Joshua Knobloch

Full Characterization of the Mechanical Properties of 11–50 nm Ultrathin Films: Influence of Network Connectivity on the Poisson’s Ratio

Precise characterization of the mechanical properties of ultrathin films is of paramount importance for both a fundamental understanding of nanoscale materials and for continued scaling and improvement of nanotechnology. In this work, we use coherent extreme ultraviolet beams to characterize the full elastic tensor of isotropic ultrathin films down to 11 nm in thickness. We simultaneously extract the Youngs modulus and Poissons ratio of low-k a-SiC:H films with varying degrees of hardness and average network connectivity in a single measurement. Contrary to past assumptions, we find that the Poissons ratio of such films is not constant but rather can significantly increase from 0.25 to >0.4 for a network connectivity below a critical value of ∼2.5. Physically, the strong hydrogenation required to decrease the dielectric constant k results in bond breaking, lowering the network connectivity, and Youngs modulus of the material but also decreases the compressibility of the film. This new understanding of ultrathin films demonstrates that coherent EUV beams present a new nanometrology capability that can probe a wide range of novel complex materials not accessible using traditional approaches.

Full-field imaging of thermal and acoustic dynamics in an individual nanostructure using tabletop high harmonic beams

We built a stroboscopic extreme UV microscope with tabletop high harmonics to make nanoscale movies of thermal and acoustic waves. Imaging charge, spin, and energy flow in materials is a current grand challenge that is relevant to a host of nanoenhanced systems, including thermoelectric, photovoltaic, electronic, and spin devices. Ultrafast coherent x-ray sources enable functional imaging on nanometer length and femtosecond timescales particularly when combined with advances in coherent imaging techniques. Here, we combine ptychographic coherent diffractive imaging with an extreme ultraviolet high harmonic light source to directly visualize the complex thermal and acoustic response of an individual nanoscale antenna after impulsive heating by a femtosecond laser. We directly image the deformations induced in both the nickel tapered nanoantenna and the silicon substrate and see the lowest-order generalized Lamb wave that is partially confined to a uniform nanoantenna. The resolution achieved—sub–100 nm transverse and 0.5-Å axial spatial resolution, combined with ≈10-fs temporal resolution—represents a significant advance in full-field dynamic imaging capabilities. The tapered nanoantenna is sufficiently complex that a full simulation of the dynamic response would require enormous computational power. We therefore use our data to benchmark approximate models and achieve excellent agreement between theory and experiment. In the future, this work will enable three-dimensional functional imaging of opaque materials and nanostructures that are sufficiently complex that their functional properties cannot be predicted.

Characterization and imaging of nanostructured materials using tabletop extreme ultraviolet light sources

Using a tabletop coherent extreme ultraviolet source, we extend current nanoscale metrology capabilities with applications spanning from new models of nanoscale transport and materials, to nanoscale device fabrication. We measure the ultrafast dynamics of acoustic waves in materials; by analyzing the material’s response, we can extract elastic properties of films as thin as 11nm. We extend this capability to a spatially resolved imaging modality by using coherent diffractive imaging to image the acoustic waves in nanostructures as they propagate. This will allow for spatially resolved characterization of the elastic properties of non-isotropic materials.

Nanoscale surface phononic crystals for characterization of complex and periodic materials using extreme ultraviolet light

Phononic crystals and acoustic metamaterials enable the precise control of elastic properties, even in ranges inaccessible to traditional materials, making them useful for applications ranging from acoustic waveguiding to thermoelectrics. In particular, surface phononic crystals (SPCs) consisting of periodic nanolines on a semi-infinite substrate can be used to generate narrow bandwidth pseudosurface acoustic waves with exquisite sensitivity to the elastic properties of the underlying substrate. Tuning the period of the surface phononic crystal tunes the penetration depth of the pseudosurface wave, and thus selectively probes different depths of layered substrates. In our experiments, we use ultrafast near infrared laser pulses to excite these waves in the hypersonic frequency range by illuminating absorbing metallic nanolines fabricated on top of complex substrates. We probe the nanoscale dynamics launched by our SPCs via pump-probe spectroscopy where we monitor the diffraction of ultrafast pulses of extreme ultraviolet light generated via tabletop high harmonic generation. We then extract the mechanical properties of the substrate by comparing our measurements to quantitative finite element analysis. Utilizing this technique, we characterize the effective elastic and thermal transport properties of 3D periodic semiconductor metalattices.

Generation of acoustic waves by an extreme ultra violet free electron laser in a transient grating experiment

The use of lasers to generate acoustic waves revolutionized the field of ultrasound and enabled numerous key developments in both fundamental research and applications. In the past decade, remarkable progress has been achieved in developing coherent sources of radiation operating in the extreme ultraviolet (EUV) and x-ray ranges, such as free electron lasers, which already yielded many breakthroughs in different fields of science. We expect that the field of ultrasonics will also greatly benefit from the availability of coherent EUV and x-ray sources. So far, a number of studies explored the use of coherent EUV and x-ray radiation for detection of acoustic waves. In this report, we describe the first experiment on the generation of surface and bulk acoustic waves in the tens of GHz range by EUV light.

Reliable characterization of materials and nanostructured systems <<50nm using coherent EUV beams

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.

Directly probing thermal and acoustic dynamics in nanostructured systems using coherent EUV beams

Utilizing coherent EUV beams, we spectroscopically probe the thermomechanical properties of nanostructured systems allowing access to new regimes of nanoscale thermal transport and to the full elastic tensor of sub-50 nm thin films