TRENDS AND FUTURE PROSPECTS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 461

For the chromium pads with gold layers on top of them, the photon energy of
the incident beam was selected to be 6.8 keV. The critical angle of total
reflection for chromium was 0.45 ° , for gold it was 0.68 ° , and for the silicon
substrate 0.26 ° . The K peaks of chromium and gold were observed with a silicon
drift detector and a reference-free quantification was performed. As expected,
the angular-dependent fluorescence intensity showed periodic oscillations for
Cr-K α below 0.26 ° , and for Au-M α below 0.68 ° , while the intensity of silicon
increased steeply at 0.26 ° . As demonstrated in Figure 7.14a, theory and
experiment generally agree; they only differ a little because of shadowing
effects. That means that the X-ray standing wave (XSW) field caused by total
reflection at the silicon substrate is hardly disturbed by the nanoscaled
structures because only 0.2% of the total area is covered. For the chromium
and permalloy structures without gold layer, an incident beam of 9.3 keV was
selected; the critical angle for chromium was 0.32 ° , for permalloy it was 0.35 ° ,
and for silicon 0.19 ° . This time, the K peaks of Cr, Fe, and Au were observed
with a silicon drift detector and a reference-free quantification was performed
again. As demonstrated in Figure 7.14b, the measured intensity/angle profiles
showed only smeared and weak oscillations; however, theory and experiment
correspond at a rough estimate [63].
The mass-deposition of chromium pads, which was in the range of
30-125 pg/cm 2 , could also be determined by reference-free quantification.
Assuming a flat top of the pads, the average values fit quite well with the nominal
values of the manufacturer with a deviation of only 6%. For the permalloy-pads,
however, the results showed deviations of about 55%. The heights of these pads
were determined afterward by SEM/energy dispersive X-ray spectrometry
(EDX) and corrected by a factor 0.42. The correction led to a nominal mass
Figure7.14. (a) Intensity/angle scan recorded by GI-XRF for an artificial park of cylindrical
metallic pads with a height of 100 nm. The pads consisting of chromium were deposited on a Si wafer
and capped with a gold layer. The solid curves represent measurements for Cr-K α , Au-M α ,
and Si-K α ; the dotted curves were calculated reference-free. The oscillations indicate nodes and
antinodes of the XSW field above the Si substrate. (b) Intensity/angle scan for Cr pads without gold
layer but with different heights of 20, 50, and 100 nm. Figure from Ref. [63], reproduced with
permission. Copyright2012, Royal Society of Chemistry.