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

At the implant energies currently of interest for device production i.e. 1 – 5 keV,
energy will be transferred mainly through nuclear interactions (elastic collisions), from
the energetic ions to the initially stationary target atoms. A collision results in an ion
being scattered and loss of energy through which the projectile slows down. Eventually
after a series of collisions with different target atoms an ion will come to rest at some
range within the Si once it has lost all its energy. For a given implant species the lower
the implant energy, the shorter the range will be. The statistical nature of the slowing
down process produces a distribution of the implanted ions within the Si about some
mean depth. The implanted ion distribution is approximately a Gaussian distribution, or
more accurately, a Pearson distribution. Driven by the relentless reduction of device
dimensions the trend has been to use increasingly lower implant energies.
Si atoms that gain sufficient energy during a collision to overcome the binding
energy are displaced from their lattice site, leaving behind a vacancy, i.e. an empty
lattice site. Si atoms and implanted dopant ions stopped on a non lattice site are termed
interstitials. Both structures are irregularities in the Si lattice and are referred to as point
defects. Conglomerations of two or more point defects, referred to as extended defects,
are also produced by ion implantation. Point defects can migrate depending on the
temperature. Regions such as the surface or internal interfaces can act as a sink to trap
migrating point defects (21).
Energetic recoil Si atoms produced by collisions with the incoming dopant ions
will also collide with other Si atoms and if sufficient energy is transferred they in turn
will be displaced. This process continues resulting in a collision cascade. The collision
cascade produced by single heavy implanted ions, such as As, Sb, and Xe, tends to
produce small amorphous zones. Once a certain implant concentration has been
exceeded these zones will spatially overlap forming a continuous amorphous layer.
Implanted light ions such as B create less damage to the Si lattice, but damage will build
up until a certain critical concentration of defects exists when the crystal structure can
collapse into an amorphous structure (20). It is difficult to produce an amorphous layer
with light ions and will require substantially higher implant doses to achieve compared
to heavy ions.
1.4 Annealing
To produce electrically active doped regions the implanted material must be
annealed to restore the crystal lattice and locate the implanted atoms onto substitutional
sites where they can become electrically active. Upon annealing, fully amorphised Si
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