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 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. Sizecompatibility and cleanliness of MEMS devices are important attributes of the polymerbased 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.

Decomposable and Template Polymers: Fundamentals and Applications

“…Polycarbonates are ideal thermally decomposable sacrificial polymers mainly owing to their favorable decomposition range (200-300 C) and nearly residue-free decomposition in both inert (N 2 or Ar) and oxygen-rich environments [27,29]. Various types of polycarbonates have been studied including poly(ethylene carbonate), poly(propylene carbonate) (PPC), poly(butylene carbonate) (PBC), poly(cyclohexene carbonate) (PCHC), poly(norbornene carbonate) (PNC), and poly(cyclohexene propylene carbonate) (PCPC) [20,27,30].…”

“…The polyether content in PPC backbone can be analyzed by 1 H-NMR. The molar concentration of polyether linkages in commercially available PPC is generally less than 10% and can be as low as 1% [29]. High-purity PPC leads to residues of less than 1% of the original film thickness.…”

“…The etch rate of BCB is significantly less than the etch rate of PPC in an O 2 plasma. The etch selectivity is about 1:36 (BCB:PPC) in an O 2 plasma (100 sccm O 2 flow rate at 300 mTorr and 25 C with 250 W alternating current power) [29]. The patterned PPC was encased by a thick, spin-coated layer of BCB.…”

Sunlight photodepolymerization of transient polymers

“…Decomposable and template polymers are of use in the fabrication of integrated circuits and packages . Polymers that can be triggered to depolymerize or decompose into volatile, monomeric units are useful as template materials for the creation of embedded‐air cavities . An emerging application for decomposable polymers is the phototriggered depolymerization of the polymer leading to the controlled vaporization and/or liquefaction of the component so as to eliminate device recovery .…”

Grafted Epoxide Functionalized Polypropylene Carbonate Porogen for Low Dielectric Constant Epoxy Films

“…29,30 In this study, it has been shown that nanoporous epoxy resins can be formed by first grafting the epoxide functionalized PPC polyol to a polymer backbone before thermal curing with the epoxy resin. Selective decomposition of the functionalized PPC polyol leads to the formation of low dielectric constant nanoporous epoxy films.…”