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

Sputtering and atomic mixing play an important role in SIMS analysis, discussed in
chapter 4, in the sense that the former is the basis of the technique and the latter places
an undesirable limit on the resolution.
3.2.3.3 Range shortening
As the implanted ion concentration increases during an implantation, the
stopping power of Si changes slightly from being the stopping in pure Si to the
combination of stopping powers of the Si and dopant, as determined by Braggs rule (see
section 4.2.1.2). An increased stopping power leads to a concept called range shortening
whereby the range of an implant reduces slightly with increasing fluence. This is most
applicable with a high concentration of implanted dopants.
3.3 Annealing
3.3.1 Introduction
Following ion implantation it is necessary to anneal the damaged wafers to
restore the crystal lattice structure and electrically activate the implanted dopant ions.
The dopant ions must be located on regular substitutional lattice sites. Traditionally
furnace annealing was used for this with temperatures from approximately 600 °C
upwards, with timescales of minutes or hours. Furnace annealing in the region of
600 °C would leave behind a large number of defects and require a second higher
temperature anneal to remove them.
Rapid thermal annealing (RTA), using for example arc or halogen lamps, has
more recently been employed. Far higher temperatures, e.g. >1000 °C and shorter
anneal durations, e.g. seconds, are common. Spike annealing (SA) is a variant, applied
to minimise diffusion, carried out using the same type of RTA equipment. SA has a
“zero” dwell time at the maximum temperature and uses rapid ramp up and down rates,
e.g. 200 °Cs -1 and 70 °Cs -1 respectively are typical.
All these types of annealing operate in the heat balance regime, which is where
equilibrium between deposited and lost power is established and results in a steady state
heat balance throughout the wafer. The timescales involved in this regime are t>10 -2 s
(36).
Other types of annealing include flash annealing, where a short duration pulse
(10 -6 < t < 10 -2 s) from flash lamps will rapidly raise the temperature, creating a
temperature gradient through the wafer. This is the thermal flux regime.
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