Epitaxial NiSi2 has been grown on (001)Si inside 50-200 nm Si3N4 openings prepared by atomic force microscope tip-induced local oxidation. The morphology of epitaxial NiSi2 was found to be significantly influenced by the opening size. For specific annealing conditions, there exists a transitional opening size below which a pyramidal faceted structure of epitaxial NiSi2 is preferred. The opening size effect is attributed to a limited supply of Ni atoms, the increased interface/volume ratio of silicides with decreasing size of openings, and the considerable stress level inside miniature openings. (c) 2005 American Vacuum Society.