RESIDUAL-STRESS BEHAVIOR OF ISOMERIC PMDA-ODA POLYIMIDES

Poly(3,4'-oxydiphenylene pyromellitimide) (PMDA-3,4'-ODA), an isomer of poly(4,4'-oxydiphenylene pyromellitimide) (PMDA-4,4'-ODA), was synthesized from pyromellitic dianhydride (PMDA) and 3,4'-oxydiphenylene diamine (3,4'-ODA). For these two polyimide isomers and their poly(amic acid) precursors in the condensed state on Si wafers, residual stress behaviour over the range 25-400-degrees-C was investigated by the dynamic measurement of wafer bending. During thermal imidization both isomers did not show any difference in stress versus temperature behaviour. Once imidized, however, one isomer exhibited a quite different stress behaviour from that of the other during cool-down: the stress of PMDA-3,4'-ODA increased rapidly from zero at 400-degrees-C to almost-equal-to 45 MPa at 40-degrees-C, whereas that of PMDA-4,4'-ODA rose gradually from zero to almost-equal-to 27 MPa. For both cured isomers, stress-temperature profile on heating was the same as that on cooling, with some deviation due to moisture uptake over the temperature range 25-150-degrees-C, indicating that their stresses were insensitive to thermal cycling or thermal annealing. From independently measured properties (thermal expansion coefficient, modulus, Poisson's ratio of 0.34) of both polyimides, the thermal stresses were calculated and compared with the measured overall stresses. It is concluded that for both polyimides the overall residual stress results primarily from the thermal stress. In comparison with PMDA-4,4'-ODA, PMDA-3,4'-ODA showed a much higher stress despite its slightly lower thermal expansion coefficient. This leads to the conclusion that the large difference between the stresses of the isomers results from the large difference in their moduli (5.0 GPa for PMDA-3,4'-ODA and 3.0 GPa for PMDA-4,4'-ODA). This behaviour is further supported by the difference in morphological structures of these two isomers as determined by wide-angle X-ray diffraction: PMDA-3,4'-ODA showed a well-developed crystalline structure, whereas the PMDA-4,4'-ODA did not. In addition, the interfacial adhesion between polyimide film and Si wafer primed with A1100 was investigated.