Copper nitride thin films were deposited on silicon wafers by reactive RF magnetron sputtering at various N-2-gas partial pressures and substrate temperatures. X-ray diffraction measurements show that the films are composed of Cu3N crystallites with anti-ReO3 structure and exhibit preferential orientation to either the [1 1 1] or [1 0 0] direction at particular N-2 pressures and substrate temperatures. The film growth prefers the [1 1 1] direction at the Nz pressures and substrate temperatures below 2.50 mTorr and 150 degreesC- respectively, and the [1 0 0] preference is achieved at 3.75 mTorr and 250 degreesC, respectively. Such preferential film growth is interpreted as being due to the variation of Cu nitrification rate with N2 pressure and substrate temperature. The pseudodielectric function of a Cu3N/Si film was measured by spectroscopic ellipsometry in the 0.8-5.0eV range. It shows strong Fabry-Perot interference oscillations at low energies due to optical transparency of Cu3N. The fundamental optical band-gap energy of the compound is determined to be 1.5 eV.