Bernard A. Lynch

Performance of a carbon nanotube field emission electron gun

A cold cathode field emission electron gun (e-gun) based on a patterned carbon nanotube (CNT) film has been fabricated for use in a miniaturized reflectron time-of-flight mass spectrometer (RTOF MS), with future applications in other charged particle spectrometers, and performance of the CNT e-gun has been evaluated. A thermionic electron gun has also been fabricated and evaluated in parallel and its performance is used as a benchmark in the evaluation of our CNT e-gun. Implications for future improvements and integration into the RTOF MS are discussed.

Simulation of a Miniature, Low-Power Time-of-Flight Mass Spectrometer for In Situ Analysis of Planetary Atmospheres

We are implementing nano- and micro-technologies to develop a miniaturized electron impact ionization mass spectrometer for planetary science. Microfabrication technology is used to fabricate the ion and electron optics, and a carbon nanotube (CNT) cathode is used to generate the ionizing electron beam. Future NASA planetary science missions demand miniaturized, low power mass spectrometers that exhibit high resolution and sensitivity to search for evidence of past and present habitability on the surface and in the atmosphere of priority targets such as Mars, Titan, Enceladus, Venus, Europa, and short-period comets. Toward this objective, we are developing a miniature, high resolution reflectron time-of-flight mass spectrometer (Mini TOF-MS) that features a low-power CNT field emission electron impact ionization source and microfabricated ion optics and reflectron mass analyzer in a parallel-plate geometry that is scalable. Charged particle electrodynamic modeling (SIMION 8.0.4) is employed to guide the iterative design of electron and ion optic components and to characterize the overall performance of the Mini TOF-MS device via simulation. Miniature (< 1000 cm3) TOF-MS designs (ion source, mass analyzer, detector only) demonstrate simulated mass resolutions > 600 at sensitivity levels on the order of 10-3 cps/molecule N2/cc while consuming 1.3 W of power and are comparable to current spaceflight mass spectrometers. Higher performance designs have also been simulated and indicate mass resolutions ~1000, though at the expense of sensitivity and instrument volume.

Microshutter array system for James Webb Space Telescope

We have developed microshutter array systems at NASA Goddard Space Flight Center for use as multi-object aperture arrays for a Near-Infrared Spectrometer (NIRSpec) instrument. The instrument will be carried on the James Webb Space Telescope (JWST), the next generation of space telescope, after the Hubble Space Telescope retires. The microshutter arrays (MSAs) are designed for the selective transmission of light from objected galaxies in space with high efficiency and high contrast. Arrays are close-packed silicon nitride membranes with a pixel size close to 100x200 μm. Individual shutters are patterned with a torsion flexure permitting shutters to open 90 degrees with minimized stress concentration. In order to enhance optical contrast, light shields are made on each shutter to prevent light leak. Shutters are actuated magnetically, latched and addressed electrostatically. The shutter arrays are fabricated using MEMS bulk-micromachining and packaged utilizing a novel single-sided indium flip-chip bonding technology. The MSA flight system consists of a mosaic of 2 x 2 format of four fully addressable 365 x 171 arrays. The system will be placed in the JWST optical path at the focal plane of NIRSpec detectors. MSAs that we fabricated passed a series of qualification tests for flight capabilities. We are in the process of making final flight-qualified MSA systems for the JWST mission.

Microshutter arrays for near-infrared applications on the James Webb Space Telescope

Magnetically actuated MEMS microshutter arrays are being developed at the NASA Goddard Space Flight Center for use in a multi-object spectrometer on the James Webb Space Telescope (JWST), formerly Next Generation Space Telescope (NGST). The microshutter arrays are designed for the selective transmission of light with high efficiency and high contrast. The JWST environment requires cryogenic operation at 45K. Microshutter arrays are fabricated out of silicon-on-insulator (SOI) wafers. Arrays consist of close-packed shutters made on silicon nitride (nitride) membranes with a pixel size of 100 × 100 m. Individual shutters are patterned with a torsion flexure permitting shutters to open 90°, with a minimized mechanical stress concentration. Shutters operated this way have survived fatigue life test. The mechanical shutter arrays are fabricated using MEMS technologies. The processing includes a multi-layer metal deposition, patterning of shutter electrodes and magnetic pads, reactive ion etching (RIE) of the front side to form shutters in a nitride film, an anisotropic back-etch for wafer thinning, and a deep RIE (DRIE) back-etch, down to the nitride shutter layer, to form support frames and relieve shutters from the silicon substrate. An additional metal deposition and patterning has recently been developed to form electrodes on the vertical walls of the frame. Shutters are actuated using a magnetic force, and latched electrostatically. One-dimensional addressing has been demonstrated.

Red blood cell sorting with a multi-bed microfabricated filter

A microfabricated fluidic chip for sorting red blood cells (RBCs) by size has been designed, fabricated and tested. The performance of the chip has been compared against a flow cytometer using samples from identical populations of cells, and statistically significant (p < 0.0005) differences in the measured cell size distributions were observed. The measurement paradigm reported here differs from previously demonstrated devices such as microfabricated Coulter counters or flow cytometers, in that the analysis is inherently parallel and is thus suitable for high throughput, point-of-care analysis. This study is empirical and semi-quantitative. However, important features of RBC trapping are characterized and indications for improved device design are described.

An Empirical Study of Boss/Seat Materials and Geometries for Ultra Low-Leakage MEMS Micro-Valves

We report work on the testing and characterization of the sealing properties of various micro-valve seat/boss interfaces. Using a custom test set-up, we have measured helium leak rates for a variety of boss materials and seat geometries. The seat geometries are micro-machined in silicon, and an orifice is DRIE etched through the chip. The test fixture allows for leak-tight edge sealing of seat chips against a viton o-ring, independent of the force used to seal the boss against the seat. Bosses are sealed against the various seat chips with forces up to 400 mN by using a precision micrometer to deflect a small spring that is coupled to the boss chip. Soft metals, such as copper and gold, and polymers such as polydimethylsiloxane (PDMS) and parylene-c, coated on silicon boss chips have been tested on hard silicon seats. In all cases, leak rates were determined as a function of sealing pressure. Seat geometries include a concentric o-ring configuration, and a silicon knife-edge. Both seats have orifice diameters varying from 60 to 110 μm. Experimental results indicate that practical MEMS-scale forces (up to several hundred mN) are sufficient to cause deformation of the soft materials coating the bosses given the small loading area, which can improve sealing capacity but not repeatability. However, uneven loading of the boss prevented a tight seal across the entire seat, which is reflected in the leak rates detected. Soft boss-materials, like PDMS, however, have shown promising results for obtaining ultra-low leak rates. Leak rates as low as 1 × 10−4 atm·cc/sec were obtained on knife-edge seats with 110 μm diameter orifices.Copyright