Reduction of Sodium Ion Density in Molybdenum‐Gate MOS Devices with the Addition of Tantalum to Gate Electrodes

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The reduction mechanism of mobile ion contamination in Ta‐Mo alloy gate MOS structures has been examined by varying the deposition and annealing conditions of gate metals. Mobile ion density in the gate for the Ta‐Mo gate structures decreases drastically after forming gas annealing at 1000°C, whereas the density increases during nitrogen gas annealing. The structure and composition analysis for the Ta‐Mo gate observed by x‐ray diffraction, SIMS, and AES indicates that nitriding of Ta‐Mo films occurs during the forming gas annealing, and the nitride, (tetragonal, structure), is stable even at 1000°C during annealing. These facts suggest that formation at the boundaries of Mo grains, which is similar to the stuffing effect observed in films, is responsible for immobilizing the sodium atoms within the gate electrodes. As a result, the diffusion of sodium from the gate electrode into the layers is suppressed.