Nanoporous low-dielectric constant polyimide films via poly(amic acid)s with RAFT-graft copolymerized methyl methacrylate side chains

Abstract

Poly[N,N′-(1,4-phenylene)-3,3′4,4′- benzophenonetetracarboxylic amic acid] (PAmA) with grafted poly(methyl methacrylate) (PMMA) side chains (PAmA-g-PMMA) was synthesized via thermally induced graft copolymerization of methyl methacrylate (MMA) with ozone-pretreated PAmA in the reversible addition-fragmentation chain-transfer (RAFT)-mediated process. The graft copolymers were characterized by nuclear magnetic resonance (NMR), elemental analysis, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and molecular weight measurements. The "living" character of the grafted PMMA side chains was ascertained in the subsequent extension of the PMMA side chains. The nanoporous low dielectric constant (low-κ) polyimide (PI) films were obtained by thermal imidization of the PAmA-g-PMMA films in argon, followed by thermal decomposition of the PMMA side chains in air. The nanoporous PI films obtained from the RAFT-mediated PAmA-g-PMMA had well-preserved PI backbones, porosity in the range of 5-20%, and pore size in the range of 5-15 nm. The pores were smaller, and the pore size distribution was more uniform than those of the corresponding nanoporous PI films obtained via graft copolymers from the conventional free radical process. Dielectric constants approaching 2.1 were obtained for the nanoporous PI film with a porosity of about 20%.