Nanotip electron gun for the scanning electron microscope

Abstract

Experimental nanotips have shown significant improvement in the resolution performance of a cold field emission scanning electron microscope (SEM). Nanotip electron sources are very sharp electron emitter tips used as a replacement for the conventional tungsten field emission (FE) electron sources. Nanotips offer higher brightness and smaller electron source size. An electron microscope equipped with a nanotip electron gun can provide images with higher spatial resolution and with better signal-to-noise ratio. This could present a considerable advantage over the current SEM electron gun technology if the tips are sufficiently long-lasting and stable for practical use. In this study, an older field-emission critical dimension (CD) SEM was used as an experimental test platform. Substitution of tungsten nanotips for the regular cathodes required modification of the electron gun circuitry and preparation of nanotips that properly fit the electron gun assembly. In addition, this work contains the results of the modeling and theoretical calculation of the electron gun performance for regular and nanotips, the preparation of the SEM including the design and assembly of a measuring system for essential instrument parameters, design and modification of the electron gun control electronics, development of a procedure for tip exchange, and tests of regular emitter, sharp emitter and nanotips. Nanotip fabrication and characterization procedures were also developed. Using a "sharp" tip as an intermediate to the nanotip clearly demonstrated an improvement in the performance of the test SEM. This and the results of the theoretical assessment gave support for the installation of the nanotips as the next step and pointed to potentially even better performance. Images taken with experimental nanotips showed a minimum two-fold improvement in resolution performance than the specification of the test SEM. The stability of the nanotip electron gun was excellent; the tip stayed useful for high-resolution imaging for several hours during many days of tests. The tip lifetime was found to be several months in light use. This paper summarizes the current state of the work and points to future possibilities that will open when electron guns can be designed to take full advantage of the nanotip electron emitters.

Physics of Schottky Electron Sources

“…It is noted that for other emitter types the effect of Coulomb interactions may actually be dominant, yielding a practical brightness largely determined by the interactions occurring between the emission site and the extractor. We expect this to be the case for the new source types based on carbon nanotubes [Jon04] and nanotips [Vla06].…”

“…have a Gaussian source intensity profile with a temperature dependent width. [Haw96b] For other source types only their shape is known, such as for liquid metal ion sources (which have a shape with much stronger tails than the Gaussian [War88]) or both shape and size are unknown, such as for the nanotube [Jon04] or the nanotip [Vla06] emitter. In that case the practical brightness of the source can only be found by measuring the source intensity profile.…”

Probe current, probe size, and the practical brightness for probe forming systems

“…Unfortunately, it is so that the higher this "intrinsic" practical brightness, the more important the effect of Coulomb interactions becomes. For Schottky emitters, it has been shown that interactions become important when the intrinsic practical brightness exceeds ϳ3 ϫ 10 8 A / m 2 sr V. 12 For the new source types based on carbon nanotubes 13 and nanotips, 14 we expect their practical brightness to be largely determined by the Coulomb interactions.…”

“…For some source types, this is known and does not need to be measured: thermionic emitters, e.g., have a Gaussian source intensity profile with a temperature dependent width. 16 For other source types, only their shape is known, such as for liquid metal ion sources, which have a shape with much stronger tails than the Gaussian, 17 or both shape and size are unknown, such as for the nanotube 13 or the nanotip 14 emitter. In that case, the practical brightness of the source can only be found by measuring the source intensity profile.…”

Beam Trajectory Analysis of Vertically Aligned Carbon Nanotube Emitters with a Microchannel Plate

“…The significant improvement was important for high current density, high resolution, high brightness, and low energy spread [ 1 , 2 , 3 ]. Considerable attention was paid to the development of the electron source for electron beam microscopy [ 4 , 5 , 6 , 7 , 8 , 9 ]. In recent years, carbon nanotubes (CNTs) have been considered field emission (FE) materials because they have remarkable properties such as high aspect ratio [ 10 ], high electrical conductivity [ 9 , 11 ], high thermal conductivity [ 12 , 13 ], and high mechanical strength [ 14 ], which make them extremely attractive as nanoscale reinforcements in high-performance composites.…”