Ion probe detection of clusters in a remotely expanding thermal plasma

The investigation of a remote depositing Ar/NH₃/SiH₄-fed expanding thermal plasma by means of an ion probe, under high SiH₄ flow rate ( \Phi_{SiH_4} >1\,sccs ) conditions, is reported here. Given the expanding nature of the plasma in the downstream region, a Gaussian-like ion flux radial profile is observed. A peculiar local off-axis ion peak for high \Phi_{SiH_4} is also observed nearby. A hypothesis for this phenomenon is proposed, on the basis of the plasma chemistry occurring in the downstream region. The local depletion of electrons, being withdrawn by silicon-containing clusters formed at the boundaries between the plasma beam and the background gas, is responsible for the local enhancement of the ion flux. This hypothesis is corroborated by further studies aiming to evaluate the effects of thermophoretic and electrostatic forces on the above-mentioned clusters. The presented work suggests the application of the capacitive probe technique for cluster detection in specific plasma chemistries.