Damage formation and annealing studies of low energy ion implants ...

Abstract
The work described in this thesis concerns studies of damage and annealing
processes in ion implanted Si, relevant for the formation of source / drain extensions in
sub 100 nm CMOS devices. Implants were carried out using 1-3 keV As, BF2 and Sb
ions and implanted samples were annealed at temperatures between 550 °C and 1130 °C.
The principal analysis technique used was Medium energy ion scattering
(MEIS), which yields quantitative depth profiles of displaced Si atoms and implanted
dopants. The results obtained have been related to comparative analyses using SIMS,
TEM and X-ray techniques.
Heavy ion damage evolution and the concomitant dopant redistribution as a
function of ion dose was investigated using As and Sb implantation into Si. It was found
that for low doses the damage build up does not follow the energy deposition function.
Instead a ~4 nm wide amorphous layer is formed initially under the oxide that grows
inwards into the bulk with increasing dose. For low doses As is seen to have migrated
into the damaged regions near the surface, where it appears to be more readily
accommodated. Both effects are ascribed to the migration of interstitials.
Various annealing studies have been carried out to investigate the regrowth
behaviour of the damaged Si and the redistribution of the dopant. Effects of
recrystallisation, dopant movement into substitutional positions, dopant segregation and
diffusion are observed. Annealing studies of implanted Silicon on Insulator (SOI)
wafers have shown a regrowth that has a wavy a/c interface unlike the layer by layer
mode that is typical in solid phase epitaxial regrowth. This effect is ascribed to localised
damage accumulation at the buried oxide layer.
Following a BF2 implant into Si, pre-amorphised by a Xe bombardment, an
interaction between Xe, F and B has been observed to occur upon annealing during
which the implanted species conglomerate at depths related to the end of range of the
BF2 implant in the amorphous Si.
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