Preparation and Characterization of Intrinsic Low-κ Polyimide Films

Three fluorinated polyimide (PI-FP, PI-FO and PI-FH) films with low dielectric constants and excellent comprehensive properties were successfully prepared using a polycondensation reaction method by incorporating p-phenylenediamine (PDA), 4-4′-diaminodiphenyl ether (ODA) and 4,4′-(Hexafluoroisopropylidene) bis (p-phenyleneoxy) dianiline (HFPBDA) into 4,4′-(Hexafluoroisopropylidene) diphthalic anhydride (6FDA), respectively. The effects of the diamine monomer structure on optical, dielectric and mechanical properties were investigated. Compared with PDA and ODA, HFPBDA can effectively improve the optical and dielectric properties of PI due to due to its special chain structure. Among the three PI films, PI-FH film presents the best optic transmission (highest transmittance = 90.2%) and highest energy gap (2.69 eV). The dielectric properties of PI-FH film improve the most. The dielectric constant and loss at 104 Hz are reduced to 2.05 and 0.0034 at 104 Hz, respectively, and remain stable up to 250 °C. The mechanical properties decrease in turn for PI-FP, PI-FO and PI-FH films due to the increase in free volume fraction. Nevertheless, PI-FH film still exhibits good mechanical properties with a tensile strength of 88.4 Mpa, a tensile modulus of 2.11 GPa and an elongation at break of 4.1%. The correlation between the dielectric and mechanical properties of PI films and their free volume characteristics is well explained with the help of positron annihilation spectroscopy.


Introduction
With the rapid development of the integration level for ultralarge-scale integration (ULSI) in the semiconductor industry, the size of electronic devices is reducing and the density of transistors is increasing, causing an increase in interconnection delays, cross-talk noise and the power dissipation of circuits [1][2][3][4]. It has become a bottleneck restricting further improvements in the speed of integrated circuits, mainly because the dielectric performance of traditional low dielectric material (SiO 2 ) between lines in microelectronic circuits cannot meet the requirements for the dielectric constant (k) = 3.9~4.2. Studies show that [5][6][7] some materials with ultra-low k are used to replace traditional low-dielectric SiO 2 , which can effectively reduce the interconnection delay, crosstalk and energy consumption. Therefore, in the context of the deep development of ULSI, the development and application of high-performance dielectric materials with low dielectric (k ≤ 2.5) and ultra-low dielectric (k ≤ 2.0) constants is urgent and inevitable. Such dielectric materials as interlayer dielectrics can reduce the capacitance between the metal interconnections, the resistance capacitance delay, the line-to-line crosstalk noise, and the power dissipation [8,9]. In recent years, polymer materials are considered as a kind of low dielectric materials with good application prospects in ULSI due to their natural low density, good flexibility, low molecular polarizability and natural microporosity. Among these polymer materials, polyimide (PI) films attracted more attention and are widely used as electronic materials the ice water mixture, and the magnetic stirrer was used to stir the solution for 8 h, after the polymerization, a viscous, homogeneous, transparent, light-yellow polyamic acid (PAA) solution was obtained. The synthesis reaction process and the photos of the PI films are shown in Figure 1. Finally, after coating on to clean glass plates, the PPA solution was heated at 300 • C for 2 h under vacuum to thoroughly remove the solvent. Three kinds of PI films were thus obtained and the thickness measured ranged from 0.019-0.021 nm. The films were coded PI-FP, PI-FO and PI-FH, where PI-FP represented 6FDA-PDA, PI-FO represented 6FDA-ODA and PI-FH represented 6FDA-HFBAPP, respectively.

