The interactions between energetic particles and the sputter gas in a magnetron sputtering system have strong effects on the growth, structures, and properties of the film. These interactions result in inhomogeneous rarefaction of the gas in the space between target and substrate and affect both the transport of particles towards the substrate and the dynamics of the plasma. A hybrid Monte Carlo and fluid model is developed to simulate three-dimensional (3D) gas rarefaction due to the sputtering of metals in argon, neon, and krypton. The governing equations are solved iteratively in a 3D space with a nonuniform grid (octree). Collision events between the sputtered particles and the process gas are assumed as the dominant source of gas heating; however, the effect of the reflected neutrals is also included in the model. Gas rarefaction profiles have been predicted for different process conditions. Model results compare well with experimental ones. The extent of rarefaction depends on process conditions as well as the thermal conductivity of the gas. Materials with high sputtering yield, such as silver, show more rarefaction at a given cathode current than those with low sputtering yield, such as tungsten and aluminum. A higher sputtering yield means more sputtered atoms, thus more energy and momentum deposited in the gas. For a 75 mm target at 300 W and 10 mTorr, a rarefaction of about 65% is obtained for the sputtering of Al in Ar gas, with the substrate plane located 10 cm in front of the target. (c) 2006 American Vacuum Society.