Andrew T. Jennings

Tensile deformation of electroplated copper nanopillars

The results are presented of uniaxial tensile testing of single crystalline electroplated copper nanopillars with diameters between 75 nm and 165 nm fabricated without the use of a focused ion beam (FIB). The experiments were performed in an in situ nanomechanical instrument, SEMentor, and reveal that the pillars’ ultimate tensile strengths follow a similar power law dependence on diameter as reported for microcompression studies on fcc metals fabricated with and without FIB. Further, these pillars are characterized by limited or non-existent initial homogeneous deformation, immediately followed by necking in the top portion of the pillar. The particular deformation attributes are discussed in the context of hardening by dislocation starvation. Site-specific transmission electron microscopy microstructural analysis of as-fabricated nanopillars indicates the presence of scarce twin boundaries in some specimens. We comment on the potential for mechanical effects due to the presence of twins.

Universal Quake Statistics: From Compressed Nanocrystals to Earthquakes.

Slowly-compressed single crystals, bulk metallic glasses (BMGs), rocks, granular materials, and the earth all deform via intermittent slips or “quakes”. We find that although these systems span 12 decades in length scale, they all show the same scaling behavior for their slip size distributions and other statistical properties. Remarkably, the size distributions follow the same power law multiplied with the same exponential cutoff. The cutoff grows with applied force for materials spanning length scales from nanometers to kilometers. The tuneability of the cutoff with stress reflects “tuned critical” behavior, rather than self-organized criticality (SOC), which would imply stress-independence. A simple mean field model for avalanches of slipping weak spots explains the agreement across scales. It predicts the observed slip-size distributions and the observed stress-dependent cutoff function. The results enable extrapolations from one scale to another, and from one force to another, across different materials and structures, from nanocrystals to earthquakes.

In situ characterization of vertically oriented carbon nanofibers for three-dimensional nano-electro-mechanical device applications

We have performed mechanical and electrical characterization of individual as-grown, vertically oriented carbon nanofibers (CNFs) using in situ techniques, where such high-aspect-ratio, nanoscale structures are of interest for three-dimensional (3D) electronics, in particular 3D nano-electro-mechanical-systems (NEMS). Nanoindentation and uniaxial compression tests conducted in an in situ nanomechanical instrument, SEMentor, suggest that the CNFs undergo severe bending prior to fracture, which always occurs close to the bottom rather than at the substrate-tube interface, suggesting that the CNFs are well adhered to the substrate. This is also consistent with bending tests on individual tubes which indicated that bending angles as large as approximately 70 degrees could be accommodated elastically. In situ electrical transport measurements revealed that the CNFs grown on refractory metallic nitride buffer layers were conducting via the sidewalls, whereas those synthesized directly on Si were electrically unsuitable for low-voltage dc NEMS applications. Electrostatic actuation was also demonstrated with a nanoprobe in close proximity to a single CNF and suggests that such structures are attractive for nonvolatile memory applications. Since the magnitude of the actuation voltage is intimately dictated by the physical characteristics of the CNFs, such as diameter and length, we also addressed the ability to tune these parameters, to some extent, by adjusting the plasma-enhanced chemical vapor deposition growth parameters with this bottom-up synthesis approach.

Electrostatic Switching in Vertically Oriented Nanotubes for Nonvolatile Memory Applications

We have demonstrated electrostatic switching in vertically oriented nanotubes or nanofibers, where a nanoprobe was used as the actuating electrode inside an SEM. When the nanoprobe was manipulated to be in close proximity to a single tube, switching voltages between 10 V – 40 V were observed, depending on the geometrical parameters. The turn-on transitions appeared to be much sharper than the turn-off transitions which were limited by the tube-to-probe contact resistances. In many cases, stiction forces at these dimensions were dominant, since the tube appeared stuck to the probe even after the voltage returned to 0 V, suggesting that such structures are promising for nonvolatile memory applications. The stiction effects, to some extent, can be adjusted by engineering the switch geometry appropriately. Nanoscale mechanical measurements were also conducted on the tubes using a custom-built nanoindentor inside an SEM, from which preliminary material parameters, such as the elastic modulus, were extracted. The mechanical measurements also revealed that the tubes appear to be well adhered to the substrate. The material parameters gathered from the mechanical measurements were then used in developing an electrostatic model of the switch using a commercially available finite-element simulator. The calculated pull-in voltages appeared to be in agreement to the experimentally obtained switching voltages to first order.

Carbon-based Nano-electro-mechanical-systems

We provide an overview of our work where carbon-based nanostructures have been applied to twodimensional (2D) planar and three-dimensional (3D) vertically-oriented nano-electro-mechanical (NEM) switches. In the first configuration, laterally oriented single-walled nanotubes (SWNTs) synthesized using thermal chemical vapor deposition (CVD) were implemented for forming bridge-type 2D NEMS switches, where switching voltages were on the order of a few volts. In the second configuration, vertically oriented carbon nanofibers (CNFs) synthesized using plasma-enhanced (PE) CVD have been explored for their potential application in 3D NEMS. We have performed nanomechanical measurements on such vertically oriented tubes using nanoindentation to determine the mechanical properties of the CNFs. Electrostatic switching was demonstrated in the CNFs synthesized on refractory metallic nitride substrates, where a nanoprobe was used as the actuating electrode inside a scanning-electron-microscope. The switching voltages were determined to be in the tens of volts range and van der Waals interactions at these length scales appeared significant, suggesting such structures are promising for nonvolatile memory applications. A finite element model was also developed to determine a theoretical pull-in voltage which was compared to experimental results.

High-throughput processes and structural characterization of single-nanotube based devices for 3D electronics

We have developed manufacturable approaches to form single, vertically aligned carbon nanotubes, where the tubes are centered precisely, and placed within a few hundred nm of 1-1.5 μm deep trenches. These wafer-scale approaches were enabled by chemically amplified resists and inductively coupled Cryo-etchers to form the 3D nanoscale architectures. The tube growth was performed using dc plasmaenhanced chemical vapor deposition (PECVD), and the materials used for the pre-fabricated 3D architectures were chemically and structurally compatible with the high temperature (700 °C) PECVD synthesis of our tubes, in an ammonia and acetylene ambient. The TEM analysis of our tubes revealed graphitic basal planes inclined to the central or fiber axis, with cone angles up to 30° for the particular growth conditions used. In addition, bending tests performed using a custom nanoindentor, suggest that the tubes are well adhered to the Si substrate. Tube characteristics were also engineered to some extent, by adjusting growth parameters, such as Ni catalyst thickness, pressure and plasma power during growth.