DIASIP – Basic Examination of Diamond-Coated Sawing Wire Processes and Wafer Qualities

Material Removal Process Optimization with New Scratch Test Stand

Duration: October 2011 - September 2014
Contracting Authority/ Sponsors: German Federal Ministry of Education and Research BMBF, German Federal Ministry of Economic Affairs and Energy BMWi
Project Partners: Wendt-3M, Steinbeis-Innovationszentrum, PVCSS, Bosch Solar Energy, SolarWorld AG
Website: www.innovationsallianz-photovoltaik.de/main/diasip/
© Fraunhofer ISE

Fig. 1: Optical micrograph of an individual diamond grain with an average diameter of 40 μm, which is embedded on the grinding wheel. © Fraunhofer ISE

© Fraunhofer ISE

Fig. 2: Optical micrograph of the scratch pattern with isolated, periodic scratches on a polished Cz silicon wafer. The geometrical parameters of the scratches were analyzed to determine the removal rate.

© Fraunhofer ISE

Fig. 3: Raman map to analyze material stress by evaluation of the wavenumber shift.

Hard materials are processed mechanically with special tools which must meet strict specifications. These include long tool lifetimes with high removal rates and premium tool surface quality. Investigation and optimization of the removal process and the tool properties are important to achieve these objectives. A test stand developed at Fraunhofer THM enables the removal process as well as the characteristics of abrasive, embedding and operating materials to be investigated cost-effectively, rapidly, precisely and under industrially relevant processing conditions. The optimization of diamond wires to saw PV solar wafers is one of its applications.

A high abrasive-particle velocity is the feature of this test stand which is of particular interest for the development of diamond wire for PV silicon wafer production. The test stand allows the industrial removal process to be reproduced in the laboratory. A single abrasive particle or a layer of abrasive grains is fixed onto a grinding wheel and accelerated to 20 m/s. By continuously transporting the test material, we have created individual scratches which can be evaluated in the laboratory by REM and Raman measurements and correlated to the scratching parameters used. This makes it possible to analyze the stress conditions and phase transitions in the silicon material with high resolution. In the centre of the scratch, we detected tensile stresses and transformation of the silicon to a microcrystalline phase (blue regions in Fig. 3). Edge regions which were not chipped showed strong compressive stress, and are visible as magenta zones in the Raman map. By correlating the material removed from the sample to grain abrasion, we predict the tool performance and determine, analyze and optimize the influence of the most important sawing parameters on the removal process. The analytical method is applicable not only to silicon wafering but can also be transferred to other hard materials.

This aspect is just one focus of the project "DIASIP". Another subject is the development and optimization of sawing wires, which is addressed in the related article "Development of Diamond-coated Wires and Applicable Sawing Processes for Cost-effective Production of Silicon Wafer for the Photovoltaic Industry".