Thermoreflectance Microscope (TRM)

• Subject Area: Electrical Engineering and Information Technology
• Term: Funded in 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 525779412

To better understand the physical processes in semiconductor power devices, especially in overload conditions (short circuit, overcurrent), it is necessary to measure the chip temperature with high spatial and temporal resolution. The measurement should be non-invasive to avoid artificially introduced inhomogeneities. In contrast to infrared-based methods, thermo-reflectance microscopy can meet these requirements and measure the temperature on the front-side metallization of the chips without prior preparation. Current filaments with high current densities in bipolar devices such as IGBTs, which occur during turn-off or short-circuit, can be visualized with this method since the filaments lead to thermal "footprints" on the chip. Until now, these filaments have only been visible in semiconductor simulators because they have very small spatial dimensions in the micrometer range and stay in place only for a few 10 to 100 nanoseconds before extinguishing. For the first time, these phenomena can be observed in real structures and possible weak spots on the chip can be localized, which has already been verified in test measurements on IGBTs under short-circuit condition. This research will contribute to further optimization and robustness improvement of the power semiconductors. Geometric weaknesses leading to current crowding can be avoided in the design. The physical understanding of filament movement can be analyzed on real chips for the first time. In addition to bipolar devices, unipolar devices such as GaN HEMTs or SiC MOSFETs can also be investigated. For SiC MOSFETs, after the growth of stacking faults, the thermal imprint shall be investigated non-invasively with short pulses. Until now, complex preparations for electroluminescence or photoluminescence measurements are necessary. Temperature inhomogeneities on SiC chips in short-circuit or avalanche condition, which can arise at (crystal) defects or at the transition to edge regions, are to be investigated. For this purpose, the thermo-reflectance microscope is synchronized with dynamic test benches in our laboratories. Several measurements can also be superimposed to improve the measurement quality (lock-in). GaN devices will be measured directly to investigate trapping effects in gate regions. So far, only electrical measurements of the dynamic RDSON have been performed on our test benches for this purpose. For special power-cycling tests, which are operated with very short on-times or using heating via switching losses, the temperature distribution at the bond feet during the short heating phases is to be investigated and gives us an improved picture of possible inhomogeneities. So far, this can only be assumed via the final failure analysis and thermo-mechanical simulations. In the field of classical failure analysis, we expect a significant improvement in the localization of small leakage currents.

• DFG Programme: Major Research Instrumentation
• Major Instrumentation: Thermoreflectance Mikroskop (TRM)
• Instrumentation Group: 8620 Strahlungsthermometer, Pyrometer, Thermosonden
• Applicant Institution: Technische Universität Chemnitz
• Leader: Professor Dr.-Ing. Thomas Basler