Substrate Loss Mechanism and Performance Enhancement via HR-Si of BAW Devices

The increase of application scenarios with high data throughput and the communication protocol of WiFi 7 have promoted the rapid development of high-frequency and wideband filters. However, the substrate loss of the bulk acoustic wave (BAW) resonators increases at high operating frequency, leading to degraded device performance. In this work, the mechanism of substrate loss was studied by the Multiphysics finite-element analysis (FEA). In addition, substrate loss was introduced into the modified Mason model to improve the accuracy of the simulated results. Furthermore, high-resistance silicon (HR-Si) was used to prepare high-performance Al<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>0.87</roman></sub>Sc<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>0.13</roman></sub>N BAW resonators with 10.09% of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>eff</i></sub><sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>2</roman></sup>, 1451 of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>max</roman></sub>-factor, and 146 of figure of merit (FOM). The BAW filters with a center frequency of 5.6 GHz were fabricated. The BAW filter explained the minimum insertion loss of -1.08 dB, a 3-dB bandwidth over 366 MHz, and exceeded -31 dB of out-of-band rejection. This study elucidates the physical mechanism of substrate loss through two simulation methods and offers valuable insights for the design and fabrication of high-frequency BAW filters, achieved through the modified Masson model and the suppression of substrate loss.