A Facile Strategy for Non-fluorinated Intrinsic Low-k and Low-loss Dielectric Polymers: Valid Exploitation of Secondary Relaxation Behaviors

High-performance low-k and low-loss circuit materials are urgently needed in the field of microelectronics due to the upcoming Fifth-Generation Mobile Communications Technology (5G Technology). Herein, a facile design strategy for non-fluorinated intrinsic low-k and low-loss polyimides is reported by fully considering the secondary relaxation behaviors of the polymer chains. A new amorphous non-fluorinated polymer (TmBPPA) with a k value of 2.23 and a loss tangent lower than 3.94 × 10−3 at 104 Hz has been designed and synthesized, which to the best of our knowledge is the lowest value amongst the non-fluorinated and non-porous polymers reported in literature. Meanwhile, TmBPPA exhibits excellent overall properties, such as excellent thermostability, good mechanical properties, low moisture absorption, and high bonding strength. As high-performance flexible circuit materials, all these characteristics are highly expected to meet the present and future demands for high density, high speed, and high frequency electronic circuit used in 5G wireless networks.


INTRODUCTION
Advances in mobile communications and microelectronics are revolutionizing our way of life. With the development of electronic products in the trend of lighter, thinner, shorter, and smaller, the integration density of flexible circuit boards is getting increasingly higher. [1−3] On the other hand, with the fast development of the emerging Fifth-Generation Mobile Communications Technology (5G Technology), signal transmission continues to develop in the direction of high frequency and high speed. [4,5] Both of the above technologies require highperformance insulated materials with low dielectric constant (D k < 2.6) and low loss tangent (D f < 5.0 × 10 −3 ) properties to reduce the resistance-capacitance delay (RC delay), the line-to-line crosstalk noise, and the power dissipation, thus to ensure the speed, the integrity, and the accuracy of signal transmission and to improve the service life of electronic components. [6−8] In principle, there are two strategies to reduce the D k and D f values of polymer materials. [9−14] One is to decrease the polymer density, which has the strongest effect on the concentration of polarizable components and the k value. [15−21] As a result, decreasing the polymer density by deliberate introduction of porosity has been the main method used by the semiconductor industry to decrease k below 3.0 (which can be as low as 1.16). [19] However, the pores with uncontrolled size and distribution exhibit high moisture absorption and usually collapse under high temperature, resulting in deterioration of the dielectric properties. [22−26] Another strategy is the introduction of strongly electronegative atoms, such as fluorine, into the polymeric structure to reduce the polarizability. [27−30] The k value of fluorinated polymers can reach 2.30, but the poor adhesion between the fluorinated polymer and the substrate remains a significant challenge. [31] Therefore, the development of nonfluorinated intrinsic polymer circuit materials with low-k and low-loss properties still requires much more efforts.
In general, the most effective way to reduce the D k and D f values of non-fluorinated and non-porous polymeric materials is to increase the intrinsic free volume. This is because the additional free volume in the polymer introduces more ultralow-k air component and subsequently dilutes the polar group concentration and weakens the interactions between the polymer chains. Most of the prior-art has focused on introducing bulky side-groups into the polymeric structure in order to enlarge the free volume between polymer chains, but the complicated polymeric structure and high manufacturing cost make it difficult to realize commercial production and industrial application. [32−34] Therefore, a facile and effective chemical structure design strategy to increase the free volume of polymers is a vital approach to obtain non-fluorinated intrinsic low-k polymers. In contrast to the static molecular design approach reported in literature, herein, a brandnew molecular design strategy is proposed to obtain a high performance intrinsic non-fluorinated low-k polymer by fully considering the secondary relaxation of the polymer chains, especially the β relaxation. For polymeric systems, when the environment temperature is below the glass transition temperature (T g ), the segmental motion is frozen; however, the side-group is still rotatable (known as the secondary relaxation or β relaxation that refers to the torsion of groups in the polymer backbone or the rotation of the pendant group), which can greatly affect the intrinsic free volume. [35−39] In order to specifically manipulate the secondary relaxation behavior of the polymer chains, we have designed a new non-fluorinated intrinsic low-k polymer (TmBPPA) (Fig. 1a) in order to improve the intrinsic free volume via the rotation of homocyclic aromatic rings in the side-groups. As-prepared TmBPPA film exhibits a k value of 2.23 with a loss tangent of 3.94 × 10 −3 at 10 4 Hz. Moreover, the film shows high thermostability with T g of 352 °C, 5 wt% decomposition temperature of 579 °C, and a residue rate of 67% at 800 °C under N 2 , which means that it certainly meets the requirement of Cu damascene metallization in microelectronics fabrication. To the best of our knowledge, the k value of TmBPPA is the lowest reported amongst the non-fluorinated and non-porous polyimides.
Besides, our product also exhibits outstanding thermo/mechanical properties.

