Preparation And Characterization Of Polybenzoxazole With Low Dielectric Constant

With the rapid development in the field of microelectronics, ultra large scale integrated circuits(ULSI) have undergone a continuous growth of integration level, and the dimensions of electronic devices utilized in ULSI have continually miniaturized. However, high integration level of ULSI would result in increased resistance-capacitance time delay, line-to-line crosstalk noise, and power dissipation, which could restrict the high-speed performance of electronic devices. Therefore, it is highly necessary to develop novel low dielectric constant materials(k<2.3) as interlayer to replace traditionally used SiO2(k=3.9~4.2) to solve these problems. Besides the low dieletctric constant, an ideal low-k material should also possess good processability, good thermal and chemical resistance, low coefficient of thermal expansion, and good mechanical properties. As such, high-performance aromatic polymers, such as polybenzoxazole(PBO), have been considered as promise alternatives for SiO2 because they met most requirements as mentioned above, but the dielectric constant(k=2.6~3.0) of these polymers was still not satisfactory. Therefore, how to reduce the dielectric constant without compromise of good intrinsic properties of aromatic polymers has become one of the hot topics in the field of microelectronics. Based on the above considerations, this dissertation had adopted two approaches to reduce the dielectric constant of PBO, those are, the introduction of branched structure into the linear PBO polymer chains and the incorporation of porous inorganic fillers into the PBO matrix. The influence of these two approaches on the thermal and mechanical properties of PBO was also evaluated.In chapter two, PBOs containing branched structures were synthesized from 2,2-Bis(3-amino-4-hydroxyphenyl) hexafluoropropan, terephthaloyl chloride and trimesoyl chloride by a two-step approach. Firstly, soluble precursors for PBO were synthesized by polycondensation in solution, then the pre-polymer film was prepared by solution-casting, PBO was finally obtained by thermal cyclization of pre-polymer films at high temperatures. Uri-viscometer, GPC, FT-IR, NMR were employed to characterize the chemical structures of PBOs containing different amount of branched structures. The microscale morphologies of these PBOs were investigated by XRD and SEM observation, while the thermal, mechanical, and dielectric properties were investigated by TGA, DSC, tensile testing, and impedance analyzer. The results suggested that the viscosity and molecular weight of polymers were reduced by the introduction of branched structure, while the thermal stability was unaffected after thermal cyclization, meanwhile, the glass transition temperatures and tensile modulus of polymers containing branched structures were both increased. More importantly, because of the decreased inter-chain interactions and packing, and increased free volume, the dielectric constant was significantly decreased with the incorporation of branched structure, which made these PBOs promise high-performance and low-k materials for electronic applications.In chapter three, through the addition of porous polyoxometalates, silicotungstic acid(SWA), into the solution of pre-polymers, and subsequent solution-casting and thermal cyclization, composite films with different amount of SWA were fabricated. The structures and morphologies of composites were characterized by FT-IR, XRD and SEM, while the thermal, mechanical, and dielectrical properties were evaluated by DSC, TGA, tensile testing, and impedance analyzer. The morphological characterization suggested that at low loading levels of fillers, the SWA dispersed homogenously in the matrix, while they tended to aggregate at higher loadings. The performance tests revealed that the good thermal stability of PBO was not affected by the incorporation of SWA,while the glass transition temperature and tensile modulus of PBO were significantly improved by the addition of SWA. Meanwhile, because of the increased cavities in the matrix induced by the porous SWA, the dielectric constant of PBO continuously decreased with the addition of SWA, and the dielectric loss maintained at a low level.