In this work, we investigated the impact of carbon ion implantation on the thermal stability of nickel silicide film and nickel-silicide-contact n +/p shallow junctions. A higher carbon ion implantation dose can prevent the nickel silicide film from agglomeration and phase transformation. However, good thermal stability does not necessarily lead to excellent junction current-voltage characteristics owing to the diffusion of nickel atoms. When the carbon ion implantation dose increases to 5 × 1015 cm -2, many crystal defects are created. Then, numerous nickel atoms diffuse along these defects into the junction depletion region during the silicide formation process, resulting in poor junction characteristics. The trade-off between thermal stability and junction electrical characteristics is discussed in this paper. Finally, two methods are suggested to solve the serious leakage current problem.