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Posted: September 3, 2007
Silicon as smooth as glass
(Nanowerk) Every irregularity no matter how small must disappear: Polishing silicon wafers for solar cells and computer chips demands the greatest preci-sion. Until now, wafers could only be inspected after polishing. A new polishing tool continuously monitors the process.
Without silicon there would be no computer industry since most computer chips consist of this semiconductor material. The same is true for solar cells: They too are predominantly silicon-based. The monocrystals are cut in round slices approximately one millimeter thick, which experts call wafers. Their surfaces must be as smooth as glass; irregularities may only be a few nanometers wide, i.e. less than one ten thousandth of a hair. Therefore, after they have been cut out of a large silicon monocrystal, the wafers must be ground and polished. Until now, whether a surface had become sufficiently smooth was only apparent after polishing. If not, the tool had to be reattached and the process repeated – a time-consuming procedure. Moreover, the tool can easily nick the silicon when it is being attached. When that happens, the expensive material of the entire wafer must be machined until the surface is even again.
Researchers at the Fraunhofer Institute for Factory Operation and Automation IFF in Magdeburg have developed a polishing tool that can constantly control the pressure on a wafer – even during polishing. Its revolutionary feature: Several piezosensors and piezoactors are integrated in the tool. If it comes across an impurity or a material defect during polishing, it intensifies the tool’s pressure on the surface of the material. The piezosensor compresses somewhat and converts this mechanical pressure into electrical voltage. This in turn signals the actor to change the tool’s pressure on the silicon and remove the uneven spot.
“The primary challenge was integrating the sensors and actors 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 Fraunhofer IFF. Another advantage: The polishing tool consists of three rings lying inside one another so that a wafer’s edge can be ground with a different pressure than the inside.
The new tool shortens machining time and simplifies the polishing process even for optical glass lenses. “The pressure with which material is machined could not be measured here until now,” says the expert. “Hence, the lenses had to be taken out of the polishing process again and again to inspect the surface with a laser. The final finish grinding was done manually.” The sensor-aided grinding system has been in industrial use for a short time.