Synthesis And Properties Of Ultra-low Dielectric Constant Polyimide/polyoxometalates Composite Thin Films

With the rapid development of electronic communication technology, especially the development of Ultra Large Scale Integrated Circuit, electronic products become more and more portable and functional, the requirement for materials with low dielectric constant increased。It is very crucial to develop next generation dielectrics. Polyimides, as high profermance polymer materials, were used as electronic medium between metal layers from 1970s. Polyimides exhibited good thermal stability, mechnical property, chemical-resistants and felting property, were widely applicated in microelectronic. The dielectric constant for ordinary polyimides were in the range of 2.9³.4, which can not meet the requirement for new dielectrics. Several methods are intoduced in order to decrease the dielectric constant of polyimides, such as incorporation of fluorinated substituent intoPIs; introduction of voids into PIs by foaming processes andembedment of nanoporous inorganic/organic hybrids like POSS into PIs. In this paper, three polyoxometalates(POMs) with different structures were firsr modification by silane coupling agents, and then those inorganic hybirds were incorporated in to polyimide(PMDA-ODA) and fluoride polyimide to fabrication series of polyimides/polyoxometalate composites thin films. FTIR, NMR, TEM, SEM, XRD, XPS, DMA, TGA and DSC were used to characterize the structures and properties of the composites.1. A mono-lancunary keggin-type decatungstosilicate (SiW₁₁) polyoxometalate modified byγ-aminopropyltriethoxysilane (KH550) was incorporated into polyimide (PI) through copolymerization. Nuclear magnetic resonance (NMR), Fourier transition infrared spectroscopy (FTIR) and wide angle X-ray diffraction (WAXD) were used to characterize the structure and composition of the polyoxometalate-organosilane hybrid (SiW₁₁KH550) and PI/SiW₁₁KH550 copolymers. The differential scanning calorimetry (DSC) studies indicate that the glass transition temperature (Tg) of PI/SiW₁₁KH550 copolymers increase from 337°C (for neat PI) to 364°C (for the copolymer sample with 10 wt% of SiW₁₁KH550). Dielectric measurement showed that the dielectric constant for copolymer thin films decreased with the increasing SiW₁₁KH550 content, and the dielectric constant is 2.10 for the copolymer sample with 10wt% of SiW₁₁KH550. On the contrary, incorporation SiW₁₁ into polyimide matrix dicreased the porperties of PI mentioned before.2. A silane-modified mono-lacunary Keggin-type polyoxometalate SiW₁₁KH560, was obtained by reaction of (3-Glycidoxypropyl)trimethoxy silane(KH560) with K8SiW₁₁O₃₉(SiW₁₁). FTIR and XPS spectrums demonstrate that KH560 is grafted onto the polyoxometalate. Then the modified polyoxometalate was added into PMDA -ODA poly(amic acid) solution, and the poly(amic acid) solution were thermally imidized to form polyimide/polyoxometalate composites. FTIR results showed that SiW₁₁KH560 can copolymerize into PI matrix and It also hydrogen bongding to PI matrix. The EDS (W-mapping) studies on the composite films reveal that the polyoxometalate clusters are well dispersed in the polyimide matrix. The incoporation of SiW₁₁KH560 into polyimide reduced the dielectric constant of the latter from 3.29 to 2.81.3. A silane-modified mono-lacunary Keggin-type polyoxometalate SiW₁₁A171, was obtained by reaction of vinyltrimethoxysilane with K8(SiW₁₁O₃₉). Then the modified polyoxometalate was physically blended with the PMDA-ODA poly(amic acid) and the blends were thermally imidized to form polyimide/polyoxometalate composites. The x-ray diffraction (XRD) analysis indicates that the polyoxometalate clusters can not form crystalline structure in the composite, suggesting that the blending lead to improved compatibility between the polymer matrix and the modified polyoxometalate. The EDS (W-mapping) studies on the composite films reveal that the polyoxometalate clusters are well dispersed in the polyimide matrix. The physical incoporation of modified SiW₁₁A171 into polyimide remarkably reduced the dielectric constant of the latter from 3.29 to 2.05 when 20 wt % of SiW₁₁A171 was used.4. PW₁₁ and P₂W₁₇ with different structures were modification by KH550 and A171, and then incorporated into PMDA-ODA PI to fubrication polyimide composites respectively. Dielectric measurement shows that dielectric constant of composites is affact by the components and structure of polyoxometalates. Experientially, PW series polyoxometalates are more effectively than SiW series ones to decrease the dielectric constant of polyimide. Furthermore, Dowson type polyoxometalates are more effectively than keggin types, and the more electronegative the POMs are, the more effective it can to decrease the dielectric constant of composites.5. Fluoride polyimide copolymers were synthesis using PMDA, ODA, 6FDA, TFDB as monomers. Dissolving testing and films fabrication process monitoring results show that fluoride polyimide has excellent properties when the fluoride PI contained 50mol% fluorine monomers. Maxwell equation, Lorenz and Vogel theories were used to calculate the dielectric constant of those copolymers, the results show that Lorenz and Vogel theories are suitable for this copolymer system. After that, P₂W₁₇KH550 were incorporated into the copolymer, the dielectric constant of those composites are decreased to 1.63 when 15wt% of P₂W₁₇KH550 was used, meanwhile, those composites can be dissolved into DMF, DMAc and NMP.