The thermal stability of nickel silicide is improved significantly by adding a thin layer of Ir or Co at the Ni/Si interface. The sheet resistance remains low after 850 degreesC annealing. The thermal stability was evaluated by measuring the junction leakage of an ultra-shallow junction with a 40 nm junction depth. With Ir, the film was stable and the reverse leakage of both N+/P and P+/N junctions remained in the picoampere range at 3 V on 100 mum x 100 mum feature after 850 degreesC annealing. With Co, the leakage from P+/N junctions was low when the temperature was as high as 850 degreesC; leakage from N+/P junction was in the picoampere range up to 750 degreesC. These films were characterized by x-ray diffraction. The improved stability and low junction leakage is attributed to a very smooth interface.