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

Chapter 7 Interaction between Xe, F and B
7.1 Introduction
The study reported in this final results chapter formed part of a much wider
investigation within the IMPULSE project to characterise the behaviour of B and find
optimum implantation and annealing conditions for CMOS devices. This was a parallel
study to the study of As implants, parts of which are reported in section 6.2.3 and 6.2.4.
B doped source/drain and extension regions in Si CMOS devices are usually produced
using BF2 ions, for reasons of higher beam and hence wafer throughput. As with the As
studies, 1 and 3 keV BF2 implants were carried out. The energy deposition rate of BF2 is
sufficient to amorphise the Si matrix at comparatively low doses, unlike B, however the
use of deeper pre-amorphising implants (PAI) is preferred, as discussed elsewhere in
this thesis. The benefits of pre-amorphising include suppression of channelling effects
and the enabling of a good spatial separation of the end-of-range defects and the
junction depth. Pre-amorphising implants using Xe were carried out instead of Ge,
which is the most commonly used PAI dopant. Xe offers several advantages, for
example it forms a sharper amorphous / crystalline interface, which can lead to a lower
concentration of hairpin dislocations (25). Other manufacturing and transistor device
advantages exist. The study in this chapter is concerned with the collective behaviour of
the Xe, F, and the implanted B.
Many previous studies have been carried out with ion implanted F in Si (1-11).
These have shown that F becomes mobile at temperatures above 550 °C and is swept in
front of the a/c interface during SPER. Other studies have shown that when BF2
implants amorphise Si, upon annealing, a small fraction of F migrates in deeper into the
bulk and is gettered in low concentrations at defects beyond the original a/c interface
(4,8,6). For non – amorphising BF2 implants, only F segregation to the surface and out
diffusion was observed (2). With high dose BF2 implants (1E16 cm -2 ), bubble formation,
as described in section 3.4.3, was observed, (9,10). F has also been found to exert an
influence on retarding the diffusion of B (3, 7), form interstitial and vacancy complexes
(1) and retard SPER. These studies have not been carried out with the added
complication of the presence of Xe. Earlier studies of Si implanted with only Xe have
shown that Xe bubbles can be formed during annealing (12). Bubble formation has also
been observed with other noble gases (13-15).
MEIS results presented in this chapter have revealed an unexpected behaviour of
the implanted Xe. While brief comments regarding Si regrowth and information
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