Development of a device for the determination of local magnetic properties of thin films on whole wafers

Abstract

The examination of local magnetic properties of thin films requires a sensible measurement device and small chip-like specimens, which fit in the machine’s magnetic field. Therefore, the carrier wafer has to be diced, which, however, renders it unusable for further processing. If more than a single spot on the wafer is of interest, multiple of these chips have to be examined one piece at a time. In contrast, the device developed within the scope of this thesis allows the measurement of local magnetic properties of continuous layers on whole wafers. For this purpose, it employs techniques known and used in the field of transformer sheet characterization. The principle of placing a magnetizing yoke on top of the specimen has been adapted in order to allow the investigation of layers with a thickness as low as 100 nm. The resulting measurement head generates the necessary field and contains all the sensors for the detection of the hysteresis loop of the specimen. Using algorithms, the flux density in the layer is calculated on the basis of the flux in the magnetizing yoke, which eliminates the need of placing a coil around the specimen. Due to the close matching of the developed hardware, electronics, and software, magnetic measurements with excitation fields of up to 2.5 kA/m in the frequency range from 50 Hz to 50 kHz are feasible. In order to obtain a high reproducibility and enable spatially resolved measurements, a motorized test stand was constructed, which is controlled by a software solution implemented with LabVIEW. The characterization of the device revealed RMS noise levels of 0.1 A/m in terms of field strength and 2·10−11 Wb in terms of flux, which corresponds to a flux density of 25 mT in a 100 nm layer. The acquired hysteresis loops of different thin film samples are compared to reference measurements obtained both with a vibrating sample magnetometer and a hysteresis loop tracer. The results demonstrate the functionality of the device but also show that there is a deviation of the measured coercivity, while the flux density matches the references well. In summary, this novel measurement method represents a fast and convenient way for the examination of the magnetic properties of thin films. Employed in a suitable test stand, it could enable fully automated test procedures like a mapping of the layer on a wafer within minutes. Since it is a non-destructive measurement method, it could be used in the clean room in between different process steps and therefore reduce development times and facilitate quality assurance.