A novel electron cyclotron resonance (ECR) reactor for plasma processing of III-V compound semiconductors is presented. The reactor is appropriate for processes which are damage sensitive or for applications in which direct overhead access to the wafer is advantageous. The reactor uses an axisymmetric magnetic geometry provided by electromagnets and locates the source region below the wafer; there is no direct line of sight to the wafer from the plasma discharge region. The geometry allows more flexible neutral gas handling capabilities and frees the area above the wafer for diagnostics, neutral gas manifolds, energetic ion beam injection, laser irradiation of the wafer or vacuum pumping access. The combination of auxiliary processing with beams or laser light simultaneous with the plasma processing step opens possibilities for new processes, increased throughput and reduced wafer handling.