Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE

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Results
Defects characterization
The depth of the defects present under the surface of the wafers was measured. This was done by
polishing a wafer sample with a slight angle in respect to its surface and observing this polished surface
by optical microscopy. The depth of each crack was measured in order to have a crack depth distribution.
This procedure was done for different chosen sets of sawing parameters to quantify the sawing
quality with respect to the sawing parameters. The so obtained distribution can be fitted to an
exponential distribution in the form of p(d)=Ae -bd , where d is the depth of the crack, A is a preexponential
parameter and 1/b is the crack characteristic depth. This last parameter determines the
diminishing rate of crack density in function of depth and allows the comparison between different
cracks depth distributions. An example of such measurement is given in Fig. 2 [1].
Figure 2: Crack depth distribution for the wafers cut with the smallest abrasive. The plain exponential
curve is fitting the measured distribution quite well. As a comparison the dashed curve represents the
fitted distribution of the wafers cut with the coarsest abrasive [1].
The quality of a cut is defined here by the quantity of cracks, particularly the deepest ones, because
they have the biggest influence on yield stress. Thus, a cut with a lot of defects near the surface but
fewer defects further deep than another cut will be said to be of better quality than the second one.
The crack depth distribution depends on the position of measurement. Actually, a measurement made
near the exit of the wire will be of better quality than a cut made near the entrance. Furthermore, the
direction on which the cracks depth measurement is done also have an influence. If it is made in a
direction parallel to the wire movement, the results will be worst than if the measurement is made perpendicular
to it. The parameter having the strongest influence on crack depth is the abrasive grit size,
but the other parameters studied also have an impact.
Roughness measurements
It has been observed that the direction of roughness measurement has a negligible influence on the
results. Thus the analysed measurements were taken perpendicular to the wire direction, as for the
crack depth distribution measurement.
As for the cracks depth, the influence of sawing conditions can clearly be seen on Figure 3, with the
same tendencies than for the crack depth distribution. The average roughness parameter Ra is ranging
from 0.3 to 1.7 µm, which means that for the worst cut, the roughness represents more than 1 % of
the total thickness of the wafer.
SIWIS: Ultra Thin Silicon Wafer Sawing by Multi-Wire Sawing, A. Bidiville, Empa Seite 63 von 288