Dielectric materials have been widely used as electric insulators in microelectronic devices and integrated circuits. The driving force for the implementation of polymeric insulators lie in their lower dielectric constant (ε), and relatively easy processing properties compared to silicon dioxide. Among a wide variety of polymers evaluated for insulators in microelectronic applications, rigid aromatic polyimides with low dielectric constant have emerged as the favored candidates for the combination of excellent thermal and mechanical properties. The present review exams the progress made in the synthesis and preparation of polyimide materials with very low dielectric constant. First, the factors such as (a) the chemistry structure of polyimides, (b) the orientation of polymer chains and moieties, (c) the thickness of polyimide film, (d) the frequency of applied electric field, and (e) the substrate the polyimide film conglutinated on dielectric properties have been briefly outlined. Then, the attempts, which have been continuously undertaken to achieve polyimide materials with very low dielectric constant to meet the higher requirements for application in micro- electronics, have been discussed. Incorporating fluorinated groups into polyimide is the most common strategy and has been proved to be the useful approach providing polyimide with super lower dielectric constant. The dielectric constant decreases almost linearly with increasing fluorine content. However, the effects of fluorine content on polyimides depend on the substitution asymmetry of fluorine groups. Furthermore, the limits of this method lie in the synthetic difficulties, the expenses of mechanic and solvent resistant property as well as thermal stability. Therefore, polyimide nanofoams have been developed in order to obtain thin film dielectric layers with very low dielectric constants. One approach for polyimide nanofoams involved the preparation of block or graft copolymers capable of self-assembly in which the continuous phase was thermally stable polyimide and the dispersed phase thermally unstable polymer. Upon high temperature treatment, the unstable component undergoes thermolysis, leaving behind pores with size and shape dictated by its initial morphology. While this technique has been demonstrated to produce nanofoams with very low dielectric constant, the synthesis procedures and processing are relatively complicated. Furthermore, thermal degradation of the labile component reduces the molecular weight as well as certain critical mechanical properties of the resulting nanofoam films. Another approach for the preparation of polyimide (PI) nanofoams with high thermal stability has been presented using polymer nanospheres as templates. The diameter of these particles was 30 to 40nm. PI nanocomposite films with various contents of nanospheres were obtained by in-situ condensation polymerization. The distribution of the nanospheres in the films and the morphology of the films were characterized by TEM. Upon thermal treatment, the thermally unstable nanospheres undergo thermolysis, leaving pores the size and shape dictated by the initial nanosphere morphology. These results revealed that nanofoams with high thermal stability and low dielectric constant approaching 2.0 could be prepared using PI nanocomposite approach.
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