Chemistry Reference
In-Depth Information
Area measurement, differential weighing, and interferometry allow a deter-
mination of the mass density of individual sublayers [78-80]:
m
layer
A
sample
t
layer
ρ
layer
(4.33)
Consequently, a stepped profile can be plotted for the density of all sublayers
dependent on the depth below the original surface.
This information of a density profile is
quiteexceptional
while a stepped
profile for the molar concentration
c
layer
by TXRF is obtained as expected. It
indicates the ratio of implanted ions and substrate atoms within the layers:
c
layer
=
n
ion
/
n
atom
. In combination with the mass density, both quantities
n
ion
and
n
atom
can be determined individually. The actual number density of
implanted ions is
c
layer
M
atom
n
ion
ρ
layer
N
A
(4.34)
M
ion
c
layer
where
M
atom
is the molar mass of substrate atoms (e.g., silicon),
M
ion
is the
molar mass of the implanted ions, and
N
A
is Avogadro's number (6.022
×
10
23
atoms per mole). The actual number density of substrate atoms is given by a
similar formula:
1
M
atom
n
Si
ρ
layer
N
A
(4.35)
M
ion
c
layer
The first factor of the latter formulas is the mass density of an individual
sublayer, which is determined separately by the combined methods. It may
differ significantly from the density of the crystalline material (e.g., of silicon,
which is
ρ
Si
10
22
=
cm
3
).
An example will be given in Section 5.4.8.2 with a step height of 3-6 nm and a
number density of 10
20
up to 10
22
As and Co ions/cm
3
. The number density of
the Si substrate was shown to vary by
±
30% relative to the nominal value,
which means a swelling or shrinking of the respective sublayers. In contrast to
wet-chemical etching plus TXRF, the method of dry-physical etching with
TXRF can be adapted to materials different from Si. It is less time-consuming
and needs only 4 h for a total density-depth profile and another 2 h for a
concentration/depth profile.
4
:
996
4.6QUANTITATIVESURFACEANDTHIN-LAYERANALYSESBYGI-XRF
The feasibility of thin-layer analysis is known from classical XRF and is based
on continuous variation of the glancing angle of incidence while fluorescence
intensity
is
recorded.
The
angle-dependent
intensity
profiles
provide
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