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

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Figure 3: Crack depth characteristic length (l/b) (top) and average roughness (bottom), sorted in
crack depth ascending order. The diamonds on the left represent the sample cut with the finest abrasive,
the ones on the right represent the cut with the coarsest abrasive. Squares are cut with small,
triangles with standard and stars with coarse abrasive [1].
As crack depth distribution and roughness are correlated with each other, the measurement of
roughness provides a good indication on the sawing quality in a non destructive way. Unfortunately, this
measurement method is global and gives an average crack depth. On the opposite, the strength of the
wafer depends on the depth of the most important crack, which is by its nature not measured in this way.
Mechanical testing
Different bending test configurations were analysed by finite element modelling, see Fig. 4 and Fig. 5.
Two examples of the geometries are presented in Fig. 4 and the results of the respective maximum
principal stress are given in Fig. 5. As expected, line bending and point bending test results are
significantly different. Considering the point ring test in Fig. 5 (a), the area with the maximum stress is
localised at the very centre of the wafer. Therefore, only the strength in the centre of the wafer and at
the vicinity of the punch is tested. The 4-points test in Fig. 8 (b), the edges are also under stress and
the strength value would also take into account defect at the edges. Hence, the final choice of the test
will be guided by the goal of the fracture test.
(a) (b)
Figure 4: Example of 2 types of tested geometries (a) point bending test and (b) 4 points bend.
(a)
Figure 5: Finite element results of the two examples of tested geometries (a) point bending test and
(b) 4 points bend. In red the maximum of the maximal principal stress and in blue location were the
stress is close to 0.
SIWIS: Ultra Thin Silicon Wafer Sawing by Multi-Wire Sawing, A. Bidiville, Empa
(b)
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