Moisture Diffusion in Dense SiO2 and Ultra Low k Integrated Stacks

The impact of moisture diffusion on two types of fully integrated stacks was investigated. One of them with dense SiO ₂ layers and the other with ultra low k (ULK), both of which are predominantly used in BEOL (Back End of Line) as inter layer dielectric films. For half of the samples of each dielectric, their surrounding seal ring was intentionally damaged. Storage of these samples was then performed either at ambient or at 85°C/85% relative humidity (RH) for five months to study the impact of moisture. Capacitance measurements and current voltages curves were used to assess moisture effect. For intact seal ring samples, no variations are observed for both dielectrics, which confirms the moisture protection offered by surrounding seal ring. For damaged seal ring structures, there is no variation neither after five months at ambient nor at 85°C/85% RH for dense SiO ₂ structures. However, very significant variations are observed on capacitance and breakdown values of ULK structures. The effect of baking on these samples was then investigated. Baking at 125°C or 250°C does not give full recovery of capacitance in the integrated stacks. Moreover, its effect is not permanent. Finally, to assess moisture diffusion path, time of flight - secondary ion mass spectrometry (Tof-SIMS) analysis was performed. Moisture seems to diffuse at ULK/SiCN interfaces.