Dielectric relaxation in polyimide nanofoamed films with low dielectric constant

Abstract

Polyimide nanofoamed films have been prepared from the polyimide precursors ͑PMDA-ODA͒ and poly͑ethylene oxide͒ ͑PEO͒ in N , N-dimethylacetamide. The dielectric properties of the films were studied over the temperature range of −150-150°C and a frequency range of 1 Hz-10 MHz. The frequency dependence of the dielectric constant for nanofoamed films with different amounts of PEO was studied. An relaxation process at below the glass transition temperature of the polyimide nanofoamed films was found. The peak value of the dielectric loss increased with increasing amount of PEO. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2840715͔In recent years, great attention has been paid to the lowk polymer and its composites as the next-generation interlayer dielectrics for multilayer interconnection in microelectronics. [1][2][3][4][5][6] In electronic packaging, low-k dielectric materials minimize cross talk and maximize signal propagation speed in devices. Aromatic polyimide ͑PI͒ has been widely used as an interlayer dielectric in microelectronic devices, thanks to its good mechanical properties, excellent chemical and thermal stabilities, high breakdown voltage, and low dielectric constant. [1][2][3][4][5][6][7][8] Simpson and St. Clair 3 have summarized the strategies used to lower dielectric constant in polyimides. In particular, fluorine is usually introduced into polyimide in order to further decrease the dielectric constant. 9 Furthermore, a high degree of free volume will offer as a low dielectric constant phase. In our recent study, silica tubes/PI composite films were investigated and the dielectric constant could be reduced to 2.9. 4 Moreover, a series of porous polyimide materials such as polyimide nanofoamed films prepared from PI and poly͑ethylene oxide͒ ͑PEO͒ have been explored. 10 One of the most suitable physical methods for the investigation of molecular mobility in low-k polymer samples under ac electrical field is the dielectric spectroscopy. This method makes it possible to obtain data on the molecular mobility of a macromolecule as a whole and of its mainchain segments, bulky side groups, etc. Dielectric losses in a polymer sample appear during thermal orientation of polar kinetic units. Although it has been reported that most of aromatic polyimide and copolyimide films exhibit relaxational processes ͑␣, ␤, and ␥ relaxation͒ in their dynamic mechanical and dielectric behaviors, 11,12 there is limited relaxation studies on polyimides. In this letter, PI-PEO nanofoamed films were prepared via in situ polymerization and thermal treatment. A low dielectric constant was achieved. The dielectric properties of these nanofoamed films over broad temperature and frequency ranges were studied and the dielectric relaxation was discussed in details.Pyromellitic dianhydride ͑PMDA͒ ͑Hangzhou Taida Ltd.͒ and oxydimethylaniline ͑ODA͒ ͑Beijing Chemical Reagent Ltd. China͒ were dehydrated prior to use. N , NDimethylacetamide ͑Beijing Yili Chemicals Reagent Co.͒ was used as received. PEO ͑Beijing Guoren Yikang Ltd...

Nanoporous polymeric materials: A new class of materials with enhanced properties

“…Polyimides (PI) are a favored material because of their high thermal stability, low stress coefficient of thermal expansion, low dielectric constant, high resistivity, and high dielectric breakdown [120,[130][131][132][133][134][135]. For instance, Lee et al [130] managed to develop a nanoporous PI films (thickness: 200 lm) with pore sizes from 10 to 40 nm, a porosity around 20%, and a dielectric constant of 2.25 (a reduction of 31% with respect to the solid).…”

The use of the final thermally stimulated discharge current technique to study the molecular movements around glass transition

“…Beside the apparent peak, a weak relaxation is entering around 337 K. This fact is better observed for E c = 18 MV/m and t c /t di = 30 min/30 min. A peak was observed in tan δ around this temperature [13] and it was attributed to absorbed water [27][28][29]. Because our measurements are carried out in a vacuum we cannot attribute the peak to absorbed water (see below).…”

“…A search of literature shows that, using the TSDC method, a peak was reported at 390 K (W = 0.19 eV) and at 420 K (W = 0.4 eV) [18], at approximately 363 K (W = 0.60 eV) and it was attributed to electric dipole orientational polarization resulting from the co-operative molecular motion of the residual reactive groups C=O [27], a peak ranging between 398 and 463 K [28]. From mechanical and dielectric measurements the activation energies for the β relaxation of polyimides have been reported in the range of 0.5-1 eV [13,17,27] . From tan δ curve the W value of the β relaxation is 1.23 eV and the peak is at 383 K [27,29].…”

“…From dielectric measurements it is assumed that the dielectric loss peak at 333 K is mainly governed by the γ-relaxation, the real part ε' of the dielectric permittivity in the temperature range 343-453 K is mainly governed by dipolar and space charge relaxations and in high temperature region (453-513 K) the increase in ε' with temperature is mainly due to interfacial polarization [14]. A recent study shows that the β transition occurs over a very wide temperature range, and the peak temperature T is around 323 K [13]. Although the exact description is still uncertain, the β transition is generally associated with local bond rotations along the polyimide backbone, showing a glasslike behavior [17].…”

“…The question about the dominant carrier species responsible for electrical conduction is not correlated with molecular movement. The molecular mobility in PI was investigated by dielectric relaxation spectroscopy [13,14] and by the thermally stimulated discharge current (TSDC) [15,16]. From dielectric measurements it is assumed that the dielectric loss peak at 333 K is mainly governed by the γ-relaxation, the real part ε' of the dielectric permittivity in the temperature range 343-453 K is mainly governed by dipolar and space charge relaxations and in high temperature region (453-513 K) the increase in ε' with temperature is mainly due to interfacial polarization [14].…”