Characterization
The microstructures were characterized by flourier transform infrared spectroscopy (FT-IR) spectra recorded in the transmittance mode on a spectrometer (VERTEX70, Bruker Company, Rudolf-Plank-Str. 27. Ettlingen, Germany). Positron annihilation lifetime spectra were measured using a conventional fast-fast coincidence lifetime spectrometer (Ortec Company, South Illinois Avenue Oak Ridge 801, TN, USA) with a time resolution of 220 ps and a channel width of 13.0 ps. A 22 Na positron source with the activity of ~20 μCi sealed between Kapton films was sandwiched between two identical samples (The superposition thickness of PI films on each side of positron source is about 1 mm.) measurements and each spectrum contains more than 1.5 × 10 6 counts and the obtained lifetime spectra were resolved using PATFIT software package. The transmittance spectra of Ultraviolet-visible for PI films in the wavelength range of 200~800 nm were measured using a UV-VIA-NIR (UH4150, Hitachi Company, 6-6, Marunouchi 1-chome, Chiyoda-ku, Tokyo, Japan) spectrometer. The dielectric properties of the PI films were determined using an Agilent 4294A Precision Impedance Analyzer and Dielectric temperature spectrum measurement system (

FT-IR Spectra and EDX
FT-IR spectra of PI films are presented in Figure 2. All of the PI films show characteristic imide absorption peaks, which are consistent with the results reported in the relevant studies [11,12,21]. The peak at 725 cm −1 represents the bending vibration of C=O in the imide ring. The peak at 1376 cm −1 represents the stretching vibration of C-N in the imide ring. The peaks at 1719 cm −1 and 1774 cm −1 represent the symmetrical stretching vibrations and the asymmetrical carbonyl stretching vibrations of C=O, respectively, in the imide ring. The peak at 1510 cm−1 represents the stretching vibration peak of the ben-zene ring, and the peak at 3072 cm−1 represents the stretching vibration of C-H on the benzene ring. The FT-IR spectra results indicate the successful reaction between the dia-mine monomers (PDA, ODA and HFPBDA) and the dianhydride (6FDA), as well as the complete imidization reaction of PAA. In addition, the C-F stretch vibration absorption peak from the trifluoromethyl (-CF3) groups is observed near 1150 cm−1 (within the pink dotted line), and

Characterization
The microstructures were characterized by flourier transform infrared spectroscopy (FT-IR) spectra recorded in the transmittance mode on a spectrometer (VERTEX70, Bruker Company, Rudolf-Plank-Str. 27. Ettlingen, Germany). Positron annihilation lifetime spectra were measured using a conventional fast-fast coincidence lifetime spectrometer (Ortec Company, Oak Ridge, TN, USA) with a time resolution of 220 ps and a channel width of 13.0 ps. A 22 Na positron source with the activity of~20 µCi sealed between Kapton films was sandwiched between two identical samples (The superposition thickness of PI films on each side of positron source is about 1 mm.) measurements and each spectrum contains more than 1.5 × 10 6 counts and the obtained lifetime spectra were resolved using PATFIT software package. The transmittance spectra of Ultraviolet-visible for PI films in the wavelength range of 200~800 nm were measured using a UV-VIA-NIR (UH4150, Hitachi Company, 6-6, Marunouchi 1-chome, Chiyoda-ku, Tokyo, Japan) spectrometer. The dielectric properties of the PI films were determined using an Agilent 4294A Precision Impedance Analyzer and Dielectric temperature spectrum measurement system (DMS2000, Partulab company, No. 602, building 9, Guanggu New Power Industrial Park, Gaoxin Road 5, Donghu New Technology Development Zone, Jiangxia District, Wuhan, China). The k value was calculated according to the equation: k = Cd k 0 S , where C is the capacitance value, k 0 is the dielectric constant of vacuum which is 8.85 × 10 −12 F/m; S and d are the electrode area and thickness of the films. The electric breakdown properties were tested by Voltage-dependent Resistor Characteristic (HG2516, Changzhou Yangzi Electronic Comany, Qingyang Road 2, Xinbei District, Changzhou, China), and the mechanical properties by an electronic universal test machine (UTM 2202, Shenzhen Suns Technology Stock Company, Heli Road 5, Shenzhen, China), and the deformation rate in tensile tests is 10 mm/min.

