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

SIMS experiments were also carried out at IMEC, Belgium, using an Atomika 4500
instrument in each case conditions for these experiments are described. Unless
otherwise stated, the SIMS measurements presented in this thesis were carried out at
ITC – IRST.
In a SIMS instrument an ion source is used to produce primary ions that are
formed into a beam by an ion optical column. Often instruments have several different
sources to allow a range of ions to be used. The most common primary beams to use are
O2 + , O - , Ar + , and Cs + . Oxygen ion bombardment enhances the secondary ion yields of
electropositive elements and produces more uniform erosion of the sample than inert
gas bombardment. O2 + is the general purpose primary beam for semiconductor samples
where sample charging is not a problem. For insulating samples when sample charging
is a problem an electron flood gun can be used to neutralise the sample and sample
charging can also be reduced using an O - beam. A beam of Cs + ions is generally used
for optimum sensitivity with electronegative elements (6). In the present studies Cs + has
been used to profile As and F, while O2 + was used for profiling B (38, 39).
Ion optics allows the ions to be accelerated and focused into a small beam that
can be rastered over an area of the sample. In these experiments a beam energy of
around 4 – 6 keV was used, but the sample was biased to create an impact energy of
0.25 – 1 keV. The beam was rastered over an area of typically around 200 µm 2 and data
recorded over an area of around 70 µm 2 depending on the particular experiment.
Incidence angles of 44 – 76° were obtained in the Cameca, which depended on the
impact energy. Normal incidence was used in the Atomika. Erosion rates of 0.2 – 0.9
Å/s were obtained (38, 39). Sputtered secondary ions are energy filtered. This is useful
for removing interference of molecular species from an atomic species with the same
mass. Secondary ions can then be mass analysed in a mass spectrometer before being
detected. Either magnetic sector spectrometers or quadrupole analysers are used. Either
positive or negative secondary ions can be monitored (6). Negative As and F ions and
positive B ions were collected (38, 39).
The number of ions sputtered from the surface of the sample provides a means
for measuring the composition of the sample. The sputtering yield is not linearly related
to the concentration of an element present in a sample since the secondary ion yield
varies with element (e.g. As or B), with changes to the matrix, (such as going from SiO2
to Si) and varies with the primary bombarding species (6). The erosion rate is also not
constant during the first couple of nm. Depth scales are often calibrated on the basis of
the final crater depth measurement and assuming a constant erosion rate. The use of a
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