Scanning Electron Microscopy

Scanning electron microscope (SEM) uses a focused beam of high energy electrons to generate variety of signals at a surface of solid specimens. Signal derived from electron sample interactions reveal information about the sample including:

1)      External morphology

2)      Chemical composition

3)      Crystalline structure and

4)      Chemical composition

In most SEM data is collected over a selected area of the surface of the sample, and a two dimensional image is generated that displays spatial variations in these properties. SEM has an electron guns such as a tungsten filament cathode and this gun thermionically emits an electron beam. The energy from emitted electron beam is in range from 0.2 keV to 40 keV, is focused by one or two condenser lenses to a spot about 0.4 nm to 5 nm in diameter.  The beam passes through pairs of scanning coils or pairs of deflector plates in the electron column, typically in the final lens, which defect the beam in the x and y axes so that it scans a rectangular area of the sample surface.

Figure 1 – Schematic of SEM

When primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and adsorption within a teardrop-shaped volume of a specimen known as the interaction volume, which extends from less than 100 nm to around 5 µm into a surface. The size of the interaction volume depends on the:

1)      Electron’s landing energy,

2)      Atomic number of the specimen and

3)      Specimen density.

The energy exchange between the electron beam and the sample results in the reflection of high-energy electrons by:

1)      Elastic scattering,

2)      Emission of secondary electrons by inelastic scattering and

3)      Emission of electromagnetic radiation.

Each of these emissions can be detected using specialized detectors.  The beam current absorbed by the specimen can also be detected and used to create images of the distribution of specimen current. Electronic amplifiers of various types are used to amplify the signals, which are displayed as variations in brightness on a computer monitor. Each pixel of computer memory is synchronized with the position of the beam on the specimen in the microscope, and the resulting image is therefore a distribution map of intensity of the signal being emitted from the scanned area of the specimen.

SEM can achieve a resolution of 1nm. Specimens can be observed in high vacuum, low vacuum and in environmental  SEM, specimens can be observed in wet conditions.

Following electron microscopy lecture explains about the Scanning electron microscopy or SEM principle and advantages.