Jia Zheng Li


March 1986

Document Type


Degree Name



Dept. of Applied Physics


Oregon Graduate Center


Field emission has long been identified with high brightness electron sources. It is particularly suitable for low voltage, sub-micron focused beam applications. To make full use of this unique feature and to overcome stability problems associated with room temperature field emitters, we investigated long life, low noise, thermal field cathodes capable of operating at a source brightness ~10[superscript 10]A/cm ²/sr in a background pressure 1X 10[superscript -8]torr. Two promising cathodes fabricated from the (100) oriented tungsten crystal, the ZrO/W and the built-up W cathodes, were investigated in a comparative manner with respect to several important characteristics including the beam spot size, current transmission, noise level, virtual source size, etc. A commercial scanning electron microscope (SEM) was modified to meet the experimental requirement for the two thermal field emitters. A special computer simulation program (SCWIM) was used to calculate the emitter surface field, current density and electron trajectories. Space charge was also included in the calculations to show its effect. The virtual source size is an important parameter for calculating the source brightness. Methods previously used for calculating the virtual source size require a particular emitter shape, e.g. the sphere and sphere-on-cone models. According to the SCWIM program any arbitrary shaped emitters (facet or other possible geometry) can be easily evaluated. Here, the virtual source size and its relationship with emitter radius and aperture angle were obtained for the both ZrO/W and built-up W(100) emitters. The experimental results, theoretical prediction and computer simulation calculations were reasonably consistent. They provide a set of practical data and considerations for the development of advanced field emission equipment with a high source angular current intensity (>1mA/sr), low beam voltage (1.kV), small beam size ( 0.2 µm), low noise level (noise/signal ~0.5 %), and relatively simple electron optical column (because of smaller virtual source size) with relaxed vacuum requirement. It is shown that the ZrO/W source is generally superior to the built-up emitter owing to its ten times less noise level and smaller energy spread.





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