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

produces a device quality regrown layer. On the other hand highly damaged Si layers,
even after annealing to very high temperatures, always contain residual defects (22).
This is one reason for using a pre-amorphising implantation (PAI) step prior to
implanting the dopant. It is energetically more favourable for Si to be in a crystal
structure rather than a damaged / amorphous state. Solid phase epitaxial regrowth
(SPER) can proceed from the interface of the crystal and amorphous region in a layerby-layer
fashion upon annealing at temperatures of 550 °C and above (22). The
regrowth rate is temperature dependant and increases with increasing anneal
temperature. The regrowth rate also varies with the crystal orientation, (100) Si regrows
faster than (110) Si, which in turn is faster than (111) orientated Si. This is one reason
for the majority of devices being made on (100) Si. (100) Si is also the easiest
orientation to grow. During the regrowth of amorphous layers the implanted dopant can
take up substitutional lattice positions depending on concentration (22). For high dopant
concentrations some dopant may not be able to be accommodated in the regrown layers
and will be swept in front of the interface between the amorphous / crystalline interface,
forming a segregated layer at the surface (23, chapter 6) or form clusters.
High anneal temperatures of > 1000 °C are typically preferred as they can result
in fewer defects and higher levels of activated dopant. The thermal budget is the integral
of the anneal temperature and time, which gives a measure of the total heat energy.
However higher thermal budgets allow for more diffusion of the dopant, adversely
affecting both the junction depth and abruptness. It is therefore favourable to optimise
the thermal budget to produce shallower junctions with high levels of activation (1).
This has led to the development of spike annealing, in which the wafers are heated to
some higher temperature (usually > 1000 °C) but are not held at this maximum
temperature. Fast ramp up and ramp down rates are used. This reduces diffusion as
opposed to the more conventional rapid thermal processing (RTP), in which the wafer is
held at the maximum anneal temperature for some duration, usually of the order of
seconds.
At the end of the range of an amorphising implant will be a region essentially
crystalline but containing a substantial number of defects. This interstitial rich defect
region is termed the end of range (EOR) and the defects in this area have an important
effect on the behaviour of the dopant ions during annealing (24, 25). These defects
evolve during annealing and form extended defects. The dissociation of these extended
defects at high anneal temperatures leads to the release of interstitials which sweep
through the lattice and interact with the dopant atoms. This results in a burst of
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