Scott D. Collins

A physical model for threshold voltage instability in Si/sub 3/N/sub 4/-gate H/sup +/-sensitive FET's (pH ISFET's)

A physical model is presented which quantitatively describes the threshold voltage instability, commonly known as drift, in n-channel Si/sub 3/N/sub 4/-gate pH ISFETs. The origin of the so-called drift is postulated to be associated with the relatively slow conversion of the silicon nitride surface to a hydrated SiO/sub 2/ or oxynitride layer. The rate of hydration is modeled by a hopping and/or trap-limited transport mechanism known as dispersive transport. Hydration leads to a decrease in the overall insulator capacitance with time, which gives rise to a monotonic temporal increase in the threshold voltage.

Microchannel Platform for the Study of Endothelial Cell Shape and Function

Microfabrication technology is implemented to realize a versatile platform for the study of endothelial cell (EC) shape and function. The platform contains arrays of microchannels, 25–225 μm wide, that are fabricated by deep reactive ion etching (DRIE) of silicon and anodic bonding to glass and within which ECs are cultured. Silicon fluidic port modules, fabricated using a combination of silicon fusion bonding and anisotropic etching in KOH, provide a simple and reversible means of coupling, via standard tubing, between an individual microchannel and off-platform devices for flow monitoring and control. For flow experiments where a well-defined flow field is required, the channels are capped with either a glass lid or a thin, self-sealing elastomer membrane that can be punctured to provide direct access to cells within the microchannels. Under static culture conditions, bovine aortic ECs (BAECs) become progressively more elongated as the channel width decreases. The shape index, a dimensionless measure of cell roundness, decreases from 0.75±0.01 (mean±SEM) for BAECs cultured in 225 μm-wide microchannels to 0.31±0.02 in 25 μm-wide channels. When cuboidal BAECs are grown in 200 μm-wide microchannels and then subjected to a fluid shear stress of approximately 20 dyne/cm2 (2 Pa), they progressively elongate and align in the direction of flow in a similar manner to cells cultured on plain surfaces. To demonstrate the utility of the microfabricated platform for studying aspects of EC function, whole-cell patch-clamp recordings were performed under static conditions in open microchannels. The platform is demonstrated to be a versatile tool for studying relationships between EC shape and function and for probing the effect of flow on ECs of different shapes. Specific future applications and extensions of platform function are discussed.

Electrochemiluminescence of Tris(2,2'-bipyridine)ruthenium in Water at Carbon Microelectrodes.

Electrochemiluminescence (ECL) of Ru(bpy)(3)(2+) in water only, without any added electrolyte or reducing agents, has been obtained at carbon interdigitated microelectrode arrays (C-IDAs) of 2 μm width and spacing. In a generation/collection biasing mode, ECL can be clearly seen with the naked eye in normal room lighting at concentrations greater than 1 mM. Using a conventional photomultiplier tube (PMT), a detection limit of 10(-)(7) M Ru(bpy)(3)(2+) has been achieved for an electrode area of 0.25 mm(2). In comparison, the ECL intensity produced at Pt-IDA of the same geometry, under identical experimental conditions, was more than 300 times less. The ECL obtained at C-IDAs is attributed to the annihilation reaction of the reduced and oxidized forms of the Ru(bpy)(3)(2+) made possible due to the small electrode spacing.

The electron beam hole drilling of silicon nitride thin films

The mechanism by which an intense electron beam can produce holes in thin films of silicon nitride has been investigated using a combination of in situ electron energy loss spectrometry and electron microscopy imaging. A brief review of electron beam interactions that lead to material loss in different materials is also presented. The loss of nitrogen and silicon decreases with decreasing beam energy and although still observable at a beam energy of 150keV ceases completely at 120keV. The linear behavior of the loss rate coupled with the energy dependency indicates that the process is primarily one of direct displacement, involving the sputtering of atoms from the back surface of the specimen with the rate controlling mechanism being the loss of nitrogen.

Development of low field nuclear magnetic resonance microcoils

A miniaturized spiral Helmholtz rf coil was fabricated using standard photolithography and electroplating. A commercial 0.6 T superconductive magnet was used to test the coil performance for both nuclear magnetic resonance (NMR) spectroscopy and imaging applications. NMR spectra of water, methanol, and 1-propanol were obtained as well as static and flow images of a water phantom. The spectral resolution was sufficient to allow chemical identification. Additionally, the viscosity of water was estimated from the experimental velocity profile and was equal to the expected value of 1 cP. Results obtained demonstrated the prospect of using the Helmholtz rf coil as part of a portable low-field NMR system for applications in analytical chemistry and process measurements in industrial settings.

Nanopore formation by low-energy focused electron beam machining.

The fabrication of nanopores in thin silicon nitride and aluminum oxide membranes by water vapor assisted, low-energy (0.2-20 kV) electron beam machining using a scanning electron microscope (SEM) is described. Using this technique, pores with diameters ranging in size from < 5 to 20 nm are easily formed. The nanopores are characterized by SEM, transmission electron microscopy (TEM) and atomic force microscopy (AFM). The mechanism of etching is briefly discussed.

Microneedle Array for Measuring Wound Generated Electric Fields

A microneedle array has been fabricated and applied to the measurement of transdermal skin potentials in human subjects. Potential changes were recorded in the vicinity of superficial wounds, confirming the generation of a lateral electric field in human skin. The measured electric field decays with distance from the wound edge, and is directed towards the wound. The measurement of endogenous fields in skin is a prelude to the study of the therapeutic efficacy of applied electric fields to chronic non-healing wounds

Fabrication and characterization of a solid-state nanopore with self-aligned carbon nanoelectrodes for molecular detection

Stochastic molecular sensors based on resistive pulse nanopore modalities are envisioned as facile DNA sequencers. However, recent advances in nanotechnology fabrication have highlighted promising alternative detection mechanisms with higher sensitivity and potential single-base resolution. In this paper we present the novel self-aligned fabrication of a solid-state nanopore device with integrated transverse graphene-like carbon nanoelectrodes for polyelectrolyte molecular detection. The electrochemical transduction mechanism is characterized and found to result primarily from thermionic emission between the two transverse electrodes. Response of the nanopore to Lambda dsDNA and short (16-mer) ssDNA is demonstrated and distinguished.

Electron beam stimulated oxidation of carbon

The patterning of carbon nanostructures by electron beam stimulated oxidation is described. Sputter deposited carbon thin films and carbon nanotubes are locally oxidized in a scanning electron microscope using injected water vapor. The resulting structures are examined with scanning electron microscopy and transmission electron microscopy. The electrical resistance obtained postprocessing is comparable to the as-deposited values. Linewidths are demonstrated down to 20 nm along with sub-2 nm nanowire fabrication in sputtered carbon films. A carbon nanowire is fabricated using this process and electrically characterized.

Integration of biaxial planar gradient coils and an RF microcoil for NMR flow imaging

A package of two planar gradient coils combined with an RF coil was microfabricated for low-field MRI measurements of velocity flow profiles. The package generates orthogonal gradients in the radial and velocity encoding directions for standard flow imaging experiments. A commercial 0.6 T superconductive magnet was used to test coil performance. For comparison flow imaging was also performed using a commercial set of gradient coils. Velocity profiles were obtained for several volumetric flow rates of water in tubes with inner diameters of 1.02 mm and 1.4 mm. Velocity resolution was 0.13 mm s−1, comparable to that achieved with commercial gradient coils. The quality of the velocity profile was sufficient for viscosity calculations, and thus permits future utilization of this coil package in the design of a portable MRI viscometer.