INTERACTION OF ELECTRON BEAM WITH CCD SENSOR

The topic of this work is the study of direct detection of electrons by Charge-Coupled-Devices (CCD). The aim is to design a detector for the angle and energy-selective detection of signal electrons in very low energy scanning electron microscopy (VLESEM), using an electron-bombarded CCD sensor (EBCCD). We concentrate upon two problems - the design of appropriate electronics and determination of an appropriate energy of the signal electrons for the CCD sensor.

Summary of the Current State of the Low Energy Microscope

Considerable attention has been paid in the last ten years to the development of low voltage scanning electron microscopy (LVSEM) in order to reduce the radiation damage of some sensitive specimens (e.g., some biological or semiconductor structures) and possibly also to reduce the local charging of a semi-insulating specimen. A radical change in LVSEM has been made by the use of very low energy electrons, with the energy below 10 eV.
The essential element of a low energy electron microscope (LEEM) that makes it different from the classical microscope (TEM) is the cathode lens. The advantages of the immersion objective (magnetic lens plus cathode lens) were fully recognized only recently. So far, experiments demonstrating capabilities of very low energy scanning electron microscopy (VLESEM) have been realized in ISI Brno. A deteriorated function of classical SEM detectors was proved by these experiments. Secondary and backscattered electrons are reaccelerated in the cathode lens and their trajectories are similar to those of the primary beam electrons. The Wien filter is suitable for the separation of the primary and the signal electron beams.

Detection of the Angular Distribution of the Signal Electrons in VLESEM

A schematic arrangement of the optics with an immersion objective, Wien separator and electron bombarded CCD (EBCCD) detector is shown in Figure. The primary electron beam (up to 10keV) from the source of electrons passes through the Wien filters, which are balanced so that the axial ray with the nominal energy is not affected. Next, the primary beam passes through the deflection system, is focused by the magnetic lens, decelerated to the desired energy by the cathode lens, and scanned over the specimen. The beam of signal electrons accelerated by the cathode lens, approximately to the primary energy, passes through immersion objective lens, and it is deflected by the Wien filter to the region of the electrostatic transpotr optics shielded from the primary beam. The transport optics directs the signal beam to the suitable detector.
The arrangement employing the Wien filter makes the angle and energy-selective detection of signal electrons possible and gives rise to the corresponding sort of contrast, if the signal is properly collected. The use of CCD technology is very suitable for this purpose. The planar CCD sensor converts the angular distribution of electrons of the signal beam into the electrical signal that can be further processed, which makes it possible to form the image corresponding to the signal electrons from the selected area of the CCD sensor.


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