Magdeburg: New way to polish silicon

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German researchers have developed a new tool for polishing silicon wafers for use in solar cells. The new tool shortens machining time and simplifies the polishing process.

To function effectively the wafers, cut in slices approximately one millimetre thick, must have surfaces as smooth as glass. Any irregularities may only be a few nanometres wide, that is, less than one ten-thousandth the thickness of a human hair.

After being cut from a large silicon monocrystal, the wafers must be ground and polished. Until now it was only possible to tell if the surface was smooth enough after polishing. If it was not, the tool had to be re-attached and the process repeated. This is a time-consuming procedure, and the tool can easily nick the silicon when it is being attached. When that happens, the surface of the entire wafer must be machined again.

Researchers at the Fraunhofer Institute for Factory Operation and Automation in Magdeburg have developed a polishing tool that can constantly control the pressure on a wafer, even during polishing. It features several piezo sensors and piezo actuators that are integrated in the tool. If the tool comes across a defect during polishing this compresses the piezosensors, which convert the mechanical pressure into electrical voltage.

This in turn signals increases the pressure the polishing tool applies to the surface, removing the uneven spot.

“The primary challenge was integrating the sensors and actuators in such a way that the tool’s surface is unaffected and the sensor is nevertheless close enough to the surface being machined,” says Susan Gronwald, project manager at the institute.