Scanning Electron Microscope: SEM

An SEM is arguably the most versatile microscope, with a magnification ranging from 5x to as high as 1000000x. It provides excellent resolution, is amenable to automation, and is user friendly.

These features have made it the most widely used of all electron beam instruments.

Sample preparation and examination are also relatively simple compared to other techniques. A wide range of nanomaterials, starting from powders to films, pellets, wafers, carbon nanotubes, and even wet samples, can be examined using an SEM. It is also possible to correlate the observations made at nanoscale to those at macroscale and draw reliable conclusions.

The use of a field-emission gun in an SEM makes it possible to image individual heavy atoms in the transmission mode by collecting scattered electrons with a sensitive detector.

When an electron beam strikes a bulk specimen, a variety of electrons, photons, phonons, and other signals are generated Fig. Three types of electrons are emitted from the electron-entrance surface of the specimen: secondary electrons with energies <50 eV, Auger electrons produced by the decay of the excited atoms, and backs cattered electrons that have energies close to those of the incident electrons. All these signals can be used to form images or diffraction patterns of the specimen or can be analysed to provide spectroscopic information.

De-excitation of atoms that are excited by the primary electrons also produces continuous and characteristic X-rays as well as visible light. These signals can be utilized to provide qualitative or quantitative information on the elements or phases present in the regions of interest. All these signals are the product of strong electron–specimen interactions, which depend on the energy of the incident electrons and the nature of the specimen.

Figure  illustrates the components of an SEM. A tungsten filament is used as the source of electrons. Since the maximum accelerating voltage for the filament is lower than that for TEM, the electron gun is smaller.

Signals generated in an SEM

The beam size is also quite small, of the order of 10 nm, which necessitates the use of two or three lenses to condense the beam to this size. The final lens that forms this very small beam is named the objective lens and its performance largely determines the spatial resolution of the instrument.

The electron beam of an SEM is scanned horizontally across the specimen in two mutually perpendicular (X and Y) directions. The X-scan is relatively fast and is generated by a sawtooth wave generator. This generator supplies scanning current to two coils, connected in series and located on either side of the optic axis, just above the objective lens.

Components of an SEM

The coils generate a magnetic field in the Y-direction, creating a force on an electron (traveling in the Z-direction) that deflects it in the X-direction. The Y-scan is much slower and is generated by a second sawtooth wave generator. The entire procedure is known as raster scanning and causes the beam to sequentially cover a rectangular area on the specimen, as shown in Fig.

Scan pattern in an SEM

The beam traces a straight line path from A to A1, which is from left to right, during its X-deflection signal. However, while taking the reverse path, the beam is deflected by a small amount in the Y-direction, and it takes a diagonal path from A1 to B. A second line scan takes the probe to point B1, at which point it flies back to C; the process is repeated until n lines have been scanned and the beam arrives at point Z1. This entire sequence constitutes a single frame of the raster scan. From point Z1, the probe quickly returns to A, as a result of the rapid flyback of both the line and the frame generators, and the next frame is executed. This process may continuously run for many frames, as happens in a raster scan terminal.

The outputs of the two scan generators can be used to generate the display on a CRT. The electron beam in the CRT scans exactly in synchronization with the beam in the SEM, so for every point on the specimen (within the rasterscanned area) there is an equivalent point on the display screen, displayed at the same instant of time. In order to introduce contrast into the image, a voltage signal must be applied to the electron gun of the CRT, to vary the brightness of the scanning spot.

This voltage is derived from a detector that responds to some change in the specimen induced by the SEM incident probe.

In recent times, the CRT display devices have become obsolete. The scan signals are generated digitally, by computer-controlled circuitry. The image is divided into a total of m × n picture elements, popularly called pixels. The SEM computer can capture an image up to the pixel level, because each pixel has an (x, y) address, which is stored in the memory. The additional information that is required is the image intensity value (also in the form of a digitized number) for each pixel. A digital image, in the form of position and intensity information, can therefore be stored in the computer memory, on a magnetic or optical disk, or transmitted over data lines (e.g., the Internet). The scanning is usually done at a rate of about 60 frames/second to generate a rapidly refreshed image that is useful for focusing the specimen or for viewing it at low magnification. At a higher magnification or when making a permanent record of an image, slow scanning (several seconds per frame) is preferred; the additional recording time results in a higher-quality image containing less electronic noise. The signal that modulates (alters) the image brightness can be derived from any property of the specimen that changes in response to electron bombardment. Most commonly, emission of secondary electrons (atomic electrons ejected from the specimen as a result of inelastic scattering) is used. 

SEM Specimen Preparation

As long as the test specimen is made of a conducting material, no special preparation is required before the microscopic examination. On the other hand, specimens of insulating materials do not provide a path to ground the specimen current Is and may undergo electrostatic charging when exposed to an electron probe. This problem is addressed by coating the surface of the specimen with a thin film of metal (gold or chromium) or conducting carbon. This is done in vacuum, using the evaporation or sublimation techniques. Films having a thickness of 10–20 nm conduct sufficiently to prevent electrostatic charging of most specimens. However, the external contours of a very thin film closely follow those of the specimen, providing the possibility of a faithful topographical image.

Applications of SEM

An important feature of an SEM is its large depth of field, which is responsible, in part, for the 3-D appearance of the specimen image. The greater depth of field of the SEM provides much more information about the specimen. Most SEM micrographs, in fact, have been produced with magnifications below 8000 diameters (8000x). At these magnifications, the SEM operates well within its resolution capabilities. In addition, the SEM is also capable of examining objects at very low magnification. This feature is useful in forensic studies as well as in other fields such as archaeology because the SEM image complements the information available from the light microscope.

Once it is in a digital form, an SEM image can be processed in a variety of ways, for example, nonlinear amplification, differentiation, and many other new and productive ways.

The availability of powerful and inexpensive computers equipped with large storage capacity, high-resolution displays, and software packages capable of a full range of processing and quantitative functions on digital images gives the user an unprecedented degree of flexibility and convenience in using the output of the SEM. Other advances in the use of an SEM involve contrast mechanisms that are not readily available in other types of instrumentation, such as electron channelling contrast produced by variations in crystal orientation and magnetic contrast from magnetic domains in uniaxial and cubic materials.

For a metallurgist, an SEM provides the capability to determine the crystal structure and grain orientation of crystals on the surface of prepared specimens. This capability makes use of the diffraction pattern of the backscattered electrons emerging from the specimen surface and is known as electron backscattering diffraction. These patterns are then analysed with a computer-assisted indexing method. Automated indexing of patterns and computerautomated crystal lattice orientation mapping allow this technique to identify phases and show misorientation across grain boundaries.

Comparison among AFM, SEM, and TEM.