Electron Gun Using a Field Emission Source

Abstract

A new electron gun has been built which features mechanical and optical simplicity. Theoretically, it can produce a focused spot having a radius smaller than 50 Å and provide 1000 times more intensity than a hot filament system having a similar final spot size. The increase in intensity is made possible by using a field emission electron source operating at a pressure of 10−9 Torr, which is provided (without baking) using commercially available pumps. The small spot is produced by using two properly shaped electrodes which accelerate and focus the electrons from the tip. It would take a hot filament gun and at least two additional lenses to replace this field emission gun when a spot radius less than 100 Å is required. Even then the brightness of the conventional source would be too low to make use of the small spot size obtained. The optical properties for the new gun were predicted on a computer and experimentally confirmed in a new scanning electron microscope. The aperture aberration coefficient was measured to be no more than a factor of two greater than the theoretical value of 1.5 cm. A spot radius of 250 Å has been measured, and this value is to be compared with the theoretical value of 150 Å. Although it was convenient to measure the spot directly only at a relatively large image distance (11.3 cm), calculations imply that the gun can provide a spot radius less than 25 Å when very small image distances are used. The gun can be used in pulsed operation because all optical properties are constant for a given voltage ratio so that application of the electrode voltages by means of a voltage divider provides automatic focusing for arbitrary changes in the applied voltage. The methods used to make and operate reliable high field emission tips are reviewed, and a technique is described for changing the required tip voltage to obtain a given emission current.

Critical issues in scanning electron microscope metrology

“…In the near future other electron sources, such as nanometer-sized field emission tips, may also become available [96,97,120,121]. [15] developed a successful cold cathode field emission source that was later introduced into commercial instrumentation [145,146]. CFE has had a relatively long history in scanning electron microscopy and SEM metrology and was the first type of field emission cathode to be applied to semiconductor processing instrumentation.…”

“…Cold field emission cathodes, developed for use m the scanning electron microscope by Crewe and co-workers [15], have an advantage of providing a relatively high-current electron probe having a low energy spread, high brightness, and a small virtual source diameter, especially at low accelerating voltages The CFE source diameter is sufficiently i'" L -^ ' ^'''^^™" S"" ^'°"^ (^s shown in Fig. 2) without any additional condenser lenses is capable of producing a 10 nm probe [17 18] From Table 2 it can be observed that depending upon the type of instrument design, better than 1 nm resolution may be reached with an instrument equipped with a field emission electron source.…”

Atomic-Resolution STEM at Low Primary Energies

“…Fig. 2 shows a schematic comparison of the emission mechanism from a cold field emission gun (Crewe et al 1968b), and the Schottky gun (Swanson and Schwind 2009). In the CFEG, the electric field at the surface of the emission tip is typically about 10x higher than in the Schottky gun.…”

Single-atom fabrication with electron and ion beams: From surfaces and two-dimensional materials toward three-dimensional atom-by-atom assembly

“…Direct observation of individual atoms in scanning transmission electron microscopy (STEM) was achieved by Crewe and co-workers in the 1960s, 6 made possible by his development of the cold-fi eldemission gun. 7 Using an annular detector, direct imaging of atomic structures became possible, 8 -10 and in the last 30 years, the progress in STEM 11 has rendered such observations routine. In comparison, scanning tunneling microscopy (STM) 12 -14 began with direct demonstration of atomic resolution on Si surfaces, even though earlier examples of current-and force-based mesoscopic profi lometry are available.…”