Bohumila Lencová

Accuracy of electrostatic lens computations with FOFEM.

The increased speed of personal computers enables fast computation of rotationally symmetric electrostatic lenses with the first-order finite element method in meshes with a large number of mesh points. In order to produce an estimate of accuracy of the computed potential, we propose a simple procedure based on doubling the number of mesh points in each coordinate. In this way, we can produce for the lower-density mesh at each point the information about error of the potential and visualize the sources of the computation errors.

Determination of analytical expansion from numerical field data

We introduce a method of calculation of the analytical expansion of the field near the axis that is based on an application of Greens theorem. The approach is demonstrated on an example of a round electrostatic unipotential lens with field computed by the finite-element method and results are compared to methods of Hermite polynomials and wavelet transformation which are used in electron optics. The work is motivated by application to calculations of aberration coefficients where the high order axial field derivatives must be known.

Experimental and simulated signal amplification in variable pressure SEM

The presence of gases (mostly water vapour with pressure range from 1 Pa to over 2000 Pa) in the specimen chamber of a variable pressure scanning electron microscope (VP-SEM) makes completely different conditions for the detection of signal electrons than for the conventional SEM, and specially designed detectors must be used. In the high pressure conditions, gas ionisation cascade amplifies the signal of secondary electrons (SE), accelerated by the applied field of detection electrode of the ionisation detector [1].

Effect of sample tilt on PEEM resolution

In electron microscopy design, the systems are usually assumed to be perfectly aligned or that possible small imperfections can be eliminated by simple multipole correctors (centering deflectors, stigmators) without loss of resolution. However, in some cases, like in the cathode lens between the sample and the objective lens in the photoemission electron microscope, even a small imperfection can impair the resolution significantly. Because of the strong field between the sample and the objective lens, even a small tilt of the sample generates a parasitic dipole field, which decreases resolution and causes image deformations. We present a simulation of the influence of a small sample tilt on the system resolution based on modern computational methods that enable simulation of the whole system including the parasitic fields, proper setting of centering deflectors and stigmators. The resolution is determined by simulating the point spread function and finding the size of its significant part. The procedure is shown on realistic data from the literature. We found out that the resolution becomes worse mainly in the direction of the parasitic dipole field.