Question

Describe, using equations, if appropriate, the compromises you will make when you change the aperture half angle in an SEM. You should discuss the signal intensity, the depth of field, aberrations and the resolution.

Answer 1

On decreasing the diameter of the aperture will:
• decrease lens aberrations and thus increase resolution.
• decrease the probe current.
• decrease the convergence angle of the beam and thus increase the depth of focus.

Resolution is the minimum spacing at which two features of the specimen can be recognized as
distinct or separate. In order to obtain high-resolution images, we need to adjust the probe diameter to
e scale of interest and ensure that a minimum level of contrast exists with the appropriate probe
settings.

Lens aberration: In reality, all lenses have defects. The defects of most importance to us are spherical aberration; chromatic aberration and astigmatism. Rather than a clearly defined focal point end up with a “disk of minimum confusion”.

1. Spherical aberration: Spherical aberration is the principle limiting factor with respect to the resolving power of the SEM To reduce the effects of spherical aberration, apertures are introduced into the beam path. Apertures are circular holes in metal disks on the micron scale. The net effect of the aperture is to reduce the diameter of the disk of minimum confusion.

2. Chromatic aberration (dC): The electron beam generated by the gun will have a certain energy spread. Electrons of different energies at the same location in the column will experience different forces. An electromagnetic lens will “bend” electrons of lower energy more strongly than those of higher energy.

3. Astigmatism: Finally, the electromagnetic lenses used in the SEM can not be machined to perfect symmetry. If the fields produced by the lenses were perfectly symmetrical, a converged beam would appear circular (looking down the column). A lack of symmetry would result in an oblong beam: the narrower diameter due to the stronger focusing plane; the wider diameter due to the weaker focusing plane. The net effect is the same as that of the aberrations above—a disk of minimum confusion rather than a well-defined point of focus.

Depth of field: Apart from its good spatial resolution, one of the most important aspects of a scanning electron microscope is its large depth of field. If we compare images taken by an optical microscope and an SEM of the same object then the difference is striking. The important consequences of the large depth of field of the SEM cannot be overemphasized. As an example, over the past thirty years, the SEM has contributed greatly to our understanding of fracture processes by providing a means of examining fracture surfaces at high resolution.