Spatially resolved ion velocity distributions in a diverging field electron cyclotron resonance plasma reactor

Electron cyclotron resonance plasma sources are gaining widespread use in plasma processing because they offer high ion flux with controllable energy and thereby high etching and deposition rates with minimal damage. However, it is unclear how ion energy distributions evolve from source to wafer as a function of plasma parameters such as pressure, microwave power, and magnetic field strength. Therefore, we used Doppler broadened and shifted laser-induced fluorescent line profiles to measure Ar+ metastable ion velocity distributions downstream from a divergent magnetic field electron cyclotron resonance source. Spatially resolved distributions, measured at positions above and across a wafer platen, differ markedly from shifted Maxwell–Boltzmann functions. Ions are accelerated along the magnetic field direction by a weak (∼0.5 V/cm), ambipolar electrostatic field. The ion energy component perpendicular to the electric field corresponds to a temperature of only 0.46±0.10 eV. On the edges of the platen, the magnetic and electrostatic fields diverge causing angled acceleration of ions.