A model of capacitively coupled radio-frequency methane/hydrogen plasmas for III-V semiconductor etching applications

A model has been developed which follows electrons and ions through assumed time varying potentials in a radio-frequency methane/hydrogen plasma. The system modelled is relevant to a low-pressure (10-90 mTorr) plasma used in the etching of GaAs surfaces. To understand the etching process, a knowledge of the flux and energy distributions of all species at the surface is required and as a first stage it is necessary to derive the spatial ionization distribution function. Inelastic and elastic collisions with neutral gas molecules are included in the model and the results give realistic electron energy distribution functions (EEDFs). These EEDFs are found to be insensitive to the inclusion of rotational modes of excitation, but vary significantly when vibrational modes of excitation are included. The EEDF is seen to change greatly with spatial position between the electrodes. Results also suggest that the form of the spatial ionization distribution function is identical for all ion species, though different in magnitude. Ion fluxes predicted at the anode are in broad agreement with experiment and are most affected by the ionization cross sections used in the simulation. Neutral fluxes to the surface are seen to make a major contribution to the particle flux, especially at the higher pressures. The secondary electron emission coefficient, though having a significant effect on the electron energy distribution function, has little effect on the mass distribution of the flux deposition on the substrate surface.