RESIDUAL-STRESS BEHAVIOR OF ISOMERIC PMDA-ODA POLYIMIDES

Poly(3,4'-oxydiphenylene pyromellitimide) (PMDA-3,4'-ODA), an isomer o f poly(4,4'-oxydiphenylene pyromellitimide) (PMDA-4,4'-ODA), was synth esized from pyromellitic dianhydride (PMDA) and 3,4'-oxydiphenylene di amine (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 dyna mic measurement of wafer bending. During thermal imidization both isom ers did not show any difference in stress versus temperature behaviour . Once imidized, however, one isomer exhibited a quite different stres s behaviour from that of the other during cool-down: the stress of PMD A-3,4'-ODA increased rapidly from zero at 400-degrees-C to almost-equa l-to 45 MPa at 40-degrees-C, whereas that of PMDA-4,4'-ODA rose gradua lly from zero to almost-equal-to 27 MPa. For both cured isomers, stres s-temperature profile on heating was the same as that on cooling, with some deviation due to moisture uptake over the temperature range 25-1 50-degrees-C, indicating that their stresses were insensitive to therm al cycling or thermal annealing. From independently measured propertie s (thermal expansion coefficient, modulus, Poisson's ratio of 0.34) of both polyimides, the thermal stresses were calculated and compared wi th the measured overall stresses. It is concluded that for both polyim ides the overall residual stress results primarily from the thermal st ress. In comparison with PMDA-4,4'-ODA, PMDA-3,4'-ODA showed a much hi gher stress despite its slightly lower thermal expansion coefficient. This leads to the conclusion that the large difference between the str esses 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 behav iour is further supported by the difference in morphological structure s of these two isomers as determined by wide-angle X-ray diffraction: PMDA-3,4'-ODA showed a well-developed crystalline structure, whereas t he PMDA-4,4'-ODA did not. In addition, the interfacial adhesion betwee n polyimide film and Si wafer primed with A1100 was investigated.