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

3.5 Dopant movement and Diffusion
3.5.1 Introduction
To produce layers with sufficiently high levels of dopant activation and to
remove the most stable defects requires annealing temperatures in excess of 1000 °C. At
high temperatures dopant diffusion occurs which can have a dramatic effect on the
depth of the junction. Diffusion of dopants has been considered a potential
“showstopper” in the production of future devices with the need for shallower junctions.
It has been observed that atoms migrate in interactions with defects. The dissolution of
extended defects introduces an influx of interstitials that can be responsible for a
dramatic enhancement, of many orders of magnitude, of the diffusion compared with
standard Fickian diffusion. As this enhancement is relatively short lived and decreases
with time it is known as transient enhanced diffusion (TED). A more detailed review of
diffusion is given in (9, 66).
3.5.2 Models of diffusion
Before describing diffusion models some notation must first be defined. When
the dopant atom occupies a substitutional site, surrounded only by Si atoms it is referred
to as As. When a vacancy V, resides next to a substitutional dopant atom it is known as
a dopant vacancy pair and designated AV. If one of the atoms in an interstitialcy defect
is a dopant atom then it will be called a dopant interstitialcy pair, AI. If the dopant atom
occupies an interstitial position, it will be referred to as an interstitial dopant and written
as Ai (9, 67). The diffusion of mainly interstitially dissolved small atoms (Ai) like
hydrogen or the 3d transition elements proceeds via interstitial lattice sites, as shown in
Figure 3.12 a). Direct diffusion of atoms on substitutional sites (As) may occur by direct
exchange or a ring mechanism, Figure 3.12 b) but this is rare and no experimental
evidence has been found (9, 67). Diffusion by indirect mechanisms involving point
defects is usually more favourable and can be expressed by the following point defect
reactions.
As + V ↔ AV (3.4)
As + I ↔ AI (3.5)
As + I ↔ Ai (3.6)
As ↔ Ai + V (3.7)
The first equation describes a vacancy mechanism for dopant diffusion and the
second equation describes an interstitialcy mechanism for dopant diffusion. Isolated
intrinsic defects approach substitutional impurities and form next – nearest AV and AI
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