In a machine for processing semiconductor wafers, a rotor includes two pairs of combs. A lock down mechanism has a lock bar, temporarily engaged and moved by a loading/unloading robot, drives a retainer against the edges of the wafers, to better hold them in place during processing. Contamination via generation of particles is reduced. Combs on the rotor have a resilient strip. The lower edges of the wafers compress slightly into or deflect the resilient strip, when urged into place by the lock down mechanism.