at the substrate contact gives the average charged particle flux density on the whole
substrate area which is dominated by electrons from the e gun. The linear
dependence of the electron current density, proven with the monitor on the
emission current
I
ESV
, enables to separate the ion part from the substrate current
I
sub
. The average ion current density
I
ion is nearly linear to the ion generation
represented by the product of
I
ESV
and
R
, but the value measured in the middle
i
ion
shows a more complicated d
ependency on the conditions. The ratio
I
ion
/
i
ion
is a
measure of whether the ion beam is focussed to the substrate center, or focused to a
point outside the center. Details of focusing are not understood and a moveable ion
monitor was installed for the ne
xt
investigations to clarify this point.
Linear Motion Ion Probe
The MBE subsystems which are relevant for the ion distribution are
sketched in Fig.
2.5
. The 100 mm diameter wafer is positioned at the chamber axis.
It is supported by a 150 mm diameter wa
fer holder made also from silicon. Slightly
below that holder is a silicon reflector ring which improves the temperature
homogeneity of the radiative heated wafer. These three silicon parts are insulated
from the heater, the rotation axis and the heat shieldings, and a substrate potential
between 0V and
-
1000V may be applied to them. Below (about 5cm) the substrate
plane the moving arm with the ion flux measuring head
is mounted on a flange and
positioned that the moving direction is perpendicular to the wa
fer flat. The
molecular beam sources are mounted at
the bottom of the chamber on separate
flanges. The shown Si electron gun
evaporator is surrounded by a heat
shielding which is set at ground potential.
An aperture allows the extraction of
neutral and ion
ized Si beams. The beam
position is off axis by around 70mm.
In Fig. 2.6
the ion current is
represented as a function of position along the moving ion probe. The substrate
middle is given in the figure. It is possible to see three distinctive features. (i)
The
low (about 20 nA/
cm
2
) but rather homogeneous flow of Si ions is measured with
no applied voltage (V=0). (ii) The strong inhomogeneous flow of positive Si ions
is measured for negative voltage of substrate several hundreds Volts. The flow is
fully concentrated on one side of the substrate from the center that is shifted to the
axis of electron
-
beam evaporator. (iii) The focusing of the ion flow becomes
stronger with the raise in voltage up to 330 nA
/
cm
2
versus 150 nA
/
cm
2
with the
sweeping field from – 400
V to
-
600
V.
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