Polymer/metal interfaces in interconnect structures: moisture diffusion and stress corrosion effects

摘要

Moisture can cause a host of reliability problems at interfaces including interface debonding. Two mechanisms can be identified. First, mooisture at an interface can reduce the interface bonding strength dramatically by altering the chemical bonds. Second, when an interface with a crack or a crack-like defect is under tensile stresses, stress corrosion may allow crack growth at stresses much lower than critical fracture would require. To avoid wet interfaces, wafers should be briefly baked or exposed to a plasma in situ before the next film deposition step. However, moisture can also reach interfaces by diffusion along interfaces from unportected edges during a wet process, such as CMP, or during storage in the ambient. In this work, the effect of moisture induced interface strength reduction was utilized to determine the diffusion distance. By using a mechanical peel technique, the diffusivity of moisture along the interface between Al and aa poly (arylene ether) based low-K material (PAE2) was measured to be 4-6 #mu#m~2/s. Stress corrosion was studied using a special 4-point bend techniqeu so that both strainenergy release rate and crack velocity can be obtained. It was found that the mechanism of stress corrosion at this interface is more complicated compared to that in a bulk material: while the chemical reaction took place at the crack tip, moisture diffusion was also occurring along the interface ahead of the crack tip, preconditioning the interface. There appeared to be a region that kinetics was limited by interfacial moisture diffusion and reaction, from which the reaction time for interface weakening was estimated to be approx 10 seconds. It was also found that even for samples saturated with moisture, the relatie humidity of the test environment was still very important.