Using diamond wire for cutting is a technology of its own – one that is being explored in depth by the Steinbeis Innovation Center for Development Technology in Oberndorf, Germany. Professor Matthias Vogel, director of both the Steinbeis center and the collaborative program of study at the Baden-Wuerttemberg Cooperative State University (DHBW), is working with his team at the development of necessary components for a complete wire connection system, the diamond wire technology and sawing machines as well as the diamond wire cutting technique itself.
Since 2010 the Steinbeis Innovation Center has been awarded funding totaling more than 1.5 million euro from programs of the Federal Ministry of Economics and Technology and the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety for three different projects. The Steinbeis experts in Oberndorf have already successfully completed their first ZIM-supported project: the development of a new generation of wire cutting machines. The construction and production of an endless loop wire saw for wires having a diameter of 0.12 millimeters is the most important result. The machine was specially designed for sawing of silicon wafers of the photovoltaic industry using diamond-coated wires. Its significance for basic research in the area of wire cutting was validated in numerous cutting trials. In addition to the determination of process parameters, the trials also included dry and wet cuts. As a result, the project team was able to define optimized cutting parameters and analyzed the effects of coolants on the tool life of a diamond-coated wire.
In a current ZIM project the Steinbeis Innovation Center is building on these results. Together with its industrial partner HK Präzisionstechnik they are developing an innovative multi-wire saw for slicing photovoltaic wafers. Those machines can cut over 2,000 wafers simultaneously – each having a thickness less than 200 micrometers – out of one silicon block using a wire field. The goal of the project is to drastically minimize costs during this step of the solar panel manufacturing process. In part this can be achieved by taking advantage of improvements in diamond wire technology, but more importantly through targeted adjustment and optimization of the mechanical design and process to the upcoming tool technology.
The Steinbeis experts are working together with notable industry partners and institutes on a BMU project, the third and youngest of their three projects. It aims on reducing production costs for photovoltaic wafers while improving their quality through the introduction of diamond wire cutting technology. The Steinbeis Innovation Center for Development Technology is particularly doing basic research regarding the cutting process, for example a detailed analysis studying the effects of coolants, cutting parameters or wire oscillation on the wafer quality. For all those tasks, the center’s high-tech equipment is frequently called into action. In addition to the endless loop wire saw, a product of the first project, which can simulate even more cutting parameters than conventional saws, a load measurement platform and a high-speed camera for analyzing wire oscillation are available. Furthermore, the team can examine the temperature profile in cut with a high-speed infrared camera and assess the wire wear or breakage with a scanning electron microscope. A brand new measuring machine to analyze wafers with an integrated confocal microscope evaluates the produced quality. The project also places emphasis on the development of a welding machine for extremely thin cutting wires. Those have to be welded with a high process reliability and stand loads of more than 35 Newton, which equates to a weight of roughly 3.5 kilograms. Considering the wire dimensions, this is quite an ambitious goal. In this case tensile strength values exceed 3,000 Newton per square millimeter.
During wire cutting, materials are cut or removed mechanically through the cutting motion of a wire on a workpiece. If the tool is a diamond coated wire, it is known as diamond wire cutting. In particular this method is employed for the separating of extremely hard materials like stone, ceramic or glass. Its most important use is in the production of wafers for the photovoltaic and semiconductor industries. In these applications slurry sawing is still the most common method: this procedure involves the cutting of a silicon block with a bare wire and the so-called slurry, a grinding emulsion made of glycol and silicon carbide grit, into thin slices, the wafers. Technically this is a cut-lapping technique, which explains why the process times can last over ten hours. This approach is losing its luster as it seems like diamond-coated wire could make the combination of wire and slurry obsolete. Why? First and foremost, it reduces process times dramatically and therefore improves cost efficiency. Advances in the manufacturing of diamond wire and its subsequent improvement in quality and sinking prices also benefit the advance of the new technology. Since know-how is lacking in the area of diamond wire cutting, experts are focusing on building up competencies before this promising method can establish itself full-scale within the industry.