Characterizing Highly-Confined Heat Flow, Elastic, and Structural Properties of Nanostructured Semiconductors and Dielectrics using Ultraviolet Light Sources

Next-generation nanoelectronic, energy, and quantum devices rely on the discovery, integration, and optimization of novel complex and nanostructured materials. As the critical dimensions of these devices shrink below 10-nm, the complex 3D geometries and physical properties—including, mechanical, thermal, and surface/interface quality—of new materials govern device efficiency and performance. However, as the complexity increases and scale decreases, conventional models fail to accurately predict the functional properties, and traditional metrology tools cannot probe the relevant behaviors on their intrinsic length- and time-scales. By harnessing tabletop sources of coherent extreme ultraviolet (EUV) light¹, novel metrology tools can characterize the elastic, thermal, and structural properties of ultrathin dielectric films and nanostructured semiconductors.<br/>Optimized ultrathin low-<i>k</i> interlayer dielectric films are critical materials to improve the speed and efficiency of nanoelectronic devices. However, precise characterization of films with thickness &lt;&lt;100nm is challenging for traditional methods. Using a tabletop dynamic EUV scatterometry technique, we nondestructively extract the full elastic properties of single- and bilayer a-SiC:H and a-SiOC:H ultrathin films with thicknesses down to &lt;5 nm—revealing surprising results^2,3. By simultaneously extracting both Young’s modulus and Poisson’s ratio in a series of hydrogenated ultrathin films, we observe an unforeseen trend in the compressibility: in the presence of strong hydrogenation which reduces the bond coordination past a critical threshold, the films undergo a transition from brittle to ductile². Additionally, we find that not only doping but also surfaces/interfaces can drastically alter the mechanical properties of ultrathin films. In very thin bilayers (&lt;5 nm) with low-hydrogen doping, surface effects induce a substantial increase in compliance—by almost an order-of-magnitude—compared with identical thicker films, an effect not observed in highly-doped systems³.<br/>Using our dynamic EUV scatterometry technique, we also nondestructively characterize the elastic properties, thickness, porosity, and thermal behavior of thin silicon metalattice layers^4-6—artificial 3D solids that are periodic on the sub-100 nm length scale. By monitoring the dispersion of hypersonic surface acoustic waves, we extract the Young’s modulus and thickness of silica-filled and empty-pore silicon metalattice films with 14 – 35 nm periods, agreeing well with nanoindentation and electron microscope images^4,5. Moreover, we nondestructively probe the porosity and interconnectivity in an empty-pore re-infiltrated metalattice and validate with electron tomography⁵. Using these characterized properties, we can subsequently analyze the exotic thermal behaviors of these nanostructured silicon materials. By tracking the cooling of laser-excited metallic nanostructures, we observe a diffusive-like heat transport behavior with an apparent thermal conductivity two-orders-of-magnitude below bulk silicon⁶, which is not predicted by traditional analytical models. To describe this behavior, we derive an analytical expression for the apparent thermal conductivity based on an analogy to rarefied gas flow in porous media⁶. We find that the physics can be separated into two components: a geometry-dependent permeability and a geometry-independent viscous component which scales universally with porosity. This analytical expression describes not only transport in silicon metalattices but also nanomeshes, nanowire networks, and porous nanowires. <br/>¹<i>Science </i><b>280</b>, 1412 (1998). ²<i>Nano Lett. </i><b>17</b>, 2178 (2017). ³<i>Phys. Rev. Mater. </i><b>4</b>, 073603 (2020). ⁴<i>Nano Lett.</i> <b>20</b>, 3306 (2020). ⁵<i>ACS AMI</i> <b>14</b>, 41316 (2022). ⁶<i>Nano Lett.</i> <b>23</b>, 2129 (2023).