Polyimide (PI) has been widely employed as interlayer dielectrics in microelectronics industry for a long time by virtue of low dielectric constant (k) [13]. However, today lower k is always demanded for its application in advanced microelectronic devices [4, 5]. Macromolecular chains of PI can form the complex with crown ether molecules via hydrogen bond, to exhibit a decrease in k as a result of shielding effect of lowpolarity crown ethers on the strong polar imide groups [68].18Crown6 induces the k of PI from 3.59 to 2.99 [9], and γallyloxymethyl 18crown6 (AC6) could even cause the k as low as 2.58 by virtue of an additional evaporation voiding during thermal imidization. The porous structure favors the decrease in the k, but it is harm to mechanical properties, and the tensile strength and fracture energy of the complex of AC6 and PI (AC6PI) decrease sharply. It becomes a great challenge to improve mechanical properties of the complex on the premise of maintaining low dielectric constant [10, 11]. In addition, the introduction of crown ethers with lower thermal stability resulted in the obvious reduction in the glass transition temperature and 5% weight loss temperature of PI. It was found that polyhedral oligomeric silsesquioxane with outstanding thermal oxidative stability could improve thermal properties of the complex of crown ether and PI to some degree [12]. But less toughness was usually induced by the structural defect originating from agglomeration of nanoparticles [13].
Improvement in both mechanical and thermal properties of PI were achieved by the introduction of crosslinked network structure of stable SiOSi skeleton [14, 15]. Vinylfunctionalized polysiloxaneblockpolyimide owned hightemperature durability and elevated tensile strength through the crosslinking of poly (hydromethylsiloxane) [16]. Moreover, the thermal stability of AC6 could be ameliorated via hydrosilylation reaction of its reactive γallyloxy group with triethoxysilane [17, 18]. It was expected that a SiOSi crosslinking approach can balance the lower k with mechanical properties and thermal stability of the AC6PI complex. In this present work the complex of AC6 and poly(amic acid) (PAA) precursor underwent the crosslinking through hydrosilylation of tetrakis(dimethylsiloxy)silane (TDSS) during thermal imidization. The mechanical, thermal properties and dielectric constant of crosslinked AC6PI (SiACPI) composites were investigated.
AC6 was synthesized in our lab according to Ref. [19, 20]. SiACPI composites were prepared through a"onepot"process.The moleratio ofAC6to4, 4'diaminodiphenyl ether(ODA)inthefeedwas0.4foreach composite. The mole ratios of SiH of TDSS to CH=CH_{2} of AC6 were changed into 0.5, 1.0 and 2.0 to obtain three composites with different crosslinked degree, named as SiACPI1, SiACPI2 and SiACPI3. Typically, SiACPI2was preparedas follows.Firstly, 1.03g(3.07mmol) AC6 and 1.57g (7.67mmol) ODA at the mole ratio of 0.4 was dissolved in 24g Nmethyl2pyrrolidone (NMP) in a threeneck flask. The mixture was stirred for 2 h at room temperature to form AC6ODA hostguest inclusion complex. Secondly, 1.71 g (7.67 mmol) 1, 2, 4, 5 pyromellitic dianhyride (PMDA) was subsequently added to form thecomplexofAC6andPAAsolution.The mixture was stirred for 6 h at room temperature. Thirdly, 0.26 g (0.77 mmol) TDSS and 0.035 g (0.03 mmol) tetrakis(triphenylphosphine) palladium (Pt(PPh3)4) were poured into AC6PAA solution, and the mixture solution was continuously stirred for 2 h. Whereafter, the obtained solution was successively heated at 80 ℃/1 h, 100 ℃/1 h, 200 ℃/1 h, 300 ℃/2 h under vacuum to accomplish thermal imidization progress. The hydrosilylation reaction between AC6PAA and TDSS occurred during thermal imidization.The content of AC6 and TDSS inSiACPI 2 was 13.87 wt% and 6.82 wt%, respectively, according to 50% inclusion rates of crown ether [9]. AC6PI complex without crosslinking of TDSS was prepared as a contrast.