FT-IR Spectra and EDX
FT-IR spectra of PI films are presented in Figure 2. All of the PI films show characteristic imide absorption peaks, which are consistent with the results reported in the relevant studies [11,12,21]. The peak at 725 cm −1 represents the bending vibration of C=O in the imide ring. The peak at 1376 cm −1 represents the stretching vibration of C-N in the imide ring. The peaks at 1719 cm −1 and 1774 cm −1 represent the symmetrical stretching vibrations and the asymmetrical carbonyl stretching vibrations of C=O, respectively, in the imide ring. The peak at 1510 cm −1 represents the stretching vibration peak of the ben-zene ring, and the peak at 3072 cm −1 represents the stretching vibration of C-H on the benzene ring. The FT-IR spectra results indicate the successful reaction between the dia-mine monomers (PDA, ODA and HFPBDA) and the dianhydride (6FDA), as well as the complete imidization reaction of PAA. In addition, the C-F stretch vibration absorption peak from the trifluoromethyl (-CF3) groups is observed near 1150 cm −1 (within the pink dotted line), and the absorption intensity of PI-FH is much stronger than that of PI-FP and PI-OP due to more -CF3 groups in the molecular chain unit of PI-FH. The EDX results of PI films are shown in Figure 3; it can be seen that the F element content is 21.94 wt%, 20.26 wt% and 32.14 wt% for PI-FP, PI-FO and PI-FH, respectively. more -CF3 groups in the molecular chain unit of PI-FH. The EDX results of PI films are shown in Figure 3; it can be seen that the F element content is 21.94 wt%, 20.26 wt% and 32.14 wt% for PI-FP, PI-FO and PI-FH, respectively.

Positron Annihilation Analysis
The discrete analysis method is widely used to analyze the positron lifetime spectrum. In this analysis, the ideal positron lifetime L(t) is the superposition of multiple exponential positron components, which can be expressed by the following formula [28]: where n is the number of components; I i is the initial intensity of component I; t is time(channel number); λ i is the annihilation rate of component I, whose reciprocal (1/λ i ) is the average life τ i of component I. In the actual measurement, considering the time resolution of the instrument, the positron lifetime spectrum (Y(t)) obtained becomes the convolution of the ideal lifetime spectrum and the instrument resolution function: where N t and B are the total count and the background count of life spectrum; R(t) is the resolution function of the lifetime spectrometer; , σ is standard deviation; and FW HM is full width at half maximum of the Gaussian function. Based on Equation (2), the positron lifetime spectrum can be fitted by the least square method to obtain λ i and I i . At present, PATFIT is a widely used spectrum resolution software package, in which POSITRONFIT can be used to obtain the lifetime τ i and its intensity I i .

