TRENDS AND FUTURE PROSPECTS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 451

Figure7.7. Ion beam sputtering for a bevel cut of a layered sample: (a) during the sputter process, a
tantalum shutter is steadily moved down; (b) the sample is etched to a bevel shape and then
subjected to a lateral scan with TXRF. Figure from Ref. [82], reproduced with permission.
Copyright1995, American Institute of Physics.
During sputtering, the sample is more and more shielded by a tantalum
shutter, which is steadily moved in front of the sample [82]. In a modified
technique, the sample is moved along a straight line behind a shield with an
aperture of about 1 cm 2 [83]. Both techniques can etch the sample to a bevel
with an inclination angle of smaller than 0.0001 ° . The vertical depth scale is
thereby transformed to a horizontal length scale at a magnification of about 10 6 ,
that is, layers of 10 nm thickness might give stripes of 1 cm width. Layers of such
a thickness are sputtered within 1-10 min.
After the sputter process, the beveled samples are moved in steps below a
TXRF detector with a diaphragm of 0.2-1 mm width. The angle of incidence is
chosen to be well below the critical angle of the relevant elements (about
0.1-0.3 ° ). The intensity for the individual elements is recorded against the
lateral position, producing a laterally resolved line scan. To derive a concen-
tration versus depth profile, both axes have to be calibrated. The length axis or
abscissa is calibrated by determining the different thicknesses of the individual
layers. This can easily be done by an additional measurement with a profil-
ometer [83], or it may be done by an additional experiment with TXRF. This
time, however, the incident angle should be set far above the critical angle [82].
The intensity axis or ordinate can be calibrated by the algorithm described in
Section 4.5.2.2. The fluorescence intensity is then calculated for the remaining
layers, dependent on the sputter depth.
The foregoing method is not only applicable to flat layered samples with
sharp interfaces as is a prerequisite for nondestructive depth-profiling by
TXRF, but it is also applicable to layered samples with diffuse interfaces. A
depth resolution of 2-3 nm and a sampling depth up to several hundred
nanometers may be obtained. Of course, the method is destructive and
time-consuming so that applications are few.