Magnetic field effects on secondary electron emission during ion implantation in a nitrogen plasma

In this work, the magnetic suppression of secondary electrons emitted during nitrogen plasma immersion ion implantation is investigated. Secondary electrons were measured by two Faraday cups with and without the presence of a magnetic field parallel to the target surface. One Faraday cup detects the electrons emerging perpendicularly to the target surface and magnetic field lines, while another cup detects electrons flowing along the field lines. Increase of magnetic field intensity resulted in a decrease of the amount of electrons detected by the perpendicular Faraday cup and in an increase of the electrons detected by the longitudinal one. This shows that secondary electrons were transversally confined by the magnetic field but diffused away from the target ends along the field lines. The secondary electron emission coefficient (γ) was estimated and the results showed that partial suppression (decrease in γ) was achieved when the plasma density was increased by an order of magnitude. We propose an explanation based on the formation of an electron layer (virtual cathode) near the target surface. Better suppression in denser plasmas would be expected because the virtual cathode would be maintained if the electron layer is formed faster than it is longitudinally lost.