Positron Annihilation Analysis
The discrete analysis method is widely used to analyze the positron lifetime spectrum. In this analysis, the ideal positron lifetime ( ) is the superposition of multiple exponential positron components, which can be expressed by the following formula [28]: where n is the number of components; is the initial intensity of component I; t is time(channel number); is the annihilation rate of component I, whose reciprocal (1/ ) is the average life of component I. In the actual measurement, considering the time resolution of the instrument, the positron lifetime spectrum (Y(t)) obtained becomes the convolution of the ideal lifetime spectrum and the instrument resolution function: where and B are the total count and the background count of life spectrum; ( ) is the resolution function of the lifetime spectrometer; , is standard deviation; and is full width at half maximum of the Gaussian function. Based on Equation (2), the positron lifetime spectrum can be fitted by the least square method to obtain and . At present, PATFIT is a widely used spectrum resolution software package, in which POSITRONFIT can be used to obtain the lifetime and its intensity .
The positron lifetime spectra for PI films are shown in Figure 4 and the detailed positron life time parameters are listed in Table 1, in which there are three lifetime components (τ1, τ2 and τ3) and their corresponding intensities (I1, I2 and I3) for PI-FP, PI-FO and PI-FH films, respectively. The τ1 is derived from the p-Ps lifetime and the free annihilation lifetime of the positrons. The 2 (424-444 ps) is due to the annihilation of free positrons trapped in vacancy-type defects on the polymer chains or free volume defects (cavities or vacancies or low-electron-density sections) of PI films without forming the metastable bound state, i.e., Ps. The τ3 in the range of 2031 ps to 2246 ps corresponds to the pick-off annihilation of o-Ps in the free volume of the amorphous phase for the PI films [27,29]. The probability of the pick-off annihilation of o-Ps is related to the size of the free volume.  Table 1, in which there are three lifetime components (τ 1 , τ 2 and τ 3 ) and their corresponding intensities (I 1 , I 2 and I 3 ) for PI-FP, PI-FO and PI-FH films, respectively. The τ 1 is derived from the p-Ps lifetime and the free annihilation lifetime of the positrons. The τ 2 (424-444 ps) is due to the annihilation of free positrons trapped in vacancy-type defects on the polymer chains or free volume defects (cavities or vacancies or low-electron-density sections) of PI films without forming the metastable bound state, i.e., Ps. The τ 3 in the range of 2031 ps to 2246 ps corresponds to the pick-off annihilation of o-Ps in the free volume of the amorphous phase for the PI films [27,29]. The probability of the pick-off annihilation of o-Ps is related to the size of the free volume. Therefore, the lifetime of o-Ps is also directly related to the size of free volume that can be estimated by the Tao-Eldrup model [30]: where τ 3 is the annihilation lifetime of o-Ps; the prefactor of 0.5 ns is the spin-averaged Ps annihilation lifetime, which is also observed in densely packed molecular crystals [8]; R is the radius of voids; ∆R is the empirical electron layer thickness parameter around the voids, normally taken as 0.1656 nm. The intensity (I 3 ) of the o-Ps pick-off annihilation is closely related to the free volume fraction of polymer. The free volume fractional FFV PALS can be estimated by the semi-empirical equation [31]: where C is a material-dependent constant which is defined as 0.0018 Å −3 [31][32][33]; V f = 4 3 πR 3 ; I 3 is the intensities of τ 3 . The free volume parameters of the PI films calculated according to Equations (3) and (4) are given in Table 2. The free volume fraction of PI-OP is almost the same as that of PI-FP. However, that of the PI-FH film is significantly higher than that of the former two films. ture due to the repulsion between the π bonds. In addition, the -CF3 group with large molar volumes can also effectively inhibit the dense stacking of molecular chain. Therefore, PI-FH has a longer molecular chain unit and more -CF3 groups than the other two PIs, which makes PI-FH film have a larger free volume size and higher free volume fraction than other two PI films.    The characteristics of the free volume defect are closely related to the molecular chain structure. The calculated results of free volume parameters for the three PI films indicate that the chain length of molecular units and the number of -CF 3 groups in the molecular chain unit have a significant effect on the free volume size and the free volume fraction of polyimide. A long molecular chain unit with more benzene rings can inhibit the dense stacking of molecular chain because the adjacent benzene rings form a non-coplanar structure due to the repulsion between the π bonds. In addition, the -CF 3 group with large molar volumes can also effectively inhibit the dense stacking of molecular chain. Therefore, PI-FH has a longer molecular chain unit and more -CF 3 groups than the other two PIs, which makes PI-FH film have a larger free volume size and higher free volume fraction than other two PI films.