Materials
3-Bromoaniline, 4-bromoaniline, 1-fluoro-4-nitrobenzene, 4-biphenylboronic acid, aniline, and cesium fluoride were purchased from Aladdin Industrial Corporation and used as received. Aliquat 336 (tricaprylylmethylammonium chloride) and tetrakis(triphenylphosphine) palladium (Pd(PPh 3 ) 4 ) were purchased from J&K company and used as received. Pyromellitic dianhydride (PMDA) was purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd. and was heated at 150 °C under vacuum for 12 h prior to use. Chromatographically pure dimethyl formamide (DMF) was purified by distillation under an inert nitrogen atmosphere. All other solvents and reagents as analytical grade were purchased from Guangzhou Dongzheng Company and used without further purification.

Synthesis of N1-([1,1':4',1''-terphenyl]-3-yl)-N1-(4-aminophenyl)benzene-1,4-diamine (A-TmBP)
N-TmBP (4.875 g, 10 mmol), one spoonful of 10% Pd/C catalyst (~0.05 g), and ethanol (100 mL) were charged into a 500 mL 3-neck round-bottom flask and then hydrazine hydrate (3 mL) was added dropwise. The reaction was followed by refluxing under nitrogen for 24 h. After removing the ethanol by rotary evaporation, the grey precipitates were collected and then purified by chromatography on silica gel with dichloromethane/ hexane as an eluent. The purified product is grey crystals with a yield of 72%. 1     obtained by thermal imidization of PAA. TmBPPA exhibits good solubility in N-methylpyrrolidone (NMP), and as shown in Fig.  1(b), the TmBPPA film possesses good flexibility. Due to the high-k of water (k water = 81), we measured the moisture absorption of the polymer film. The TmBPPA film exhibits moisture absorption of less than 0.78% after being soaked in deionized water at 25 °C for 24 h, which perfectly meets the requirement of ideal low-k materials. Such low moisture absorption is preferred for low-k materials and is attributed to the hydrophobic groups in the TmBPPA polymer chains. Scanning electron microscopy (SEM) image (Fig. 1c) shows that the film section is uniform and compact. Atomic force microscopy (AFM) image (Fig. 1d) shows that there are no apparent pores on the film surface, and the surface roughness (R q ) is 0.386 nm, which indicates the polyimide film contains no nanoporous structures after solution casting and subsequent solvent evaporation. The k value of the TmBPPA film was measured by the capacitance method at various frequencies. The parallel plate capacitor was fabricated using Cu sheets (1 cm × 1 cm) as the electrodes sticking on the two surfaces of the TmBPPA film by silver paste (Fig. S1 in electronic supplementary information, ESI). The completed parallel plate capacitor underwent heat treatment at 120 °C for 2 h to remove the residual water and chemical solvents. All the capacitance data (C) was measured by an SI1260 impedance analyzer (Solartron, USA) in a constant temperature and humidity environment. The k value of the TmBPPA film was subsequently calculated from its capacitance (C) by Eq. (1):

