Bias thermal stress characterization for porous ultra low-k materials

New ultra low-k materials are now approaching dielectric constants as low as k = 2. Reliability data for interconnect structures using these materials are starting to be reported in terms of electromigration (EM) properties, but very little has been reported on Bias Thermal Stress (BTS), which is another important reliability test. BTS testing evaluates the structure's ability to resist degradation under various temperatures and voltages. Until recently low-k materials have been oxide like in nature and hence have shown few BTS related issues. The most recent low-k reliability work, however, has shown significant issues with the current diffusion barriers on copper dual damascene structures, and mechanical properties related to thermal stress. Since these are issues that could be detected with BTS testing, BTS testing could become a key reliability test for estimating the lifetime of a part and comparing the effectiveness of various low-k materials. BTS testing is a test technique typically for gate stack technology and is best understood at thin film thickness. There are several BTS Models available, but they are all typically used for oxide materials less than 40A thick. The most commonly accepted BTS model is the E-model. This is one of the first times this model has been used for low-k materials especially at film thicknesses greater than 200A.