The distribution and transport of particles in materials processing plasmas has been studied with a rastered laser light scattering technique. Contrary to expectation, the distribution of particles in a plasma processing tool is rarely random. Instead, structured clouds of particles form at the plasma/sheath boundary. The effect is attributed to trapping of the particles by weak electric field nonuniformities and the characteristic negative charge of isolated particles in a plasma. Field nonuniformities appear to be influenced by the topography and material design of the tool. For example, the presence of a Si wafer often induces significant particle trapping. Examples of particle trapping in a laboratory system are given, and similar phenomena are also verified in a manufacturing sputter deposition tool operating in a class 100 cleanroom. The implications of particle trapping in plasma processing are discussed.