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

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The test methodology was optimized in order to have results as relevant as possible [2]. It was found
that the edges of the wafers were paramount for the fracture stress, see Fig 6a where the effect of
etching the edges (i.e. removing the cracks on the edges) is shown. In consequence, the test chosen
was the ring-on-ring test, which stresses a large area in the center of the wafer but leaves the edges
free of stress. Finally, the importance of the contact between the testing tool and the wafer was
investigated, and it turned out that the use of carbon paper at the contacts helped improving the
fracture strength measured, as it can be seen in Fig. 6b. An explanation comes from the fact that
without carbon paper, the contact between the tool and the wafer is made on the hills present on the
rough surface of the latter. This localized contact may induce stress concentrators and prevent the
relative sliding of the two bodies needed for bending the wafer. This added stress will in turn diminish
the fracture stress and introduce a random breaking.
The influence of sawing parameter on fracture strength can be seen on Fig. 7, where the cut sawn
with the coarsest abrasive is compared with the cut made with the thinnest abrasive.
Probability of failure [%]
100
90
80
70
60
50
40
30
20
10
0
Edges not etched
Edges etched
50 75 100 125 150 175
Stress-to-Rupture [MPa]
0
100 200 300 400 500 600
Stress-to-Rupture [MPa]
(a)
(b)
Figure 6: Influence of the test methodology on the fracture strength distribution. a) Influence of edges
defects. The wafers with the etched edges are stronger than the wafer without etched edges.
b) Influence of carbon paper: without carbon paper, the fracture stress distribution is poorly fitted with
a Weibull distribution and the mean fracture stress is lower than with carbon paper, where the fracture
stress distribution is well fitted with a Weibull distribution [2].
Probability of failure [%]
100
90
80
70
60
50
40
30
20
10
Without carbon paper
With carbon paper
Figure 7: Yield stress distribution for two sets of wafers sawn with different conditions, in gray with
the coarsest, in black with the finest abrasive. The other sawing parameters remain the same [1].
SIWIS: Ultra Thin Silicon Wafer Sawing by Multi-Wire Sawing, A. Bidiville, Empa Seite 65 von 288