CHEM02200704003 Nilamadhab Pandhy - Homi Bhabha National ...

Chapter 3
ionized. However, differences arise in the method of ionization, and in the formation of gas phase
of the required species. The plasma from the ion source has a positive charge, and is extracted by
accelerating them from the source by applying a negative potential. The ion generation and
extraction system affects significantly the final beam qualities, thus careful selection of the process
parameters are required. The generated ions are mass analyzed according to their e/m ratio, for
allowing the single species in a defined charge state for the implantation purpose. The mass
analyzer magnet is positioned along the beam path between the source and the process chamber.
As the ions travel through the analyzer, magnetic field moves the ions in a circular path and the
ions of mass to charge ratio that have equal centrifugal and centripetal force pass through the
chamber. The magnetostatic field does not change the kinetic energy of the ions, but only changes
the direction. The dose control system for an ion implantation system mounts the specimen, and
also moves in a radial direction relative to an ion beam. The control system senses the beam
current by a Faraday cup. This current is integrated using Eq. (1) over time of implantation to
obtain the flux of charge (ions/cm 2 ) required for the desired ion dosage [57].
(1)
Where, is the flux of charge for the required ion dose, q is charge on the ion, A is the
area of the ion beam, I is the beam current, and t is the time required for implantation.
As the ions travel inside the target it undergoes series of collisions with host atoms until it
finally stops at some depth known as projected range (R p ). The energy loss is due to elastic
collision of the positive ions with the nuclei of the target material, and inelastic collision with the
electron cloud of the target atoms. Thus, the total stopping power (S) of the target material is
defined as the energy loss per unit path length travelled by the ion as represented in Eq. (2) [57].