PERFORMANCE OF TXRF and GI-XRF ANALYSES - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 262

ratio c AL /( c AL + c AP ). From Equation 2.24 it can be shown that this quantity is
given by

3
2

N 2
N 1

100 % (4.31)

c L 1

The corresponding percentage of the particulate type c P can be defined by the
ratio c AP /( c AL + c AP ), given by

3
2

N 2
N 1

1
3

c P 100 % (4.32)

Both values sum up to 100%. The calculations presuppose that samples or
wafers are free from bulk-type contamination and ideally flat and even.
Otherwise, the results only represent approximate values.

4.5.2CharacterizationofThinLayersbyTXRF

Thin layers embedded in or deposited on a flat and even substrate can be
characterized by recording angle-dependent intensity profiles, that is, by GI-
XRF, which is nondestructive, but requires a mechanical tilting and above all it
is indirect. However, there are two variants combined with classical TXRF that
can compete without such drawbacks. They are based either on wet-chemical
etching or dry-physical etching. Both methods are destructive but only a small
piece of a rectangular sample is needed (about 1 cm 2 ).

4.5.2.1MultifoldRepeatedChemicalEtching

A flowchart in Figure 4.24 demonstrates the method of wet-chemical etching
and TXRF applied to Si wafers [72,73]. It starts with the determination of the
wafer area, which is measured by a graduated sample stage of an optical
microscope (e.g., Ortholux, Ernst Leitz GmbH, Wetzlar, Germany) at a
magnification of 100-fold. The values of 2-3cm 2 can be determined with
high precision (RSD < 0.01%). Chemical etching is performed by oxidizing
this piece with a solution of hydrogen peroxide for some minutes (30% H 2 O 2
solution of high purity, some milliliter). Afterward, the thin oxidized sublayer is
removed while dipping the sample in hydrofluoric acid (4% HF dilution of high
purity, some milliliter). These steps are similar to vapor-phase decomposition
(VPD, see Section 5.4.7.2). However, in contrast to VPD, oxidation is induced
by H 2 O 2 and not by ambient air. Moreover, etching is carried out by an aqueous
solution and not by the vapor of HF.
Before oxidation and after etching, the wafer piece is dried and weighed
thoroughly on a calibrated microbalance (see Section 4.5.2.2). The difference
gives the mass of the removed sublayer (0.1-1 μ g). With help of the density of