Figure of merit for environmental SEM and its implications

Abstract

Summary A recently introduced figure of merit for environmental and low vacuum scanning electron microscopes has now been computed in the full operational pressure range for one commercial instrument. The direct simulation Monte Carlo method has been used in lieu of experimental measurements. The theory of this figure of merit is further consolidated. It is shown that a thin pressure limiting aperture can indeed be used as an optimum reference system for all instruments employing differential pumping in the transfer of an electron beam from high vacuum to high pressure. The implications of the results obtained are discussed both in relation to existing commercial instruments and associated literature to pave the way for future progress in the field.

ESEM modifications to LEO SUPRA 35 VP FESEM

“…where q c is a characteristic value for each corresponding diameter of a thin aperture exhausting in vacuum as provided by Danilatos (2009Danilatos ( , 2011. This figure is independent of scattering cross-sections and accelerating voltages but, again, we can see the very significant improvement in performance by way of electron beam loss at greatly improved chamber pressure levels now becoming possible.…”

“…We immediately see the big improvement by the much lower values of q min0 for all modified cases against the unmodified case. However, the comparison is done best by invoking the figure of merit ε Danilatos, 2011) …”

“…As predicted in previous reports Danilatos, 2011Danilatos, , 2012, a simple and practically cost-free modification is possible to a microscope offered as a "variable pressure" SEM (VP-SEM) originally specified and restricted to operate under 133 Pa of nitrogen gas. First, it was demonstrated that the same machine can be drastically improved by way of image quality, range of kV, working distance and pressure level operation.…”

“…It should be noted again that the values of scattering crosssections involved in any calculations in this paper have been taken from theoretical calculations used previously in related works Danilatos, 2011) for easy comparison. Experimental measurements of scattering cross-sections have found lesser values (Wight and Konicek, 2012;Rattenberger et al, 2009), but they are not used until they can be shown to be definitive.…”

Experimental evaluation of environmental scanning electron microscopes at high chamber pressure

“…Of particular importance in this context is the additional stagnation gas thickness (aSGT) [m], which is the distance the primary electron beam travels inside the imaging gas above the pole piece (environmental distance + aSGT = θ). The aSGT is closely related to the excess particle thickness factor (Danilatos, ), for which results based on DSMC simulations of the same microscope (Quanta 600), as used in this paper, are available (Danilatos et al., ). The environmental distance is equal to the working distance minus the aperture distance (Danilatos, ).…”