Uncovering new thermal and elastic properties of nanostructured materials using coherent EUV light

Author
Abstract
Advances in nanofabrication have pushed the characteristic dimensions of nanosystems well below 100nm, where physical properties are often significantly different from their bulk coun-terparts, and accurate models are lacking. Critical technologies such as thermoelectrics for energy harvesting, nanoparticle-mediated thermal therapy, nano-enhanced photovoltaics, and efficient ther-mal management in integrated circuits depend on our increased understanding of the nanoscale. However, traditional microscopic characterization tools face fundamental limits at the nanoscale. Theoretical efforts to build a fundamental picture of nanoscale thermal dynamics lack experimental validation and still struggle to account for newly reported behaviors. Moreover, precise character-ization of the elastic behavior of nanostructured systems is needed for understanding the unique physics that become apparent in small-scale systems, such as thickness-dependent or fabrication-dependent elastic properties. In essence, our ability to fabricate nanosystems has outstripped our ability to understand and characterize them.
In my PhD thesis, I present the development and refinement of coherent extreme ultraviolet (EUV) nanometrology, a novel tool used to probe material properties at the intrinsic time- and length-scales of nanoscale dynamics. By extending ultrafast photoacoustic and thermal metrol-ogy techniques to very short probing wavelengths using tabletop coherent EUV beams from high-harmonic upconversion (HHG) of femtosecond lasers, coherent EUV nanometrology allows for a new window into nanoscale physics, previously unavailable with traditional techniques. Using this technique, I was able to probe both thermal and acoustic dynamics in nanostructured systems with characteristic dimensions below 50nm with high temporal (sub-ps) and spatial (< 10pm vertical) resolution, including the smallest heat sources probed (20nm) and thinnest film (10.9nm) fully mechanically characterized to date.

By probing nanoscale thermal transport (i.e. cooling) of periodic hot nanostructures down to 20nm in characteristic dimension in both 1D (nanolines) and 2D (nanocubes) geometries, I un-covered a new surprising regime of nanoscale thermal transport called the “collectively-diffusive regime. In this regime, nanoscale hot spots cool faster when placed closer together than when far-ther apart. This is a consequence of the interplay between both the size and spacing of the nanoscale heat sources with the phonon spectrum of a material. This makes our technique one of the only ex-perimental routes to directly probe the dynamics of phonons in complex materials, which is critical to both technological applications and fundamental condensed matter physics. I developed a proof of concept model and used it to extract the first experimental differential conductivity phonon mean free path (MFP) spectra for silicon and sapphire, which compare well with first-principles calcula-tions. However, a complete picture of the physics is still elusive. Thus, I developed a computational solver for the phonon Boltzmann transport equation in realistic experimental geometries. Using this approach, I successfully found confirmation of the influence of the period in thermal transport from periodic heat sources: a smaller periodicity can enhance the heat dissipation efficiency. This result is qualitatively consistent with the results of the “collectively-diffusive regime”, but more work is needed for a full theoretical quantitative picture of the experimental results.

In other work, I used coherent EUV nanometrology to simultaneously measure, in a non-contact and non-destructive way, Young’s modulus and, for the first time, Poisson’s ratio of ultra-thin films. I successfully extracted the full elastic tensor of the thinnest films to date (10.9nm). Moreover, by using our technique on a series of low-k dielectric sub-100nm SiC:H films, I uncovered an unexpected transition from compressible to non-compressible behavior. This new behavior is observed for materials whose network connectivity had been modified through hydrogenation (that breaks bonds in order to decrease the dielectric constant of these materials). This finding demonstrates that coherent EUV nanometrology provides a valuable, quantitative new tool for measuring nanomaterial properties with dimensions an order of magnitude smaller than what was possible with traditional techniques.

I also present here some of my written work on science and technology policy studies. I present my thoughts on the Kuhnian model of scientific revolutions and how it relates to my own experience. I also discuss two case studies to illustrate the critical importance of defining appropriate metrics to measure science policies by looking at the design of metrics for the American Reinvestment and Recovery Act, and the results of exploring a novel modality of funding for large complex scientific and technological challenges: the US Department of Energy Innovation HUBs.

Coherent EUV nanometrology presents an exciting new window into nanoscale phonon dy-namics, making measurements of the phonon MFP spectrum of materials and the full elastic tensor of ultra-thin films possible. It is now a robust technique that is already having impact in many areas of materials science and condensed matter physics, and it will continue to do so in the future.
Year of Publication
2017
Academic Department
Department or Physics
Degree
Ph.D.
Number of Pages
180
Date Published
2017-05
University
University of Colorado Boulder
City
Boulder
Advisors - JILA Fellows
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