Design and Fabrication of AlN Bulk Acoustic Resonator for mm-Wave 5G RF Filter Applications

Millimeter-wave 5G RF filters are becoming increasingly crucial for future wireless communication applications. In this work, we present the design and fabrication of a bulk acoustic wave (BAW) resonator tailored for high-frequency applications. The resonator adopts a concise sandwich structure comprising a suspended bottom electrode, an aluminum nitride (AIN) piezoelectric layer, and a top electrode, enabling a straightforward fabrication process. We conducted detailed simulation analyses of the frequency characteristics of this structure with core area of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10 \mu \mathrm{m} \times 10 \mu \mathrm{m}$</tex>, considering factors such as the metal electrode materials and device dimensions. The results demonstrate the BAW resonator's efficient resonant characteristics at frequencies exceeding 30 GHz. Additionally, we constructed a mm-Wave 5 G RF filter with a center frequency of 40.5 GHz and a relative attenuation exceeding 16 dB with only two small resonators by combining the modified Butterworth-van Dyke (mBVD) model with RF circuit simulation tools. The core area of the filter is merely <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$425 \mu^{2}$</tex>. This can validate the feasibility of constructing a simple and high-performance mm-Wave 5G RF filter using the designed resonators and offer a new co-design strategy for SAW/BAW devices and filter circuits. Finally, we developed a simple and innovative planar process flow to fabricate the BAW resonator and characterized the fabricated devices. The core area measured only <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$18 \mu ~\mathrm{m} \times 18 \mu ~\mathrm{m}$</tex>.