A novel cross-linked polyimide film: synthesis and dielectric properties Hu ¨seyin Deligo ¨z a, * , Saadet O ¨ zgu ¨mu ¨s ¸ a , Tuncer Yalc ¸ınyuva a , Saffet Yıldırım b , Deniz Deg ˇer b , Kemel Ulutas ¸ b a Chemical Engineering Department, Faculty of Engineering, Istanbul University, 34320 Avcilar-Istanbul, Turkey b Physics Department, Faculty of Science, Istanbul University, 34459 Vezneciler-Istanbul, Turkey Received 12 March 2004; received in revised form 13 December 2004; accepted 14 February 2005 Available online 7 April 2005 Abstract A novel cross-linked polyimide (CPI) has been prepared by imidization of cross-linked poly(amic acid) (CPAA). In this work, the Ac conductivity and dielectric properties of this polyimide are presented comparitively with those of conventional polyimide (PI), in the 0.2– 100 kHz frequency range and 300–463 K temperature interval. Although the frequency and temperature dependencies of dielectric constant of both conventional and cross-linked polyimides show the same behaviour, the dielectric constant of CPI takes lower values. The Ac conduction studies suggest that electron hopping is responsible for conduction of the PI and CPI films. The activation energy calculated in 296–353 K temperature interval and the ß-relaxation was also observed for CPI. q 2005 Elsevier Ltd. All rights reserved. Keywords: Polyimide; Dielectric constant; AC conductivity 1. Introduction Polyimides have many desirable characteristics such as excellent mechanical properties, low dielectric constant, low relative permittivity, high breakdown voltage, inertness to solvent and radiation resistance. They are distinguished from other high performance polymers by the solubility of poly(amic acid) precursor form, which can be casted into uniform films and quantitatively conversion to polyimide. Polyimides have been especially used widely in micro- electronic, film, adhesive and membrane industry [1–7] due to these prior properties. One of the most important applications of polyimide thin films is used as the inter level dielectric insulators within LSI. The polyimide films that are most widely used in the microelectronic industry with low relative permittivity (dielectric constant) have values in the range of 2.9–3.4 [7]. The utmost importance for the microelectronic applications is the lowness of dielectric constant and high dielectric break- down voltage of polyimides. In electronic packing, low dielectric materials minimize cross talk and maximize signal propagation speed in devices. Hence, the development of polyimides with increasingly lower dielectric constants has been the focus of several investigations [7–13]. In particular, fluorinated polyimides have received extensive attention due to their lower dielectric constants suitable for the microelectronic applications. Simpson and St Clair [14] well described the routes for lowering dielectric constants. Among the strategies used to lower dielectric constants in polyimides are as follow: incorporating diamine and dianhydride reactants which minimize polarizability, incor- porating diamine and dianhydride reactants which impart a high degree of free volume and incorporating fluorine atoms into the molecular structure of the polyimide. Hougham et al. [7,8] and Onah et al. [9] mainly interested in fluorine introduction to polyimide structure in order to decrease dielectric constant. They reported that the incorporation of fluorine into polyimide structure has been intensively explored in the past decade and also fluorine amount increasing has been found to generally lower the dielectric constant and moisture absorption. On the other hand Cassidy et al. has reported the effect of fluorination on polyamide whose dielectric constant nearly in the range of 2.6–3.0 [10]. Polymer 46 (2005) 3720–3729 www.elsevier.com/locate/polymer 0032-3861/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2005.02.097 * Corresponding author. Present address: Universita ¨t zu Koeln Institute fu ¨r Physikalische Chemie, Luxemburger str.116 D-50939 Koeln, Germany. Tel.: C90 212 591 2479; fax: C90 212 591 1997. E-mail addresses: h.deligoz@uni-koeln.de (H. Deligo ¨z), hdeligoz@ istanbul.edu.tr (H. Deligo ¨z).