Chemistry Reference
In-Depth Information
10
16
ions/cm
2
. The molar fraction of implanted ions and Si atoms was deter-
mined on the nanometer scale. The corresponding stepped profiles are dem-
onstrated in Figure 5.18a.
Both profiles represent a bell-shaped distribution of ions in a depth normal
to the surface plane. The step height is 6 nm for the arsenic and 3 nm for the
cobalt sample. Both profiles show a maximum in a certain depth of 60 nm (for
As) and 30 nm (for Co) and with a certain width. They have an offset or surface
value at
z
=
0 and are skewed to the right. Both profiles were compared with
RBS measurements carried out independently at Bochum and Munich Uni-
versity. TXRF and RBS profiles match quite well, with differences only
occurring at the surface of both implantations and at the maximum of the
As implantation. Detection limits for the molar fraction are about 0.01. The
depth resolution is 10-15 nm for RBS and only 3 nm for TXRF (four times
better).
The differences of TXRF and RBS profiles are caused by an assumption
simply and solely made for RBS—that the original number density of crystal-
line silicon remains unchanged during implantation. This assumption, however,
is not permitted. During implantation, the original crystalline structure is
damaged and the crystal is amorphized. A swelling (expansion) of the crystal
can generally be observed at a depth of some 10 nm and a shrinking (compres-
sion) usually occurs at a depth of 10-50 nm. For even deeper layers, the
crystalline structure of silicon remains unchanged. In contrast to RBS, the
application of TXRF allows an individual determination of both quantities
n
ion
and
n
Si
(see Section 4.5.2.2), in accord with Equations 4.34 and 4.35. The
respective results are shown in Figure 5.18b. The values of
n
ion
for RBS are
directly proportional to
c
layer
of Figure 5.18a and show a similar profile;
however, respective values for TXRF lead to changed profiles that are less
smoothed because of uncertainties of mechanical measurements. Both implan-
tations show a good match of RBS and TXRF; differences only appear on their
increasing flank down to the maximum and can be explained by the different
depth resolution of RBS and TXRF [168,169]. Both curves correspond very
closely on their decreasing flank, that is, in a depth where the crystalline
structure is nearly undestroyed.
5.4.8.2Thin-LayerStructuresbyDirectGI-XRF
X-ray fluorescence at grazing incidence can be employed to characterize thin-
layer structures in the near-surface range of 1-500 nm with respect to their
composition and thickness
nondestructively
. This requires, however, an angle
scan of the layered wafer in order to record and interpret angle-dependent
intensity profiles.
A variety of thin layers deposited on flat surfaces (mainly silicon wafers)
have been analyzed by GI-XRF [170-184]. These single- and double-layer
systems consisted of pure metals, metal alloys, metal oxides, or nitrides. The
mass fraction of the individual elements covered the total percentage range, the
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