Cathodes fabricated by these techniques provide a stable electron beam current, but their brightness depends on the shape and size of the deposited films.Ī miniature MEMS electron microscope is being developed at the Wroclaw University of Science and Technology 18. A CNT layer can be deposited on the surface of a cathode by spray coating 15, drop drying 16, or electrophoretic deposition 17. It is easier to fabricate electron sources using prefabricated CNTs suspended in solution. It requires expensive equipment and experience. Although CNTs are perfect electron emitters, the fabrication of a single carbon nanotube on a silicon surface is a challenging process. Furthermore, the energy spread of the emitted electron beam is low. According to the authors, such an electron source is characterized by a stable current and a long lifetime. A single carbon nanotube has been used as a point electron source in high-resolution electron beam instruments 14. Moreover, CNTs are very good conductors, which makes them ideal candidates for electron field emitters. Recently, carbon nanotubes (CNTs) were investigated as a material for electron field-emission cathodes 9, 10, 11, 12, 13, 14, 15, 16, 17. Using silicon microengineering techniques, it is also possible to fabricate sharp silicon structures with good field-emission properties 5, 6, 7, 8. Tungsten wire can be electrochemically etched to form a sharp field emitter 4. Others have used different materials, e.g., tungsten 3. 2 used molybdenum to form sharp cones as electron emitters. Field-emission cathodes are usually formed into sharp protrusions with a high aspect ratio. For SEM applications, RMS fluctuations of a few percent over 30 min at a relatively low angular emission current density of <10 ♚/sr are generally considered acceptable 1. Field cathodes are best suited for the fabrication of microcolumns due to their high brightness and small initial source size 1. ![]() The choice of the electron gun is especially important in the realization of miniature electron-optical columns (microcolumns). These parameters depend on the electron gun. The electron beam parameters of an electron microscope, i.e., a very small spot size with high beam density and stability, determine the quality of the image obtained. Cathodoluminescence of the phosphor layer (ZnS:Ag) deposited on the thin silicon nitride membrane (anode) was observed. The developed sharp silicon/CNT cathodes were used to test the MEMS electron source demonstrator, a key component of the MEMS electron microscope, operating under atmospheric pressure conditions. Moreover, the electron beam current fluctuations at the anode could be reduced to ~1% by using a feedback loop circuit that controls the gate voltage, regulating the anode current. The gate limited the emission current and elongated the lifetime of the gun when the current limit was set. In the triode configuration, an extraction electrode (gate) control function was reported. In the diode configuration, the spot size was the same as the emission area (~10 µm), which is a satisfactory result. The electron beam spot of the source was observed on the phosphor screen. After 30 min of operation at 900 V, the emission current decreased to 1.6 ♚ and was stable for at least 40 min, with RMS fluctuation in the anode current lower than 10%. For the diode configuration, a low threshold voltage <1000 V and a high emission current that reached 90 ♚ were obtained. It was tested in diode and triode configurations. ![]() The electron gun contains a silicon cathode with a single very sharp protrusion and a bundle of disordered CNTs deposited on its end (called a sharp silicon/CNT cathode). Its fabrication process follows the technology of a miniature device under development built from silicon electrodes and glass spacers. This article presents a field-emission electron gun intended for use in a MEMS (microelectromechanical system) electron microscope.
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