Focused Ion Beam Nano-thermometry

Microelectronic components are increasingly fabricated on the nanoscale to unlock new functionalities and increase computational power. In heterogeneously integrated (HI) microelectronics, the increase in nanoscale features can result in an expected decrease in thermal conductivity from the truncation of carrier mean free path [1]. This decrease in thermal conductivity can lead to so-called “hot-spots” where failure occurs from overheating. To mitigate these thermal failure pathways, novel nanoscale thermal characterization is needed to accurately assess heat dissipation and its relation to nanoscale features. Bulk measurements such as thermoreflectance techniques [2, 3] are diffraction limited and therefore cannot provide in-plane thermal information with nanoscale resolution. X-ray techniques can provide some nanoscale thermal information, but they are time-intensive and require rare equipment. In this contribution, we present opportunities for the dual focused ion beam scanning electron microscope (FIB-SEM) for nanoscale thermometry, building on work that demonstrated a temperature dependence on secondary electron emission in the SEM [4]. Dual beam FIB-SEMs hold significant promise for nano-thermometry due to their milling capabilities which allow access to sub-surface information with high spatial resolution. Additionally, sample excitation with an ion beam generates secondary electrons but not backscattered electrons suggesting a more reliable temperature-dependent signal can be obtained. First attempts on intrinsic silicon to replicate results in ref. [4] using a simple Peltier heater set-up in a Zeiss Gemini SEM proved challenging due to the significant impact of carbon build-up and charging artifacts. Attempts in an FEI Ga+ D401 FIB-SEM revealed similar charging and contamination issues, but more reliably showed a temperature dependent signal, likely due to the higher overall signal obtained compared to an SEM. Solutions to these issues included plasma cleaning of the chamber, milling of the sample surface to reduce carbon, and more effective charging mitigation through silver paint usage. Figure 1 demonstrates one of the temperature dependent signal maps obtained in the FIB-SEM. These results demonstrate the potential of nano-thermometry in the FIB-SEM, paving the way to thermal failure analysis methods for microelectronics. Future efforts will focus on multi-material system and eventually full devices [5]. Temperature dependent secondary electron yield in dual focused ion beam scanning electron microscopy (FIB-SEM)