Origami-Inspired Fabrication of High-Performance Wafer-Level Packaged Three-Dimensional Radio-Frequency Inductors

Abstract

Planar radio-frequency (RF) on-chip inductors suffer from large electromagnetic losses, stemming from the low resistivity silicon substrate. Though it is well-known that 3-dimensional (3D) MEMS-based RF inductors offer significant performance enhancement by reducing the substrate proximity effects, they are prone to mechanical failures and often come without any appropriate packaging, leading to reliability issues. Herein, we report an approach to fabricate highly robust packaged 3D RF inductors, while relying on the concept of stress-induced self-assembly. By leveraging the large residual stresses in evaporated chromium thin films at low thicknesses (below 50 nm), we assemble the planar copper coils coated with chromium nano-layers into out-of-plane folded RF inductors, directly improving their <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$</tex-math> </inline-formula> -factor. Through measurements, we show more than three-fold ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 300%) improvement in the inductor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$</tex-math> </inline-formula> -factor after the bending. Considering the inherent fragility of suspended inductors, we then add a dedicated wafer-level polymer packaging to our inductors, to improve their mechanical robustness. Specifically, we coat a thick SU-8 layer using a quasi-static volumetric dispensing process, to embed the inductors without deforming them. Upon baking, the polymer layer solidifies, making the buried inductors extremely rigid. We show that though our inductors have <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$</tex-math> </inline-formula> -factors similar to existing MEMS inductors, their low design complexity and high strength gained via polymer packaging make them stand out among the others. This packaging technique essentially paves the way for the widespread commercialization of suspended inductors, which has been hindered in the past by their poor mechanical reliability. 2023-0018