MATERIALS AND INTERFACES Nanoporous Low-Dielectric Constant Polyimide Films via Poly(amic acid)s with RAFT-Graft Copolymerized Methyl Methacrylate Side Chains G. D. Fu, B. Y. Zong, E. T. Kang,* and K. G. Neoh Department of Chemical and Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 119260 C. C. Lin and D. J. Liaw Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 106 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%. 1. Introduction The use of ultralow dielectric constant (ultralow κ) interlayer materials can greatly reduce the resistance- capacitance time delay, cross-talk, and power dissipa- tion in the new generation of high-density integrated circuits. 1-4 In addition to exhibiting low dielectric constants, the next generation of interlayer dielectrics for submicron and nano-level electronics must also satisfy a variety of requirements, such as chemical inertness, good thermal stability, low moisture adsorp- tion, and good adhesion to semiconductor and metal substrates. Polyimides (PIs) have been widely used as dielectric and packaging materials in the microelectronic industry because of their unique physicochemical properties: excellent thermal stability, good radiation and chemical resistance, good mechanical strength, low moisture adsorption, and good adhesion to semiconductor and metal substrates. 5-8 However, with dielectric constants (κ’s) of about 3.1-3.5, the conventional PIs are insuf- ficient in meeting the requirement of κ < 2.5 for the dielectrics of the near future. 1 In recent years, the introduction of air gaps into interconnect structures 9,10 and nanopores into polymers 11-13 to reduce their dielec- tric constants has been demonstrated. The incorporating of air, which has a dielectric constant of about 1, can greatly reduce the dielectric constant of the resulting porous structure. The approaches to preparation of porous PI films include microwave processing 14 and incorporation of foaming agents 15,16 and hollow micro- sphere. 17,18 A more recent approach to the preparation of porous low-κ PIs is through the creation of voids by thermal degradation of the labile block or graft chains in the PI copolymers. 19-22 Other related fine works include the preparation of nanoporous low dielectric constant poly(silsesquioxane)s 23,24 and organosilicates. 12,25 The pore size and size distribution are of great importance to the mechanical and dielectric properties of the porous materials. A better control of the pore size and pore size distribution in nanoporous PI film can probably be achieved through a better control of the molecular weight and polydispersity of the thermally labile components in various copolymers. Recent progress in polymer synthesis techniques has made it possible to produce well-defined graft chains (polymer brushes) with well-controlled length and chain architecture. 26 * To whom correspondence should be addressed. Tel.: +65- 68742189. Fax: +65-67791936. E-mail: cheket@nus.edu.sg. 6723 Ind. Eng. Chem. Res. 2004, 43, 6723-6730 10.1021/ie0498807 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/17/2004