Silicided shallow p+-n junctions, formed by BF2+ implantation into thin Co films on Si substrates and subsequently annealed, show a reverse anneal of junction characteristics in the temperature range between 550 and 600°C. The reverse anneal means a behavior showing the degradation of considered parameters with increasing anneal temperature. A higher implant dosage causes a more distinct reverse anneal. The reverse anneal of electrical characteristics was associated with the reverse anneal of substitutional boron. A shallow p+-n junction with a leakage current density lower than 3 nA/cm2, a forward ideality factor better than 1.01, and a junction depth of about 0.1 μm was therefore achieved by just a 550°C anneal.