Using a light microscope to study sawed silicon wafer surfaces. In the foreground: a silicon block with sawed wafers, coils of wire, and a finished solar cell. (Fraunhofer IWM)
A filigree wire is standardly used to slice silicon blocks (ingots) into paper-thin wafers for solar cells. This wire can cut through the ingot at a speed of up to 60 km/h. Several hundred kilometers long, the wire is arranged in such a way that the ingot is sliced into hundreds of wafers simultaneously. The process takes around six hours and the resultant slices are approximately 180 µm thick. Researchers are setting out to reduce this thickness, and therefore reduce waste.

Dr. Rainer Kübler of Fraunhofer Institute for Mechanics of Materials IWM, explains, “When slicing the wafers, the challenge is to reduce the saw gap width.” The space between two wafers is governed by the thickness of the wire. The steel wire is wetted with a type of paste (slurry), a mixture of silicon carbide and polyethylene glycol. This is harder than silicon and cuts through the ingot. The gap arises where the silicon is reduced to powder during cutting. “Gap widths are currently around 180 µm,” says Kübler, “which means that given a wafer thickness of 180 µm, we generate the same amount of waste for each silicon slice. That’s inefficient.”

The researchers want to achieve smaller saw gap widths of around 100 µm, which are also suitable for industrial applications. In a project funded by the federal ministry for the environment, they are currently studying the abrasion process and contact regimes using a single-wire saw, and are principally interested in the interactions between the wire, slurry, and silicon. They are also using computer modeling to simulate different configurations. The researchers are currently striving to achieve gap widths of 90 µm, which would represent a huge increase in efficiency as waste would be halved.