To understand the correct mechanism of removing nano-sized contaminant particles (CPs) by momentum transfer, the collision process between a cleaning fragile bullet particle (BP) and a target CP adhered to a wafer substrate was simulated using molecular dynamics. The breakup of the BP and the time change of the velocity of the CP were obtained and analyzed. After collision, the CP started to move by sliding first; rolling was superimposed later, and the CP departed from the surface only after the rolling motion began. The fraction of kinetic energy transferred from the BP to the CP was <0.1 irrespective of BP size and velocity, but was nearly proportional to the cosine of the angle theta between the BP trajectory and the surface. The factor that most affected particle removal was the average acceleration of the CP. A dimensionless critical acceleration was estimated to be 0.1; this value is calculated from the diameter and velocity of the BP, and the cosine of theta. (C) 2013 Elsevier Ltd. All rights reserved.