GENERATION AND BEHAVIOR OF PARTICULATES IN A RADIO-FREQUENCY EXCITED CH4 PLASMA

The behavior of particulates in a capacitively coupled, 13.56 MHz radi o frequency (rf) excited CH4 plasma was investigated by time-resolved laser:light scattering (LLS). The ballistic motion of the large partic ulates causes the LLS signal to fluctuate with peaks at the plasma she ath boundary. In an Ai plasma with a substrate which had been coated b y diamondlike carbon film in the CH4 plasma process, a quasistatic par ticulate cloud formed and the LLS signal, the self-bias voltage, and-t he optical emission oscillated slowly all with the same period. Also, the shape of the particulate cloud changed periodically. The time vari ation of the particulate's mean size in the cloud was obtained by the angular dissymmetry measurement of scattered light intensities. The be havior of the particulates seemed to depend on the particulate size. T he, time-resolved plasma potential obtained from the heated, fast scan ning Langmuir probe Indicates that the formation of particulates influ ences the plasma state. The optical microscope and scanning electron m icroscopy were used to observe the morphology and the size of the part iculates. Transmission electron microscopy and electron probe x-ray mi croanalysis studies indicate that they have an armorphous structure. T ogether with the observation of the collected particulates, the result s of LLS studies showed that two different groups of particulates are generated in CH4 plasma. The optical emission spectra of the particula te-contaminated Ar plasma and the normal clean Ar plasma were obtained and compared. The difference between the two spectra indicates that t he mean energy of the secondary electrons which come out of the powere d electrode changes as a result of the interaction with the particulat e cloud formed around the plasma-sheath boundary. The distribution bf; the particulate cloud was highly dependant on the rf power. The ion d rag force due to the ambipolar diffusion flux from the central high pl asma density region to the wall is thought to play an important role i n particulate cloud distribution. A modified turn-off process is sugge sted to prevent particulates from falling on the wafer surface. (C) 19 95 American Vacuum Society.