Session: 11-05-02: Thermophysical Properties: Characterization and Modeling Across Scales

Paper Number: 145459

145459 - Nanoscale Temperature Mapping for Operating Devices With Reflection Electron Elastic Energy Thermometry 

Nanoscale thermal characterization is crucial for optimizing chip thermal management and validating advanced non-diffusive thermal transport models, but current metrologies face limitations to achieve sub-50nm resolution with high compatability. Far-field optical techniques, such as thermoreflectance imaging and Raman thermometry, are fundamentally constrained by diffraction, limiting their spatial resolution to hundreds of nanometers and hindering the exploration of short mean free path phonons. Scanning probe methods, including scanning thermal microscopy (SThM), can achieve ~10 nm resolution but suffer from complex tip-sample thermal interactions and perturbation of the thermal field. While transmission electron microscopy (TEM) based techniques, such as plasmon energy expansion thermometry (PEET) and vibrational spectroscopy, have demonstrated sub-20 nm resolution, they require impractically thin samples, restricting their applicability to real-world devices on bulk substrates. Existing scanning electron microscopy (SEM) approaches using secondary electrons, electron backscatter diffraction, and cathodoluminescence have yet to demonstrate nanoscale temperature mapping capabilities and face challenges such as large interaction volumes and material-specific limitations.

In this work, we develop Reflection Electron Elastic Energy Thermometry (REEET), a novel thermal imaging method employing Reflection Electron Energy Loss Spectroscopy (REELS) in SEM. REEET measures the mean kinetic energy of thermally vibrating atoms via the quasi-elastic peak in REELS spectra. The technique's physical model, based on individual electron-nucleus scattering under the impulse approximation, is validated through calibration experiments and first-principles calculations. REEET exhibits a relative temperature sensitivity of ~0.09%/K for light elements (C, Si) at 300-600 K, surpassing other electron-beam thermometries such as PEET and vibrational spectroscopy, which typically require ultra-high energy resolution and monochromated electron beams due to their lower temperature sensitivities.

We demonstrate REEET's capability by mapping non-contact temperature distributions of nanostructured graphitic thin film devices with 300-400 nm feature sizes and a 240-nm diameter multiwalled carbon nanotube under Joule heating. Visualized local hotspots display peak temperature rises of 500-800 K with an estimated spatial resolution of ~22 nm, primarily limited by e-beam noise and the energy analyzer's small range. Analytical and finite element modeling based on Fourier's law effectively describes the measured temperature profiles using suppressed thermal conductivities of ~450 and ~410 W m^-1 K^-1 for graphite and MWCNT, respectively. These results align with prior reports, attributable to Casimir size effects, material defects, and interface scattering.

REEET offers several advantages over existing methods, achieving nanoscale spatial resolution (estimated at 20-80 nm), probing depth (<10 nm), flexible bulk sample compatibility, element sensitivity in compound materials, and well-understood physics while relaxing electron monochromacy demands compared to other electron-beam techniques. These attributes position REEET as a versatile and powerful tool for investigating nanoscale thermal transport phenomena in operating devices, opening new avenues for advancements in thermal management and fundamental understanding of non-diffusive heat transfer mechanisms at the nanoscale.

Presenting Author: Qiye Zheng The Hong Kong University of Science and Technology

Presenting Author Biography: Dr. Qiye Zheng is current an Assistant Professor of Mechanical and Aerospace Engineering at The Hong Kong University of Science and Technology. He worked at UC Berkeley and Lawrence Berkeley National Lab from 2019-2022 after obtaining his PhD in 2017 and a 1-year postdoc in Materials Science and Engineering from the University of Illinois at Urbana-Champaign. His current research interests include nanoscale heat transfer, non-invasive thermal wave characterization, switchable thermal materials, dynamic thermal insulation, and thermal metrology. He has published 31 papers in prestigious journals such as Science, Applied Physics Review, Advanced Functional Materials, and Applied Energy with 2000+ citations and a h-index of 19. He serves as an independent reviewer for 55 distinct journals including Physical Review Letter, Advanced Functional Materials, Nano Letter, and Applied Energy, a Topic Editor for Frontiers in Mechanical Engineering, and a Topical Advisory Panel member of the journal Crystals. He also served as session chair and poster judge for conferences such as ASME IMECE and International Conference on Multiscale Simulation of Thermal Physics.

Authors:

Menglong Hao Southeast University
Qiye Zheng The Hong Kong University of Science and Technology
Junqiao Wu University of California, Berkeley
Chris Dames University of California, Berkeley