Uncovering new thermal and elastic properties of nanostructured materials using coherent EUV light
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 dynamics, 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.
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Department of Physics
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University of Colorado Boulder
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