Processing and encapsulation of silver patterns by using reactive ion etch and ammonia anneal

Silver is a potential candidate for interconnect materials in advanced interconnect technology due to its excellent conductivity and better electromigration resistance than aluminum. In this study, the fabrication of 10 μm-linewidth silver patterns has been carried out by applying reactive ion etch in an oxygen plasma. The influence of rf-power supply and etch time on the etch effect has been investigated by the use of scanning electron microscopy (SEM) and Rutherford backscattering spectrometry (RBS). In this particular study, the etch condition of 50 W and 5 min can lead to complete removal of the Ag film (100 nm thick) uncovered by the photoresist. The etch mechanism has been discussed. The essential point for Ag etch in the oxygen plasma is that the Ag film is oxidized and then the oxidized Ag film flakes off due to the strain-induced cracking and ion bombardment. In an attempt to address some major existing issues of silver as a good interconnect material, an encapsulation process has been successfully implemented by annealing silver patterns (with Ti/SiO₂/Si stack structure as substrate) in a flowing ammonia ambient. Upon anneal, the silver patterns are encapsulated with a thin TiN(O) layer due to the diffusion of the underlying Ti through the Ag layer to the surface and the reaction of Ti with ammonia. In the meantime, part of Ti reacts with SiO₂ to form Ti₅Si₃ and Ti(O) at the original Ti/SiO₂ interface.