Coherent extreme ultraviolet beams from tabletop high harmonic generation offer revolutionary capabilities for observing nanoscale systems on their intrinsic length and time scales. By launching and monitoring acoustic waves in such systems, we fully characterize sub-10nm films and find that the Poisson’s ratio of low-k dielectric materials does not stay constant as often assumed, but increases when bond coordination is bellow a critical value. Within the same measurement, by following the heat dissipation dynamics from nano-gratings of width 20-1000nm and different periodicities, we confirm the effects of the newly identified collectively-diffusive regime, where close-spaced nanowires cool faster than widely-spaced ones.
Coherent extreme ultraviolet beams from tabletop high harmonic generation offer several revolutionary capabilities for
observing nanoscale systems on their intrinsic length and time scales. By launching and monitoring hypersonic acoustic
waves in such systems, we characterize the mechanical properties of sub-10nm layers and find that the material densities
remain close to their bulk values while their elastic properties are significantly modified. Moreover, within the same
measurement, by following the heat dissipation dynamics from 30-750nm-wide nanowires, we uncover a new thermal
transport regime in which closely-spaced nanoscale heat sources can surprisingly cool more efficiently than widelyspaced
heat sources of the same size.
Photoacoustic nanometrology using coherent extreme ultraviolet (EUV) light detection is a unique and powerful tool for probing ultrathin films with a wide range of mechanical properties and thicknesses well under 100 nm. In this technique, short wavelength acoustic waves are generated through laser excitation of a nano-patterned metallic grating, and then probed by diffracting coherent EUV beams from the dynamic surface deformation. Both longitudinal and surface acoustic waves within thin films and metallic nanostructures can be observed using EUV light as a phase-sensitive probe. The use of nanostructured metal transducers enables the generation of particularly short wavelength surface acoustic waves, which truly confine the measurement within the ultrathin film layer of interest, to thicknesses < 50 nm for the first time. Simultaneous measurement of longitudinal and transverse surface wave velocities yields both the Young’s modulus and Poisson’s ratio of the film. In the future, this approach will make possible precise mechanical characterization of nanostructured systems at sub-10 nm length scales.
Photoacoustic spectroscopy is a powerful tool for characterizing thin films. In this paper we demonstrate a new
photoacoustic technique that allows us to precisely characterize the mechanical properties of ultrathin films. We focus an
ultrafast laser onto a nano-patterned thin film sample, launching both surface acoustic waves (SAWs) and longitudinal
acoustic waves (LAWs). Coherent extreme ultraviolet pulses are then used to probe the propagation dynamics of both the
SAWs and LAWs. The resulting photoacoustic signal on both short (picosecond) and long (nanosecond) time scales
yields important information. In the first 100ps, a fast oscillation followed by an echo signal corresponds to LAWs
traveling inside the nanostructures and the thin film, from which the LAW velocities in the two materials can be
extracted. On longer time-scales, SAW oscillations are observed. By combining the measured SAW frequency with the
wavelength (determined by the nanostructure period) the SAW velocity can be accurately determined, even for very
short wavelength surface acoustic waves with very small penetration depths. Using this technique, the elastic properties,
including the Young's modulus and Poisson ratio for the thin film, can be obtained in a single measurement, this
technique can be extended to sub-10nm thin films.