The "textbook" phonon mean free path of heat carrying phonons in silicon at room temperature is ∼40  nm. However, a large contribution to the thermal conductivity comes from low-frequency phonons with much longer mean free paths. We present a simple experiment demonstrating that room-temperature thermal transport in Si significantly deviates from the diffusion model already at micron distances. Absorption of crossed laser pulses in a freestanding silicon membrane sets up a sinusoidal temperature profile that is monitored via diffraction of a probe laser beam. By changing the period of the thermal grating we vary the heat transport distance within the range ∼1-10  μm. At small distances, we observe a reduction in the effective thermal conductivity indicating a transition from the diffusive to the ballistic transport regime for the low-frequency part of the phonon spectrum.

Direct Measurement of Room-Temperature Nondiffusive Thermal Transport Over Micron Distances in a Silicon Membrane

Plasma-aided nanofabrication is a rapidly expanding area of research spanning disciplines ranging from physics and chemistry of plasmas and gas discharges to solid state physics, materials science, surface science, nanoscience and nanotechnology and related engineering subjects. The current status of the research field is discussed and examples of superior performance and competitive advantage of plasma processes and techniques are given. These examples are selected to represent a range of applications of two major types of plasmas suitable for nanoscale synthesis and processing, namely thermally non-equilibrium and thermal plasmas. Major concepts and terminology used in the field are introduced. The paper also pinpoints the major challenges facing plasma-aided nanofabrication and identifies some emerging topics for future research.

http://iopscience.iop.org/article/10.1088/0022-3727/40/8/S01/pdf

Plasma-aided nanofabrication: where is the cutting edge?

The use of carbon nanotubes for piezoresistive strain sensors has acquired significant attention due to its unique electromechanical properties. In this comprehensive review paper, we discussed some important aspects of carbon nanotubes for strain sensing at both the nanoscale and macroscale. Carbon nanotubes undergo changes in their band structures when subjected to mechanical deformations. This phenomenon makes them applicable for strain sensing applications. This paper signifies the type of carbon nanotubes best suitable for piezoresistive strain sensors. The electrical resistivities of carbon nanotube thin film increase linearly with strain, making it an ideal material for a piezoresistive strain sensor. Carbon nanotube composite films, which are usually fabricated by mixing small amounts of single-walled or multiwalled carbon nanotubes with selected polymers, have shown promising characteristics of piezoresistive strain sensors. Studies also show that carbon nanotubes display a stable and predictable voltage response as a function of temperature.

/pdf/a-review-carbon-nanotube-based-piezoresistive-strain-sensors-tulig9lhts.pdf

A Review: Carbon Nanotube-Based Piezoresistive Strain Sensors

Carbon science in 2016: Status, challenges and perspectives

Most materials shrink laterally like a rubber band when stretched, so their Poisson's ratios are positive. Likewise, most materials contract in all directions when hydrostatically compressed and decrease density when stretched, so they have positive linear compressibilities. We found that the in-plane Poisson's ratio of carbon nanotube sheets (buckypaper) can be tuned from positive to negative by mixing single-walled and multiwalled nanotubes. Density-normalized sheet toughness, strength, and modulus were substantially increased by this mixing. A simple model predicts the sign and magnitude of Poisson's ratio for buckypaper from the relative ease of nanofiber bending and stretch, and explains why the Poisson's ratios of ordinary writing paper are positive and much larger. Theory also explains why the negative in-plane Poisson's ratio is associated with a large positive Poisson's ratio for the sheet thickness, and predicts that hydrostatic compression can produce biaxial sheet expansion. This tunability of Poisson's ratio can be exploited in the design of sheet-derived composites, artificial muscles, gaskets, and chemical and mechanical sensors.

https://science.sciencemag.org/content/sci/320/5875/504.full.pdf

Sign Change of Poisson's Ratio for Carbon Nanotube Sheets

Carbon-nanotube (CNT) -based strain sensors have the potential to overcome some of the limitations in small-scale force/displacement sensing technologies due to their small size and high sensitivity to strain. A better understanding of the dominant and limiting causes of high strain sensitivity is needed to enable the design and manufacture of high-performance sensor systems. This paper presents the theoretical framework that makes it possible to predict the strain sensitivity of a carbon nanotube based on it chiral indices $(n,m)$. This framework is extended to capture the behavior of sensors composed of multiple CNTs in a parallel resistor network. This framework has been used to predict that a parallel resistor network of more than 100 randomly selected CNTs should have a gauge factor of approximately $78.5\ifmmode\pm\else\textpm\fi{}0.4$. This is within the experimental error of the measured gauge factor of $75\ifmmode\pm\else\textpm\fi{}5$ for such CNT resistor networks.

