PERFORMANCE OF TXRF and GI-XRF ANALYSES - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 264

The concentration of a sublayer defined by the molar ratio c layer = n ion / n Si can
simply be given as a result in mole per mole and with a precision of < 5% and an
accuracy < 8%. The sputter depth is relevant for the thickness of the previous
sublayer (result of the etching solution) while the information depth of the
primary beam is relevant for the next sublayer (result of the freshly etched
surface). The information depth is about 3 nm due to total reflection of X-rays
and both depth values should approximately be equal.
The latter five steps are repeated many times (about 20 or 30 times), until the
implanted layer is completely removed (order of 10-200 nm). The terminus is
controlled by a visual inspection (changing blaze of the wafer) or by TXRF
(vanishing spectral peaks of implanted ions). Finally, two concentration/depth
profiles are plotted showing the number density of the implanted ions versus
the depth in nanometer. The first profile is based on TXRF of the etching
solution and the second profile represents TXRF of the freshly etched surface.
The number density of silicon in the implanted wafer is assumed to remain
nearly constant during implantation (about 5.0 × 10 22 /cm 3 ).
It has to be emphasized that all these steps necessitate extreme cleanliness.
Gloves have to be worn and the wafer sample should only be touched with
tweezers as little as possible and only from the side edges. Between the
different steps the sample has to be kept in a Petri dish in order to prevent
contamination by air dust and humidity.
An example of such a chemical etching is given in Ref. [72]. It provides the
number density of implanted ions directly in a range of 10 18 up to 10 21 /cm 3 . The
individual steps are between 0.5 and 5 nm wide, which is very favorable.
However, a clear disadvantage is the troublesome chemical etching. Each
sublayer needs about an hour, a total profile with 30 steps needs about 4 days.
The step height cannot easily be reproduced and the requirement of a constant
density of Si in the different sublayers is hardly fulfilled. Besides, this kind of
chemical etching can only be applied to the class of Si wafers. The next variant
is able to overcome these restrictions.

4.5.2.2StepwiseRepeatedPlanarSputterEtching

This method replaces the wet-chemical etching by a physical variant based on
planar ion sputtering [73,74]. Figure 4.25 demonstrates the method of dry-
physical etching by means of a flowchart [75,76]. Of course, the conditions of
extremely clean working demanded for wet-chemical etching have also to be
observed. The starting point again is a small rectangular piece of a wafer with
an area of about 3 cm 2 (0.4-0.75 mm thick). Its actual dimensions are precisely
determined by an optical microscope as already mentioned. Afterward, surface
analysis is carried out by TXRF of the upmost layer as described earlier
(Section 4.5.2.1).
The next step is sputter-etching of thin sublayers by a special ion source of
the Kaufman type (RR-ISQ 40, Roth & Rau, Wüstenbrand, Germany). A
sectional view and a photo of the complete device are represented in