Investigation of space charge neutralization effects in high-current positive ion beams

• Main Author: Fiala, J.
• Published: University of Salford 2008
• Subjects: 530.44
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.494741

Through the experience gained during the industrial development of low energy implanters, it is commonly believed that improvements in forced space-charge neutralization are responsible for the maintenance of high currents down to an energy of about 3 keV. However, despite major improvements in plasma flood neutralizer design, drift mode currents below this energy are still too low to have the same performance as obtained in accel/decel (A/D) mode. It has become clear that neither the nature of neutralization process nor the role of the "so called" beam plasma that is created in the beam environment are fully understood. The work presented was carried out on the PLUTO ion implanter beamline at the University of Salford fitted with a high density (HD) plasma flood source (PFS). The beam and flood plasmas were studied independently prior to the investigation of the coupled system. Langmuir probe measurements have been carried out on 1-10 keV Ar + beams at the wafer position in the PLUTO machine, which is a specially adapted version of a commercial ion implanter (Applied Materials xR LEAP). The main parameters of the beam plasmas, electron and ion densities, electron temperatures, and plasma and floating potentials have been measured as functions of beam energy, beam current, beam-line pressure and beam tuning settings. It has been found that in common with the plasmas in ion source and plasma flood plasmas, the electrons in the beam plasma are characterized by two temperatures. All plasma parameters depend critically on the details of the tuning parameters and the beamline pressure. The PFS plasma using mainly Xe was studied and the two operational modes (A/D and BIAS) compared. The results from mass energy analyser suggest that the plasmas are very similar after correcting the plasma potentials for the different electrical setups. When the ion implanter and the PFS are operated simultaneously, the beam current transmitted to the wafer changes in a manner that depends on the operating conditions (ion beam current, PFS arc voltage, PFS arc current, PFS gas flow rates). This dependence is the strongest when the A/D mode is used, due to the presence of a positive potential close to the ion beam.