Two designs of a microscale electron ionization ͑EI͒ source are analyzed herein: a 3-panel design and a 4-panel design. Devices were fabricated using microelectromechanical systems technology. Field emission from carbon nanotube provided the electrons for the EI source. Ion currents were measured for helium, nitrogen, and xenon at pressures ranging from 10 −4 to 0.1 Torr. A comparison of the performance of both designs is presented. The 4-panel microion source showed a 10ϫ improvement in performance compared to the 3-panel device. An analysis of the various factors affecting the performance of the microion sources is also presented. SIMION, an electron and ion optics software, was coupled with experimental measurements to analyze the ion current results. The electron current contributing to ionization and the ion collection efficiency are believed to be the primary factors responsible for the higher efficiency of the 4-panel microion source. Other improvements in device design that could lead to higher ion source efficiency in the future are also discussed. These microscale ion sources are expected to find application as stand alone ion sources as well as in miniature mass spectrometers. © 2010 American Institute of Physics. ͓doi:10.1063/1.3429220͔
I. APPLICATIONS AND LITERATURE REVIEW OF MICROSCALE ION SOURCES
A. Introduction to ionization sourcesIon sources are important scientific devices that find application in a variety of areas. They are used in particle accelerators, ion implantation systems for the semiconductor industry, semiconductor processes such as reactive ion etching, giant neutral beam injectors in experimental fusion reactor devices, and in analytical instruments such as mass spectrometers.
1Gas phase ionization techniques include electron ionization ͑EI͒, field ionization, and chemical ͑or charge transfer͒ ionization. Electron impact ionization or electron bombardment ionization is the process in which an electron with suitable energy collides with a neutral atom or molecule resulting in the formation of an ion and an electron. In field ionization, an electron is removed by tunneling from the molecule in the presence of a strong electric field, creating an ion. Field ionization tends to result in fewer fragmentation events than does EI. Chemical ionization uses a reagent gas, such as methane. The reagent gas is ionized using an EI source, for example, and then reacted with the analyte molecules, ionizing them in a lower energy state, also resulting in less fragmentation than in EI.Liquid and solid phase ionization techniques are most commonly used for mass spectroscopy. Liquid phase techniques include atmospheric pressure chemical ionization, field desorption, and electrospray ionization ͑ESI͒. These techniques require the analyte compound to be nebulized into the gas phase and the solvent removed through a series of stages of increasing vacuum ͑typically atmosphere to 10 −6 Torr or greater͒ as the ions travel into the mass spectrometer. Solid phase techniques include matrix-assisted laser des...
