Formation kinetics and control of dust particles in capacitively-coupled reactive plasmas

Abstract

Formation kinetics and behavior of dust particles below about 10 nm in size, referred to as clusters, in silane capacitively-coupled RF plasmas are studied using double pulse discharge and photon-counting laser-light-scattering methods. Even under so-called device quality conditions, clusters of ∼1011 cm-3 high compared to a plasma ion density of ∼10-9 cm3 are found at t ∼ 50 ms after the discharge initiation. Clusters begin to be composed of two size groups at t ∼ 10 ms. The ones in the small size group have an almost constant average size of ∼0.5 nm during the discharge period, while the ones in the large size group grow at a rate of ∼4 nm/s. This result indicates that the large clusters are nucleated by the small ones containing 3–4 Si atoms.Various methods for suppressing cluster growth have also been studied using two in situ cluster detection methods. Since species contributing to the initial growth of clusters are principally produced in the radical production region around the plasma/sheath boundary near the rf electrode, the pulse discharge modulation which has the discharge-off period in one modulation cycle longer than the diffusion time of clusters through the radical production region is effective in reducing the growth of clusters. Thermophoretic force due to heating of the grounded electrode drives neutral clusters above a few nm in size toward the cool RF electrode which is at room temperature. Periodical pulse discharge modulation is much more effective in reducing the cluster density when it is combined with grounded electrode heating. Hydrogen dilution of a high H2/SiH4 concentration ratio above about 5 is useful for suppressing cluster growth especially in the radical production region near the RF electrode.