UV-Visible Transmission Spectra
The UV-VIS transmittance spectra in the range of 400-650 mm −1 of PI-FP, PI-FO and PI-FH are shown in Figure 5, and their highest transmittance is shown in Table 3. The transmittance increase in turn for the three PI films and accordingly, the color gradually fades from orange to light yellow (Figure 1b). In general, the UV-Vis transmittance and coloration of aromatic polymers depends on the formation of their charge transfer complexation (CTC) and/or conjugated aromatic structures [34]. The -CF 3 has a negative effect on the conjugation degree of aromatic structure. The fluorine atoms with strong ability of electron-withdrawing can inhibit the formation of the intermolecular and intramolecular CTC and increase the transmittance of PI [35].

Dielectric Properties
The dependence of the dielectric properties on the frequencies of PI films at room temperature is shown in Figure 7, and the measured dielectric constant dielectric loss at 10 4 Hz is listed in Table 4. The films exhibit low dielectric constants (k) of 2.06-2.92 at 10 4 Hz, which are much lower than the dielectric constant of a commercial PI film (3.49 at 10 4  In addition, the energy gap (E g ) is an important optical parameter, which can be obtained (according to Figure 6) by Equation (5): where α is the absorption coefficient, which can be obtained in terms of: α = (1/d) ln(1/T), d is the film thickness, T is the transmission, K is a coefficient, h is the Planck constant, and ν is the photon frequency. The E g value can be obtained based on the linear approximation of the absorption edge to the energy axis, i.e., the intercept at the energy axis. The estimated E g value is 2.56 eV, 2.58 eV and 2.69 eV for PI-FP, PI-FO and PI-FH films, respectively. The effect of the conjugation degree of the aromatic structure on the optical property of the polymer can be determined by the energy gap, since the length of conjugation within the polymer chain is related to the E g . The -CF 3 group with a large volume located on the side of the polymer chain leads to the torsion of molecular chains and reduces conjugation length in the molecular chain, thus reducing the conjugation absorption. In addition, the oxyether (-O-) bond can break the conjugation of the molecular chain and weaken the large π conjugation absorption of the aromatic polymer chain. Therefore, the increase in the number of -CF 3 groups and oxygen ether bonds in the molecular chain can lead to the increase in E g .

Dielectric Properties
The dependence of the dielectric properties on the frequencies of PI films at room temperature is shown in Figure 7, and the measured dielectric constant dielectric loss at 10 4 Hz is listed in Table 4. The films exhibit low dielectric constants (k) of 2.06-2.92 at 10 4 Hz, which are much lower than the dielectric constant of a commercial PI film (3.49 at 10 4 Figure 6. The relation between absorption coeffi-cient spectra and the energy gap.

