The SMIL: an immersion lens allowing Ultra Low Energy on
magnetic sector
The highly superior sensitivity
of the CAMECA
magnetic sector SIMS starts with a high secondary ion collection due to
a strong electrostatic extraction field above the sample. For ULE implants
of electro-positive elements (B, …), the IMS 7f provides benchmark results
in routine. For example, to obtain a 45° incidence angle at 500 eV impact
energy, the extraction (sample voltage) is set to +600 V, with ion source
at +1.1 kV.
For electro-negative elements
(As, P,…) where Cs+ is chosen to enhance the ionization yield,
the analysis of ULE implants also requires the use of Ultra Low impact
Energy (e.g. < 500 eV). In this case, the reduction of the sputter yield
at low energy can lead to a too high cesium surface coverage reducing the
ionization yield. Large sputter angle are then required for the obtention
of reliable measurements. This ULE/ large angle capability is not available
on the IMS 7f.
The new Shielded Multi-mode Immersion Lens (SMIL) of the CAMECA IMS Wf/ SC ULTRA provides
a field free space from the last primary lens to the secondary extraction
field area to minimize the deflection effect of this field on the primary
beam trajectory. The SMIL consists of 4 electrodes (a shielding electrode
at the sample voltage, an extraction electrode, a focusing electrode and
a grounded electrode) to uncouple the extraction and focusing functions.
Thus, a 250 eV primary impact energy with a secondary extraction voltage
of 3 kV can be achieved, at 58° impact angle, whatever the secondary
ion polarity. High depth resolution and high transmission capabilities
can therefore be combined.
The IMS Wf or SC Ultra can be simultaneously equipped with one column
at 58° (Cs+ and/ or O2+) and one column
at 36° (O2+).
