6:10 PM - ST03.04.02
Mechanical and Structural Properties of Nanostructured Metalattices Probed by Coherent EUV Beams
Begoña Abad Mayor1,2,Joshua Knobloch1,Travis Frazer1,Jorge Nicolas Hernandez Charpak1,Hiu Yan Cheng3,Alex Grede3,Noel Giebink3,Tom Mallouk3,Pratibha Mahale3,Nabila Nova3,Andrew Tomaschke1,Virginia Ferguson1,Vincent Crespi3,Venkatraman Gopalan3,Henry Kapteyn1,John Badding3,Margaret Murnane1
University of Colorado Boulder1,University of Basel2,The Pennsylvania State University3
Phononic crystals represent a very promising route for tuning the properties of next-generation nanoelectronics, thermoelectrics, and ultralight materials. These consist of periodic arrays embedded in an elastic medium, arranged in a specific lattice symmetry [1,2,3]. Nanofabrication techniques can now produce nanoscale phononic crystals with dimensions <<100 nm (referred to as metalattices) which make it possible to engineer new elastic and transport properties [4,5]. To fabricate metalattices, nanospheres are first assembled into a colloidal crystal with face centered cubic order. This base structure can be tuned from monolayer to microns in thickness, with sphere sizes from the nanometer (nanoscale opals) to microns (opals) [1,6]. The interstitial space between the nanospheres of the colloidal crystal is then infiltrated with another material, forming a metalattice structure for periodicities in the sub-100nm. To understand the mechanical properties of metalattices, studies to date have focused only on one component of the metalattice—either the template or the etched-out structures, and always for periodicities >100 nm. In this work, we present a nondestructive method to accurately extract, for the first time, the mechanical and structural properties of metalattices with much smaller feature sizes. Specifically, we probe silicon metalattices fabricated from sphere diameters of 14 nm and 30 nm, with periodicities of 19 nm and 42 nm, respectively. These metalattices contain feature sizes that are an order of magnitude smaller than opals that have been characterized to date. We use an ultrafast laser pulse to heat a set of transducer gratings, which impulsively launch surface acoustic waves (SAW) in the metalattice. The wavelength of the SAW can be tuned by varying the transducer grating periodicity, which also changes the SAW penetration depth into the metalattice and the silicon substrate. We then monitor the SAW frequency from the time-dependent change in extreme ultraviolet (EUV) light diffracted off the grating. This method allows us to simultaneously extract the acoustic dispersion, as well as the Young’s modulus, thickness and filling fraction of the metalattice. Interestingly, the extracted mechanical and structural properties agree well with macroscopic predictions, while the transport properties of the same metalattices do not agree with bulk models. Additionally, the measured metalattice thicknesses agree with scanning electron microscopy images and the extracted Young’s moduli agree with nanoindentation measurements, while achieving higher accuracy. Finally, this technique represents the only approach to date to nondestructively validate the filling fraction of deep-nanoscale metalattices. It also has advantages over destructive electron imaging because it probes over large areas and does not suffer from material contrast issues. These results can enable precise fabrication, characterization and understanding of materials with tailored mechanical and transport properties.
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