Low-stress polyimide block copolymers

In order to produce polyimide films in situ on silicon substrates: while minimizing the thermally induced residual stresses on cooling from the cure temperature, we have prepared various polyimide- polydimethylsiloxane based block copolymers derived from pyromellitic dianhydride (PMDA) and p,p'-oxydianiline (ODA) via the poly(amic-alkyl ester) precursor route. This approach allows more synthetic flexibility than the poly(amic acid) route because it permits the use of a wider range of solvents and solvent mixtures, allowing copolymers with considerable variety in the polyimide backbone, as well as in the coblock, block length and composition. PMDA/ODA containing 20 wt% polydimethylsiloxane blocks of molecular weight 5400 g mol(-1) was found to show no residual thermal stress after curing at 350 degrees C, although its Young's modulus and yield stress remained high (about half those of the PMDA/ODA homopolymer, which showed residual stresses of about 40MPa for comparable film thicknesses). The rapid stress relaxation is attributed to the relatively good phase separation and minimal perturbation of the molecular ordering and orientation of the PMDA/ODA matrix in this copolymer. (C) 1997 Elsevier Science Ltd.