Stress induced self aligned contact failure during tungsten-poly gate process in sub-60 nm memory device

We investigated the mechanism of stress-induced self-aligned contact (SAC) failure in the sub-60 nm W-dual poly metal gate process in DRAM devices. It was found that during NH₃ pre-purge step of gate capping nitride deposition, amorphous WN_x barrier and side of gate etched W electrode were transformed into tensile crystallized W₂N, which relieves high compressive stress of inner gate W. Asymmetrical relief of W stress could create torque leading to gate leaning which is a main culprit of SAC failure during reliability test. Therefore, by reducing NH₃ pre-purge time, we successfully could reduce gate leaning which endures good reliability characteristics.