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

Abstract

Probe size, shape, and current are important parameters for the performance of all probe forming systems such as the scanning ͑transmission͒ electron microscope, the focused ion beam microscope, and the Gaussian electron beam lithography system. Currently, however, the relation between probe current and probe size is ill defined. The key lies in a lacking definition of "size." This problem is solved with the introduction of the "practical brightness." In literature, many different definitions of "brightness" can be found, but for systems in which the whole of the virtual source is imaged onto the target, it is the practical brightness of a source that determines how much current is in the probe. This means that only with the practical brightness the performance of a probe forming system can be calculated quantitatively. The beauty of the practical brightness is that this source property is unaffected by the quality of the column: without interactions between electrons in the beam, the practical brightness is conserved down to the target. This makes it the only relevant brightness for probe forming systems to be used to compare different sources. The practical brightness can be measured, but can also be calculated when the source intensity profile is known. The Gaussian source intensity profile of thermionic, Schottky, and cold field emitters yields a practical brightness of 1.44ej / ͗͘, where j is the current density on the emitting surface and ͗͘ is the average tangential electron energy.

Review of ZrO/W Schottky Cathode

“…If one assumes a Gaussian emission distribution from the virtual source, the diameter d v 50 containing 50% of the current is given by 46 44 is not included in the calculation of d v 50. The results, given in Table 1.3 for I′ = 0.20 mA/sr, indicate that brightness levels in the range of low 1 × 10 8 A / m 2 sr V can r r decreases.…”

Off-axis emission properties for the extended Schottky electron source

“…It was pointed out by Bronsgeest et al 19 that a proper definition of d v must take into account the current density distribution in the source image plane. A useful definition for d v that avoids assumptions regarding the intensity profile is the diameter containing a specified fraction of the current ͑FC͒.…”

Brightness measurements of a gallium liquid metal ion source

“…The average width of all edges, after discarding extreme values, we take as the d 25-75 probe size. 15 This size can conveniently be converted to the FW50 probe size without having to know the shape of the probe, 16 d FW50 = 1.76d . ͑9͒…”