8 Concluding Remarks
The IMPETUS II code is intended to be a useful computational
tool for engineers engaged in using SIMS or SNMS techniques.
The software is very versatile in that any number of material
systems may be attached. During the execution of IMPETUS II
the software gives an on-screen display of the redistribution
of material within the solid as well as the yield history.
The yield-dose and yield-depth curves may be viewed on the
linear or logarithmic scales. The results from the run are reproduced
in a file. The yield curves produced by IMPETUS II
aids the engineer in deciding whether a given sputter profile
is compatible with the presumed original material structure.
In this paper the methods that are central to the IMPETUS II
code have been described in detail and results given.
The data that describes the material system
is derived from the results of other software packages such
as TRIM or it is empirical. Discrepancies between the
results from SIMS or SNMS experiments could be as a result
of insufficiently accurate data. For some material
systems the model itself may need extending since the
present model only includes the fundamental
bombardment-related processes. Nevertheless, the atomic
mixing model provides us with a realistic representation
of the underlying process in SIMS and SNMS for many
material systems under a range of experimental conditions.
This work has focussed on improving the efficiency and
consistency of the computational solution of the original
atomic mixing model.
The replacement of the explicit finite difference method in
the original IMPETUS
by an implicit method is the main reason for the dramatic improvement
in the execution of IMPETUS II in comparison to
the original codes. Moreover, in the implicit method there
is not a strong connection between fineness of the spacial
mesh and the dose step length, as there is with the explicit method.
This means that if the spacial mesh requires refinement in
the implicit method (in the case, for example, of a thin layer
in the structure), it is not necessary to also severely
reduce the dose step.
There are still many challenging areas in the area of the computational
modelling of the atomic mixing process.
For example the original mathematical model is
likely to need extending to include the effect of the chemical bonding
characteristics of some species. The inverse problem to the
one considered in this paper, that of recovering
the original material structure from the sputter yield curves, is
of great importance and would be a powerful tool
for SIMS engineers. Clarifying the computational modelling,
improving the methods and hence significantly decreasing run-time and
having one central program serviced by material system libraries,
consist an important step in making IMPETUS II an efficient
and versatile code. Such improvements are also important
before extending the model and tackling the inverse problem.