/pdf/carbon-nanotubes-as-piezoresistive-microelectromechanical-404d4ld0sp.pdf

Carbon nanotubes as piezoresistive microelectromechanical sensors: Theory and experiment

One of the biggest challenges in microscale additive manufacturing is the production of three-dimensional, microscale metal parts with a high enough throughput to be relevant for commercial applications. This paper presents a new microscale additive manufacturing process called microscale selective laser sintering (μ-SLS) that can produce true 3D metal parts with sub-5 μm resolution and a throughput of greater than 60 mm3/hour. In μ-SLS, a layer of metal nanoparticle ink is first coated onto a substrate using a slot die coating system. The ink is then dried to produce a uniform nanoparticle layer. Next, the substrate is precisely positioned under an optical subsystem using a set of coarse and fine nanopositioning stages. In the optical subsystem, laser light that has been patterned using a digital micromirror array is used to heat and sinter the nanoparticles into the desired patterns. This set of steps is then repeated to build up each layer of the 3D part in the μ-SLS system. Overall, this new technology offers the potential to overcome many of the current limitations in microscale additive manufacturing of metals and become an important process in microelectronics packaging applications. A new 3D fabrication process could greatly accelerate the manufacture of finely-detailed microelectronic devices for a plethora of applications. Additive manufacturing strategies such as 3D printing are becoming commonplace in many industries, but these are generally ill-suited for producing the tiny metallic structures required for microelectronic devices. Researchers led by Michael Cullinan at the University of Texas at Austin have now developed a powerful process for 3D printing metal nanoparticles to generate precision structures with resolution below 5 microns. The authors demonstrate their microscale selective laser sintering (μ-SLS) technique in the context of producing lightweight lattice structures and metallic pillars that can serve as next-generation interconnects for microelectronics. Future iterations of μ-SLS could tackle even more aspects of the device fabrication process, or facilitate the production of specialized materials for plasmonics, microfluidics or other applications.

/pdf/a-novel-microscale-selective-laser-sintering-m-sls-process-td2gtoij5c.pdf

A novel microscale selective laser sintering (μ-SLS) process for the fabrication of microelectronic parts.

Traditional MEMS sensing systems do not scale down well to the nanoscale due to resolution and fabrication limitations. Therefore, new sensing systems need to be developed in order to meet the range and resolution requirements of nanoscale mechanical systems. Several nanoscale mechanical sensing systems have emerged that take advantage of nanoscale phenomena to improve the quality of nanoscale sensors. In this paper, we will discuss some of the fundamental limitations in scaling mechanical sensors down to the nanoscale and some of the emerging technologies for nanoscale sensing.

/pdf/scaling-electromechanical-sensors-down-to-the-nanoscale-3bvuirtvhp.pdf

Scaling electromechanical sensors down to the nanoscale

A high electrical and thermal conductivity coupled with low costs make copper (Cu) an enticing alternative to aluminum for the fabrication of interconnects in packaging applications. To tap into the benefits of the ever-reducing size of transistors, it is required to increase the input/output pin count on electronic chips, and thus, minimize the size of chip to board interconnects. Laser sintering of Cu nanoparticle (NP) inks can serve as a promising process for developing these micron sized, 3D interconnect structures. However, the exact processing windows for Cu NP sintering are not well known. Therefore, this paper presents an extensive experimental investigation of the sintering processing window with different lasers including femtosecond (fs), nanosecond (ns), and continuous-wave (CW) lasers. The dependence of the processing window on Cu layer thicknesses and laser exposure durations has also been investigated. A simplified model to estimate optimum laser sintering windows for Cu NPs using pulsed lasers is presented and the predicted estimates are compared against the experimental results. Given the simplicity of the model, it is shown to provide good estimates for fluence required for the onset of sintering and the processing window for good sintering of Cu NPs. [DOI: 10.1115/1.4038455]

A Comprehensive Study of the Sintering of Copper Nanoparticles Using Femtosecond, Nanosecond, and Continuous Wave Lasers

Long range, high precision, XY stages have a multitude of applications in scanning probe microscopy, lithography, micro-AM, wafer inspection and other fields. However, finding cost effective precision motion stages with a range of more than 12 mm with a precision better than one micron is a challenge. This study presents parametric design of the two XY flexure-based stages with a travel ranges of up to 50 mm and sub-micron resolution. First, the fabrication and testing of a two-axis double parallelogram flexure stage is presented and the results obtained from FEA and experimental measurements are shown to be in good agreement with the analytical predictions for this stage. A modified stage design with reduced higher order modes and same range, is also presented. This modified design is shown to be capable of achieving an open loop resolution of 100 nm with a travel range of greater than 50 mm. Higher order modes of the modified stage have been shown to be shifted from 25 Hz in the double parallelogram flexure (DPF) stage to over 86 Hz in the modified DPF stage making it much simpler to design a high speed (>10 Hz) controller for the modified stage.

Michael Cullinan

Papers

Carbon nanotubes as piezoresistive microelectromechanical sensors: Theory and experiment

A novel microscale selective laser sintering (μ-SLS) process for the fabrication of microelectronic parts.

Scaling electromechanical sensors down to the nanoscale

A Comprehensive Study of the Sintering of Copper Nanoparticles Using Femtosecond, Nanosecond, and Continuous Wave Lasers

Design and characterization of a two-axis, flexure-based nanopositioning stage with 50 mm travel and reduced higher order modes