PERFORMANCE OF TXRF and GI-XRF ANALYSES - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 269

Figure4.27. Schematic diagram of a GI-XRF instrument suitable for surface and thin-layer
analyses of flat samples like wafers. The specific components are X-ray tube, monochromator,
sample positioning device, wafer robot, and solid state detector. All steps for positioning and
measurement may be computer-controlled. To avoid contaminations, the equipment should be
placed in a clean-bench. Figure from Ref. [4], reproduced with permission. Copyright1996, John
Wiley and Sons.
The incident beam must be monochromatized in order to produce an
unambiguous dependence of the intensity and the glancing angle. X-ray photons
of different energies would blur this relationship. Details on multilayers, Bragg
crystals, and monochromators were given in Sections 3.4.2 and 3.4.3.
The wafer robot is used for loading and changing up to 50 wafers with a
diameter of about 100-200 mm (4-8 in.). The wafers are placed automatically on a
glass plate in rear-contact and cling to it by either an electrostatic or a vacuum
chuck. Instruments are designed for 24 h unattended operation and measurement.
The sample-positioning device is the most important component (already
described in Section 3.5.3). By its means, the sample or wafer is first adjusted in
the observation plane. Mechanical sensors should not be employed in order to
avoid any physical contact with the sample. The adjustment can simply be
controlled by the fluorescence signal generated by the sample itself. Thereafter,
the sample or wafer is tilted stepwise in order to run an angle scan. Starting
from a setting close to zero, the incident angle is increased stepwise and
fluorescence spectra are recorded during time intervals between steps. Usually,
only about three to six spectral peaks or bands, which are set in so-called ROIs
are recorded, rather than recording the entire spectrum. The step width may be
chosen between 0.001 ° and 0.05 ° , and the counting or acquisition time between
5 and 100 s. About 50-200 steps are taken to reach an angle of about twice the
critical angle of the sample material, which is generally < 1 ° . The total measur-
ing time is commonly between 5 min and 5 h. After that, the peak intensities of
those elements that have been detected are plotted against the incident
glancing angle. These angle-dependent intensity profiles are used for a quali-
tative and quantitative characterization of the samples.