We present results on superconducting and crystalline properties of NbN films with a thickness smaller than 10 nm. The films were deposited using reactive magnetron sputtering on heated silicon substrates. Zero resistance critical temperatures of about 9 K have been measured for films with a thickness of about 5 nm and reaches values approx12 K for 10 nm thick films. A value of the superconducting coherence length of about 4 nm was estimated from the measurements of the second critical magnetic field. High-resolution transmission electron microscopy accompanied with electron-spectroscopy techniques was used to analyze the structure, thickness, and film-substrate interface of fabricated films. The interrelations between fabrication conditions, superconducting and crystalline properties of NbN films on Si substrates are presented and discussed.