The FTIR spectra of PI films were recorded on a Bruker Vector22 Fourier transform infrared spectroscopy (FTIR). The fracture surface morphology of PI films was observed on a Nova NanoSEM430 scanning electron microscopy (SEM). According to ASTM D88212, mechanical properties of the films were analyzed with a universal material testing machine. The glass transition temperature (T_{g}) and the storage modulus (E') of PI films were obtained from a Netzsch 242C dynamic mechanical analyser (DMA) at a heating rate of 5 ℃/min. Thermal stability of the samples was conducted under nitrogen with a Netzsch 209F1 thermogravimetric analyzer (TGA) at a heating rate of 20 ℃/min. The morphological structure of the samples was performed using a Bruker D8ADVANCE wideangle Xray diffractometer (XRD) and Cu Kα (l = 1.542 Å) was chosen as the radiation source. Capacitance of PI films was measured by an Agilent 4284ALCR meter at a frequency of 1 MHz. The film density was measured at 23 ± 0.5 ℃ via liquid pyknometer method according to ISO 11831:2012. The dielectric constant (k) of PI films was calculated according to the following equation: k = Cd/ε_{0}S, where C is capacitance (F), d is thickness of the films (m), S is the area of the electrodes (m^{2}), ε_{0} is the permittivity of vacuum which equals to 8.854 －10^{12} F/m.
AC6 is a kind of crown ethers involving reactive γallyloxy group. Like 18crown6, it can form the interlocking complex with the macromolecular chain of PI to decrease the k. It is more difficult for AC6 to evaporate from the matrix than 18crown6. The escape of small amounts of unlocked AC6 at nearly 300 ℃ leaves pores in the matrix after imidization of AC6PAA precursor. The density of the obtained AC6PI is only 1.250 g/cm^{3}, which is 13% lower than the density of PI (1.441 g/cm^{3}). The presence of pores can be confirmed by the comparison of crosssectional SEM micrographs of PI (Fig. 1a) and AC6PI (Fig. 1b). In fact, the porous structure favors the decrease in k, which drops to 2.58 from 2.99 of the complex of 18crown6 and PI. But it is detrimental to mechanical properties of PI. The tensile strength and fracture energy of AC6PI sharply decrease to 67.5 MPa and 3.1 MJ/m^{3} from 119.3 MPa and 20.9 MJ/m^{3} of PI, respectively. It is necessary to overcome the undesirable results of evaporation voiding.
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Fig. 1. Crosssectional SEM images of (a) pristine PI, (b) AC6PI, (c) SiACPI1, (d) SiACPI2, and (e) SiACPI3. 
Here TDSS containing four SiH bonds was applied for hydrosilylation with γallyloxy group of AC6 during thermal imidization of AC6PAA. For one thing, TDSS can fasten unlocked AC6 to alleviate evaporation voiding. For another, it acts as a crosslinking agent for macromolecular chains of AC6PI. The preparation process of AC6PI complex and its crosslinked composites (SiACPI) was shown in Scheme 1a. The structure of SiACPI composites was diagramed in Scheme 1b.
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Scheme 1. (a) The preparation process of AC6PI complex and SiACPI, (b) the structure diagram of SiACPI composites. 
FTIR spectra and section spectra of AC6PI, SiACPI2 and TDSS are shown in Fig. 2. The structure of SiACPI composites can be proved to some degree from their FTIR spectrum variation (Fig. 2a). The absorption bands of C＝C at 3100–3050 cm^{1} (Fig. 2b) are characteristics of γallyloxy group, and the absorption bands of SiH at 22502090 cm^{1} cannot be observed in the spectrum of SiACPI2, indicating the reaction occurrence between AC6 and TDSS. Besides, C–H stretching absorption of CH_{3} from TDSS at 2960 cm^{1} (Fig. 2b) becomes quite evident.
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Fig. 2. (a) FTIR spectra and (b) section spectra from 3200 cm^{1} to 2900 cm^{1} of AC6PI, SiACPI2 and TDSS. 
