Wafer Level Curing of Polymer Dielectrics with Variable Frequency Microwaves

Variable Frequency Microwave (VFM) technology has reduced the full cure temperatures of 350degC for PI films to as low as 175degC with only one hour cycle times. PBO films, with standard cure temperatures of 350-380degC, can be fully cured at 185-200degC in one hour with VFM. The resulting VFM cured film properties are equivalent or better than the conventionally cured films and chemically identical. VFM technology uses electronically swept frequency rather than a single fixed frequency. This eliminates hot spots and arcing damage caused by single frequency microwave energy. Existing production polymer cure applications and ongoing wafer level studies of next generation circuits show no negative effects on device performance. BCB, photo-resists and epoxy-based films have also been evaluated for WLP applications despite their inherent low elongation, high shrinkage and brittleness. VFM curing of of these highly cross-linked materials, again at lower temperatures and faster times, has been shown to lower modulus and decrease shrinkage and stress of these materials. The use of lower cure levels is becoming common to reduce thermal budgets. The risks of under-curing these layers include loss of interfacial adhesion, film cracking, and latent product failure modes.