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

Fiala, J

Abstract

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.

Citation

Fiala, J. Investigation of space charge neutralization effects in high-current positive ion beams. (Thesis)

• Thesis Type: Thesis
• Deposit Date: Oct 3, 2012
• Award Date: Sep 1, 2008