DIFFERENT FIELDS OF APPLICATIONS - Total-Reflection X-Ray Fluorescence Analysis and Related Methods - page 340

The detection of low-level metallic contamination on wafers may lead to the
sources of contamination brought in by the different steps. The aim is to reduce
the contaminants to a minimum, to remove the remainder by effective cleaning
steps, and finally to improve the manufacturing process. TXRF can be applied
to monitor the entire process automatically and to serve as a means of
production control. Two modes of operation can be distinguished. The first
is direct TXRF, which can be applied quickly and nondestructively to deter-
mine trace-metal contamination above 10 9 atoms/cm 2 and even to make a
complete map of the total wafer surface. For each point of the surface, only a
single measurement at one fixed angle position is necessary. It is even possible
to distinguish between particulate and thin-layer-type contaminations. For this
purpose, however, at least two measurements at two distinct angle positions are
required.
The second mode of operation involves contamination down to the level of
10 7 atoms/cm 2 . They can be determined if they are first collected from the
entire surface of a wafer and then concentrated on a small spot. This technique
is based on etching of the wafer surface—and consequently requires that the
usual goal of nondestructiveness be abandoned in this case. This mode of
operation is called vapor-phase decomposition (VPD) TXRF.
5.4.7.1WafersControlledbyDirectTXRF
The so-called direct TXRF is the commonly used nondestructive method
applied to a quick contamination control of wafers. Usually, five to nine
different spots of a wafer with a diameter of about 0.8 cm are analyzed directly,
that is, without any preconcentration. A triple beam excitation from a twin
X-ray source is advisable in order to detect all elements from sodium to
uranium simultaneously. The measuring time is chosen to be 50 to 500 s per
spot. Detection limits for the critical transition metals, such as Cr, Mn, Fe, or Ni
are on the level of 10 9 atoms per cm 2 , which is equivalent to ∼ 0.1 pg.
A device is needed for wafer positioning, especially for shifting and fine-
angle adjustment, as already demonstrated in Figure 4.27. A W tube is
preferably chosen and the W-L β instead of the L α peak is selected by a
multilayer monochromator in order to excite the transition metals, including
zinc, and to avoid the excitation of germanium in case of Ge wafers or gallium
and arsenic in case of GaAs wafers. A single measurement is needed for the
determination of contaminants at each spot of the surface chosen for the
control. For this purpose, a fixed glancing angle has to be adjusted that should
amount to about 70% of the critical angle of the wafer material according to
Equation 4.28. The angle adjustment can be controlled by the fluorescence
intensity of the wafer material itself (e.g., silicon), as described in Section 4.6.1.
Calibration can be carried out by an external standard (e.g., a nickel-plated
wafer), as described in Section 4.5.1.
Figure 5.13 shows the results of a wafer mapping for different elements [149].
The complete mapping with about 225 spots needs about 12 h measuring time.