Synthesis and microwave characterizations of crosslinked oligoimide L. Bes a,* , B. Boutevin a , A. Rousseau a , R. Mercier b , B. Bellini c , D. Decoster c , J.F. Larchanche c , J.P. Vilcot c a Laboratoire de Chimie Macromole Âculaire-UMR 5076, Ecole Nationale Supe Ârieure de Chimie de Montpellier, 8 rue de l'Ecole Normale, 34296 Montpellier, France b Laboratoire des Mate Âriaux Organiques a Á Proprie Âte Âs Spe Âci®ques-UPR 9031, B.P. 24, 69390 Vernaison, France c Institut d'Electronique et de Microe Âlectronique du Nord (CNRS UMR 9929), D.H.S., Avenue Poincare  B.P. 69, 59652 Villeneuve d'Ascq, France Abstract In this paper, we present the microwave characterization of polymers for electro-optic devices, carried out as a collaboration between chemical and physical research groups. We begin by reporting the synthesis and characterization of telechelic oligoimides functionalized with trialkoxysilane end-groups. Addition of a crosslinking agent, tris(4-hydroxyphenyl)ethane, and thermal curing leads to network formation, with the trialkoxysilyl end-groups as crosslinking sites. The network formation is con®rmed by FTIR spectroscopy. The resulting material exhibits a glass transition temperature above 3008C and a thermal decomposition temperature above 4508. These oligomers offer an attractive processing alternative as low viscosity precursors to polyimide networks. The permittivity from 250 MHz to 40 GHz is determined by spin-coating the polymer on a structure as close as possible to a travelling wave electro-optic modulator. The permittivity is obtained from the measured scattering parameters and a spectral domain approach (S.D.A.)-based simulation The scattering parameters (measured), constant propagation (simulated) and characteristic impedance (simulated) are linked together using two alternative physical representations, the multiple re¯ection method and the attenuation method. This approach was ®rst validated with a commercial polymer and then applied to the oligoimides. # 2000 Published by Elsevier Science S.A. Keywords: Crosslinked oligoimide; Permittivity; Microwave 1. Introduction Polymers are of great interest for microelectronics and especially optoelectronics in view of their large electro-optic coef®cient, low dispersion, ¯exibility in the fabrication process and low cost [1]. Yet one of the trickiest require- ments is to achieve thermal stability [2]. Polymers, such as polyimides have undoubtedly good potential for elevated temperatures; moreover their thermal and oxidative stability, their mechanical properties, solvents and acid resistance, their electrical resistance and good coating properties make them interesting materials [3,4]. In this paper, we ®rst present the synthesis and character- ization of optoelectronic-dedicated polymers. Then, we introduce the method of determination of the permittivity from 250 MHz to 40 GHz, which is a critical material characteristic for devices, such as electro-optic travelling wave modulators. Eventually, after validating the technique with a commercial polymer, we report measurements of our polymers. 2. Synthesis 2.1. Structure and physico-chemical characterization of the oligoimides with trimethoxysilane end-groups The synthesis of the oligoimide has been described else- where [5]. The structure is given in Fig. 1. The physico- chemical characterizations of the oligoimide OIP are given in Table 1. 2.2. Crosslinking mechanism The trialkoxysilyl end-groups increase the solubility in the common organic solvents of the oligoimide. But they are mainly use as sites for the formation of crosslinked net- works. 1,1,1-Tris(4-hydroxyphenyl)ethane (TRIOH) was Synthetic Metals 115 (2000) 251±256 * Corresponding author. 0379-6779/00/$ ± see front matter # 2000 Published by Elsevier Science S.A. PII:S0379-6779(00)00321-0