Functional Polyimide/Silicon Films: Fabrication And Properties

Polyimides,in particular aromatic polyimides,are extensively used in aerospace and microelectronic applications etc.,owing to their inherent strong mechanical strength,high–temperature stability,good chemical resistance and desirable dielectric properties.In aerospace applications,polyimides are commonly used onboard spacecrafts mainly as surface protective materials,thermal blankets and lightweight flexible substrates in solar cells.However,polyimides are rapidly degraded in the highly oxidative environment of low earth orbit(LEO),where atomic oxygen(AO),micrometeoroids,thermal cycle,ultra–violet(UV)and vacuum ultra–violet(VUV)radiation are present.The primary mechanism for destroying organic hydrocarbon polymers is oxidation by atomic oxygen(AO),which can cause significant mass loss and remarkably roughen the surface morphology and consequently,giving rise to reduction in physical,mechanical and optical properties and shortening the lifetime of spacecrafts.Therefore,rational design focusing on the development of AO resistant polyimides is of great significance for aerospace applications.In microelectronic applications,polyimides have served as electronic packaging and dielectric materials for at least 40 years.Regrettably,most commercially available polyimides have a high relative permittivity(Dk)in the range of 3.0-4.0,which cannot meet the rapidly developing microelectronic requirements.In this regard,to increase the processing speeds of highly integrated circuits in portable communication devices and reduce electronic signal interference(line–to–line crosstalk noise)and power dissipation,the development of interlayer dielectric materials possessing low Dk has attracted increasing attention.Against the above background,the current investigation would mainly focus on the development of AO resistant and low–Dk polyimides.The main contents and conclusions are as follows.A series of polyimide/silicon hybrid thin films have been designed and prepared by co–polymerizing or physically blending 4,4'–diaminodiphenyl ether(ODA)and pyromellitic dianhydride(PMDA)with various silicon–containing compounds.In order to elucidate the relationship between the erosion yields of polyimide/silicon hybrid films and their structures,octa–aminopropylsilsesquioxane(OAPS),amine–functionalized polysiloxane(APSi),polydimethylsiloxane(PDMS),silica sols and silica nanoparticles were chosen as silicon–containing modifiers.It is found that silicon–containing units of lower oxidation states,for example,PDMS,cannot effectively protect the underlying polymer from AO attack,while those of higher oxidation states,such as POSS and APSi,can help speed the formation of the Si O2 passivating layer and impart remarkably enhanced AO resistance to the resulting hybrid thin films.POSS and APSi are conducive to improving the AO durability of polyimide films.Thus a novel polyhedral oligomeric silsesquioxane(POSS)surrounded by two amine groups,namely POSS–diamine,was prepared via hydrolytic co–condensation reactions from the hydrolysis of phenyltriethoxysilane and ?-aminopropyltriethoxysilane(APS).Afterwards,a series of POSS–based polyimide films were fabricated by co–polymerizing POSS with PMDA and ODA.The simulated AO exposure experiments demonstrate that the resulting POSS polyimide films possess greatly enhanced AO resistance,with the lowest AO erosion yield as little as 3% that of the pure polyimide after irradiation at an AO fluence of 3.87 × 1020 O atoms×cm-2.XPS characterization reveals that the enhancement in AO resistance is attributed to the formation of a Si O2 passivating layer on the film surface upon AO exposure.Regrettably,when the mass fraction of POSS reaches 29.7 wt%,the resulting hybrid films exhibit great brittleness.In order to endow both high AO survivability and desirable mechanical property with the resulting polyimides,a novel hybrid polyimide has been prepared by first synthesizing an amine–functionalized polysiloxane with a hyperbranched architecture(HBPSi)and second incorporating HBPSi into polyimide chains via copolycondensation reactions.The AO erosion yield of 29.7 wt% HBPSi polyimides was as low as 7.9% that of pure polyimide,while its tensile strength and break elongation still maintain at 80 MPa and 15%,respectively,which contributes this novel material to a candidate of “drop–in” replacement for the widely used Kapton on spacecrafts functioning in space environment.It is found that when the mass fraction of POSS and HBPSi exceeds 14.4 wt%,the AO resistance is significantly improved,indicating that there is a percolation threshold of POSS and HBPSi addition in improving the AO resistance of polyimides.With the aim of elucidating the corresponding mechanism and further improving the AO resistance,a higher molecular weight of hyperbranched polysiloxane,namely HBPSi',was synthesized through the hydrolytic co–condensations of tetraethoxysilane(TEOS),diphenyldimethoxysilane(DPDMS)and APS.A series of polyimide hybrid thin films with different HBPSi' contents were fabricated by copolymerizing HBPSi' and PMDA and ODA.The mechanism of the “percolation threshold” was revealed by investigating the evolution of surface chemistry and morphology of HBPSi' polyimides in simulated AO environment.TEOS and DPDMS incorporation have improved the AO durability of HBPSi' in AO environment,thus imparting higher AO survivability to the resulting polyimides with the AO erosion yield as low as 6.3% that of the pure polyimide.The rapid development of microelectronics technology requires interlayer dielectrics to possess low enough Dk.Introducing fluorine or pores into polyimides has been reported to efficiently obtain low–dielectric property,but unavoidably deteriorate the mechanical and/or thermal properties.According to the polarization mechanism of polymer in an alternating electric field,decreasing the dipole polarization strength and/or reducing the number of dipoles are efficient to acquire low dielectric property.Herein,a series of robust polyimide films were fabricated by first synthesizing a huge and rigid amine–functionalized hyperbranched polysiloxane(HBPSi'')and then copolymerizing HBPSi'' with PMDA and ODA.The outstanding dielectric properties were achieved in 35 wt% HBPSi'' polyimide film,which exhibits a Dk as low as 2.24(1 MHz),about 33.3% reduction as comparing with pure polyimide,but not significantly sacrificing its mechanical and thermal properties,promising HBPSi'' polyimide a strong candidate for the future interlayer dielectrics.Additionally,to endow desirable processability with low–Dk polyimides,hyperbranched architecture has been incorporated into polyimides by using 2,4,6–triaminopyrimidine(TAP)as the “branching center”.The rigidity of the backbone chemistry is herein tailored by using one flexible diamine ODA,in combination with another rigid diamine 2,2'–dimethylbenzidine(DMBZ).As a consequence,a series of mechanically strong and crack–resistant hyperbranched polyimide(HBPI)films were prepared.The Dk of the resulting HBPIs was tuned by tailoring the DMBZ fraction in the polyimide backbone.The hyperbranched architecture renders the resulting polyimides can be easily dissolved in not only high–boiling–point polar solvent but also low–boiling–point solvent like chloroform,which guarantees good processability of HBPIs.This research provides a new idea for fabricating polyimide films with desirable comprehensive properties for microelectronic applications.