Organic passivation of copper for MCM-D packaging

Abstract

In the fabrication of multilevel thin film packages a polyimide precursor, such as a polyamic acid, a polyamic ester, a polyisoimide or a pre-imidized polyimide, is spin-coated onto a substrate consisting of metal and polyimide. The precursor is then cured at high temperature to obtain a polyimide film with good mechanical/chemical properties. If a polyimide precursor is directly coated onto copper, either adhesion of polyimide to copper is weak or if it is a polyamic acid, it reacts with copper leaving copper oxide particles in the cured polyimide layer, which results in the increase of dielectric constant and the decrease of material properties. To prevent such a reaction and to enhance adhesion to copper, a metal such as Cr, Ti or Ta is currently used. To simplify this process in manufacturing, an organic coating has been developed. The organic layer consisted of polyisoimide (4,4′-oxydiphthalic anhydride- 1,3-aminophenoxy benzene) with 8-azaadenine end group which is thermally stable up to 350°C and contains triazole functional groups that are known to interact with copper. Using a 1.0-μm thick polyisoimide layer followed by a 20-μm thick PMDA-ODA coating directly on Cu gave peel strengths in the range of 50-80 g/mm. TEM cross-sections of the Cu-polymer interface show no copper oxide precipitates in polyimide as expected. Thus the ODPA-APB-8-azaadenine polyisoimide can be used as a copper passivation layer as well as an adhesion promoter for the polyimide-to-copper interface.