Power coupling and utilization efficiencies of silicon-depositing plasmas in mixtures of H₂, SiH₄, Si₂H₆, and Si₃H₈

Adding Si₂H₆ or Si₃H₈ additives to SiH₄/H₂ discharges increases the growth rates for thin films of microcrystalline and amorphous silicon, but the reasons for this increase are not well understood. To better distinguish the chemical and physical from electrical effects of these additives, a comprehensive electrical study was performed for mixtures of H₂, SiH₄, Si₂H₆, and Si₃H₈. The power coupling efficiency, power utilization efficiency, voltage, current, impedance, and phase were measured as a function of total pressure, electrode gap, gas mixture, rf power, and time. The measurements identified a regime of pressure and gap in which the electrical behavior is optimized. In this regime, the power coupling efficiency is quite high and insensitive to gas mixture, and the power utilization efficiency also does not vary dramatically with mixture. Therefore, in this regime, chemical or physical effects of additives on growth rates predominate over electrical effects. Impedance models of the plasma and sheaths provide explanations for the optimized regime and its correlation with impedance phase. In addition, electrical signals were identified that can be used to detect a transient in the gas-phase density of silicon-containing molecules during deposition as well as other transient phenomena. The signals show promise for use in process monitoring and control.