Characteristics of an ECR ion beam source for low-pressure etching

We have investigated an ECR ion beam source operated with a pair of coils and a two-grid extraction optics. The most important parameter for the source operation at low pressure (p smaller than 10 -3 mbar) is the magnetic field in the source. A basic understanding of the physics occurring in this source can be obtained by simply registrating the total ion beam current through the first grid as a function of the magnetic field from the coils. From these characteristics we find that for the wave-propagation through the plasma column the magnet next to the microwave feed (rear magnet) is the important one. The other one (front magnet) controls the electron losses if the plasma generation is mainly located in the rear magnet. For certain magnetic field distributions we find harmonics of the ECR resonance, indicating nonlinear plasma wave interactions occurring in the source.