Preparation And Properties Of Side Chain Crosslinked Polyimide Films With Low Dielectric Constant

With the development of times and technology,microelectronics and communication technology are also developing rapidly.In recent years,the emergence of 5G communication technology makes the signal of electronic equipment develop towards the direction of high speed and high frequency.The application of 5G communication will greatly promote the development of medical care,transportation,agricultural and educational fields.However,the miniaturization and high integration of 5G communication equipment will result in resistance-capacitance time delay(RC delay)and high power consumption.The most straightforward way to solve this problem is to use low-dielectric(low-k)materials.As the mainstream dielectric material of 4G,commercial polyimide has been unable to meet the requirements of 5G due to its relatively high dielectric constant and loss.Nevertheless,polyimide(PI)dielectric materials still have obvious advantages in cost control and manufacturing process.Therefore,modified polyimide(MPI)dielectric materials remain important candidates for 5G.The dielectric constant of conventional polyimide films is generally between 3.0 and3.6,which is mainly caused by the close packing of segments and the formation of intermolecular electron transfer complexes(CTCs).In addition,the hydrophilic group(—O—)in the molecular structure usually makes the polyimide have a high water uptake.When the polyimide water absorption,the dielectric constant will be significantly increased.In order to reduce the dielectric constant,dielectric loss,water absorption,and improve the dielectric stability of polyimide.The structure of polyimide can be diversified and specific properties can be achieved by changing the molecular structure.Therefore,this paper prepared a series of cross-linked polyimides from the molecular structure and constructed hydrophobic cross-linked network through thermal crosslinking,so as to reduce dielectric constant and improve dielectric stability and other properties.The main research contents are as follows:Firstly,a diamine monomer(4-(2,3,5,6-tetrafluoro4-vinylphenoxy)benzene-1,3-diamine;TFVPDM)with tetrafluorostyrene side group was prepared.A series of crosslinkable polyimides(PIs: PI-10;PI-20;PI-30)were synthesized by polymerization of diamine monomer TFVPDM with commercial diamine monomer(4,4’-diaminodiphenyl ether;ODA)and dianhydride monomer(4,4’-biphenyl ether dianhydride;ODPA).The cross-linkable polyimide is thermally crosslinked to obtain crosslinked polyimides(CL-PIs: CL-PI-10;CL-PI-20;CL-PI-30).It was found that the dielectric constant and water absorption of PIs decreased due to the introduction of tetrafluorostyrene side group and m-benzene structure.The dielectric constant of PI-30 is3.17 and the water absorption is 2.09%.After crosslinking,the formation of hydrophobic crosslinking network not only limits the orientation of dipoles in the electric field,but also improves the free volume,resulting in a gradual decrease of dielectric constant.The phenomenon is continuously enhanced with the increase of crosslinking density.The dielectric constant of CL-PI-30 is 2.93 and the water absorption is 1.29%.It proves that the introduction of crosslinking network structure can effectively reduce the dielectric constant.However,the dielectric properties of the prepared polyimide films still have large development space.Secondly,in order to further reduce the dielectric constant of polyimide films,a series of high fluorine content(28.6%-30.5%)cross-linkable polyimides(FPIs: FPI-10;FPI-20;FPI-30)were synthesized by polymerization of diamine monomer TFVPDM with(2,2’-bis(trifluoromethyl)-4,4’-diaminobenzene(TFDB)and(4,4’-(hexafluoroisopropene)diphthalic anhydride(6FDA).The FPIs are thermally crosslinked to obtain crosslinked fluorinated polyimides(CL-FPIs: CL-FPI-10;CL-FPI-20;CL-FPI-30).It is found that the dielectric constant of FPIs decreases greatly under the combined action of higher fluorine content and larger free volume.The dielectric constant of FPI-30 is 2.46 and the water absorption is 0.265%.After crosslinking,the formation of hydrophobic crosslinking network can further restrict the dipole orientation and improve the free volume.The dielectric constant of CL-FPI-30 is 2.22,and the water absorption is0.128%.This proves that the synergistic action of fluorinated unit and crosslinking structure can further reduce the permittivity of polyimide.Finally,when the fluorine content exceeds a certain limit,the dielectric constant of polyimide is difficult to decrease with the increase of fluorine content.The introduction of pore structures in polyimide films is an effective way to reduce the dielectric constant.However,the introduction of pore structures usually leads to an increase in the water uptake of polyimide.In order to avoid a significant increase in water uptake while introducing pore structures,a series of cross-linkable porous fluorinated polyimide films(PFPIs: PFPI-10;PFPI-20;PFPI-30;PFPI-40;PFPI-50)were prepared by solution casting and removal of porogenic agents,choosing FPI-20 with a high fluorine content as the substrate and adding different mass fractions of diphenyl sulfone.Then,a series of crosslinked porous fluorinated polyimide films(CL-PFPIs: CL-PFPI-10;CL-PFPI-20;CL-PFPI-30;CL-PFPI-40;CL-PFPI-50)were obtained by thermal crosslinking treatment.It was found that the pore structures of polyimide films before thermal crosslinking treatment could effectively reduce the dielectric constants,which ranged from 1.92-2.54 for PFPIs,0.0128-0.0146 for dielectric loss,and 0.39%-3.23% for water uptake.After thermal crosslinking treatment,most of the pore structure can still be preserved.The crosslinked network formed inside the matrix can effectively restrict the molecular chain sliding and thus prevent the collapse of the pores inside the polyimide.In addition,the crosslinked network can restrict the dipole orientation and prevent the entry of water molecules,thus reducing the dielectric constant and dielectric loss while improving the water resistance.Among them,CL-PFPIs have dielectric constants between 1.86-2.40,dielectric losses between 0.0067-0.0084,and water uptake between 0.30%-2.90%.It proves that the synergistic effect of the pore structure,fluorinated units and cross-linked structure can further reduce the dielectric constant.