Electron Microscopy: TEM and STEM Walter J. Botta F.

Output signals generated by electron interaction with a thin specimen

(Electron – atom interaction)

(Interaction volume)

Interaction volume

illumination in TEM and SEM

illumination in TEM

illumination in TEM

illumination in SEM

illumination in SEM

illumination in SEM

illumination in SEM

illumination in SEM

objective lens The objective lens forms an inverted initial image, which is subsequently magnified. In the back focal plane of the objective lens a diffraction pattern is formed. The objective aperture can be inserted here.

objective aperture

The objective aperture is placed in the back focal plane of the image. Its function is to select those electrons which will contribute to the image, and thereby improve the contrast of the final image.

objective aperture

By inserting the aperture or tilting the beam, different types of images can be formed. The most common conditions are: No aperture – the diffraction pattern is centered on the optical axis. Aperture is centred on the optical axis. Aperture displaced, selecting a diffracted beam.

objective aperture

Beam is tilted so that the diffracted beam is on the optical axis.

Objective lens - SEM

Image formation in TEM

Resolution Diffraction Contrast

resolution

RESOLUTION: ability to distinguish closely spaced points as separate points. RESOLUTION LIMIT: smallest separation of points which can be recognized as distinct. RESOLVING POWER: resolution achieved by a particular instrument under optimum viewing conditions.

Abbe criteria of resolution ● The fundamental nature of light poses limits on the detail that can be resolved. ● Abbe (1893) showed that the smallest resolvable distance is about 1/2 the wavelength of light used. ● Thus, 1/2 the wavelength of the radiation used is the ultimate resolving power of any instrument. ● This limits the usable magnification of optical microscopes to