Three-dimensional interconnect with excellent moisture resistance for low-cost MMICs

The moisture resistance of three-dimensional (3-D) interconnects using organic insulator films and Au metals has been investigated to ascertain the feasibility of housing monolithic-microwave integrated circuits with these interconnects in inexpensive nonhermetic packages. By comparing polyimide and benzocyclobutene (BCB) for organic insulator films, it was found that although polyimide has higher moisture absorption than BCB, it has a greater moisture resistance. This suggests that moisture absorption is not the dominant factor in moisture resistance and that BCB has higher water permeability than polyimide. As an adhesion layer between Au metal and insulator film, W and WN have better moisture resistance than WSi and WSiN; adhesion layer compositions containing Si oxidize easily. Further, the metal patterning method has an effect on moisture resistance in terms of leakage current. Reactive ion etching (RIE) with SF₆ gas is necessary in order to completely remove the metal atom residue left after ion milling with Ar gas. An interconnect using polyimide insulator film, W or WN adhesion metal, and metal patterned by ion milling and RIE, did not fail in terms of contact resistance and leakage current under stress of 85°C and 85% relative humidity with a bias for 1000 h.