Stability of contacts in shallow junction devices against high current density is a reliability issue for very large scale integration technology. We have observed a strong polarity effect on failure at nickel and nickel silicide contacts on both n- and p-type Si under high stress conditions. In a pair of cathode and anode contacts the Ni/n+-Si contact pair fails at the anode, while the Ni/p+-Si pair fails at the cathode. The Ni/Ni2Si/n+-Si and Ni/Ni2Si/p+-Si were found to fail preferentially at the cathode. Microbeam Rutherford backscattering spectrometry and Auger electron spectroscopy depth profiles show that a silicide reaction occurs between Ni and Si during current stressing, especially at the failed contacts. In situ resistance data indicate that the resistance of the failed contact increases with time while that of the other contact in the pair remains constant. Transmission electron microscopy shows that the silicide formation is not uniform at the damaged contacts. A mixture of dominant epitaxial NiSi2 and a minor amount of polycrystalline NiSi2 phases was identified. We have proposed mechanisms to explain the polarity effect on failure: wear-out mechanism for the damaged positive contacts of Ni/n+-Si, electromigration enhanced suicide formation for the damaged negative contacts of Ni/Ni2Si/n+-Si and electron-hole recombination mechanism for the damaged negative contacts of Ni/p+-Si and Ni/Ni2Si/n+-Si.