Boyd M. Evans

Large Discrete Resistance Jump at Grain Boundary in Copper Nanowire

Copper is the current interconnect metal of choice in integrated circuits. As interconnect dimensions decrease, the resistivity of copper increases dramatically because of electron scattering from surfaces, impurities, and grain boundaries (GBs) and threatens to stymie continued device scaling. Lacking direct measurements of individual scattering sources, understanding of the relative importance of these scattering mechanisms has largely relied on semiempirical modeling. Here we present the first ever attempt to measure and calculate individual GB resistances in copper nanowires with a one-to-one correspondence to the GB structure. Large resistance jumps are directly measured at the random GBs with a value far greater than at coincidence GBs and first-principles calculations. The high resistivity of the random GB appears to be intrinsic, arising from the scaling of electron mean free path with the size of the lattice relaxation region. The striking impact of random GB scattering adds vital information for understanding nanoscale conductors.

Cytometric catheter for neurosurgical applications

Implantation of neural progenitor cells into the central nervous system has attracted strong interest for treatment of a variety of pathologies. The replacement of dopamine-producing neural cells in the brain appears promising for the treatment of patients affected by Parkinsons disease. Previous studies of cell replacement strategies have shown that less than 10% of implanted cells were viable 24–48 hours following implantation. We present the design of an instrumented cell-delivery catheter that has been developed to facilitate the quantification of the cells delivered and determination of viability. The catheter uses a fibre optic probe to perform fluorescence-based cytometric measurements on cells exiting the port at the catheter tip. Results of fluorescence testing data are presented and show that the device can characterize the quantity of cell densities ranging from 60 000 to 600 000 cells ml−1 with a coefficient of determination of 0.93 (p < 0.05, n = 6).

Optical readout of uncooled thermal detectors

We investigated microposition sensing of micro-electro- mechanical systems (MEMS) that is based on optical readout techniques. We determined the parameters that affect or limit the performance of optical readout techniques especially as they apply to detection of infrared radiation. Such microposition sensing schemes are very important as readout mechanisms for large arrays of microstructures which are required for imaging. In addition, we explored the performance of uncooled micromechanical IR sensors using Fresnel zone plates (FZP). This type of diffractive feature diffracts along the optical axis and not perpendicular to that axis. We found that temperature fluctuation noise and background fluctuation noise, are currently the limits to the performance of uncooled micromechanical IR detectors. The noise at the output of the optical readout includes amplified noise from the micromechanical structures and noise added by the optical readout itself. However, the added noise is negligible compared to the amplified temperature fluctuation noise inherent in the microstructures. In this context an optical readout is nearly an ideal, noiseless readout method.

Comparison of materials for use in the precision grinding of optical components

Precision grinding of optical components is becoming an accepted practice for rapidly and deterministically fabrication optical surface to final or near-final surface finish and figure. In this paper, a comparison of grinding techniques and materials is performed. Flat and spherical surfaces were ground in three different substrate materials: BK7 glass, chemical vapor deposited silicon carbide ceramic, and sapphire. Spherical surfaces were used to determine the contouring capacity of the process, and flat surfaces were used for surfaces finish measurements. The recently developed Precitech Optimum 2800 diamond turning and grinding platform was used to grind surfaces in 40mm diameter substrates sapphire and silicon carbide substrates and 200 mm BK7 glass substrates using diamond grinding wheels. The results of this study compare the surface finish and figure for the three materials.

Piezoresistive microcantilever optimization for uncooled infrared detection technology

Uncooled infrared sensors are significant in a number of scientific and technological applications. A new approach to uncooled infrared detectors has been developed using piezoresistive microcantilevers coated with thermal energy absorbing material(s). Infrared radiation absorbed by the microcantilever detector can be sensitively detected as changes in the electrical resistance as a function of microcantilever bending. These devices have demonstrated sensitivities comparable to existing uncooled thermal detector technologies. The dynamic range of these devices is extremely large due to measurable resistance change obtained with only nanometer level cantilever displacement. Optimization of geometrical properties for selected commercially available cantilevers is presented. Additionally, we present results obtained from a modeling analysis of the thermal properties of several different microcantilever detector architectures.

