Novel low-κ polyimide/mesoporous silica composite films: Preparation, microstructure, and properties
Introduction
In the past decade, miniaturized ultra large-scale integrated circuit (IC) chips and semiconductor devices with improving performance have become a main orientation in microelectronics industry and been pursued as a global trend. However, the technicians and engineers have to face lots of serious technological problems in IC designs, such as the resistance capacitance time delay, cross-talks, power dissipation, etc. Fortunately, these problems can be effectively resolved by using inter-dielectric materials with low-dielectric constant (low-κ) [1], [2], [3]. Besides having a low-κ value, inter-dielectric materials should possess good thermal stability, low moisture uptake, excellent radiation and chemical resistance, high mechanical strength, and good adhesion to semiconductor and metal substrate. Considering these requests, a variety of polymers have been reported as potential low-κ materials for use in the development of advanced IC chips, which include polyimides, heteroaromatic polymers, polyaryl ethers, fluoropolymers, nonpolar hydrocarbon polymers, polysilsesquioxanes, etc. [4].
As a species of the most important high-performance polymers, polyimides (PIs) are well known for their high-temperature durability, good mechanical properties, excellent chemical and thermal stabilities, low thermal expansion coefficient, and low-dielectric constant. They have been widely used as inter-dielectric materials in microelectronics and large-scale IC industry, as electrical insulation for conventional appliances, and as functional materials for other industrial applications. However, with dielectric constants of about 3.1–3.5, the conventional PIs are insufficient for meeting the requirement of microelectronic and insulating applications. In recent years, the preparation of PIs with low-κ and high performance has become one of the research focuses. There are several approaches to reduce the dielectric constant of PIs, which include incorporation of fluorinated substituents into polymers [5], [6], [7], [8], thermal degradation of the labile block or graft chains in the PI copolymers [9], [10], [11], and introduction of air gaps into interconnected structures and nanopores into polymers [12], [13]. Considering that the incorporation of air having a dielectric constant of about 1 can decrease the dielectric constant remarkably by resulting in porous structure, most studies were focused on the last two approaches. Fu et al. prepared nanoporous low-κ polyimide films via grafting poly(amide acid) (PAA) with thermally degradable side chains by a reversible addition-fragmentation chain-transfer-mediated process [14]. Zhang et al. incorporated the hollow silica tubes into polyimide through ultrasonic dispersion and in situ polymerization so as to reduce the total dielectric constant of the composites (κ = 2.952, containing 3 wt% nano-scale silica tubes) [15]. Chen et al. introduced methacrylated-poly(silsesquioxane) (POSS) into the polyimide matrix to generate a PI/POSS semi-IPN-like nanocomposite, which resulted in an steady decrease of the dielectric constant as the methacrylated-POSS content increases (κ = 2.51, containing 15 wt% methacrylated-POSS) [16]. Wang et al. synthesized poly[4′,4′′-(hexafluoroisopropylidene)bis(4-phenoxyaniline)4,4-(hexafluoroisopropylidene)diphthalic anhydride]–montmorillonite (MMT) nanocomposites from modified MMT and PAA, whose dielectric constant reached a value of 2.6 at temperature over 100 °C [17].
Recently, mesoporous silica materials have attracted considerable interests in applications of molecular sieves, catalysts, adsorbents, optical devices, and sensor devices due to their highly ordered and uniform mesoporosity [18]. Most importantly, the mesoporous silica films synthesized via a surfactant templating process could provide large pore sizes (5–30 nm), high porosities (45–75%) and controlled pore structures [19]. This makes it possible to introduce voids into the bulk so that the low-κ air (κ = 1) can be utilized to reduce the dielectric constant of the materials. Many studies showed that the mesoporous silica films have low-dielectric constants in the range of 1.42–2.1 [20], [21], [22], which is lower than most of other low-dielectric materials, such as silisequioxane based dielectric, fluorine doped silica film, carbon doped silica film, and organic polymer dielectrics [23]. The low-κ mesoporous silica films can meet the requirements of the new dielectric films with low-dielectric constant (κ < 2.5) for the applications in the microelectronics and the insulations. However, owing to the poor processability of the mesoporous silica films, it is difficult to endue them with low-dielectric constant, low moisture uptake, and high mechanical strength simultaneously [24]. Based on the low-dielectric constant of the mesoporous silica, incorporation of the mesoporous silica into the PIs would be expected to reduce the dielectric constant of the PIs.
In order to obtain the low-κ materials with good performance through a sample method, we develop a new approach in this study to prepare the low-κ PI composite films by incorporating the mesoporous silica into PI matrix to combine the low-dielectric constant of mesoporous silica and the excellent properties of PI. A significant reduction of the dielectric constants for the PI composite films can be achieved with this method. We also evaluate the influence of the mesoporous silica materials on the PI composite films on basis of the dielectric, thermal, dynamic mechanical and mechanical properties. This method provides a sample and effective means to prepare the low-κ PI composite films.
Section snippets
Materials
Poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (EOn-POm-EOn) triblock copolymers, EO20PO70EO20 (Pluronic P123) and EO106PO70EO106 (Pluronic F127) were commercially obtained from BASF Company. Tetraethoxysilane (TEOS) used as silica source and (γ-aminopropyl)triethoxysilane (APTS) used as coupling agent were purchased from Aldrich Chemical Company. Pyromelltic dianhydride (PMDA), 4,4′-oxydianiline (ODA), and N,N′-dimethylacetamide (DMAc) were purchased from Beijing Chemical Reagent
Microstructure and properties of the synthesized mesoporous silica
Two kinds of typical mesoporous silica materials, SBA-15 and SBA-16, were synthesized by templating with the EO20PO70EO20 and EO106PO70EO106 triblock copolymers as templates, respectively, via a sol–gel process. Fig. 1 shows the XRD patterns of SBA-15 and SBA-16. One can find a well-resolved single diffraction peak at 2θ = 0.93° and 0.71°, respectively, as well as a series of broad diffraction peaks with low intensity for each mesoporous silica. Through the Bragg's law and the corresponding
Conclusion
A series of the novel PI composite films containing the SBA-15 or the SBA-16 type mesoporous silica were successfully prepared via the in situ polymerization and following thermal imidization. The dielectric constants of the PI composite films can be reduced from 3.34 of the pure PI to 2.73 and 2.61 by incorporating 3 wt% SBA-15 and 7 wt% SBA-16, respectively. The reduction of the dielectric constant is attributed to the incorporation of the air voids (κ = 1) stored within the mesoporous silica
Acknowledgements
The authors greatly appreciate financial support from the National Nature Science Foundation of China (Grant No.: 50573006).
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