Page 81 Leica EM TIC 3X Operating Manual 07/17
5.6.2.2
Function
The triple ion source consists of 3 independent controllable saddle-field ion guns.
The ion energy is adjustable between 1 … 10 keV. The ion source will be fed with
a process g
as, preferable Argon. A high voltage (1 … 10 kV) will be applied on
the anode. The cathodes and Wehnelt cylinders are on earth potential. Due to the
electrical field between anode and cathode the process gas will be ionised (Ar
+
)
and the plasma ignites. Positive charged ions will be accelerated towards the
cathodes and generate electrons. This bombardment leads to an abrasion of the
cathode. Negative charged electrons will be accelerated towards the anode. On
this way they collide with gas atoms and generate ions. Due to the shape of the
electrical field between anode and Wehnelt cylinders (saddle field) 2 ion beams
are built and accelerated towards both cathodes. One beam is blocked by the
(blind) rear side cathode. The other beam will be lead through the beam exit at
front side cathode. The energy of the ions came out matches to the acceleration
voltage.
5.6.2.3
Ion Source Lifetime vs. Replacement Cycle
The ion source lifetime defines the lifetime of the cathodes only. The lifetime of
cathodes is limited because of the structure and physics of the saddle-field-
configuration. Due to permanent abrasion (sputtering) of cathode material, they
will worn-out after time of use. Depending on the used beam energy, the lifetime
of aluminium cathodes is approx. 350 hours at 8 kV.
Depending on the process parameter (e. g. voltage, process time) and the
environment (e. g. Ar purity, vacuum level, residual gas, sample material)
replacement or cleaning of the ion source might be required earlier. Additionally
the sputtered Al from the cathodes forms a layer on the anodes and Wehnelt
cylinders. The adhesion of that Al-layer strongly depends on impurities and layer
thickness. For example short process times in combination with high energies
yield to a multi-layer system of thin and highly stressed Al-films, which leads to
delamination and short circuits. This effect can be enhanced by impurities inside
the Al-layers caused by residual gas or contaminated process gas (leak).
The following items advance ion source contamination and lead to shorter
cleaning cycles:
•
Short process times (< 10 min) in combination with high process energies
(> 8 kV)
•
Contaminated or low order process gas (purity less than 5.0)
•
Long open times with vented chamber
•
Insufficient basic chamber pressure (residual gas in chamber)
•
Out gassing sample materials
•
Sample materials including hydrocarbons, oxygen and water