Dielectric Properties
The dependence of the dielectric properties on the frequencies of PI films at room temperature is shown in Figure 7, and the measured dielectric constant dielectric loss at 10 4 Hz is listed in Table 4. The films exhibit low dielectric constants (k) of 2.06-2.92 at 10 4 Hz, which are much lower than the dielectric constant of a commercial PI film (3.49 at 10 4 Hz). The dielectric constant of PI-FO is only slightly lower than that of PI-FP. However, the dielectric constant of PI-FH film is much lower than that of the former two films.  The dielectric constant mainly depends on the number of polarized molecules per unit volume of polymer [26,36]. The relation between the dielectric constant and polarizability can be explained by the Debye equation [21].
where k is dielectric constant, N is the number density of dipoles, a e the electric polarization, a d is the distortion polarization, µ is the orientation polarization related to the dipole moment, and T and k b are temperature and the Boltzmann constant, respectively. The influence of -CF 3 groups located at the side molecular chain on the dielectric properties of PI includes two aspects. On the one hand, -CF 3 groups can hinder the dense stacking behavior of molecular chains, increase the free volume fraction, and then reduce the number of polarized molecules (N) per unit volume. On the other hand, less bondconjugated electrons can lead to the reduction in a e , a d and µ due to the strong ability of the electron-withdrawing of fluorine atoms [21]. The free volume fraction of PI-FO is slightly higher than that of PI-FP, resulting in a higher dielectric constant of PI-FO than that of PI-FP. Compared with PI-FP and PI-FO films, the PI-FP film presents the lowest dielectric constant (k = 2.05) due to the most -CF 3 groups contained in the molecular chain units of PI-FP. According to the definition of ultra-low dielectric constant (≤2.0) material, the PI-FP film can be approximately regarded as an ultra-low dielectric PI film.
In addition, low dielectric loss (D) is an important index to evaluate the dielectric properties of dielectric materials. Generally, the total energy loss in dielectric materials including the loss from the dielectric relaxation process and loss from DC conductivity can be expressed as Equation (7) [37]: where w is the frequency; ε s and ε ∝ are the static and high-frequency permittivity; τ is relaxation time of the polarized molecule; ε 0 is permittivity of the vacuum; and σ dc is DC conductivity. At low frequencies, ωτ 1, the dielectric loss mainly dominated by σ dc and at high frequencies, σ dc = 0, the dielectric loss mainly dominated by τ. The high free volume fraction is conductive reducing σ dc , and then reducing the dielectric loss from leakage conduction at low frequencies. The low molecular polarizability is conductive to reduce the dielectric loss at high frequencies from the relaxation of polarization molecular. Therefore, the high free volume fraction and low molecular polarizability are beneficial to reduce the dielectric loss of polymers [38]. PI-FP has extremely low dielectric loss values (D = 0.0034) at 10 4 Hz, which are very important for reducing the energy consumption of devices when applied as an interlayer medium applied in ULSI. The temperature dependence of the dielectric properties at 10 4 Hz for the three PI films is shown in Figure 8. All the curves of the dielectric constant and loss versus temperature display a very good stability of up to 250 • C. The excellent temperature stability of dielectric properties suggests a good potential for microelectronic applications as dielectric and insulating materials.
The electrical breakdown strength is an important parameter to evaluate the insulating performance of dielectric materials. Because breakdown strength can lead to a short circuit which seriously damages the electronic devices. The measured DC breakdown strength values of the three PI films are shown in Table 4. The breakdown strength decreases successively, which can be ascribed to the gradual increase in the average size of free volume and the fraction of free volume for PI-FP, PI-FO and PI-FH films.

Mechanical Properties
The mechanical property of the PI-based film is identified as an important index to evaluate its application as an interlayer material. The measured stress-strain curves of films subjected to tensile tests at room temperature are displayed in Figure 9. The measured tensile strength, tensile modulus, and elongation at break of films are listed in Table  5. The tensile strength decreases gradually from 122.5 MPa for the PI-FP film, to 109.6 MPa for PI-FO film, and to 88.4 MPa for PI-FH film. Accordingly, the tensile modulus decreases gradually from 2.86 GPa for the PI-FP film, to 2.42 GPa for PI-FO film, and to 2.11 GPa for PI-FH film; the elongation at break decreases from 5.1% for the PI-FP film, to 4.6% for the PI-FO film and to 4.1% for the PI-FH film. Generally, the mechanical properties of polymers are affected by many factors, such as free volume defect, structure of molecular

