As device dimensions continue to shrink, charging damage during reactive ion etching (RIE) has become a serious concern. Fast neutral beams with energies of tens to hundreds of eV may be useful for mitigating charging damage. For neutral beam processing to be viable, however, the beam energy, flux and directionality must be comparable to those in RIE. Characterization of neutral beams in terms of flux and energy distributions is therefore of critical importance. In the neutral beam source implemented in this study, positive ions generated in an inductively coupled argon plasma were extracted through a metal (or silicon) neutralization grid with high aspect ratio holes (or trenches). Ions suffering grazing angle collisions with the inside surface of the grid holes turned into fast neutrals.

The fast neutral energy distribution (NED) was measured by ionizing a small fraction of the fast neutrals using an electron beam, and detecting the resulting ions with an electrostatic parallel-plate ion energy analyzer. Calorimetry was employed to deduce the flux of fast neutrals and residual ions. The neutral energy distribution shifted to lower energies compared to the corresponding residual ion energy distribution. The neutralization efficiency increased with power, decreased with the imposed plasma potential (controlled by an electrode). The residual ion flux decreased with increasing hole diameter and hole aspect ratio. The fast neutral flux first increased and then decreased as the hole diameter was increased. These results were explained based on plasma molding inside the grid holes.

The effect of surface roughness of the grid walls on the energy distribution and flux of fast neutrals and residual ions was also studied. With a nearly atomically smooth Si grid, a small fraction of fast neutrals was observed at energies nearly equal to the maximum ion energy. For the metal grids, with rougher surfaces, the highest energy neutrals were well below the maximum ion energy. These observations were explained in terms of the type of scattering (specular vs. non-specular) that occurs when ions are converted into fast neutrals.

A new diagnostic based on fast atom-atom ionization was developed to measure the energy distribution of a fast neutral beam. The ionized species (produced by fast atom-atom ionization in chamber of controlled background pressure) current was measured as a function of energy with a gridded energy analyzer.

A pulsed-plasma technique was implemented to achieve a nearly monoenergetic ion beam. Ion energy was controlled by a DC bias, applied on an electrode in contact with the plasma, during part of the afterglow period. The energy spread was 3.3 eV (FWHM) for a peak residual ion beam energy of 101.5 eV. For a grid with smooth surfaces, NED peak at 86 eV with an energy spread of 14 eV. Energy shifts and broadening of NED were explained by specular reflection and distribution of angle of incidence of ions at the grid hole surfaces.