Simulation of a Bessel box electron energy analyser for analysis of secondary electrons
Authors: Suri Ashish , Pratt Andrew , Tear Steve , Walker Christopher , El Gomati Mohamed 
Published: December 2016

Our understanding of the surface topography, texture, elemental composition etc. have not only improved the reliability of the fabrication procedure of micro circuits but also the working of the final device. Low‐voltage scanning electron microscopes (LVSEM) have played a major role in the ever improving quality control of Si devices. ­The smaller interaction volume of the low‐energy electrons with the specimen leads to the ability to observe finer details. Lowering the beam energy to a few hundred eV results in higher yield of secondary electrons (SE) relative to the number of backscattered electrons [1]. A conventional SEM uses the widely used Everhart‐Thornley (ET) detector for detection of secondary electrons [2], however it acquires secondary electrons over a large angular and energy range. This means that information regarding angularly resolved and energy resolved data is lost. What is needed is a small scale device that is relatively little affected by ambient electric and magnetic fields that can access this information. Furthermore demands of low voltage SEM at short working distances require a device of small dimensions to be placed near to the sample.

In this study we propose the lesser known Bessel box Electron Energy Analyser (EEA) [3] to detect and energy analyse secondary electrons emanating from the specimen. The Bessel box is a very simple EEA with a cylindrical geometry. It comprises a hollow cylindrical electrode coupled with two input/output electrodes. The acceptance angle of the Bessel box is determined by the slit on the input cap electrode. Focusing/defocussing action of the Bessel EEA depends on the voltage difference applied to the cylindrical and i/o cap electrodes. The Bessel EEA is essentially a band pass filter with a retarding field effect on the electrons. We examine the properties of a Bessel box EEA by simulation over a range of take off angles with SIMION 8.1 and COMSOL 5.2 Multiphysics. Figure 1 demonstrates the working of such a Bessel filter, lower energy electrons lose energy as they travel through the Bessel EEA and are repelled back before they reach the output slit (fig. 1a). In contrast, higher energy electrons are not focussed by the Bessel EEA (fig. 1b) and only a narrow band of energies are allowed to pass through (fig. 1c). Its size can be much smaller than other EEAs and so could potentially be inserted into existing SEMs. Its small size also makes it less affected by the disturbing effects of external electric and magnetic fields.

The Bessel box was simulated using the conditions: Cylindrical and output electrodes set to ‐22.2V and input cap at ground. The transmission of the Bessel EEA for a range of incoming electron energies is shown in fig. 2. We have investigated a range of cylindrical and output cap electrode voltages and determined energy resolution and transmission for several geometries with the aim of optimising the design for use in a SEM. In addition the effects of magnetic fields from the objective lens of the SEM on the characteristics of the Bessel box are studied.