Preparation And Properties Of Polyimide Films With Low Dielectric Constant

With the rapid development of semiconductor industry of ultralarge-scale integration(ULSI)in the semiconductor industry,the integrated circuits is gradually miniaturing and the chip wire spacing becomes smaller and smaller.Thus,the signal hysteresis caused by capacitive coupling and cross interference greatly increased.Therefore,it is urgent to develop insulating materials with lower dielectric constant.Polyimide(PI)is one of the few insulating materials that can meet these requirements.The aim of this paper is to obtain polyimide films with excellent properties and low dielectric constant.The 4,4'-(hexafluoroisopropylphthalic anhydride)and phthalic anhydride(PMDA)were used as dianhydride monomers,the p-phenylenediamine(PDA),4-4'-diaminodiphenyl ether(ODA)and 2,2-bis[4-(4-aminophenoxybenzene)]hexafluoropropane(HFPBDA)were used as diamine monomers.A series of intrinsic polyimide films and sesimiloxane(POSS)/polyimide composite films were prepared by using ocaphenyl octemiloxane(POSS)as a filler.The structure and properties of polyimide films were characterized by the Fourier infrared spectroscopy,X-ray energy dispersive spectroscopy(EDS),impedance analyzer,UV-visible spectrophotometer,tensile tester and the positron annihilation spectrometer.The internal pore size was calculated by the theoretical model of Tao-Eldrup positron spectroscopy,and the influence of pore structure characteristics on its performance was analyzed as follows:Four kinds of intrinsic polyimide films(Kapton,PI_6FDA-PDA,PI_6FDA-ODAand PI_6FDA-HFPBDA)were prepared by polycondensation reaction.The dielectric properties,optical properties and mechanical properties of polyimide were studied,the results show that the PI_6FDA-HFPBDAfilm has the best dielectric properties and optical properties,the dielectric constant is 2.05 at 10⁴Hz,the highest transmittance is 90.5%.Through the comparison of the properties of the above four intrinsic PI films,it was found that the most important factor causing the performance difference of the four films may be the presence of trifluoromethyl in the molecular chain.Therefore,a series of polyimide(FPI)films with different fluoride content were prepared in order to further explore the influence of trifluoromethyl on PI matrix.The internal pore structure changes were studied by using positron annihilation technique.The results of positron annihilation lifetime spectrum test show that the internal pore size increases from 2.32A of PI-2 to 2.90A of PI-5.With the increase of trifluoromethyl content,the larger of inner pore size of the film.The results show that trifluoromethyl can effectively break the close stacking of molecular chains,cause the increase of the free volume of space,and reduce the number of polarized molecules per unit volume,which resulting in a lower dielectric constant.A low-cost physical blending method was designed to prepare a series of Kapton type polyimide/phenyl-POSS hybrid composite films because of the high price of fluoric anhydride or diamine monomer.When the doping amount of phenyl-POSS reaches 25wt%,the dielectric constant is 2.37(10⁴Hz)and the tensile strength is56.4MPa.The introduction of the cage hollow molecular structure in phenyl-POSS into the matrix can effectively reduce its.The decrease of dielectric constant because of the introduction of the cage hollow molecular structure in phenyl-POSS into the matrix.At the same time,other excellent performance is maintained.However,when the doping amount is excessive,the agglomeration and interface polarization will occur resulting in the rise of dielectric constant.Finally,in order to further obtain the polyimide with low dielectric constant,the polyimide/phenyl-POSS hybrid film was prepared by adding phenyl-POSS into the PI_6FDA-HFPBDAmatrix.The composite films with low dielectric constant and excellent mechanical properties were prepared due to the phenyl-POSS hollow molecular structure.When the doping amount of phenyl-POSS reaches 5wt%,The dielectric constant is 1.77(10⁴Hz)and the tensile strength is 76.12MPa.