Chester G. Wilson

Spectral detection of metal contaminants in water using an on-chip microglow discharge

This paper reports on the detection of trace contaminants in water by spectroscopy of micro glow discharges that operate in air or at moderate vacuum using liquid electrodes. A liquid electrode spectral emission chip (LEd-SpEC) has been developed to perform this function. The device is fabricated by a four mask process, and provides a reservoir and channels in a glass substrate, along with electrodes that bias the water sample. Liquid from the cathode is sputtered into the discharge, for spectroscopic detection of impurities. Using a commercial spectrometer, Na concentrations <10 ppm, and Pb concentrations of 5 ppm, and Al and Cr concentrations of 10 ppm have been measured. The ratio of Na spectral intensity to that of ambient N/sub 2/ is shown to be a suitable measure of Na impurity concentration over several orders of magnitude. The addition of HNO/sub 3/ to lower the pH of the liquid solution increases this ratio by almost an order of magnitude. By selectively doping the solution, the device can also be used as a customizable optical source for UV and visible wavelengths.

Silicon micromachining using in situ DC microplasmas

This paper reports on the generation of spatially confined plasmas and their application to silicon etching. The etching is performed using SF/sub 6/ gas and dc power applied between thin-film electrodes patterned on the silicon wafer to be etched. The electrodes also serve as a mask for the etching. The typical operating pressure and power density are in the range of 1-20 Torr and 1-10 W/cm/sup 2/, respectively. The plasma confinement can be varied from 1 cm by varying the electrode area, operating pressure, and power. High power densities can be achieved at moderate currents because the electrode areas are small. Etch rates of 4-17 /spl mu/m/min., which enable through-wafer etching and varying degrees of anisotropy, have been achieved. The etch rate increases with power density, whereas the etch rate per unit power density increases with operating pressure. Scaling effects are explored for varying sized mask openings. Plasma resistance measurements and electric field modeling are used to provide an initial assessment of the microplasmas.

Exploring microdischarges for portable sensing applications

AbstractThis paper describes the use of microdischarges as transducing elements in sensors and detectors. Chemical and physical sensing of gases, chemical sensing of liquids, and radiation detection are described. These applications are explored from the perspective of their use in portable microsystems, with emphasis on compactness, power consumption, the ability to operate at or near atmospheric pressure (to reduce pumping challenges), and the ability to operate in an air ambient (to reduce the need for reservoirs of carrier gases). Manufacturing methods and performance results are described for selected examples. FigureSide-view photograph of an ultraviolet light source that uses microdischarges

Profiling and modeling of dc nitrogen microplasmas

This article explores electric current and field distributions in dc microplasmas, which have distinctive characteristics that are not evident at larger dimensions. These microplasmas, which are powered by coplanar thin-film metal electrodes with 400-μm minimum separations on a glass substrate, are potentially useful for microsystems in both sensing and microfabrication contexts. Experiments in N2 ambient show that electron current favors electrode separations of 4 mm at 1.2 Torr, reducing to 0.4 mm at 10 Torr. The glow region is confined directly above the cathode, and within 200–500 μm of its lateral edge. Voltage gradients of 100 kV/m exist in this glow region at 1.2 Torr, increasing to 500 kV/m at 6 Torr, far in excess of those observed in larger plasmas. Numerical simulations indicate that the microplasmas are highly nonquasineutral, with a large ion density proximate to the cathode, responsible for a dense space-charge region, and the strong electric fields in the glow region. It is responsible for ...

Microfluidic electrodischarge devices with integrated dispersion optics for spectral analysis of water impurities

This paper reports a microfluidic device that integrates electrical and optical features required for field-portable water-chemistry testing by discharge spectroscopy. The device utilizes a dc-powered spark between a metal anode and a liquid cathode as the spectral source. Impurities are sputtered from the water sample into the microdischarge and characteristic atomic transitions due to them are detected optically. A blazed grating is used as the dispersion element. The device is fabricated from stacked glass layers, and is assembled and used with a charge-coupled device (CCD) sensing element to distinguish atomic spectra. Two structural variations and optical arrangements are reported. Detection of Cr and other chemicals in water samples has been successfully demonstrated with both devices. The angular resolution in terms of angular change per unit variation in wavelength (/spl part//spl theta///spl part//spl lambda/) is experimentally determined to be approximately 0.10 rad//spl mu/m, as opposed to the idealized theoretical estimate of 0.22 rad//spl mu/m. This is because the microdischarge is uncollimated and not a point source. However, this is sufficient angular resolution to allow critical spectra of metal impurities to be distinguished.

