Multilayer micromachined bandpass filter for L/S band satellite communication systems

This paper presents the design and fabrication of a 2nd order L/S band filter used as a test vehicle for the development of a fabrication technology for cavity microwave filters based on micromachining in order to preliminary explore all the technological constraints on a simpler structure. The multilayered 2nd order pseudo-elliptic L/S band filter is based on λ/4 TEM mode resonators which are patterned on a dielectric layer. For convenience 500 μm thick Si wafers have been used even if this limits the simulated Q factor of the 2nd order L/S band filter to about 200. The test structures presented here amount to the more sophisticated 4th order filters in an extended technological concept (i.e. 1500 μm thick Si wafer and two additional modules) but still based on similar resonating elements aiming to replace the existing bulky metallic waveguide filters installed in many satellite transceivers. A five mask fabrication process is employed for the realization of the elements of said filter which is based on three modules. Module A and B are fabricated on the same wafer while module C which served as ground is fabricated on a separate wafer. A 2 μm high sealing ring is etched on the back of module A and B by DRIE (Deep Reactive Ion Etching) while cavities and TSVs (Through Silicon Vias) are etched by TMAH (TetraMethylAmmonium Hydroxide). The surface mounting compatibility of the filter is obtained by adopting vertical via holes to connect the external feeding lines (e.g. microstrip or coplanar) with the filter resonators. Such a transition separates the input/output from the filter input/output coupling mechanism. The final wafers are diced and specimens are vertically stacked and bonded through thermocompression bonding. The overall filter dimensions are 48x20x1.5 mm3.