We use room temperature ion beam assisted sputtering to deposit niobium nitride thin films. Electrical and structural characterizations were performed by electric transport and magnetization measurements at variable temperatures, X-ray diffraction, and atomic force microscopy. Compared to reactive sputtering of niobium nitride, films sputtered in the presence of an ion beam show a remarkable increase in the superconducting critical temperature Tc, while exhibiting lower sensitivity to nitrogen concentration during deposition. Thickness dependence of the superconducting critical temperature is comparable to films prepared by conventional methods at high substrate temperatures and is consistent with behavior driven by quantum size effects or weak localization.