Optimization of Micromachined Photon Devices

The Oak Ridge National Laboratory has been instrumental in developing ultraprecision technologies for the fabrication of optical devices. We are currently extending our ultraprecision capabilities to the design, fabrication, and testing of micro-optics and MEMS devices. Techniques have been developed in our lab for fabricating micro-devices using single point diamond turning and ion milling. The devices we fabricated can be used in micro-scale interferometry, micro-positioners, micro-mirrors, and chemical sensors. In this paper, we focus on the optimization of microstructure performance using finite element analysis and the experimental validation of those results. We also discuss the fabrication of such structures and the optical testing of the devices. The performance is simulated using finite element analysis to optimize geometric and material parameters. The parameters we studied include bimaterial coating thickness effects; device length, width, and thickness effects, as well as changes in the geometry itself. This optimization results in increased sensitivity of these structures to absorbed incoming energy, which is important for photon detection or micro-mirror actuation. We have investigated and tested multiple geometries. the devices were fabricated using focused ion beam milling, and their response was measured using a chopped photon source and laser triangulation techniques. Our results are presented and discussed.

Direct Label-Free Electrical Immunodetection of Transplant Rejection Protein Biomarker in Physiological Buffer Using Floating Gate AlGaN/GaN High Electron Mobility Transistors

Monokine induced by interferon gamma (MIG/CXCL9) is used as an immune biomarker for early monitoring of transplant or allograft rejection. This paper demonstrates a direct electrical, label-free detection method of recombinant human MIG with anti-MIG IgG molecules in physiologically relevant buffer environment. The sensor platform used is a biologically modified GaN-based high electron mobility transistor (HEMT) device. Biomolecular recognition capability was provided by using high affinity anti-MIG monoclonal antibody to form molecular affinity interface receptors on short N-hydroxysuccinimide-ester functionalized disulphide (DSP) self-assembled monolayers (SAMs) on the gold sensing gate of the HEMT device. A floating gate configuration has been adopted to eliminate the influences of external gate voltage. Preliminary test results with the proposed chemically treated GaN HEMT biosensor show that MIG can be detected for a wide range of concentration varying from 5 ng/mL to 500 ng/mL.

Structural Dependence of Grain Boundary Resistivity in Copper Nanowires

We report the direct measurement of individual grain boundary (GB) resistances and the critical role of GB structure in the increased resistivity in copper nanowires. By measuring both intra- and inter-grain resistance with a four-probe scanning tunneling microscope, large resistance jumps are revealed owing to successive scattering across high-angle random GBs, while the resistance changes at twin and other coincidence boundaries are negligibly small. The impurity distributions in the nanowires are characterized in correlating to the microstructures. The resistance of high symmetry coincidence GBs and the impurity contributions are then calculated using a first-principle method which confirms that the coincidence GBs have orders of magnitude smaller resistance than the high-angle random GBs.

Optimization of cell suspension media for use in a cytometric neuro-catheter

Parkinsons disease is a neurodegenerative disorder that impairs motor and speech skills due to the degeneration of dopamine-producing cells in the substantia nigra. It is anticipated that future treatments will involve replacing the degenerated cells with cultivated dopamine-producing multipotent cells. Previous studies involving these treatments encountered complications due to death of the majority of injected cells. It is currently unknown if cell death occurs during delivery or after injection. A neuro-catheter has been designed that is capable of taking fluorescence-based cytometric measurements using fiber optic probes. This device will allow researchers, and eventually clinicians, to quantify the number of viable cells delivered. During delivery, a uniform distribution of cells is essential to ensure the viability of the delivered cells. In order to accomplish this, cell suspension materials are required. In order to investigate the effects of suspension materials on cell delivery, a cuvette experiment was designed and implemented. The goal of this experiment was to determine the extent of cell settling issues for several different suspension materials. The 3RT1 rat gliomal cell line was used as a model for neural cell delivery. The viability of the cells was also examined in each of the given suspension materials over a two hour period. Of the tested materials, 1% methyl cellulose in phosphate buffer saline was determined to be the best material for uniform cell delivery.

Finite element modeling of micromachined MEMS photon devices

The technology of microelectronics that has evolved over the past half century is one of great power and sophistication and can now be extended to many applications (MEMS and MOEMS) other than electronics. An interesting application of MEMS quantum devices is the detection of electromagnetic radiation. The operation principle of MEMS quantum devices is based on the photoinduced stress in semiconductors, and the photon detection results from the measurement of the photoinduced bending. These devices can be described as micromechanical photon detectors. In this work, we have developed a technique for simulating electronic stresses using finite element analysis. We have used our technique to model the response of micromechanical photon devices to external stimuli and compared these results with experimental data. Material properties, geometry, and bimaterial design play an important role in the performance of micromechanical photon detectors. We have modeled these effects using finite element analysis and included the effects of bimaterial thickness coating, effective length of the device, width, and thickness.