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

⎛ 〈 C ⎞ ⎛ ⎞
I 〉 〈 C V 〉
D = D
⎜
⎟ + ⎜ ⎟
I D
* V ⎜ * ⎟
(3.12)
⎝ C I ⎠ ⎝ C V ⎠
where DI and DV are the intrinsic diffusivities due to interactions with self interstitials
(I) and vacancies (V) and / Cx * and the local / equilibrium concentration of point
defects (70). B is known to diffuse by interstitial mechanisms, Sb by vacancy
mechanisms and As is thought to diffuse by both interstitial and vacancy mechanisms
(71).
3.5.2.2 Transient enhanced diffusion
Anomalous diffusion of dopants has been observed where the diffusivity can be
increased by several orders of magnitude from that calculated for normal diffusion. This
enhancement was first attributed to the release of Si interstitials during the dissolution
of {113} defects during annealing at temperatures from 670 °C to 815 °C (58, 61).
However it has since been shown that it is not just the {113} defects that cause TED
(59). TED occurred in samples where the implant dose was too low to form {113}
defects. In this instance the activation energy for TED was in close agreement with the
dissolution of very small clusters of interstitials or the activation energy of migration of
free self Si interstitials. Additional experiments had also shown that the maximum
diffusivity occurred in timescales before the dissolution of {113} defects starts.
Therefore the dissolution of several defect types causes an injection of interstitials
responsible for TED.
A model to describe TED in PAI Si assumes a surface interstitial concentration
which increases with depth to the EOR region where there is a constant concentration of
interstitials which decreases with time (72). Essentially all of the emitted interstitials
arrive by diffusion at the Si surface where they are trapped. The flux to the surface is
inversely related to the depth of the EOR band, so can be beneficial to separate EOR
from dopant implant. TED causes the segregated As buried underneath the oxide to
diffuse to greater depths.
References
1 S.M. Sze, Physics of Semiconductor devices, Second edition, John Wiley and
Sons, New York, 1981.
2 C. Kittel, Introduction to solid state physics, Seventh Edition, John Wiley and
Sons, New York, 1996.
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