Langmuir probe and mass spectrometric measurements in inductively coupled CF4 plasmas

Abstract

Electron and ion energy distribution functions and other plasma parameters such as plasma potential (Vp), electron temperature (Te) andelectron and ion number densities (ne and ni) in low-pressure CF4 plasmas have been measured. The experiments were conducted in a GEC cell using an inductively coupled plasma device powered by a 13.56 MHz radiofrequency (rf) power source. The measurements were made at 300 W of input rf power at 10, 30 and 50 mTorr gas pressures. Langmuir probe measurements suggest that ne, ni and Vp remain constant over 60% of the central electrode area, beyond which they decrease. Within the limits of experimental error (±0.25 eV), Te remains nearly constant over the electrode areaand peaks towards the electrode edge before falling rapidly. Te and Vp increase with a decrease in pressure. ne and ni are not affected as significantly as Te or Vp by variation in the gas pressure. The electron energy distribution function measurements indicate a highly non-Maxwellian plasma. CF3+ is the most dominant ion product of the plasma, followed by CF2+ and CF+. Significant amounts of etch products, SiFx/COFx(x = 0-3), of the quartz window were also detected. The concentrations of CF2+ and CF+ are much larger than that is possible from direct electron impact ionization of the parent gas. The cross-section data suggest that the direct electron impact ionization of fragment neutrals and negative ion production by electron attachment may be responsible for increase of the minor ions.