Paper PS2-ThM1
Power Dynamics in Low Pressure Capacitively Coupled Plasma Discharges

Thursday, November 12, 2009, 8:00 am, Room B2

As feature sizes shrink and feature aspect ratios increase in advanced microelectronics devices, critical dielectric etching processes in capacitively coupled plasmas (CCP) are generally transitioning towards lower gas pressures (< 30 mTorr). Long electron mean free path and large bias voltages in this regime means that kinetic effects play an important role in the power dynamics in these low pressure plasma discharges. A coupled one-dimensional particle-in-cell (PIC) and fluid model is used to understand power dynamics in low pressure CCP discharges in this investigation. Our PIC model for charged species is based on the well-established computational techniques developed by Birdsall and colleagues (C. K. Birdsall and A. B. Langdon, Plasma Physics via Computer Simulation, IOP Publishing, Bristol, 1991) and includes a Monte Carlo based model for charged species collisions. Since multiple neutral species are present in plasmas of typical etching gases, the PIC model is coupled to a fluid model for neutral species that takes into account species transport in the plasma bulk, chemical reactions, and surface processes. The PIC + fluid model is applied to understanding power dynamics in a variety of etching-relevant single and dual frequency plasmas including Ar, O₂, and CF₄. Substantial fraction of applied power is consumed by the ions in the sheaths, which is dissipated at the electrodes. In Ar, electrons primarily gain energy at the sheath edge during sheath expansion, which results in highly energetic electrons. These energetic electrons stream through the plasma towards the opposite electrode, causing excitation, dissociation and ionization in their path. At low pressures (< 50 mTorr), these energetic electrons are able to reach the opposite sheath and lose some their energy while decelerating in the sheath. This behavior is consistent with the fact that plasma density is lower at lower pressure in the 5 – 50 mT range. The situation becomes more complicated in molecular gases due to electron collision processes with low threshold energies. Secondary electrons play an important role in sustaining the plasma at low frequencies (< 30 MHz), but sheath heating of electrons is sufficient for plasma sustenance at higher frequencies (> 60 MHz). Simulation results will be compared to experimentally measured ion densities.