Sample Preparation and Resistivity Measurements for Operando Transmission Electron Microscopy During Heating and...

Characterizing microstructures of materials is essential for understanding the relationships with their properties. However, the microstructure can experience changes during operation at elevated temperatures and electrical biasing. Such conditions are standard for thermoelectric devices, which generate electricity from a temperature differential. To characterize these microstructural changes in situ in a transmission electron microscope (TEM), commercialized MEMS-based technology has been utilized. Operando conditions of thermoelectric devices, such as heat loads and electrical biasing, can be replicated by the chips during in situ TEM observation. Here, we show our approach to prepare TEM samples on in situ MEMS chips, suited for heating and biasing, using focused ion beam (FIB). The sample is first lifted out from a bulk material and then connected to the positive and negative electrodes of the chip. The contacts are established by ion beam deposition of a Pt-C composite. Finally, the TEM sample is thinned directly on the chip using FIB. To measure the electrical properties of the TEM sample, a voltage is applied between two electrodes. The current through the sample is measured to derive the total resistance at each temperature. Afterward, the resistivity of the sample and the contact resistance are determined from the measured resistances at different sample dimensions during different stages of FIB thinning. The results demonstrate that heating up to 200 °C causes a reduction in the contact resistance after each FIB thinning. After annealing of the contact, the electrical resistivity can be measured reliably with the presented approach from high temperature down to room temperature, free from the influence of contact resistance.