Optical and Dielectric Anisotropy in Polyimide Nanocomposite Films Prepared from Soluble Poly(amic diethyl ester) Precursors** By Youngkyoo Kim,* Wan Hee Goh, Taihyun Chang, Chang-Sik Ha,and Moonhor Ree Rigidrod-likearomaticpolyimideshavebeenwidelyused asapassivationorinter-dielectriclayerinmicroelectronicde- vices due to their high mechanical modulus, excellent ther- mal stability, low thermal expansion, low solvent swelling, and low dielectric constant. [1±5] Although this outstanding property comes from the rigid phenyl and imide moieties in the polyimide backbone, the drawback of the rigid rod-like characteristicsincludespooradhesiontometallicsurfaceand large dielectric anisotropy, leading to a catastrophic device failure and signal deformation. [1,4,5] Therefore, the rigid rod- like polyimide cannot be used further for highly advanced devices with ultra large scale integration that require much moreprecisesignaltransmission. Inthisregard,ithasbeenattemptedtoimprovetheweak- ness of rigid rod-like polyimides through a molecular com- positethatcanbepreparedbymolecularlevelmixingofrigid rod-likeandflexiblecoil-likepolyimidesbecauseflexiblecoil- likepolyimidesexhibitnormallygoodadhesion,highelonga- tion, and low dielectric anisotropy (see Fig. 1). [6±11] Although optically transparent polyimide composite films were suc- cessfully prepared using soluble precursors, poly(amic acid) and/or poly(amic dialkyl ester), which are well dissolved in common weak base solvents, careful studies showed that the resulting polyimide composite films were phase segregated on a scale of several nanometers, meaning a nanocomposite rather than a molecular composite, because rigid rod-like polyimides have inherently high tendency to intermolecular chain packing and preferential in-plane molecular orienta- tion. [9,11] In particular, these previous reports focused on the physical and structural analysis in order to study the forma- tion of molecular composite. However, the in-plane and out- of-planeopticalanddielectricpropertieshavenotbeeninves- tigated yet, even though this is one of major causes to make the polyimide molecular composite because the dielectric constantofthepackagingorinter-dielectriclayershasahuge influenceonthedelayofsignalpropagationandcrosstalkbe- tweenothersignallines. [12] In this work, we report on the high frequency optical and dielectric anisotropies of the polyimide nanocomposite films prepared from poly(p-phenylene biphenyltetracarboxamic diethyl ester) (BPDA-PDA ES) and poly(4,4¢-oxydiphenylene biphenyltetracarboxamic diethyl ester) (BPDA-ODA ES) through the in-plane and out-of-plane refractive indices mea- surement.Thepristineprecursorfilmsshowedalmostsimilar dielectric constant for in-plane and out-of-plane, whereas the pristine polyimide exhibited large optical and dielectric an- isotropy. The dielectric anisotropy dependence of the BPDA- PDAPI/BPDA-ODAPInanocompositefilmsonthecomposi- tion was in good agreement with the structural anisotropy investigated by the wide angle x-ray diffraction measure- ment. COMMUNICATIONS ADVANCED ENGINEERINGMATERIALS 2004, 6, No. 1±2 DOI: 10.1002/adem.200300546  2004 WILEY-VCH VerlagGmbH & Co. KGaA, Weinheim 39 ± [*] Dr.Y.Kim DepartmentofPhysics,BlackettLaboratory,ImperialCollege London, PrinceConsortRoad,LondonSW72BW,UK E-mail:y.kim@ic.ac.uk Prof.M.Ree,Mr.W.H.Goh,Prof.T.Chang DepartmentofChemistry,CenterforIntegratedMolecular Systems,PolymerResearchInstitute,DivisionofMolecular andLifeSciences,andBK21Program, PohangUniversityofScienceandTechnology, Pohang790-784,RepublicofKorea E-mail:ree@postech.edu Prof.C.S.Ha DepartmentofPolymerScienceandEngineering, PusanNationalUniversity, Pusan609-735,RepublicofKorea [**] ThisstudywassupportedbytheKoreaScience&Engineering Foundation via the Center for Integrated Molecular Systems andbytheMinistryofEducation(BK21Project).Y.K.person- ally thanks Prof. D. D. C. Bradley for arranging financial support. Fig. 1. Schematic illustration for the preparation of molecular composite consisted of rigidrod-likeandflexiblecoil-likepolyimidesthatareconvertedfromsolubleandcoil- likeprecursorsbythermalimidization(D).