Mechanical Properties
The mechanical property of the PI-based film is identified as an important index to evaluate its application as an interlayer material. The measured stress-strain curves of films subjected to tensile tests at room temperature are displayed in Figure 9. The measured tensile strength, tensile modulus, and elongation at break of films are listed in Table 5. The tensile strength decreases gradually from 122.5 MPa for the PI-FP film, to 109.6 MPa for PI-FO film, and to 88.4 MPa for PI-FH film. Accordingly, the tensile modulus decreases gradually from 2.86 GPa for the PI-FP film, to 2.42 GPa for PI-FO film, and to 2.11 GPa for PI-FH film; the elongation at break decreases from 5.1% for the PI-FP film, to 4.6% for the PI-FO film and to 4.1% for the PI-FH film. Generally, the mechanical properties of polymers are affected by many factors, such as free volume defect, structure of molecular chain, crystallinity, test temperature and so on. Firstly, the free volume factor is very important. It is considered that the tensile strength of the PI film decreases with the increase in free volume size and free volume fraction [29]. Secondly, the molecular chain structure factor cannot be ignored. When the -CF 3 groups are introduced into the molecule chain, the electron cloud of the C-C bond will transfer to the side of the C atom close to -CF 3 due to the strong electron-withdrawing ability of fluorine atoms, which reduces the bond energy of C-C bond, and then reduces the tensile strength. In addition, the bond energy of C-O (326 kJ/mol) is slightly lower than that of C-C (332 kJ/mol), which is the reason in the decline of mechanical properties of PI-FO compared with PI-FP. Therefore, according to the free volume parameters and the structure of molecular chain units of the three PI films, it is not difficult to understand that the mechanical properties decrease in turn for PI-FP, PI-FO, and PI-FH films. In addition, the existence of an -O-bond in the molecular chain increases the flexibility of the molecular chain and reduces the tensile modulus of the PI film [25]. Therefore, the tensile modulus decreases in turn for PI-FP, PI-FO, and PI-FH films. Nevertheless, it is worth noting that the PI-FH film still exhibits good mechanical properties compared with the other fluorinated PI films, as reported in studies [20,39] on the case of excellent dielectric properties (Figure 9). increases the flexibility of the molecular chain and reduces the tensile modulus of the PI film [25]. Therefore, the tensile modulus decreases in turn for PI-FP, PI-FO, and PI-FH films. Nevertheless, it is worth noting that the PI-FH film still exhibits good mechanical properties compared with the other fluorinated PI films, as reported in studies [20,39] on the case of excellent dielectric properties (Figure 9).

Conclusions
In summary, three PI films were synthesized by means of polycondensation, taking 6FDA, PDA, ODA and HFPBDA as raw materials. The optical and dielectric properties of PI film were improved by changing of diamine monomer structure. Among the three PI films, the PI-FH film presents the best optic transmission (highest transmittance = 90.2%) and the best dielectric properties (k = 2.05, D = 0.0034 at 10 4 Hz). Compared with PDA and ODA residues, HFPBDA residue has more -CF3 groups and a longer chain length. The -CF3 groups can effectively weaken the intermolecular and intramolecular CTC effect and the conjugation degree of the aromatic structure, and then increase the optic transmission and energy gap. Compared with PDA and ODA residues, the HFPBDA residue in the molecular chain can more effectively increases the free volume fraction of PI, and then reduce the number of polarized molecules per unit volume. Additionally, the -CF3 can

Conclusions
In summary, three PI films were synthesized by means of polycondensation, taking 6FDA, PDA, ODA and HFPBDA as raw materials. The optical and dielectric properties of PI film were improved by changing of diamine monomer structure. Among the three PI films, the PI-FH film presents the best optic transmission (highest transmittance = 90.2%) and the best dielectric properties (k = 2.05, D = 0.0034 at 10 4 Hz). Compared with PDA and ODA residues, HFPBDA residue has more -CF 3 groups and a longer chain length. The -CF 3 groups can effectively weaken the intermolecular and intramolecular CTC effect and the conjugation degree of the aromatic structure, and then increase the optic transmission and energy gap. Compared with PDA and ODA residues, the HFPBDA residue in the molecular chain can more effectively increases the free volume fraction of PI, and then reduce the number of polarized molecules per unit volume. Additionally, the -CF 3 can reduce the polarizability due to the strong electron-withdrawing ability of fluorine atoms. This is the main reason that the dielectric properties of PI-FH film are the most improved. The mechanical properties weaken in turn for PI-FP, PI-PO and PI-FH films. Nevertheless, the PI-FH film still exhibits a tensile strength of 88.4 Mpa at room temperature considering its excellent dielectric properties. This study establishes a novel method of understanding the dielectric and mechanical properties of PI films based on their free volume defect characteristics and the correlation between them, aided by positron annihilation spectroscopy.