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This letter evaluates Ru and W capping layers for MoTa metal gate electrodes in MOS capacitor applications. The authors report that the oxygen diffusion from the capping layer plays an important role in determining the MoTa alloy effective work function value on SiO2. A MoTa alloy metal gate with Ru capping exhibits stable effective work function up to 900°C annealing but is not stable with W capping. Auger electron spectroscopy and Rutherford backscattering spectroscopy analyses show minimal oxygen diffusion into MoTa gate stacks with Ru capping while severe oxygen diffusion into the gate is observed with W capping metal after 900°C annealing. Current-voltage analysis also demonstrates different barrier heights of MoTa on SiO2 with Ru or W capping layer after 900°C annealing, confirming the effective work function value change.