High-purity silicon is used in computer chips and solar cells. A new analytical method developed at the Department of Ion Physics and Applied Physics in collaboration with Wacker Chemie AG can detect impurities in real time and reduces resource consumption during the production of high-purity polysilicon.
Impurities with boron can render large quantities of high-purity silicon unusable. Since the element has one less electron than silicon in its outer shell, a boron atom causes a so-called hole in the electron structure of the silicon semiconductor, which negatively affects the electronic properties. In the production of high-purity silicon, liquid silicon compounds are distilled using a significant energy input. Until now, it was only possible to check at the end of the process whether the required purity had been achieved, in an elaborate test deposition. The new method makes it possible to measure the boron concentration during distillation. This allows the required high purity of the silicon to be achieved with lower energy and material consumption.
Turning a disadvantage into an advantage
The process developed at the Department of Ion Physics and Applied Physics by Brigitte Fox-Beyer, Martin Beyer, and Milan Ončák in collaboration with Wacker Chemie exploits precisely the chemical property of boron that causes problems in semiconductors: "In gaseous compounds, such as those present in distillate, boron forms only three chemical bonds, meaning it still has room for an electron pair or a fourth bond in its outer shell", explains Martin Beyer, head of the research group. "In the flow reactor of a mass spectrometer, negatively charged ions that have a free electron pair therefore react preferentially with the boron compound." This allows the boron compound to be specifically isolated of a mixture in a targeted manner, even if only a few molecules of it are present in a billion molecules of the silicon compound.
The team implemented the method using a modified PTRMS mass spectrometer from the Innsbruck-based spin-off company Ionicon Analytik GmbH. The analytical method has since been filed for patent .
