Thermal Effects on Plasma Characteristics and Spatial Uniformity of Neutral and Ion Species in Ar/C4F8 Inductively Coupled Plasmas

Inductively coupled plasmas (ICPs) are widely used in oxide etching because they produce high-density plasmas at low pressure. In fluorocarbon-based ICP etching, the densities and spatial distributions of CxFy, CFx radicals, F atoms, and ions are important plasma-phase quantities for characterizing plasma chemistry and the transport of reactive species toward the wafer. Many conventional fluorocarbon plasma simulations, however, assume a constant neutral-gas temperature and therefore do not account for neutral-gas heating and the resulting gas rarefaction. Recent thermally coupled ICP studies have shown that gas heating influences gas flow, electron characteristics, and reactive-species transport in Ar and Ar/O2 discharges. This study examines how neutral-gas heating and rarefaction affect the plasma characteristics and species-specific spatial distributions of fluorocarbon neutrals and ions in an Ar/C4F8 ICP. A two-dimensional axisymmetric fluid model was developed in COMSOL Multiphysics by coupling the neutral-gas energy equation to the gas density, reaction rates, and transport properties, and its results were compared with those of a conventional isothermal model over a range of RF powers, pressures, and Ar/C4F8 mixture ratios. Under the baseline condition of 20 mTorr, 500 W, and Ar/C4F8 = 50/50, the maximum gas temperature reached 604 K. Within the thermally coupled model, the neutral-gas density varied by 51.7% between its spatial maximum and minimum. Neutral-gas heating and the resulting gas rarefaction decreased the maximum electron density and increased the maximum electron temperature. These changes modified the electron-impact reaction rates and species transport, producing species-dependent changes in the densities and spatial distributions of CxFy, CFx, F, and the major ions. Radial density profiles were evaluated at 10 mm above the wafer to characterize this spatial redistribution. Under the baseline condition, thermal coupling reduced the radial non-uniformity of the CxFy, CFx, and F density profiles by approximately 33–57% relative to the isothermal model. These results show that neutral-gas heating and rarefaction change the predicted plasma characteristics and species-specific spatial distributions of fluorocarbon neutrals and ions in Ar/C4F8 ICPs.