Dielectric and Thermal Properties of Polyimide Poly(ethylene oxide) Nanofoamed Films

Transcription

1 Journal of ELECTRONIC MATERIALS, Vol. 41, No. 8, 2012 DOI: /s Ó 2012 TMS Dielectric and Thermal Properties of Polyimide Poly(ethylene oxide) Nanofoamed Films YI-HE ZHANG, 1,2,3 LI YU, 1 LI-HANG ZHAO, 1 WANG-SHU TONG, 1 HAI-TAO HUANG, 2,4 SHAN-MING KE, 2 and H. L. W. CHAN 2 1. State Key Laboratory of Geological Processes & Mineral Resources, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing , People s Republic of China. 2. Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hong Kong, People s Republic of China zyh@cugb.edu.cn aphuang@polyu.edu.hk Polyimide nanofoamed films have been prepared by incorporating poly(ethylene oxide) (PEO) into poly(amide acid) (PAA) precursors with subsequent imidization of PAA precursors at high temperature. The porous structure, thermal decomposition temperature, and dielectric property of nanofoamed films were investigated by scanning electron microscopy, thermogravimetric analysis, and impedance spectroscopy. Nanopores with sizes around 40 nm to 200 nm were formed in nanofoamed films by pyrolysis of PEO during the imidization progress. The decomposition temperature of nanofoamed films decreased slightly with increasing volume fraction of nanopores and maintained the high decomposition temperature of C when the volume fraction of nanopores was 10.9 %. The dielectric constant of nanofoamed films decreased from 3.4 for pure PI to 2.4 at 10 3 Hz through the introduction of nanopores with volume fraction of 10.9 %. Key words: Dielectric properties, thermal properties, composites, thin films INTRODUCTION Polyimide (PI), as one of the high-performance polymers, has been widely used as a dielectric and packaging material for integrated circuits in the microelectronics industry due to its outstanding mechanical, thermal, and dielectric properties. 1,2 In recent years, increasing attention has been paid to organic inorganic hybrid materials based on PI. It has been reported that room-temperature mechanical, thermal, and anticorrosive properties of PI hybrid films can be improved by incorporation of fillers such as carbon nanotubes, 3 montmorillonite, 4 11 silica, and mica. 17 In particular, aromatic PI is used for dielectric layers in multichip packaging for semiconductors. Although PI meets most of the material requirements for microelectronic applications, a lower dielectric constant of PI is needed. Low dielectric constant is one of the most (Received October 13, 2011; accepted March 1, 2012; published online April 25, 2012) attractive properties of dielectric materials for microelectronic and insulating applications. Most PI materials possess a dielectric constant of about 3.4. However, this is too high to meet the requirement for microelectronic applications in the future. To overcome these drawbacks, several methods for preparation of low-k PI have been developed. 8,18 20 Although layered silicate/pi nanocomposites display much lower water absorption, leakage current density, and thermal expansion coefficient than those of pristine PI, the dielectric constant of the nanocomposites is larger than that of matrix PI, and increases with the silicate content. 8 Moreover, the dielectric constant of PI composites can be tailored by adding fillers with hollow structure such as silica tubes. 19 An alternative way to substantially reduce the dielectric constant while maintaining reasonable thermal or mechanical properties of PI is to fabricate foamed PI. Decrease of the dielectric constant can be achieved by replacing a significant part of the polymer with air, which has dielectric constant of 2281

