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

stopped at depths beyond the observed profiles. This implies a movement of dopant
species back to a depth closer to the surface and indicates that As and Sb is more readily
accommodated in the amorphous layer than the damaged crystalline layer. Implantation
carried out into an amorphous matrix does not show this movement of the dopant back
towards the surface, in agreement with the proposed mechanism.
A range of annealing studies was performed using As implants. The first strand
focused on implantation and annealing conditions that are similar to current device
manufacturing conditions, i.e. using predominantly spike annealing above 1000 °C.
MEIS results showed that for many anneals, complete regrowth of damaged layers was
achieved. For temperatures of 700 °C and above, the regrowth rate was sufficient to
complete regrowth within a timeframe of several seconds. Segregation of As occurred,
as did the disappearance of the dopant from view of the He + beam. Combined SIMS
studies showed the full retention of the As, and hence the As lost from view is equated
to As taking up substitutional positions. For higher temperature anneals, i.e. > 900 °C
diffusion of the As to greater depths was observed. Any improvements in depth using a
1 keV implant compared to a 3 keV implant, which lead to shallower implant depths,
were found to be negated by the greater effect of diffusion during subsequent annealing.
Diffusion, which is influenced by the level and nature of the implant damage,
has been identified as a limiting factor in reducing the junction depth in future devices.
Further SPER annealing studies were carried out using low anneal temperatures (550 °C
– 700 °C). MEIS results captured various stages of regrowth within these samples. This
highlighted the layer-by-layer nature of the regrowth process. Changes in the As
profiles were observed that matched the depth of the amorphous / crystalline interface,
showing how As became substitutional or segregated out with the movement of the
interface. With these low anneal temperatures, importantly, deeper As diffusion was not
observed. A reduction in the regrowth rate was observed with increasing As
concentration and proximity to the surface.
Finally annealing studies using SOI wafers were carried out. For samples where
the implant damage was well away from the buried oxide layer, MEIS results showed
that the regrowth behaviour of SOI samples was the same as for bulk Si samples. For
samples where a small amount of implantation damage extended to the depth of the
buried oxide layer, or a depth close to the buried oxide, MEIS results showed that the
amorphous / crystalline interface of annealed samples could be much wider than those
observed for bulk Si samples. In SOI samples the interface was observed to be
distributed over a depth of approximately 14 nm. This unusual behaviour was attributed
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