Low dielectric polyimide/poly(silsesquioxane)-like nanocomposite material
Introduction
Polyimides have been widely utilized as packaging material and dielectric layers for the electronic and microelectronic industry because of their outstanding characteristics such as low dielectric constant [1], [2], [3], [4]. Interlayer dielectrics of ULSI multilevel interconnections require a low dielectric constant because of many problems in circuit performance, such as signal transmission rates and power dissipation [5]. A low dielectric constant is one of the most attractive properties of polyimide materials for electronics applications. It was expected that a low dielectric constant could be achieved by having large substituted groups or perfluorgroups [6], [7], [8], [9], [10], [11] in a molecular skeleton. The former would lead to a higher free volume content and the latter would reduce the polarization under an electric field. Introducing fluorine atoms into polyimides is one of the most popular ways of achieving this purpose. However, the fluoro-polyimides may have low adhesion strength, low glass transition temperature, low mechanical strength or high thermal expansion coefficient (CTE). Moreover, they are expensive.
Several studies have been carried out on the preparation of polyimide–silica [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34] hybrid material with silica particles dispersed in the polyimide matrix. Iyoku et al. introduced methyltriethoxysilane (MTES) [32] and phenyltriethoxysilane (PhTES) [33] into the polyamic acid (ODA–PMDA precursor). Furthermore, for polyimide (ODA–PMDA) hybrids the MTES component was partially replaced with dimethyldiethoxysilane (DMDES) [34]. They are mostly produced by sol–gel technique, which can be viewed as a two-step network forming process, the first step being the hydrolysis of a metal alkoxide and the second consisting of a polycondensation reaction. The hybrids have been aimed directly at improving the properties of polyimide in terms of better mechanical strength and uniform nanocomposite. Nevertheless, there is little research focusing on dielectric properties [35]. The main reason is that the dielectric constant of the polyimide–silica increases with the loading content of silica (about k=3.9) [35]. Poly(silsesquioxane) (PSSQ) is an important low dielectric material in microelectronics [20], [36], [37]. The silica component based on phenyltrimethoxysilane (PTS), PSSQ-like with phenyl substitutes on the silicon atom, not only provides high thermal stability but also shows less volume contraction upon condensation than TMOS-based ones [20].
In this study, we intend to prepare low dielectric polyimide/PSSQ-like (PI/PSSQ-like) nanocomposite material by employing p-aminophenyltrimethoxysilane (APTS) to provide bonding between the PSSQ-like and polyimide ODA–ODPA phase through a sol–gel process. There are three unique features in the synthesis. First, it makes use of acid groups of PAA, rather than an additional acid or base, as the catalyst to carry out self-catalyzed condensation reaction. Second, the hydrolysis of methoxysilyl is processed by water from air and imidization of the polyamic acid APTS–PAA or PAA–PTS. Third, the hybrid films can maintain excellent transparency even under high content of PTS and reasonable mechanical properties. Without adding catalyst, the hybrid films may maintain high clarity. Moreover, the synthesis method is simple and easy to apply to large-scale production. Although whether PAA is capable of self-catalyzing is still being disputed [28], it has been proved that adding acid or base and water creates larger silica domains and thus reduces the mechanical property of hybrid films [28], [31]. This may reduce the transparency of the film.
Because the PI/PSSQ-like hybrid composite film is a new type of material, there are few systematic studies on its synthesis and characteristics to correlate with the chemical composition and physical structure. The dynamic mechanical properties of PI/PSSQ-like hybrid films have been discussed in our other paper [38]. In this article, we intend to correlate these properties with the silica component, the PSSQ-like content, the polyimide block chain length and the cross-linking density to understand the influence of composition of the low dielectric PI/PSSQ-like nanocomposites on their thermal stability, phase transitions, dielectric constants, CTE, moisture absorption, and optical transparency in the visible region. The morphology is also studied using a transmission electron microscope (TEM) and a scanning electron microscope (SEM).
Section snippets
Materials
4,4′-Diaminodiphenylether (ODA, 98%) from Loncaster was dried in a vacuum oven at 120°C for 3 h prior to use. 3,3′-Oxydiphthalic anhydride (ODPA, 98%) from Tokyo Chemical Industry was purified by recrystallization from acetic anhydride and then dried in a vacuum oven at 125°C overnight. N-methyl-2-pyrrolidone (NMP) from Tedia Company was dehydrated with molecular sieves. APTS (95% para and 5% meta) from Gelest Inc. and PTS (98%) from Lancaster were used as supplied.
Synthesis of aminophenyltrimethoxysilyl-terminated polyamic acid (APTS–PAA) oligomers
The reaction is shown in
Synthesis characterization and morphology
The PI/PSSQ-like hybrid films were obtained using the sol–gel process. The APTS–PAA oligomers were prepared by the reaction of 4,4′-diaminodiphenyl ether (ODA) with the 3,3′-oxydiphthalic anhydride (ODPA) in the presence of APTS to control the polyimide block chain length (X) ranging from 3000 to 20 000 g mole−1 and end-group functionality. The inherent viscosities of APTS–PAA oligomers ranged from 0.29 to 0.57 dl/g. The monomer composition and inherent viscosity of pure ODA–ODPA polyamide acid
Conclusions
PI/PSSQ-like hybrid material has been successfully prepared by a self-catalyzed hydrolysis/condensation process. The synthesis is creative to avoid adding external catalyst or water, using only self-acid groups as catalysts and water from air. The PSSQ-like nanocomposite hybrid films have excellent transparency in visible light even under high content of PTS, and the PSSQ-like domain sizes depending on the composition are in the range 30–600 nm by both TEM and SEM observation. The PSSQ-like
Acknowledgements
The authors would like to express their appreciation to the National Science Council of the Republic of China and China Petroleum Company for financial support of this study under grant NSC 89-CPC-E-009-008. We would like to thank Mr Chin-Lung Hung for assistance with the scanning electron microscopy, Ms Su-Jen Chang for providing the dielectric measurement, Mr Yun-I Tien and Ms Mei-Li Chang for assistance with the sample preparation and transmission electron microscopy.
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