Size-Compatible, Polymer-Based Air-Gap Formation Processes, and Polymer Residue Analysis for Wafer-Level MEMS Packaging Applications

This study aims at investigating a polymer-based air-gap creation method for the packaging of microelectromechanical systems (MEMS), and exploring the chemical composition of the polymer residue on the final package. Polymer-based air-gap formation utilizes thermal decomposition of a sacrificial polymer, poly(propylene carbonate) (PPC), encapsulated within an overcoat polymer. BCB (Cyclotene 4026-46) was used as the overcoat material because decomposition products of sacrificial polymer are able to permeate through it, leaving an embedded air-gap structure around the MEMS device. Size-compatibility and cleanliness of MEMS devices are important attributes of the polymer-based air-gap MEMS packaging approach. This study provides a framework for size-compatible and clean air-gap formation by selecting the type of PPC, optimizing thermal treatment steps, identifying air-gap formation options, assessing air-gap formation performance, and analyzing the chemical composition of the residue. The air-gap formation processes using photosensitive PPC films had at least twice the residue compared to processes using nonphotosensitive PPC films. The major contribution to the residue in photosensitive PPC films was from the photoacid generator (PAG), which was used to catalyze the thermal decomposition of the PPC. BCB is compatible with PPC, and provides mechanical stability during creation of the air-gaps. The polymer-based air-gaps provide a monolithic, low-cost, integrated circuit compatible MEMS packaging option.