Evaluation of Ionic Salt Photodefinable Polyimides As Mcm-D Dielectrics with Copper Metallization

Abstract

The excellent film properties given by BPDA/PPD make it a prime candidate for the dielectric material in MCM-D fabrications, and photodefinable versions offer the additional advantages of reduced labor and material costs. However, less than adequate adhesion to the preferred copper metallurgy coupled with prolonged development times may limit widespread adoption of this chemistry. Modifications to the BPDA/PPD backbone were made in a series of photodefinable polyimides prepared as the ionic salt precursors, and key performance and processing parameters were determined. Significant adhesion of the highly rigid BPDA/PPD backbone to copper metal was only achieved after substantial incorporation of either more flexible co-monomers, especially those containing fluorine substituents, or by incorporation of an assumed surface reactive, hydroxyl-substituted co-monomer. These modifications generally also resulted in reduced development times. Self-adhesion of polyimide to polyimide layers was optimized by using a partial cure process for the bottom polyimide layer, and appropriate cure temperatures were determined for each structure to prevent solvent-induced cracking of the bottom layer upon application of the top layer while retaining self-adhesion. Operable processing windows were found for several candidates which maximized adhesion and minimized cracking.