A Robust Thin-Film Encapsulation for RF-MEMS With Aluminum Nitride/Molybdenum Cap

Wafer-level thin-film packaging offers high yield, low cost, and small packaging size and can be easily integrated into components within microelectromechanical systems (MEMS). In this work, a fabrication process of thin-film encapsulation (TFE) with aluminum nitride (AlN)/molybdenum (Mo) cap is proposed for the packaging of film bulk acoustic wave resonators and filters. The high Young’s modulus of AlN and the high toughness of Mo fully guarantee the stability of the packaging structure. By optimizing the sputtering parameters, AlN exhibits compressive stress, which compensates the tensile stress of Mo, resulting in an average stress distribution of 5.763 MPa in the capping layer, thereby avoiding the bending of the packaging structure. Besides, polyimide (HD8820) was spin-coated onto the capping layer to further improve the airtightness of the packaging. At the same time, network signal tests were conducted on the devices at every step of the packaging process to ensure that the packaging structure had minimal impact on device performance. Finally, the devices underwent the unbiased highly accelerated stress test (UHAST) (JESD22-A118B), resulting in only 0.2 dB of loss. This method may hold immense commercial value in enhancing the performance of radio frequency (RF) MEMS devices.