The interplay of gas flow and depletion by plasma dissociation determines the spatial distribution of species and the deposition uniformity in a plasma source. Many plasma reactors use a gas showerhead and the design of the flow dynamics is a critical aspect of the reactor performance. In this paper, plasma deposition is considered as chemically reacting gas flow in an ideal showerhead reactor. The gas fluid flow is described by finite-gap stagnation-point creeping flow. The distribution of neutral species across the electrode gap is determined by diffusion equations, whereas their lateral transport is purely convective. Parameters relevant to large-area radio-frequency plasma deposition are particularly suitable for a complete analytical solution of the multi-component transport. A representative reaction scheme for hydrogen/silane plasma deposition is used for an analytical example from first principles which shows good agreement with numerical simulation. For a laterally uniform plasma, apart from edge effects, the deposition uniformity is limited only by the lateral uniformity of the pressure: if the electrode gap is very small in a large-area reactor, the pressure and deposition rate will be non-uniform even for a uniform showerhead. The deposition mass flux is self-consistently accounted for by the Stefan velocity for arbitrary levels of gas concentration and depletion, and its influence on streamlines and fluid velocity is shown.

A reactor using localized remote plasma in a grid electrode is presented in this study. The aim is to reduce the ion bombardment energy inherent in RF capacitively coupled parallel plate reactors used to deposit large area thin film silicon solar cells. High ion bombardment energy could cause defects in silicon layers and deteriorate electrical interfaces, therefore, by reducing the ion bombardment energy, lower defect density might be obtained. In this study, the low ion bombardment energy results from the reactor design. By inserting a grounded grid close to the RF electrode of a parallel plate reactor, the electrode area asymmetry is increased while retaining the lateral uniformity required for large area deposition. This asymmetry causes a strong negative self-bias voltage, which reduces the time-averaged plasma potential and thus lowers the ion bombardment energy. In addition to the self-bias, the time evolution of plasma light emission and plasma potential RF waveform are also affected by the grid, thereby further reducing the time-averaged plasma potential and ion bombardment energy. Finally, a good correlation between the measured time-averaged plasma potential and measured low ion bombardment energy is found in a broad range of RF voltages.

The boundaries of capacitively coupled radio-frequency (rf) plasma reactors generally include at least one grounded metal grid or perforated plate for purposes of gas flow or diagnostic access. When increasing the rf power, an intense localized plasma (a plasmoid) can spontaneously ignite in a hole of a grounded surface. Experiments described here show that the plasmoid funnels rf current through the hole to the other side of the grounded plate, thereby increasing the effective grounded area in contact with the plasma. Hence, plasmoid ignition is always accompanied by a drop in the dc self-bias voltage of the rf electrode.The small area of the plasmoid aperture means that the rf current density passing through the plasmoid is very high, causing intense optical emission and strong local heating. Plasmoid ignition can therefore cause a loss of process reproducibility and potentially lead to melting and eventual destruction of reactor components.