The crosslinked density (v_{e}) of SiACPI composites can be estimated from the storage modulus (E') of DMA test in accordance with MooneyRivlin equation v_{e} ＝ E'/3RT [21, 22]. Here, T is T_{g} + 40 ℃, and E' is storage modulus at the temperature of T_{g}+40 ℃ in E'temperature curves (Fig. 3a). v_{e} of SiACPI1 rises to 2.25 mmol/cm^{3} from 1.34 mmol/cm^{3} of AC6PI (Table 1), evidencing the increase of crosslinking degree with the introduction of TDSS. v e of SiACPI3 rises slightly to 2.36 mmol/cm^{3} from 2.33 mmol/cm^{3} of SiACPI2, indicating the occurrence of full crosslinking among AC6PAA chains at equal mole ratio of Si—H to CH＝CH_{2}.
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Fig. 3. (a) E'temperature curves, (b) stressstrain curves, (c) tan δtemperature curves, and (d) weighttemperature curves of five PI films. 
The mechanical strength of AC6PI enhances remarkably via crosslinking. Young's modulus, tensile strength and fracture energy of PI, AC6PI and three SiACPI composites are listed in Table 1, and stressstrain curves are shown in Fig. 3b. It is found that several mechanical properties of SiACPI1 evidently excel over AC6PI. The properties of the composites become higher with the increase in crosslinking degree. Compared with AC6PI, Young's modulus and tensile strength of SiACPI2 are increased by 12% and 87%. It is more significant that fracture energy of SiACPI2 rises to 25.3 MJ/m^{3} from 3.1 MJ/m^{3} of AC6PI, which overmatches pristine PI. This should be attributed to the evident abatement of evaporation voiding (Figs. 1c–e). Less number and smaller size of pores in the matrix can be observed from crosssectional SEM micrographs of SiACPI2 (Fig. 1d). The density of SiACPI2 goes up to 1.361 g/cm^{3}. This modulus and strength variation of three SiACPI composites can be obtained well explanation from ve. It is worth noting that two properties of SiACPI3 are slightly superior to those of SiACPI2.
As the crosslinked structure restricts the movement of PI chain segments, the glass transition temperature (T_{g}) of SiACPI2 goes up to 341.9 ℃ from 327.4 ℃ of AC6PI, quite close to 352.4 ℃ of pristine PI (Fig. 3c). The 5% weight loss temperature (T5%) of SiACPI2 increases from 414.3 ℃ to 453.1 ℃ (Fig. 3d). It can be seen from the figures that the crosslinking can actually elevate thermal stability of AC6PI.
The dielectric constant is an important parameter for the application of PI in microelectronic materials. Less number and smaller size of pores in the matrix usually induce the increase of k value, while the introduction of SiOSi skeleton broadens the interchain distance of PI, which is beneficial to the reduction of k. The interchain distance (d) of PI can be calculated from XRD diffraction patterns (Fig. 4) according to Bragg equation 2dsinθ ＝ nλ [23, 24]. The results indicate that d of AC6PI (4.81 Å) is evidently larger than that of pristine PI (4.23 Å), and d of SiACPI composites further increases, and d of SiACPI2 is 4.85 Å. It is found that the k of SiACPI1, SiACPI2 and SiACPI3 is 2.59, 2.61 and 2.63 at 1 MHz, near to 2.58 of AC6PI, indicating the primary effect of SiOSi skeleton.
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Fig. 4. XRD diffraction patterns of five PI films. 
The SiACPI composites were prepared through hydrosilylation crosslinking of AC6PAA precursor with TDSS during thermal imidization. Mechanical and thermal properties of AC6PI were simultaneously enhanced with the formation of crosslinked structure. Especially, the fracture energy was elevated from 3.1 MJ/m^{3} to 25.3 MJ/m^{3} owing to the evident abatement of evaporation voiding. Young's modulus, tensile strength and fracture energy of the composites overmatched pristine PI, with the k as low as 2.6. An approach is presented to decrease the dielectric constant as well as upgrade mechanical properties of PI. It shows great application prospects in microelectronic materials.
AcknowledgmentsThis research is supported by the National Natural Science Foundation of China (No. 51573054) and the Project for Science and Technology of Guangzhou (No. 201604016080).