RESULTS AND DISCUSSION
where C is the obtained capacitance, d is the thickness of the TmBPPA film, S is the surface area of the Cu sheet electrode, and k 0 is vacuum permittivity (8.854 × 10 −12 F·m -1 ). Fig. 2(a) shows the relationship of k and loss tangent with frequency in the range from 10 2 Hz to 10 6 Hz. The k values of the TmBPPA film kept below 2.3 and the loss tangent values of the TmBPPA film kept below 0.008 at these frequencies, which are much lower than those of other traditional intrinsic low-k non-fluorinated polymers (e.g., 2.25−2.35 for polyethylene, 2.45−3.10 for polystyrene), and even some fluorinated polymers. More importantly, the k value of the TmBPPA film is extremely stable even when the temperature reaches 300 °C (Fig. 2b). There are two aspects that contribute to the reduction of k of the TmBPPA film. Firstly, the amorphous form of the TmBPPA molecular chains in the film, detected from wide angle X-ray diffraction (WAXD) (Fig.  S2 in ESI), can effectively decrease orientation polarization and subsequently reduce the k value compared with a crystalline polymer. The second aspect is the additional free volume present in the polymer chains, which can greatly decrease the number density of the dipoles. On the dynamic mechanical analysis (DMA) curve (Fig. 3), there is an obvious broad peak ranging from 50 °C to 200 °C that represents the β relaxation behavior of the polymer chains. For the TmBPPA film, the β relaxation behavior mainly corresponds to the rotation of side terphenyl unit connected to the nitrogen atom (blue unit of the structure inset in Fig. 3). The blue unit keeps a constant angle (60°) with the rotational axis, and the rotation of this side terphenyl unit connected to the nitrogen atom will create additional free volume like a cone. The additional free volume will not be influenced by the segment parking below T g and can observably reduce the k value of the TmBPPA film.
To clearly verify the formation mechanism of additional free volume in the TmBPPA film, we synthesized two other polyimides (TPPA and TpBPPA) as references. As shown in Fig. 4(a), TPPA has the same polymer backbone structure as TmBPPA, but only contains a phenyl ring attached to the ni- trogen atom as the pendant group, i.e., with no biphenyl unit attached to the meta-position of the phenyl ring. On the other hand, TpBPPA also has the same backbone structure as TmBPPA, but in this case, the phenyl ring is substituted with a biphenyl unit at the para-substitution. With regards to the chemical structure, the β relaxation behavior may occur in all these three polyimides (T β TPPA : 139 °C, T β TpBPPA : 145 °C, T β TmBPPA : 150 °C, shown in Fig. S3 in ESI), but different molecular structures in the side-group will lead to different sizes of free volume created by the self-rotation of the side-group. In these three polyimides, only TmBPPA with meta-substitution shows more free volume by the rotation of its terphenyl unit in the pendant group. The increase in free volume can be verified by WAXD measurements and density testing compared to the TPPA film and the TpBPPA film. The WAXD peaks at 2θ may be assigned to the interlayer distance of the polymer. The interlayer distance calculated from 2θ data indicates that the TmBPPA film (d = 0.564 nm) has larger interlayer distance than the TPPA film (d = 0.541 nm) and the TpBPPA film (d = 0.524 nm) (see Fig. S2 and Table S3 in ESI). The density of the TmBPPA film (ρ = 1.2627 ± 0.0044 g•cm −3 ) is lower compared with those of the TPPA film (ρ = 1.3263 ± 0.0027 g•cm −3 ) and the TpBPPA film (ρ = 1.2901 ± 0.0030 g•cm −3 ). The interlayer distance and density clearly demonstrate that the introduction of a biphenyl unit with meta-substitution in the side-group of the polymer chains (TmBPPA, k = 2.23, tanδ = 3.94 × 10 −3 , 10 kHz) can effectively increase the free volume due to the self-rotation of the terphenyl unit with meta-sub-stitution in the side-group. The same tendency was found in the MD simulation of fractional free volume (FFV) (the details of MD simulation process are described in ESI). As shown in Table 1 and Fig. 5, the TmBPPA film has the largerest FFV in these three polyimides, which also indicates the introduction of a biphenyl unit with meta-substitution as a side group to the polymer chains can effectively improve the free volume. Thanks to the more free volume, TmBPPA shows the lowest k value (2.23, 10 kHz) and tanδ (3.94 × 10 −3 , 10 kHz) compared with TPPA (k = 3.48, tanδ = 8.97 × 10 −3 , 10 kHz) and Tp-BPPA (k = 2.75, tanδ = 5.31 × 10 −3 , 10 kHz) (Figs. 4b and 4c).
The thermostability of TmBPPA was evaluated by DMA (Fig. 3), thermal mechanical analysis (TMA) (Fig. S4 in ESI), and thermal gravimetric analysis (TGA) (Fig. S5 in ESI). The DMA results show that T g of the TmBPPA film is about 352 °C. The TMA results show that the coefficient of thermal expansion (CTE) of the TmBPPA film is about 46 ppm•K −1 , and the TGA measurements show that the 5 wt% decomposition temperature for TmBPPA is 579 °C with a residual rate of 67% at  800 °C under N 2 . The TmBPPA film exhibits excellent thermostability because of the imide ring structure. In the microelectronics processing field, such thermostability of TmBPPA meets the requirement in structure interconnect fabrication of Cu metallization using the damascene process, during which the low-k insulating materials must be stable in a temperature range from 350 °C to 400 °C.
The mechanical properties of the TmBPPA film were measured using a tensile testing machine with pneumatic clamps. The results show that the TmBPPA film has a tensile modulus of 2.43 GPa and a tensile strength of 73 MPa. Different from the low-k fluorinated materials, TmBPPA shows a great bonding strength of 140 MPa with the monocrystalline wafer that was measured by scratch tests, which shows attractive application prospects in the field of flexible microelectronics, such as flexible circuit materials and flexible substrates.

CONCLUSIONS
In summary, by full consideration of the secondary relaxation behaviors of the polymer macro-chains, we have designed and synthesized a new amorphous non-fluorinated and non-porous polymer (TmBPPA), which shows a k value of 2.23 with a loss tangent of 3.94 × 10 −3 at 10 4 Hz. Moreover, the TmBPPA film shows k values below 2.30 in the range from 10 2 Hz to 10 6 Hz. Such k value is lower than those of the previously reported lowk non-fluorinated and non-porous polyimides. In comparison with porous and fluorinated low-k materials, TmBPPA shows a higher modulus and bonding strength, which makes it more suitable for applications as flexible circuit materials with the characteristics needed for present and future high density, high speed, and high frequency electronic circuit designs for 5G wireless networks.

Electronic Supplementary Information
Electronic supplementary information (ESI) is available free of charge in the online version of this article at http://dx.doi.org/ 10.1007/s10118-020-2339-4.