Preparation And Properties Of Low Dielectric Constants Polyimide Films

The integrated circuits with high density,high transmission speed,and multi-layer interconnection required the use of low dielectric constant materials as interlayer media,thereby reducing parasitic capacitance,noise,and delay time of transmission signals.The commonly used interlayer medium silicon dioxide had the drawbacks of high dielectric constant（4.0）,high film forming temperature,and poor flatness,which limited its further development in integrated circuits.The dielectric constant of general-purpose polyimide was generally about 3.1,the dielectric loss was less than 0.004,and the volume resistivity was1×10¹⁶Ωcm.Polyimide was an ideal interlayer medium with good insulation,simple molding process and low production cost,but it still cannot meet the requirements for interlayer dielectric constant（<2.5）for very large scale integrated circuits.Intrinsic polyimides can reduce the electron density by introducing fluorinated groups,and increase the free volume between molecular chains by introducing large steric groups or flexible groups,thus reducing the dielectric constant.However,this weakened the force between molecular chains,and significantly reduced the thermal and mechanical properties of polyimides.Although the porous polyimide had a mesoporous structure that significantly reduced the dielectric constant,the discontinuous internal structure would significantly reduce the mechanical properties.Both intrinsic polyimides and polyimides with mesoporous structure were difficult to give consideration to low dielectric constant,good thermal and mechanical properties.In this paper,the research direction was to obtain polyimides with excellent comprehensive properties.New polyimides with low dielectric constant,good thermal stability and mechanical properties were prepared by copolymerization or compounding of various amine monomers and anhydride monomers.The molecular simulation software was used to simulate the transformation process of polyimide molecular chain conformation,and the potential energy contour map of polyimide molecular chain conformation transformation was drawn.The specific research content was as follows:1.The three amine monomers,2,2’-bis（4-（4-aminophenoxy）phenyl）propane（BAPP）,1,4-bis（4-aminophenoxy）benzene（TPE-Q）,and 1,3-bis（4-aminophenoxy）benzene（TPE-R）,were synthesized using bisphenol A,hydroquinone,resorcinol,p-chloronitrobenzene,and hydrazine hydrate as the main raw materials.It was characterized using techniques such as infrared spectroscopy,¹H-NMR,and ¹³C-NMR.2.Four kinds of homopolyimides（polyimides 1～4）and three kinds of ternary copolyimides（polyimide 5～7）were prepared with pyromellitic dianhydride（PMDA）as the acid anhydride monomer,and 4,4’-diaminodiphenyl ether（ODA）,BAPP containing side methyl,TPE-Q containing para substituted structure of benzene ring and TPE-R containing meta substituted structure of benzene ring as the amine monomer.By the thermal mechanical analyzer and universal material testing machine,the dielectric constant,dielectric loss,thermal and mechanical properties were studied.The results showed that for homopolyimides,the side methyl structure and the para/meta substituted structure of the benzene ring can reduce the dielectric constant of the corresponding polyimides,of which the side methyl structure had the greatest impact,followed by the meta substituted structure,and the para substituted structure had the smallest impact.Their decomposition temperature and glass transition temperature were maintained above 511℃and 320℃,respectively.The maximum tensile strength and elongation at break can reach 129.1 MPa and 64%.For ternary copolyimides,the dielectric constant was between homopolyimides with similar structure,in the range of 2.49～2.90（1 MHz）.The thermal decomposition temperature and glass transition temperature of the seven polyimides were kept above 527℃and 319℃respectively,and the maximum tensile strength and elongation at break were 89.8 MPa and 47.3%.3.Two homopolyimides（polyimide 8 and polyimide 9）and two ternary copolyimides（polyimide 10 and polyimide 11）were prepared with 4,4′-（4,4′-isopropylidenediphenoxy）-bis（phthalic anhydride）（BPADA）and PMDA containing side methyl as acid anhydride monomers,ODA and BAPP containing side methyl as amine monomers.The molecular simulation software was used to simulate the transformation process of polyimide molecular chain conformation,and the influence of bisphenol A