Plasma density oscillations in an ion implanter source and their effect on ion beam transmission

High productivity in ion implanters require that extracted beams from the plasma ion source are well focused to minimize attenuation during travel to the wafer.  Noise in the extracted beam can reduce transmission and beam angle control.

The plasma in this hot cathode ion source is made stable with an axial magnetic field that confines electrons that come from the hot cathode and from ionized atoms and molecules.  This creates a plasma in the ion source from which ions are extracted and transmitted along the implanter beamline to the wafer.  However, this extracted beam exhibits cyclical beam density modulations with a frequency in the range of 100 kHz.

We embarked on tests to determine the cause of these oscillations and ways to mitigate the effect.  Facing the cathode is an anti-cathode at the same potential as the cathode.  Both are at a negative potential relative to the plasma chamber.  These experiments utilized a specially designed anti-cathode with multiple probes for measuring plasma fluctuations within the ion source.  Using this set of probes, oscillations in the plasma were measured in the ion source at frequencies similar to that measured in the extracted beam.  Phase shifts measured at the probes in the plasma oscillations indicate rotational motion of a single node around the confining magnetic field axis.

With the cause of the beam noise identified, efforts are now focused on reducing the effect of these plasma oscillations on beam transmission to the wafer.