[1] 
C.M. Leu, Y.T. Chang, K.H. Wei, Macromolecules 36 (2003) 91229127. DOI:10.1021/ma034743r 
[2] 
Y.H. Xiao, Y. Shao, X.X. Ye, et al., Chin. Chem. Lett. 27 (2016) 454458. DOI:10.1016/j.cclet.2015.12.018 
[3] 
X. Zhao, Q.F. Geng, T.H. Zhou, X.H. Gao, G. Liu, Chin. Chem. Lett. 24 (2013) 3133. DOI:10.1016/j.cclet.2012.11.013 
[4] 
G. Maier, Prog. Polym. Sci. 26 (2001) 365. DOI:10.1016/S00796700(00)000435 
[5] 
J.N. Myers, Z. Chen, Chin. Chem. Lett. 26 (2015) 449454. DOI:10.1016/j.cclet.2015.01.016 
[6] 
T. Iijim, S.A. Vignon, H.R. Tseng, et al., Chem.Eur. J. 10 (2004) 63756392. 
[7] 
J.G. Hansen, N. Feeder, D.G. Hamilton, et al., Org. Lett. 2 (2000) 449452. DOI:10.1021/ol991289w 
[8] 
S. Saha, M.N. Roy, J. Mol. Struct. 1147 (2017) 776785. DOI:10.1016/j.molstruc.2017.07.017 
[9] 
Y. Li, J.Q. Zhao, Y.C. Yuan, et al., Macromolecules 48 (2015) 21732183. DOI:10.1021/acs.macromol.5b00307 
[10] 
J.L. Hedrick, T.P. Russell, Macromolecules 29 (1996) 36423646. DOI:10.1021/ma950903q 
[11] 
M.Q. Liu, J.P. Duan, X.Z. Shi, J.J. Lu, W. Huang, Express Polym. Lett. 9 (2015) 1422. DOI:10.3144/expresspolymlett.2015.3 
[12] 
C.Q. Shi, S.M. Liu, Y. Li, et al., Compos. Sci. Technol. 142 (2017) 117123. DOI:10.1016/j.compscitech.2017.02.002 
[13] 
J. Choi, R. Tamaki, S.G. Kim, R.M. Laine, Chem. Mater. 15 (2003) 33653375. DOI:10.1021/cm030286h 
[14] 
H.B. Hsueh, C.Y. Chen, C.C. Wang, T.J. Chu, J. Appl. Polym. Sci. 89 (2003) 28652875. 
[15] 
X.F. Lei, Y.H. Chen, M.T. Qiao, L.D. Tian, Q.Y. Zhang, J. Mater. Chem. C 4 (2016) 21342146. DOI:10.1039/C5TC03391H 
[16] 
N. Furukawa, M. Yuasa, Y. Yamada, Y. Kimura, Polymer 39 (1998) 29412949. DOI:10.1016/S00323861(97)006149 
[17] 
Y.Y. Chen, G.P. Yuan, Chem. J. Chin. Univ. 4 (1983) 739744. 
[18] 
Y.Y. Chen, J.Q. Luo, X.R. Lu, Chin. J. Appl. Chem. 6 (1989) 1621. 
[19] 
FavreRéguillon A., N. Dumont, B. Dunjic, M. Lemaire, Tetrahedron 53 (1997) 13431360. DOI:10.1016/S00404020(96)010721 
[20] 
W.Y. Huang, Y.M. Wu, J.T. Liu, Chin. J. Chem. 13 (1995) 251262. 
[21] 
H. Koerner, R.J. Strong, M.L. Smith, et al., Polymer 54 (2013) 391402. DOI:10.1016/j.polymer.2012.11.007 
[22] 
Z. Rasheva, L. Sorochynska, S. Grishchuk, K. Friedrich, Express Polym. Lett. 9 (2015) 196210. DOI:10.3144/expresspolymlett.2015.21 
[23] 
K. Takizawa, J. Wakita, K. Sekiguchi, S. Ando, Macromolecules 45 (2012) 47644771. DOI:10.1021/ma300497a 
[24] 
J.R. Weidman, S. Luo, C.M. Doherty, et al., J. Membrane Sci. 522 (2017) 1222. DOI:10.1016/j.memsci.2016.09.013 