anhydride monomer with side methyl structure on the transformation of polyimide molecular chain conformation and its relationship with dielectric constant were discussed.The results showed that the thermoplastic polyimide could be prepared by the reaction of BPADA containing side methyl with ODA and BAPP respectively.The polyimide obtained by the reaction of BAPP with the same structure containing side methyl bisphenol A and PMDA had no thermoplastic property.The dielectric constant decreased from 3.10（1 MHz）for ODA/PMDA to 2.84（1 MHz）for ODA/BPADA,while the dielectric constant of BAPP/BPADA further decreased to 2.32（1MHz）.For ternary copolyimide,the dielectric constant of ODA/BPADA-PMDA and BAPP/BPADA-PMDA increased to 2.95 and 2.62（1 MHz）respectively.The decomposition temperature of the four polyimides remained above 516℃,and the maximum tensile strength and elongation at break reached 99.65 MPa and 11.36%.The transformation process of polyimide molecular chain conformation was simulated by molecular simulation calculation software,the transformation potential energy and bond angle change of polyimide molecular chain conformation were obtained,and its potential energy contour map was drawn.The introduction of side methyl structure and phenyl ether structure can make the angle range of polyimide molecular chain conformational transformation larger and the energy barrier lower,that was,the polyimide molecular chain conformational transformation was easier.The potential energy contour map effectively reflects the difficulty of polyimide molecular chain conformational transformation,and was consistent with the changing trends of the corresponding polyimide dielectric constant.4.In order to obtain low dielectric constant polyimides with excellent comprehensive properties,four kinds of homopolyimides（polyimide 3,4,12 and 13）were prepared using TPE-Q and TPE-R with benzene ring para substitution as amine monomers and BPADA with side methyl as acid anhydride monomers.The results showed that TPE-Q containing para substituted phenyl ring and TPE-R containing meta substituted phenyl ring as amine monomer could react with BPADA of bisphenol A type respectively to prepare thermoplastic polyimides.The dielectric constant of TPE-R/PMDA was 2.49（1 MHz）lower than that of TPE-Q/PMDA,which was 2.61（1 MHz）.When the anhydride monomer was BPADA containing side methyl groups,they decreased to 2.42（1 MHz）and 2.53（1 MHz）respectively.The decomposition temperature of the four polyimides was kept above 521℃,and the maximum tensile strength and elongation at break were 129.1 MPa and 64.0%.The transformation process of polyimide molecular chain conformation was simulated by molecular simulation software.For polyimides containing benzene ring pair/meta substitution structure,their molecular chain length,density,molar volume,van der Waals volume and fractional free volume were similar,and the potential energy contour map obtained from bond angle and potential energy can effectively reflect the influence of both on the conformational transformation of polyimide molecular chain,which was consistent with the changing trends of polyimide dielectric constant.In order to test the effectiveness of the potential energy contour map,the potential energy contour maps of polyimides with different structures were compared to reflect the influence of side methyl structure,phenyl ether structure and benzene ring pair/meta substitution structure on the conformational transformation of polyimide molecular chain,which was consistent with the changing trends of dielectric constant.5.Polyimide solid microspheres were prepared by non-aqueous lotion polymerization using BAPP as amine monomer and PMDA as anhydride monomer.Polyimide composites containing 10 wt%～50 wt%polyimide solid microspheres were prepared with ODA/PMDA polyimide as matrix and polyimide solid microspheres as filler,respectively.The results showed that the dielectric constant of polyimide composites decreased from 3.10（1 MHz）to2.26（1 MHz）,and the decomposition temperature and glass transition temperature remained above 449℃and 295℃respectively with the increase of polyimide solid microspheres.When the content of polyimide solid microspheres was 30 wt%,the comprehensive mechanical properties were the best,the dielectric constant was 2.32（1 MHz）,the decomposition temperature and glass transition temperature were 449℃and 329℃,respectively,and the tensile strength and elongation at break were 128.50 MPa and 9.18%,respectively.