Moisture Uptake of Bisbenzocyclobutene (BCB) Films for Electronic Packaging Applications

Abstract

ABSTRACTElectronic packaging has been changing.rapidly to meet the often conflicting demands of higher speed and lower cost. In the multichip module (MCM), an array of semiconductor chips is mounted on a substrate and interconnected via a network of multilevel metal lines with polymer layers used as a dielectric between orthogonal metal signal paths. A potential reliability problem that must be investigated is the absorption of moisture by the dielectric in the MCM package. Moisture absorption may lead to long term problems such as loss of adhesion, corrosion of the metal interconnect and increased insulator conductivity.Bisbenzocyclobutene (BCB) is an insulating polymer that is being used in the MCM package as a dielectric material. In this work, the moisture uptake in BCB is investigated gravimetrically using a Cahn-1000 microbalance with a resolution of I pgram. The BCB films are spincoated onto silicon substrates and cured yielding film thickness ranging from 7 to 20µm. The moisture uptake of the BCB film is monitored in situ and non destructively to reveal the performance of the film as used in an integrated circuit environment. The moisture uptake of the films was monitored at different ambient relative humidity (RIH) values between 20% and 80% at 23°C. The transient behavior of the moisture absorption in BCB films has been characterized. Moisture uptake in BCB films increases as the relative humidity is increased, and reaches a maximum of less than 0.2% by weight at 80% RH. By comparison, this moisture uptake is significantly lower than that of fully cured PMDA-ODA polyimide, which is about 2.5% by weight under similar conditions. The transient data show that the equilibration time for a 22.8 µ.m film is approximately 220 seconds, yielding a diffusion coefficient of about 4.5µm2/s.