Particle formation from Si and SiO2 substrates in Ar sputtering plasmas and CCl2F2/Ar reactive ion etching (RIE) plasmas has been investigated using laser light scattering, electron microscopy, and optical emission spectroscopy. Nearly spherical particles with sizes down to 50 nm are collected downstream from the substrate even before the onset of light scattering or at pressures below the threshold for cloud formation. Si atoms are seen readily by optical emission during sputtering, but not during RIE, of both Si and SiO2. The rate of particle formation correlates with the density of Si atoms in the plasma. Hence, it is suggested that Si atoms are probably the species responsible for particle nucleation in sputtering and RIE, and formation of a particle cloud occurs only when the transport of Si atoms out of the sheath edge region becomes inefficient, as evidenced by redeposition in sputtering and by polymerization in RIE.