2 2282 Zhang, Yu, Zhao, Tong, Huang, Ke, and Chan 1.0. In general, the routes to prepare PI foams include incorporation of foaming agents or hollow microspheres, and microwave processing. Traditionally, a commonly utilized route to fabricate foamed PI is to use a foaming agent and reactive system. However, most PI foams prepared via these methods have several drawbacks such as fairly large pore size and open pore structure. For thinfilm or coating applications in microelectronics, the pore size should be much smaller than the film thickness in order to maintain the integrity of the insulating layer and circuitry deposited on the film. In this study, thermally labile polymer poly(ethylene oxide) (PEO) was selected as a foaming agent to prepare PI nanofoamed films. PI derived from pyromellitic dianhydride (PMDA) and 4,4-oxydianiline (ODA) was selected as the polymer matrix due to its ease of film casting, excellent chemical stability, mechanical properties, and insulating performance Nanofoamed films were prepared from PI monomers with different amounts of PEO. The frequency and temperature dependences of the dielectric constant of nanofoamed films with different PEO contents were systemically investigated. EXPERIMENTAL PROCEDURES For the experiments, PMDA and ODA were purchased from Hangzhou Taida Ltd. and Beijing Chemical Reagent Ltd., China, respectively. N,N- Dimethylacetamide (DMAC) purchased from Beijing Yili Chemical Reagent Co. was used as received. PEO was purchased from Beijing Guoren Yikang Ltd, Beijing, China. The typical synthetic process of nanofoamed films is illustrated in Fig. 1. PMDA, ODA, and PEO were first dried in an oven for a few hours. Then, PMDA, ODA (PMDA: ODA = 1.015:1 molar ratio), and DMAC were mixed together in a flask equipped with a mechanical stirrer. The mixture was stirred at room temperature for 6 h to synthesize poly(amide acid) (PAA) precursors. After the preparation of PAA, PEO was mixed together with PAA and stirred for 6 h. Nanofoamed PI films were prepared by casting the solution onto a glass plate with step-like heating to complete the imidization. The preparation method for pure PI film was the same as that for nanofoamed films but without PEO incorporation. Cross-sectional images of the PI nanofoamed films were taken by scanning electron microscopy (SEM, JEOL JSM-6335F) operating at 15 kv. The thermal stability of the PI nanofoamed films was studied in nitrogen atmosphere by thermogravimetric analysis (TGA, HCT-2, Beijing, China) at heating rate of 10 C/min. The dielectric constant was determined using a frequency-response dielectric analyzer (Novocontrol Alpha-analyzer) at frequencies from 1 Hz to 10 MHz at temperature from 150 C to 150 C. All dielectric measurements were conducted under nitrogen flow (20 ml/min) with sample thickness between 30 lm and 40 lm. RESULTS AND DISCUSSION Figure 2 shows cross-sectional SEM images of PI nanofoamed films containing 0 wt.% (pure PI film), 5 wt.%, 7 wt.%, and 10 wt.% PEO. The porous structure of the nanofoamed films was clearly observed compared with the dense structure of pure PI film. It can be clearly seen that voids are uniformly distributed in PI film. This is due to the fact that the PEO nanoparticles disperse well in the PI film, and closed voids form from degraded PEO phase during the imidization of PI. Figure 2b d shows that the size of closed voids ranges from 40 nm to 200 nm and there are almost no interconnected voids in the PI nanofoamed films. This can be explained based on stress generated by small molecules (created from thermally decomposed PEO) as they escape from the PI matrix, resulting in deviation of the void shape from the expected spherical morphology. This volatilization process changes the pore s original (spherical) shape and size. When small molecules escape from the film, interconnected channels and a rougher fracture surface are obtained, resulting from the fact that interconnected channels form weak links during the fracture process. Moreover, the pore sizes of the 7 wt.% and 10 wt.% composite films are larger than that of the 5 wt.% composite film. Fig. 1. Scheme of the preparation of PI PEO nanofoamed film.

3 Dielectric and Thermal Properties of Polyimide Poly(ethylene oxide) Nanofoamed Films 2283 Fig. 2. SEM images of PI nanofoamed films with (a) 0 wt.%, (b) 5 wt.%, (c) 7 wt.%, and (d) 10 wt.% PEO. Fig. 3. TGA curves of PI nanofoamed films with 0 wt.%, 7 wt.%, and 10 wt.% PEO. Fig. 4. Dielectric constant as a function of frequency for PI nanofoamed films with different PEO contents. Figure 3 exhibits TGA curves of PI and PI PEO nanofoamed films. It can be seen that the thermal decomposition temperature of PI nanofoamed films decreases with increasing PEO content. Thermal decomposition temperatures of 0 wt.%, 7 wt.%, and 10 wt.% composite films are C, C, and C, respectively. Although the decomposition temperature decreases slightly with increasing PEO content, nanofoamed films still possess high thermal stability up to temperature of 500 C. Figure 4 shows the frequency dependence of the dielectric constant of PI nanofoamed films with different PEO content. It can be observed that the dielectric constant of composite films is weakly frequency dependent from 1 Hz to 10 MHz. The dielectric constant of pure PI is about 3.4 at 1 Hz. The dielectric constant of nanofoamed films decreases with increasing PEO content. The dielectric constants of the 5 wt.% and 10 wt.% nanofoamed films are 3.0 and 2.6, respectively. The decrease of the dielectric constant of the PI

