Vertically Stacked Millimeter-Wave LTCC Filters With Cavity-Mode Suppression for System-in-Package Application

This article presents a series of vertically stacked millimeter-wave (mm-wave) bandpass filters (BPFs) based on low-temperature cofired ceramic (LTCC) technology. First, a vertically stacked coupling topology with multiple ground planes is proposed for LTCC filters. As its benefits, the unwanted electric coupling between adjacent resonators is avoided, while the electric coupling to ground planes is enhanced, contributing to higher stopband suppression levels. Moreover, over 50% size reduction is also realized by using the vertical topology. Second, a common issue of high-frequency harmonics induced by an LTCC-based parallel-plate cavity is addressed in this work. If multiple ground planes are employed in LTCC circuits, the cavity resonances will be excited, weakening the stopband performance seriously for LTCC filters. To this end, the LTCC-based parallel-plate cavity is analytically investigated, and the suppression techniques are accordingly proposed by miniaturizing, staggering, or notching the ground planes. For demonstration, two 27-GHz LTCC BPFs are designed, fabricated, and measured. The results indicate an extremely small size of 0.132 mm2, a small insertion loss (IL) of 1.32 dB, and a wide stopband are all realized.