DIASIP – Silicon Wafer Production with Diamond-Coated Sawing Wire

Development of Diamond-Coated Wires and Applicable Sawing Processes for Cost-Effective Production of Silicon Wafer for the Photovoltaic Industry

Duration: October 2011 - September 2014
Contracting Authority/ Sponsors: German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU), German Federal Ministry of Education and Reserach (BMBF), Saxon State Ministry for Sciences and Arts (SMWK), Sächsische AufbauBank (SAB), European Regional Development Fund (ERDF)
Project Partners: PV Crystalox Solar Silicon GmbH, Solar World Innovations, BOSCH Solar Energy AG, Wendt GmbH / Winterthur / 3M, Steinbeis-Innovationszentrum Entwicklungstechnologie
Website: http://www.innovationsallianz-photovoltaik.de/main/diasip/
© Fraunhofer ISE

Fig. 1: High-resolution scanning electron micrograph of the crosssection through a diamond-coated sawing wire.

© Fraunhofer ISE

Fig. 2: Scanning electron micrograph of a wafer cross-section to determine the depth of micro-cracks.

© Fraunhofer ISE

Fig. 3: Maximum crack depth as a function of surface crystal orientation for a multicrystalline silicon wafer.

Wafer production with diamond-coated sawing wire is a promising sawing technique offering an alternative to the conventional slicing process with loose abrasives. The main technological difference is the coating of the sawing wire with fixed, abrasive diamond particles. This results in a microscopically different ablation process. Research and further development of this slicing technology is a major field of work at the Fraunhofer Technological Centre for Semiconductor Materials THM. Our results show that surface damage of monocrystalline and multicrystalline silicon material depends on crystal orientation.

The application of diamond-coated wires to sawing silicon wafers is relatively new, such that there is a great need for process development. For industrial application of this technology, investigations, including some fundamental research, and innovative process optimisation are needed. The technical and analytical equipment of Fraunhofer THM is oriented toward this research goal. For the sawing experiments, we use a technically optimised, industrial multi-wire saw and have equipped it with additional measurement technology. It was used to produce wafers of monocrystalline and multicrystalline silicon and characterise their surface, micro-crack and fracture properties with specially adapted or optimised optical and mechanical measurement systems. Analysis of these parameters allows not only the sawing process to be improved, but also the resistance to fracture of wafers to be determined and optimised on an industrial scale. Our investigations have demonstrated that the maximal crack depth on a wafer varies as a function of the crystal orientation by a factor of 2 (Fig. 3). For surfaces with a {110} orientation, two cleavage planes are present perpendicular to the surface. Depending on the angle between the sawing wire and the cleavage planes, the crack depth can increase by 123 % in the most unfavourable case. This can result in a reduced fracture stress, which was confirmed by fracture tests.

With the purpose of improving the production efficiency of such sewing wires, scientists at THM developed an innovative scratch test stand that allows for detailed industry-related research on material and processes and enables further optimization of material as well as processes. Details on this part of the DIASIP project are available in the related article "Material Removal Process Optimization with New Scratch Test Stand".