of temperature and humidity on the mechanical properties of BCB. After testing in relatively harsh conditions, the BCB seal stays gross leak tight and shear strengths as high as 30 MPa are measured. BCB turns out to be a very robust and reliable material to encapsulate MEMS devices.