Abstract
The high negative bias of a sample in a scanning electron microscope constitutes the “cathode lens” with a strong electric field just above the sample surface. This mode offers a convenient tool for controlling the landing energy of electrons down to units or even fractions of electronvolts with only slight readjustments of the column. Moreover, the field accelerates and collimates the signal electrons to earthed detectors above and below the sample, thereby assuring high collection efficiency and high amplification of the image signal. One important feature is the ability to acquire the complete emission of the backscattered electrons, including those emitted at high angles with respect to the surface normal. The cathode lens aberrations are proportional to the landing energy of electrons so the spot size becomes nearly constant throughout the full energy scale. At low energies and with their complete angular distribution acquired, the backscattered electron images offer enhanced information about crystalline and electronic structures thanks to contrast mechanisms that are otherwise unavailable. Examples from various areas of materials science are presented.
Highlights
Historical IntroductionThe idea of immersing the sample under observation in a strong electric field by means of electrons is one of oldest principles appearing in the development of electron microscopy
When using an ultrahigh vacuum (UHV) device, we normally clean the sample by rinsing in suitable solvents or even etchants but still, after loading it to the sample chamber, we find it covered with a thin adsorbed layer of air gases and hydrocarbons
The secondary electrons (SE) are emitted only from a shallow subsurface layer the thickness of which is governed by the rate of absorption of hot electrons and is significantly thicker for nonconductors but still not exceeding 10 to 20 nm, the backscattered electron (BSE) can escape from substantial part of the interaction depth that approaches 50% of this depth at low keV [21]
Summary
The idea of immersing the sample under observation in a strong electric field by means of electrons is one of oldest principles appearing in the development of electron microscopy. In 1941, a theoretical analysis [2] showed an image resolution proportional to the ratio of the initial emission energy and the energy acquired between the cathode and anode This configuration later received the name emission electron microscope, and the immersion objective lens was called the cathode lens. The second was a historical moment when the functional sample of the low energy electron (reflection) microscope appeared [5] with the sample surface bombarded by a coherent plane wave of very slow electrons so that the electron illumination column had to be similar to that of a SEM This approach led to low energy electron microscopy (LEEM) instrumentation as one of the most successful pieces of equipment in surface science [6]. The SLEEM method has been developed and demonstration experiments have authenticated its performance in various branches of materials science [11,12,13]
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