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

Abstract

This study aims at investigating a polymerbased airgap creation method for the packaging of microelectromechanical systems (MEMS), and exploring the chemical composition of the polymer residue on the final package. Polymerbased airgap formation utilizes thermal decomposition of a sacrificial polymer, poly(propylene carbonate) (PPC), encapsulated within an overcoat polymer. BCB (Cyclotene 402646) was used as the overcoat material because decomposition products of sacrificial polymer are able to permeate through it, leaving an embedded airgap structure around the MEMS device. Sizecompatibility and cleanliness of MEMS devices are important attributes of the polymerbased airgap MEMS packaging approach. This study provides a framework for sizecompatible and clean airgap formation by selecting the type of PPC, optimizing thermal treatment steps, identifying airgap formation options, assessing airgap formation performance, and analyzing the chemical composition of the residue. The airgap 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 airgaps. The polymerbased airgaps provide a monolithic, lowcost, integrated circuit compatible MEMS packaging option.