Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE
Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE Programm Photovoltaik Ausgabe 2008 ... - Bundesamt für Energie BFE
5/6 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
Seite 66 von 288 National Cooperation Project Co-ordinator: � Empa – Materials Science and Technology, Laboratory for mechanics of materials and nanostructures Partners: � Applied Materials Switzerland S.A, Cheseaux (formely HCT Shaping Systems SA) � Ideal Chimic S.A., Geneva � Institute of microtechnology, IMT, Neuchâtel � EPFL - Fluid Mechanics Laboratory (LMF), Lausanne Project evaluation and perspectives In this project, the influence of several chosen sawing parameters was investigated. For this, a large number of cut with different sawing parameters were performed and analysed. In order to minimize the number of cut that had to be made to explore the influence of the chosen parameters, a Doehlert experimental design was used. This allowed to test the influence of four sawing parameters in only 21 different cuts. A good correlation between the average roughness and the crack depth distribution was found. The same tendency was found for wafer strength measurement. From this, the paramount importance of abrasive particle size in sawing quality was highlighted, as well as the relative importance of other sawing parameters. Out of all the cut realized, it was possible to increase the mean fracture stress from 571 to 793 MPa, i.e. an increase of 36 %. With this higher strength, it will be possible to cut thinner wafers for the end of the project – namely 120 µm thick wafers – with still an acceptable strength. Despite of the end of this CTI project, the collaboration between Applied Materials Switzerland and Empa will go on. A strong insight into wire sawing has been acquired, which opens new doors towards a further improvement of the process. The characterisation tools developed during this project allows to thoroughly analyse silicon wafers. Publications from this project [1] A. Bidiville, K. Wasmer, J. Michler, C. Ballif, M. Van der Meer, P. M. Nasch: Towards the correlation of mechanical properties and sawing parameters of silicon wafers, Proceedings of the 22 nd EUPVSEC, Milan, Italy, 2007. [2] K. Wasmer, A. Bidiville, J. Michler, C. Ballif, M. Van der Meer, P. M. Nasch: Effect of strength test methods on silicon wafer strength measurements, Proceedings of the 22 nd EUPVSEC, Milan, Italy, 2007. Other reports and results are confidential SIWIS: Ultra Thin Silicon Wafer Sawing by Multi-Wire Sawing, A. Bidiville, Empa 6/6
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Seite 66 von 288<br />
National Cooperation<br />
Project Co-ordinator:<br />
� Empa – Materials Science and Technology, Laboratory for mechanics of materials and<br />
nanostructures<br />
Partners:<br />
� Applied Materials Switzerland S.A, Cheseaux (formely HCT Shaping Systems SA)<br />
� Ideal Chimic S.A., Geneva<br />
� Institute of microtechnology, IMT, Neuchâtel<br />
� EPFL - Fluid Mechanics Laboratory (LMF), Lausanne<br />
Project evaluation and perspectives<br />
In this project, the influence of several chosen sawing parameters was investigated. For this, a large<br />
number of cut with different sawing parameters were performed and analysed. In order to minimize the<br />
number of cut that had to be made to explore the influence of the chosen parameters, a Doehlert experimental<br />
design was used. This allowed to test the influence of four sawing parameters in only 21<br />
different cuts. A good correlation between the average roughness and the crack depth distribution was<br />
found. The same tendency was found for wafer strength measurement. From this, the paramount importance<br />
of abrasive particle size in sawing quality was highlighted, as well as the relative importance<br />
of other sawing parameters.<br />
Out of all the cut realized, it was possible to increase the mean fracture stress from 571 to 793 MPa,<br />
i.e. an increase of 36 %. With this higher strength, it will be possible to cut thinner wafers for the end of<br />
the project – namely 120 µm thick wafers – with still an acceptable strength.<br />
Despite of the end of this CTI project, the collaboration between Applied Materials Switzerland and<br />
Empa will go on. A strong insight into wire sawing has been acquired, which opens new doors towards<br />
a further improvement of the process. The characterisation tools developed during this project allows<br />
to thoroughly analyse silicon wafers.<br />
Publications from this project<br />
[1] A. Bidiville, K. Wasmer, J. Michler, C. Ballif, M. Van der Meer, P. M. Nasch: Towards the correlation of mechanical<br />
properties and sawing parameters of silicon wafers, Proceedings of the 22 nd EUPVSEC, Milan, Italy, 2007.<br />
[2] K. Wasmer, A. Bidiville, J. Michler, C. Ballif, M. Van der Meer, P. M. Nasch: Effect of strength test methods on silicon<br />
wafer strength measurements, Proceedings of the 22 nd EUPVSEC, Milan, Italy, 2007.<br />
Other reports and results are confidential<br />
SIWIS: Ultra Thin Silicon Wafer Sawing by Multi-Wire Sawing, A. Bidiville, Empa<br />
6/6
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