It will be recalled that an electrostatic lens is in essence formed in the following way: one or more plate electrodes, which are each pierced with a hole through which the electron beam passes, are placed on the path of the electron beam. Potentials are applied to the electrodes in order to induce different electric fields upstream and downstream of each electrode. The interaction between these electric fields and the electron beam deviates the electrons and directs them toward a focal zone or point. Focal length may be calculated quite simply from the values of the electric fields and the energy of the electrons.
For an electron beam of given energy, increasing the strength of the electric fields, and therefore the voltages applied to the electrodes, increases focal strength. However, focal strength decreases as the energy of the beam increases. This leads to difficulties focusing high-energy beams as it becomes necessary to apply very high voltages between closely spaced electrodes, thereby running the risk of breakdown.
Electrostatic lenses for focusing electrons moreover have a drawback in that they focus both electrons, and ions that have the same energy. As a result, if the electron beam emitted by a cathode and focused on a surface tears positive ions from this surface, these positive ions can stream back along the beam from downstream to upstream and bombard the cathode, on which the electrostatic lens tends to focus them; this creates a risk of deterioration of the cathode, or quite simply a risk of contamination affecting its properties. For this reason it is sometimes preferable to focus the electron beam with electromagnetic lenses, which do not have this drawback because they act differently on electrons and ions.
However, in certain applications electrostatic lenses are preferable. Specifically, electromagnetic lenses require coils, high currents and electromagnetic shields, thereby making them expensive. Less expensive focusing systems, which are notably applicable to beams of relatively low energy (less than 100 keV), rather use electrostatic lenses. This is the case for low-energy lithography systems, spectrometers, the electron guns of cathode-ray tubes, etc. Moreover, it would be difficult to use electromagnetic lenses in multibeam e-beam lithography systems because of the bulk of electromagnetic devices, and these systems must therefore use electrostatic lenses.
Lastly, it should be recalled that electrostatic lenses are affected by geometric aberrations and chromatic aberrations, or even astigmatic aberrations. These aberrations need to be taken into account and complex multipolar systems have already been proposed for correcting them, such as for example in patent publications EP0500179, EP1492151 and EP1811540. Curved conductive foils have also been suggested as a way of deforming the equipotentials around a beam-passing aperture in an accelerating electrode of an electron gun, with the aim of correcting spherical aberrations (U.S. Pat. No. 4,567,399).