Polyimides generally possess excellent thermal and mechanical properties, making them attractive candidates for high performance applications. To be useful for microelectronic applications, however, these materials must also be good insulators, as well as be readily processable.
The incorporation of flexible polysiloxane segments into the polyimide backbone structure has been shown to yield soluble, processable copolyimides with good thermal and mechanical properties. In addition, the siloxane component imparts a number of other significant benefits for electronic applications. These include reduced water sorption, surface modification, good thermal and ultraviolet stability, and resistance to degradation in oxygen plasma environments. For polar polyimide systems, siloxane incorporation will also reduce the dielectric constant. The use of other less polar, more hydrophobic monomers will consistently yield soluble systems with lower dielectric constants as well.
In this work, a series of high molecular weight, soluble polyimide homopolymers and segmented polysiloxane-polyimide copolymers were synthesized by a solution technique. The solution procedure, conducted at lower temperatures (˜170°C) than the classical bulk thermal imidization (300°C), has been shown to yield polyimides of enhanced solubility. In order to further enhance processability, molecular weight was controlled through the incorporation of monofunctional reagents such as phthalic anhydride and maleic anhydride, yielding nonreactive or potentially reactive endgroups, respectively. A series of maleic anhydride terminated imide oligomers with varying molecular weights were synthesized based upon the hexafluoropropane linked dianhydride and bisaniline diamine. In their oligomeric state, they exhibited enhanced solubility compared with their linear high molecular weight analogue. As these monomers were relatively nonpolar and hydrophobic, they afforded polyimides of low dielectric constant and a low level of water sorption. After thermally crosslinking the endgroups, the advantages of insoluble network systems could be realized. Particular advantages for electronic applications include thermal and dimensional stability over a wide temperature range, good mechanical properties, and chemical resistance. Structure-property characterization, including water sorption, dielectric constants, solubility behavior and thermal/mechanical properties will be reported.