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.
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.
A poly-silicon piston micro-mirror array, which has been enhanced with a multilayer coating to exhibit special reflective properties at Cu Kα emission line of 1.54 Å is presented. The micro-mirror array is fabricated using the MEMSCAP PolyMUMPs process and packaged in a ceramic package. The packaged array is coated using atomic layer deposition with an Al2O3/W multilayer. The first Al2O3 layer is thicker than for a normal bilayer pair and prevents the mirror coating from creating an electrical short. This device was tested before and after coating. The snap-down voltage was reduced by half, but qualitatively the mechanical motion remained similar. The fabrication process presented for the Cu Kα wavelength at 1.54 Å can be easily adapted to other optical MEMS and for other wavelengths.