LOW-STRESS POLYIMIDE BLOCK-COPOLYMERS

In order to produce polyimide films in situ on silicon substrates: whi le minimizing the thermally induced residual stresses on cooling from the cure temperature, we have prepared various polyimide- polydimethyl siloxane based block copolymers derived from pyromellitic dianhydride (PMDA) and p,p'-oxydianiline (ODA) via the poly(amic-alkyl ester) prec ursor route. This approach allows more synthetic flexibility than the poly(amic acid) route because it permits the use of a wider range of s olvents and solvent mixtures, allowing copolymers with considerable va riety in the polyimide backbone, as well as in the coblock, block leng th and composition. PMDA/ODA containing 20 wt% polydimethylsiloxane bl ocks 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 mod ulus and yield stress remained high (about half those of the PMDA/ODA homopolymer, which showed residual stresses of about 40MPa for compara ble film thicknesses). The rapid stress relaxation is attributed to th e relatively good phase separation and minimal perturbation of the mol ecular ordering and orientation of the PMDA/ODA matrix in this copolym er. (C) 1997 Elsevier Science Ltd.