Investigation of Field Emission Ionization Sources Using Spindt Cathodes, 15-9159

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Principal Investigators
Martin P. Wüest
Gregory P. Miller

Inclusive Dates: 09/15/99 - 02/15/02

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 a continuing goal is to reduce power consumption as much as possible. Fiel.d 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. Field emission is used to extract the electrons without heating the cathodes. As a solid-state device, the field emission cathodes should be perfect for a space application. However, one issue that needs to be investigated 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 blunt the emission point and increases the required voltage. The sputtering is highly dependent on the gas environment. Oxidation of the emission surface changes the electron emission properties. Hydrogen-rich molecules tend to increase the emission, while oxygen-based molecules tend to decrease the emission.

Approach - Because the lifetime depends on the gases present and on the microtip material, the team tested three different types of field emission arrays. The test arrays include an original Spindt-type cathode array, in which the tips are made of molybdenum and an array in which the microtip material is silicon and a diamond-like carbon cathode.

Accomplishments - The team assembled the system to test the microtips and performed the tests. The best experience is with the molybdenum based field emitters. In addition, the team designed a mass spectrometer ion source using microtips.

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