Study on a 3D packaging structure with benzocyclobutene as a dielectric layer for radio frequency application

A new wafer-level 3D packaging structure with benzocyclobutene (BCB) as interlayer dielectrics (ILDs) for multichip module fabrication is proposed for the application in radio frequency. The packaging structure consists of two layers of BCB films and three layers of metalized films, in which the monolithic microwave IC (MMIC), thin film resistors, strip lines and microstrip lines are integrated. Wet etched cavities fabricated on the silicon substrate are used for mounting the active and passive components. BCB layers cover on the components and serve as ILDs for interconnections. Gold bumps are used as electric interconnections between different layers, which eliminate the need of preparing vias by costly dry etching and deposition process. In order to get highly qualified BCB films for the subsequent chemical mechanical planarization (CMP) and multilayer metallization processes, the BCB curing profile is optimized and the roughness of the BCB film after CMP process is controlled lower than 10 nm. The thermal, mechanical and electrical properties of the packaging structure are investigated. The thermal resistance can be controlled below 2degC/W. The average shear strength of the gold bumps on the BCB surface is around 70 N/mm². The performances of MMIC and interconnection structure in high frequency are optimized and tested. The S-parameters curves of the packaged MMIC shift slightly showing perfect transmission character. The insertion loss (S₂₁) change after the packaging process is less than 1 db range at the operating frequency. And the return loss (S₁₁) is less than -8 dB from 10 GHz to 15 GHz.