Particle beam-induced reactions versus thermal degradation in PMDA-ODA polyimide

Chemical reactions induced by heat and particle bombardment at low energy in PMDA-ODA have been studied by XPS and REELS. Different chemical reactions involving the chemical state of oxygen and nitrogen are shown to occur with different products and rates depending on the peculiar way of energy deposition. In particular, thermal treatments proceed mainly by decarbonylation of imidic rings, leaving the ether linkage in the PMDA-ODA unit essentially unaffected. Evidence is found that thermal treatment results in the formation of large condensed aromatic systems, possible precursors for true graphitization at higher temperature. Also, a characteristic REELS signature is observed, diagnostic of the formation of a hydrogenated amorphous carbon phase. In the case of particle bombardment, the destruction of imide and phenyl rings occurs through a random fragmentation mechanism, leaving a detectable concentration of the imidic moiety at the steady state. A specific reaction channel has been identified, involving the production of recoiling oxygen atoms, which react with the carbon backbone to form new additional ether or hydroxyl groups. Also the electronic structure of the irradiated samples is shown to be different from that of the pyrolyzed samples. In particular, no evidence is found for the formation of large delocalized aromatic systems observed in the thermal case, while the predominant effect is the formation of highly disordered a-C:H phases.