Characterization of Vertical Interconnects in 3-D System in a Package

A novel RF MEMS integration technology was developed to achieve the three-dimensional integration of microwave and millimeter-wave components and sub-circuits with improved performance using multilayer vertical interconnects. This technology, which allows planar circuits to be monolithically stacked vertically in three dimensions (3D), provides one of the major initial steps in the realization of a “system in a package.” To process high-aspect-ratio via interconnects in 3D MMIC multilayer circuitry, combination of unique microelectronic and traditional MEMS microfabrication technologies were used. Based on these techniques, a set of test structures were successfully fabricated to facilitate the vertical interconnect characterization. Experimental results revealed that at microwave frequencies, e.g. X band (8–12 GHz), the vertical interconnect discontinuities contributed significantly to the insertion loss and the phase change. With the available advanced conductor loss models, lumped-element equivalent circuit models were derived from the via module measurements. Using quarter wavelength T-junction resonator structures, polyimide was also characterized for its microwave properties over a wide frequency range. Multilayer vertically interconnected transmission line circuits were monolithically processed and used to verify the derived electrical models.