4 2284 Zhang, Yu, Zhao, Tong, Huang, Ke, and Chan Fig. 5. Dielectric constant as a function of temperature of PI nanofoamed films with 7 wt.% PEO (measuring frequencies from bottom to top are 10 MHz, 1.23 MHz, 116 khz, 10.9 khz, and 1.03 khz). Fig. 6. Comparison between experimental values and theoretical predictions of dielectric constant for the nanofoamed films at 1 khz. nanofoamed films originates from the increased voids in the foam s porous structure. Figure 5 shows the dielectric constant of the PI nanofoamed films with 7 wt.% PEO content as a function of temperature. A dielectric constant maximum for PI nanofoamed film is observed at around 0 C. The dielectric constant maximum may be attributed to freezing of absorbed moisture in the PI nanofoamed films. However, the detailed mechanism cannot be given at this stage, and further investigation is needed. It can also be found that, at whatever temperature, the dielectric constant of the PI nanofoamed films decreases with increasing measuring frequency. Considering spherical voids (trapped air) distributed in a matrix with dielectric constant e matrix, the dielectric constant of the material can be calculated by the Maxwell model as 2 v matrix e matrix þ v air e air e ¼ 3 þ e air 3e matrix þ v air ; (1) v matrix 2 3 þ e air 3e matrix where v air and v matrix are the volume fractions of air and matrix, respectively. e air is the dielectric constant of air and taken as 1.0. The theoretical value of the dielectric constant of PI nanofoamed films with various PEO contents of 0 vol.%, 5.5 vol.%, 7.7 vol.%, and 10.9 vol.% (equivalent 0 wt.%, 5 wt.%, 7 wt.%, and 10 wt.%) can be calculated on the basis of Eq. 1. A comparison between experimental values and theoretical predictions of the dielectric constant for the nanofoamed films is presented in Fig. 6. It can be seen from Fig. 6 that the general trend of decreasing dielectric constant with increasing PEO content follows the Maxwell model quite well. However, the experimental values of the dielectric constant are lower than the theoretical values suggested by Eq. 1. This might be caused by two reasons. One reason is that the Maxwell model assumes that the voids are spherical and uniformly distributed in the matrix. However, from the SEM cross-section images, it can be found that the shape of voids left inside the matrix is ellipsoidal. The major axis of the ellipsoidal voids is usually parallel to the surface plane of the composite film, and ellipsoidal voids decrease the dielectric constant more effectively than spherical voids would. The other reason is that the actual volume of voids is larger than that used in the calculations. This is ascribed to the fact that additional voids (for example, empty channels) can be formed when thermally decomposed PEO escapes from the matrix. Therefore, the theoretical dielectric constant of the nanofoamed films is overestimated. CONCLUSIONS Polyimide nanofoamed films with pore sizes ranging from 40 nm to 200 nm were successfully prepared by decomposition of PEO. This was an effective approach to lower the dielectric constant of the polymeric material. The dielectric constant of the nanofoamed films decreased greatly with the introduction of pores. PI nanofoamed films possess low dielectric constant of 2.4 and high thermal stability up to about 500 C. Therefore, it can be suggested that PI nanofoamed films have potential application in the microelectronics industry. ACKNOWLEDGEMENTS This research was supported by the Key Project of Chinese Ministry of Education (No ), Hong Kong Polytechnic University Postdoctoral Fellowship Scheme (G-YX70), and Fundamental Research Funds for the Central Universities (2011PY0180, 2011PY0181).

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