A Field Emission Ionization Source Using Spindt Cathodes, 15-9159

Principal Investigators
Martin P. Wüest
Gregory P. Miller

Inclusive Dates: 09/15/1999 - Current

Background - Electron impact is the most common form of ionization for the neutral gas analysis of small molecules by mass spectrometry. Traditionally, thermionic electron emission from a filament is used to generate the electrons. Power is a premium resource on scientific space vehicles, and researchers strive to reduce power consumption as much as possible. Field emission cathodes provide an alternative approach at generating electrons without the heating power needed in a filament-based design. Field-emitter cathodes are electron sources, in the form of arrays of microfabricated sharp tips (see top photograph). Field emission is used to extract the electrons without heating the cathodes. As a solid-state device, the field-emission cathodes should be ideal for a space application. However, an issue that needs investigation is their lifetime. Emission depends upon tip radius and work function, which may change during operation and degrade the device characteristics. Sputtering of the tip by residual gas molecules ionized by the emitted electrons blunts the emission point and increases the required voltage. The sputtering is highly dependent on the gas environment. Hydrogen-rich molecules increase the emission, while oxygen-based molecules decrease the emission. No report could be found in the literature concerning nitrogen, the most abundant gas in the Earth's atmosphere and the second most abundant gas on Mars's atmosphere.

Approach - Because the lifetime depends on the gases present and on the microtip material, the research team plans to test three types of field-emission arrays. These arrays include an original Spindt-type cathode array, in which the tips are made of molybdenum; an array in which the microtip material is silicon; and a diamond-like carbon cathode. The team will subject the different microtip array to different gases that are relevant to planetary neutral gas mass spectrometry and investigate the emission characteristics as functions of time and gas exposure. In addition, the team will develop and build an ion source using microtips to get actual performance data from a microtip-based electron source.

Accomplishments - The team has assembled the system to test the microtips and has performed the first test. The team has also designed an orthogonal ion source, which will be manufactured and tested. A cross section of the source can be seen in the bottom illustration.

Scanning electron micrograph of a field emission array with silicon microtips.

Cross-section of orthogonal ion source

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