The
density of the ion
flux
impinging on the surface de- pends on three
mechanisms:
first the number of generated ions in the e gun
(generation),
second
the probability for an ion to leave the e-gun aperture before recombinating or hit-
ting the shielding
(extraction),
and third the chance for an ion to hit the substrate
center
(focusing).
Extraction,
focusing,
and the ion energy can be substantially
modified by applying a negative voltage
U
sub to the
substrate,
see
Fig.
2.3. Ion
beam focusing and the relative position of substrate and e gun can
cause a density
distribution over the substrate.
The
ion
flux
superposed by electrons from the e
gun causes a current
I
sub that can be measured at the substrate contact during
growth.
For
exact
measurements electron and ion currents must be separated.
Ion flux
measurement
To
measure the ion and electron densities separately a monitoring system
was developed and attached in the
MBE
growth chamber.
Silicon
atoms,
ions,
and
electrons are separated by electrodes with opposite potential ±
U
pl
in a shielded
box,
see
Fig.
2.4.
Fig. 2.4. Separation of atoms, electrons, and ions in
the measuring head
Assuming that every
electron and ion hits the
electrodes,
the densities of ions and electrons
are given by the currents on the electrodes
I
i
and
I
c
and the area of the aperture of the
box.
This
monitoring system was mounted in the
center position below the
substrate,
see
Fig.2.3.
The
substrate potential was
applied to the
box
and arm to disturb the electric
field as little as possible
compared to growth conditions without the monitoring system.
The
ion
flux
impinging on the substrate surface depends on the net rate of
ions generated in the electron beam evapo
rator,
on the punch through of the
electric field through the aperture for
extraction
of ions from the source
region,
and on focussing the
extracted
ion beam by the applied potential
U
sub
The
generation of ions is restricted to a rather small volume above the molten
Si
where
the evaporated
Si
atoms may be hit by the magnetically
deflected emission
current electrons
(Fig.1).
According to
Eq.
(1)
the generation of ions is
proportional to
Fe
and
n
Si
.
The
generation of ions can be described in a simple
model assuming uniformity in a volume with the base area
A
of the molten
Si
and a height
h
defined by the electron beam deflection.
Considering
the relations
for the emission current
I
ESV
:
I
ESV
=
A
•
F
e
•
q
(2)
A monitoring system was developed and attached
to the MBE system to
measure ion and electron density in the substrate center. The Si growth rate
R
depending on the emission currents
I
ESV
was determined. The current
I
sub
measured
113
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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