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

acquired were converted into damage depth distributions in terms of number of
displaced Si atoms or dopant atoms cm -3 , using a standard calibration procedure (22).
The backscattered ion yield was referenced to the random level measured on a Si
sample, amorphised by high dose self ion bombardment. In addition the energy scales
were converted into depth scales using established inelastic energy loss data (23) and
applying the surface approximation (22, 24). Implanted As / Sb and displaced Si
distributions can be detected down to levels of ~ 1 × 10 19 cm -3 and 10 21 cm -3 ,
respectively.
Additional SIMS As profiles were obtained in an Atomika 4500 instrument at
IMEC using a 0.5 keV 02 + primary beam at normal incidence. In these measurements a
crater size of 250 µm was used and mass Si 30 was monitored to ensure a better than 1%
current stability. Depth scale calibration relied on a constant ion beam current and was
performed using the erosion rate obtained on one deep crater in combination the erosion
time for each crater. Relative sensitivity factors (RSF, from implant standard) were
repeatable within 1%.
5.3 Results and discussion
Considering first the damage evolution during As ion bombardment, a series of
Si samples was implanted through the native oxide with 2.5 keV As ions to doses
ranging from 3 × 10 13 to 1.8 × 10 15 cm -2 . The dose dependence of the MEIS spectra
obtained for these samples is shown in Figure 5.1. Peaks resulting from the As implant,
the displaced Si atoms and the oxide, respectively, are indicated in the figure. For
comparison Figure 5.1 also contains the spectrum for a virgin Si sample as well as a
random spectrum. The virgin Si sample shows two peaks due to scattering off Si (edge
at 171 keV) and O atoms (edge at 153 keV) both contained in the native oxide. The
thickness of the native oxide thickness is calculated as ~1.5 nm. It should be noted
however that in the case of the Si peak there is also a contribution from disordered Si
atoms at the oxide / Si interface (19, 25). The virgin spectrum is the base against which
any additional backscattering from displaced Si atoms that results from the As implant
is measured. The figure shows the changes in the energy spectrum in terms of the
growth of the As and Si scattering peaks for increasing As implant dose. It is seen that
the Si damage peak and also the As peak, spread towards lower energies, i.e. greater
depths, although the movement of the latter is less pronounced.
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