Electron field emission (FE) from broad-area metal surfaces is known to occur at much lower electric field than predicted by Fowler-Nordheim law. Although micron or submicron particles are often observed at such enhanced field emission (EFE) sites, the strength and number of emitting sites and the causes of EFE strongly depend on surface preparation and handling, and the physical mechanism of EFE remains unknown. To systematically investigate the sources of this emission, a DC scanning field emission microscope (SFEM) has been built as an extension to an existing commercial scanning electron microscope (SEM) equipped with an energy-dispersive spectrometer (EDX/EDS) for emitter characterization. In the SFEM chamber of ultra high vacuum (~10-9 Torr), a sample is moved laterally in a raster pattern (2.5 mm step resolution) under a high voltage anode micro-tip for field emission detection and localization. The sample is then transferred under vacuum by a hermetic retractable linear transporter to the SEM chamber for individual emitter site characterization. Artificial marks on the sample surface serve as references to convert x, y coordinates of emitters in the SFEM chamber to corresponding positions in the SEM chamber with a common accuracy of ±100–200 mm in x and y. Samples designed to self-align in sample holders are used in each chamber, allowing them to retain position registration after non–in situ processing to track interesting features. No components are installed inside the SEM except the sample holder, which doesn’t affect the routine operation of the SEM. The apparatus is a system of low cost and maintenance and significant operational flexibility. Field emission sources from planar niobium—the material used in high-field RF superconducting cavities for particle accelerator—have been studied after different surface preparations, and significantly reduced field emitter density has been achieved by refining the preparation process based on scan results. Scans on niobium samples at ~ 140 MV/m are presented to demonstrate the performance of the apparatus.

This work was supported by DOE contract DE-AC05-84ER40150.

Author(s): Tong Wang, Charles E. Reece, Ronald M. Sundelin

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