Uniform High Current Field Emission of Electrons from Si and CNF FEAs Individually Controlled by Si Pillar Ungated FETs

J. Microelectromech. Syst.

Gassend

Akinwande

2010

We report the fabrication and experimental characterization of a carbon nanotube (CNT)-based MEMS/NEMS electron impact gas ionizer with an integrated extractor gate for portable mass spectrometry. The ionizer achieves low-voltage ionization using sparse forests of plasma-enhanced chemical-vapordeposited CNTs as field emitters and a proximal extractor grid with apertures aligned to the CNT forests to facilitate electron transmission. The extractor gate is integrated to the ionizer using a high-voltage MEMS packaging technology based on Si springs defined by deep reactive ion etching. The ionizer also includes a high-aspect-ratio silicon structure (μfoam) that facilitates sparse CNT growth and also enables uniform current emission. The devices were tested as field emitters in high vacuum (10 −8 torr) and as electron impact ionizers using argon at pressures of up to 21 mtorr. The experimental data show that the MEMS extractor gate transmits up to 66% of the emitted current and that the ionizers are able to produce up to 0.139 mA of ion current with up to 19% ionization efficiency while consuming 0.39 W. [2009-0246] Index Terms-Carbon nanotubes (CNTs), deep reactive ion etching (DRIE), electron impact ionization, field emission, gas ionizer, portable mass spectrometry (MS), 3-D MEMS packaging. I. INTRODUCTION M ASS SPECTROMETERS are powerful analytical tools that are helpful to quantitatively determine the chemical composition of a sample. Unfortunately, conventional mass spectrometry (MS) hardware is bulky and power hungry, which limits the range of applications that the technology can satisfy. The development of gas chromatography and MS (GC/MS) systems that are small, light, mechanically and thermally robust, and low-powered would enable their portability and therefore would expand the applicability of MS techniques. Portable GC/MS systems, either as individual units or as part of massive networks, can be used in a wide range of in situ applications including geological survey, law enforcement, environ-Manuscript

Section: Device Structure and Designmentioning

confidence: 99%

CNT-Based MEMS/NEMS Gas Ionizers for Portable Mass Spectrometry Applications

J. Microelectromech. Syst.

Gassend

Akinwande

2010

“…Finally, the data shows that our ionizer is currentlimited, presumably because of velocity saturation of electrons within the µfoam, resulting in more uniform current emission [12], [13]. Un-ballasted field emitter arrays don't work uniformly because of the broad tip diameter size distribution and the strong dependence of the current emission on the emitter tip diameter as shown by Eq.…”

Section: Device Structure and Designmentioning

confidence: 98%

“…The current limitation effect seems to be related to the high aspect-ratio of the µfoam structure, and the high resistivity (> 50 Ω.cm) of the silicon substrate used to create it. The µfoam acts as an ungated field-effect transistor (FET) that limits the current due to velocity saturation of electrons in silicon [12], [13]. The linear part of the FN plot predicts a field factor β = 3.3×10 5 /cm.…”

Section: Experimental Characterizationmentioning

confidence: 99%

CNT-based gas ionizers with integrated MEMS gate for portable mass spectrometry applications

TRANSDUCERS 2009 - 2009 International Solid-State Sensors, Actuators and Microsystems Conference

Gassend

Akinwande

2009

We report the fabrication and experimental characterization of a novel low-cost carbon nanotube (CNT)-based electron impact ionizer (EII) with integrated gate for portable mass spectrometry applications. The device achieves low-voltage ionization using sparse forests of plasma-enhanced chemical vapor deposited (PECVD) CNTs field emitter tips, and a proximal gate with open apertures to facilitate electron transmission. The gate is integrated using a deep reactive ion etched (DRIE) springbased high-voltage MEMS packaging technology. The device also includes a high aspect-ratio silicon structure (µfoam) that facilitates sparse CNT growth and limits the electron current per emitter. The devices were tested as field emitters in high vacuum (10 -8 Torr). Electron emission starts at a gate voltage of 110 V, and reaches a current of 9 uA at 250 V (2.25 mW) with more than 55% of the electrons transmitted through the gate apertures. The devices were also tested as electron impact ionizers using Argon. The experimental data demonstrates that the CNT-EIIs can operate at mtorr-level pressures while delivering 60 nA of ion current at 250 V with about 1% ionization efficiency. KEYWORDSCarbon nanotubes (CNTs), DRIE, gas ionizer, electron impact ionization, field emission, portable mass spectrometry, 3D MEMS packaging.

“…Field emitters quantum tunnel electrons to vacuum when the electrostatic field on the emitter tip surface is at least ~3×10 7 V/cm [6]; nanosharp tips can generate such high electric fields at a low bias voltage (< 100 V). Arrays of emitters can be used to greatly increase the total emission current of the field emission cathode [7], and individual regulation of the emitters can be implemented to achieve high array utilization and emission uniformity [8], [9]. Field emission cathodes operate at room temperature, consume less power to produce the same electron current compared to thermionic electron sources, and they can also operate at lower vacuum [10] and in atmospheres with residual reactive gases [11].…”

Section: Introductionmentioning

confidence: 99%

Nanostructured silicon field emitter array-based high-vacuum magnetic-less ion pump for miniaturized atomic spectroscopy sensors

Basu

Perez

2015 Transducers - 2015 18th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS)

2015

We report the design, fabrication, and characterization of a novel magnetic-less ion pump architecture for generation and maintenance of high vacuum in small systems that is compatible with alkali-vapor cells for lasercooled atomic sensors. The pump uses a nanostructured silicon field emitter array to generate a beam of electrons that fragments the gas molecules into ions, which are captured by a high-voltage getter. In a proof-of-concept demonstration with a 200 cm 3 vacuum chamber, pressures as low as 3.0×10 -7 Torr were generated, corresponding to a 25% decrease of the pressure inside the chamber. The field emission cathode does not degrade when operated in the presence of rubidium vapor at a pressure as high as 7×10 -6 Torr, making the pump technology compatible with laser-cooled alkali atom clocks and inertial sensors. KEYWORDSAtomic clocks, atom interferometry, electron impact ionization, field emission, MEMS/NEMS ion pump, vacuum generation.