Microfluidic discharge-based optical sources for detection of biochemicals

This paper reports a discharge-based optical source for fluorescence of biochemicals in microfluidic systems. Its efficacy is demonstrated using a stacked microchip that integrates a microfluidic wavelength-tunable optical source, a biochemical sample reservoir and optical filters. It is shown to excite fluorescence in l-tryptophan and DNA samples labeled by SYBR green dye. The discharge is struck in ambient air, between a metal anode and a cathode cavity that is filled with an aqueous solution, which is doped with a metal salt selected for its emission characteristics. The characteristic line spectra, which arise from energetic transitions of the metal ions that are sputtered into the glow region of the discharge, are optically filtered and guided to the biochemical sample that resides in a separate on-chip reservoir. For DNA fluorescence, a barium chloride solution is used to emit light at 454 and 493 nm. For tryptophan fluorescence, the cathode contains lead (ii) nitrate solution to provide a 280 nm emission. The resulting fluorescence from the DNA and tryptophan samples is compared to reference data. This technique can also be used to excite other fluorophores by using appropriately doped liquid cathodes having the desired emission characteristics.

A Multichannel Nanoparticle Scintillation Microdevice with Integrated Waveguides for Alpha, Beta, Gamma, X-Ray, and Neutron Detection

This paper reports on a glass microdevice with a new patterned nanoparticle composite resin that detects and discriminates all species of radiation emitted from fissionable bomb making materials. Tailored charge conversion nanoparticles doped into a fast electron scintillating resin produce different optical pulses specific to the radiation species. The resins are integrated into a glass substrate where deep cavities are made using microsandblasting, forming independent optical paths leading to fiber-optic attachments. A separate, off the shelf Photo-Multiplier (PM) tube measures the light pulse.

Miniaturized magnetic nitrogen DC microplasmas

This paper explores the use of miniaturized magnets to enhance the parameters of dc microplasmas. The microplasmas are powered by thin coaxial electrodes and are enhanced by a coaxial magnetron configuration machined from niobium composite magnets. At operating pressures of 1-4 torr, a glow region that is confined to the volume directly over the cathode, forms a traditional magnetron-type annular ring. Three coaxial magnets, ranging in total size from 3.2 to 7.2 mm in outside diameter generate measured magnetic fields up to 3030 G. The magnetic field structure is profiled with a small Hall probe and is modeled by finite-element analysis. The plasma currents for various applied voltages are measured, and the plasma breakdown/termination voltages are determined. In nitrogen ambients at 1.2 torr and 370-V bias, the current changes from 9.3 mA in the absence of a magnetic field to 17.6 mA with the addition of the largest magnet. The sheath region decreases with the addition of the magnetic structures, illustrating an effect on the Debye length and, therefore, the local plasma density. The dimensions of the sheath are found to vary radially within the annular microplasma. The smallest sheath corresponds to the region of highest magnetic field over the south pole for the largest magnetic configuration. This effect is used to generate a microplasma in SF/sub 6/ on a silicon wafer producing a localized etch. The etch rate in the region of the brightest glow is three times greater than the weakest etch rate, allowing spatially localized etch selectivity without masking.

Integrated System for Wireless Radiation Detection and Tracking

This paper reports on an integrated system of wirelessly linked radiation detectors. A small radiation detector is combined with a pulse counting circuit to record radiation pulses in the area of the detector. Multiple sensors are mesh networked together using wireless radios. Mesh networking allows for a large network of battery powered nodes with only one receiving hub. Radiation data is communicated to a central location where it is compared to the location of the node and its relative position to the other nodes. This information is used to locate and track a radiation source.

A microfluidic ultra-violet emission source for direct fluorescence of tryptophan

This paper describes a fluidic microchip for observing the direct fluorescence of tryptophan and other amino acids, which are used in studying protein structure and dynamics. Since the excitation wavelengths of these are in the 250-290 nm ultra-violet range, it is a major challenge to find appropriate light sources that can be integrated onto micrototal analysis systems. This effort demonstrates that illumination from a microdischarge can be used to observe the direct fluorescence of tryptophan. The discharge is ignited across an air gap between an on-chip metal anode and a liquid cathode made of a saturated solution of lead nitrate. Atomic transitions in Pb atoms that are consequently sputtered into the discharge provide the wavelengths necessary to excite amino acids. Unwanted wavelengths are rejected by an optical filter that separates the microdischarge from the tryptophan sample. Measured results include the unfiltered and filtered spectral output of the microdischarge, as well as that of the resulting tryptophan emission, which has the characteristic broad peak from 300-450 nm.