Rapid Production of Ultralow Dielectric Constant Porous Polyimide Films via CO 2 -tert-Amine Zwitterion-Induced Phase Separation and Subsequent Photopolymerization Kentaro Taki,* Kazunori Hosokawa, Shota Takagi, Hiroyuki Mabuchi, and Masahiro Ohshima Department of Chemical Engineering, A4-021, Katsura Campus, Kyoto University, Katsura, Kyoto, 615-8510, Japan ABSTRACT: Porous polymeric films are promising materials for the production of ultralow-dielectric constant materials. A high porosity polyimide thin film was prepared via the phase separation of a polyimide precursor in an N,N-dimethylaceta- mide (solvent)/2-(diethylamino)ethyl methacrylate/photoini- tiator system. A novel technique involving high-pressure CO 2 (5 MPa) gas injection was used to form CO 2 -2- (diethylamino)ethyl methacrylate zwitterion salt and induce the immediate phase separation and solvent droplet formation (within 60 s) of a wet precursor film on a metal substrate. The film was exposed to UV light through quartz windows for 30 s to polymerize the 2-(diethylamino)ethyl methacrylate while maintaining a constant CO 2 pressure. The cured thin film containing numerous pores with an average diameter of approximately 1 μm ± 1.0 μm was treated at 320 °C for 1 h under a continuous flow of nitrogen. The obtained film was 30 μm thick and exhibited pores with an average diameter of approximately 1 μm ± 0.9 μm. The ultralow-k level minimum relative dielectric constant for the optimal polyimide film was 1.536, and the porosity was 74% with open porous structure. ■ INTRODUCTION Polyimide (PI) films and membranes have been investigated not only for the electronics industries but also for a wide range of applications, such as gas purification, 1-9 the fundamental physics of diffusion and sorption in gas purification, 10-16 and fuel cells. 17-20 In the electronics industry, PI films are utilized in the production of flexible printed cable (FPC) in electronic devices because of the inherently high heat resistance, flexibility and chemically stability of PI. 21 With the increasing trans- mission speed of mobile information devices, e.g., cellular and smart phones, PI FPCs require a lower relative dielectric constant to decrease the signal attenuation. 22-25 There are two strategies for reducing the dielectric constant of PI. One strategy involving chemical modifications of the PI backbone and side chains has been examined in a recent review article. 25 For example, it is well-known that the incorporation of fluorinated substituents into polymers decreases their dielectric constant because of the low dipole moment and the low polarizability of the C-F bond. 25 The dielectric constant of the fluorinated PI is limited to 2.7-3.0 and the poor adhesion between the copper substrate and the fluorinated polymer remains a significant challenge. An alternative strategy involves the introduction of voids into PI films. 25 Pioneering studies on the introduction of 10 nm- sized pores into PI were conducted by Hedrick, 26 who designed a block copolymer system that could be pyrolyzed at high temperatures to form voids in a PI matrix for applications such as insulator in integrated circuits. Voids were successfully formed in the PI, although the porosity was limited by the small number of pores. Another approach for forming pores involves the physical foaming technique, wherein CO 2 gas dissolved in a polymer matrix is thermally phase-separated to induce bubble nucleation in the glassy and rubbery state of the polymer. 27,28 The dielectric constant achieved using this method is 1.77, which is known as the ultralow-k level. 28 A supercritical fluid- assisted extraction technique has been applied to remove mesoscale domains in PI matrix, while the remaining domains are allowed to become voids. 29 Although the process used to create mesosized pores is highly sophisticated, it is assumed that the complete extraction of all domains is not possible. The resulting incomplete extraction may be unfavorable for the application of such materials in electronic devices. Water-borne porous PI has been prepared via the condensation of water vapor onto the PI solution spun on a substrate. 24 The micrometer-seized water droplets on the substrate correspond to the porogen. This method resulted in a decrease of the dielectric constant of porous films to 1.7, although a 48 h process was required for the water droplets to form in the porous films. The formation of meso-sized pores in a PI matrix by the addition of polyhedral oligomeric silsesquioxane (POSS) as an organic porogens has been reported by several authors. 30-33 Inorganic porogens such as hollow silica particles and mesoporous silica were effectively used to reduce the dielectric constant. 34-39 Compounding the porogens produces films with relatively low porosity; however, it is difficult to achieve Received: November 21, 2012 Revised: February 17, 2013 Published: March 5, 2013 Article pubs.acs.org/Macromolecules © 2013 American Chemical Society 2275 dx.doi.org/10.1021/ma302406m | Macromolecules 2013, 46, 2275-2281