Single-ended-to-differential and differential-to-differential channel-select filters based on piezoelectric AlN contour-mode MEMS resonators

This paper reports on the first demonstration of single-ended-to-differential and differential-to-differential (S2D and D2D) channel-select filters based on single-layer (SL) and dual-layer-stacked (DLS) AlN contour-mode MEMS resonators. The key filter performances in terms of insertion loss (as low as 1.4 dB), operating frequency (250-1280 MHz), and out-of-band rejection (up to 60 dB) constitute a significant advancement over all other state-of-the-art RF MEMS technologies. The fabrication process, namely stacking of two piezoelectric AlN layers (600 nm each) and three Pt electrode layers (100 nm each), is fully compatible with the previously demonstrated AlN RF MEMS switch process (also post-CMOS compatible), which makes it possible to implement multi-frequency switchable filter banks on a single chip. The S2D configuration is also able to combine the balun, filter, and impedance transformer functions in a single MEMS structure and only takes on a very small form factor (60 × 200 μm). These unique features will potentially revolutionize the field of RF and microwave IC design by enabling MEMS-IC co-design and the development of unconventional and low-power RF architectures.