Wafer-Level Packaging of Microelectromechanical Systems Based on Frame Structure

Abstract

Modern microelectromechanical systems (MEMS) are devices that incorporate microelectronic components and micromechanical structures on a single chip. Packaging is a mandatory stage in MEMS manufacturing. It ensures mechanical protection, sealing and transmission of electric energy and signals. The present work was aimed at developing a MEMS packaging method as a part of the consolidated manufacturing process. The method is developed on the example of a microwave MEMS switch. The switch manufacturing scheme includes conventional technologies used for producing gallium arsenide integrated circuits: optical lithography, liquid etching, electron-beam and magnetron deposition of metallic, resistive and dielectric films. The work presents a new inter-plate MEMS packaging based on a frame structure with a passivating film. The main purpose of the package frame layer is mechanical support for an upper layer of the sealing material. The frame layer should have the structure allowing for unimpeded removal of the sacrificial photoresist and be impermeable for the sealant. To satisfy the requirements stated, a metallic thin copper-film spatial frame was fabricated by galvanic deposition. The frame structure is a geodesic dome comprised of a complex network of triangle cells arranged in rows. The connected triangles create a self-supporting durable framework. The measurement and modeling results demonstrate that the round frame structure is more durable than a square frame with the same maximum cell dimensions. The stress-strain state for the round framework considerably alters depending on the number of rows of triangle cells. In addition to the mechanical support, the cell structure of the framework – with adequate selection of cell dimensions, solvent and sealant viscosities – allows for unimpeded penetration of the solvent (N-methyl-2-pyrrolidone, NMP) and removal of ma-P1225 photoresist sacrificial layers. At the same time, the layer structure is impermeable for the sealant (benzocyclobutene, BCB). The proposed MEMS switch packaging enables mass fabrication of GaAs integrated circuits in a single process, which expands their frequency range. The new plate-level packaging technology is absolutely compatible with MEMS fabrication technology without specific materials and equipment which reduces the dimensions and cost of MEMS.