Barrier stack with improved barrier properties

A capacitor over plug structure comprising:

• a capacitor having first and second electrodes separated by a capacitor dielectric layer;
• a plug coupling the first electrode of the capacitor;
• and
• a barrier stack disposed between the plug and the first electrode comprising:
• a first barrier layer having a top surface;
• an intermediate layer on the top surface of the first barrier layer formed by a thermal process in an oxidizing ambient, the thin intermediate layer providing elements to stuff the grain boundaries on or near the top surface of the first barrier layer;
• and
• a second barrier layer on the thin intermediate layer, wherein the grain boundaries of the first and second barrier layer are mismatched, the stuffed grain boundaries of the first barrier layer and mismatched grain boundaries of the first and second barrier layers enhance the barrier properties of the barrier stack to reduce oxidation of the plug.