TRENDS AND FUTURE PROSPECTS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 449

If a synchrotron is not available, excitation of light elements can be done
by the M α peak of a W tube (1.775 keV). Unfortunately, the background
below the aluminum peak is dominated anyway by resonant inelastic Raman
scattering [56,57,74]. Intensity and width of the Raman band depend on the
glancing angle of the incident beam and disappear for glancing angles below
0.06 ° [73]. For the detection of aluminum contaminants, either a small
glancing angle has to be chosen or the intensity of the Raman band has
to be subtracted carefully. It can also be eliminated by deconvolution of the
spectrum, which can lead to a detection limit of about 3 × 10 9 Al atoms/
cm 2 [57,74]. The residue of droplets with 1 μ g of boron could be determined
leading to a detection limit of some ng [73].
7.2.2AblationandDepositionTechniques
Apart from instrumental modifications, special techniques of sample treat-
ment were developed with the aim of trace enrichment or improved spatial
resolution. Trace elements from an aqueous solution can be deposited on a
TXRF carrier by an electrochemical reaction. The carrier should serve as a
cathode in a DC cell with a continuous flow of the electrolyte. A conductive
material, such as glassy carbon, is suitable as both the cathode and the TXRF
carrier. Such electrochemical enrichment has been in its infancy for a long
time [75-77].
The opposite process is the anodic decomposition of a metallic surface, also
called electropolishing. It may be combined with a subsequent TXRF analysis
of the electrolyte. A high current density and a low temperature are beneficial
to the fine etching of thin metallic layers. However, the depth resolution seems
to be restricted to a layer thickness of about 0.1 μ m [78].
Native oxide layers on silicon surfaces can be decomposed by hydrofluoric
acid, as noted in Section 5.4.7.2. The aqueous converted solution can be analyzed
by TXRF. A continually repeated oxidation, ablation, and analysis of the upper-
most layer can give a depth resolution of 1-3 nm (see Section 4.5.2.1).
In addition to such chemical or electrochemical methods used for depth
profiling, physical methods have been proposed. An initial suggestion has come
from Schwenke etal . [58], and a patent application has been filed [79].
Figure 7.6 depicts the procedure. The layered sample is first etched or eroded
by an ion beam of a sputter device, usually by an Ar + beam. This process is
commonly used in combination with methods of surface and thin-layer analy-
ses, for example, Auger electron spectrometry (AES), X-ray photoelectron
spectrometry (XPS), or secondary ion mass spectrometry (SIMS). The sput-
tered material emitted as an atomic vapor partly passes a shielding slit
positioned above the sample. The vapor is deposited as a thin layer on a
substrate suitable as a TXRF carrier.
This substrate is horizontally shifted during the sputter process, and thereby
the vertical concentration profile is transformed to a horizontal one. Afterward,
the thin layer with its lateral distribution of atoms is subjected to TXRF