Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) is similar to optical microscopy with exception that electrons are used instead of photons and the image is formed in a different manner, which will be described next. An SEM consists of an electron gun, a lens system, scanning coils, an electron collector, and cathode ray display tube (CRT). Electrons emitted from an electron gun pass through a series of lenses to be focused and scanned across the sample. The most common electron gun is a tungsten hairpin filament emitting electrons thermionically with an energy spread of around 2 eV. Tungsten sources have been largely replaced by lanthanum hexaboride (LaB 6) sources with higher brightness, lower energy spread (~ 1 eV) and longer life. Field emission guns are about 100× brighter than LaB 6 sources and 1000×

brighter than tungsten sources, respectively and energy spread of about 0.2 to 0.3 eV can be achieved with even longer lifetime than the other sources. The emitted electrons are

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accelerated through a voltage up to ~30 kV, and the resulting beam is finely focused by a series of magnetic coils to form a spot on the specimen. A scan generator moves this spot across the specimen via two sets of scan coils. The electrons that escape from the sample comprise the signal and can be collected by various electron detectors depending on the applications to monitor some emission (or property of) the specimen. The resultant signal is amplified and transferred to the display device.

The electron energy used in SEM is in the range of 10 - 30 keV for most samples, but for insulating samples the energy can be as low as several hundred eV. The use of electrons has two main advantages over optical microscopy such as the higher magnification possible using electron wavelengths and the greater depth of field. The electron wavelength, λe, depends on the electron velocity, v, and the accelerating voltage, V, can be written as

𝜆ₑ =

mv

=

√2qmV

=

√V (3-3)

As an example, a voltage of 10 kV results in the wavelength of 0.012 nm. This wavelength, significantly below the 400 - 700 nm wavelength of visible light, allows for making a resolution of SEM much greater than that of optical microscopy. The focused beam of electrons is either scanned across the surface of the specimen to form an image or stopped on a fixed location to perform one of a variety of spectrographic or analytical functions. The interaction of the beam with the specimen results in the generation of secondary electrons, backscattered electrons, Auger electrons, characteristic x-rays, and photons of various

energies. Electrons and photons are emitted at each beam location and subsequently detected.

Secondary electrons from the conventional SEM image, backscattered electrons can also form an image; X-rays are used in the electron microprobe, emitted light is known as cathode luminescence, and absorbed electrons are measured as electron beam induced current. Figure 3.3 shows the various signals that are emitted by the electron beam, along with the spatial region of the sample from which each signal is emitted. Pertinent analysis modes used in this

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research are secondary electrons and electron beam induced current.

Secondary electrons – Secondary electrons are used in imaging and provide surface topographic information. The high energy incident beam electrons interact with loosely bound conduction band electrons in the specimen giving up some of their energy. The amount of energy given to these secondary electrons is small, so they have a very limited range (a few nm) in the sample. Moreover, only those secondary electrons excited near the surface have sufficient energy to be emitted from the surface and detected. Therefore, the imaging via secondary electrons is the “standard” SEM mode of imaging since it provides better resolution versus plotting backscattered electron concentrations or the x-ray signal as the secondary electrons are largely emitted from a region relatively near the surface. Primarily, secondary electrons provided topographical information for studies on large scale defects that included etch pits counting for EPD measurements as well as surface morphology information. As a secondary tool, it was used in conjunction with the other SEM modes for sample orientation and identification of topographic artifacts.

Electron Beam Induced Current – Materials characteristic can also be obtained as a result of the beam injecting charge carriers into the specimen. By making electrical connections to the sample, the induced current from these carriers can be collected, amplified, and, via the SEM scan circuitry, displayed on a CRT. The sample can be inspected in both planar and cross-section geometries providing both surface and depth information. By combining this technique with other SEM viewing modes, the position of crystalline defects, p/n junctions, and other electrically active characteristics can be correlated to the surface topography. EBIC images are a plot of the current flowing through a p-n junction or Schottky barrier due to the electron beam-induced electron-hole pairs (ehp’s) vs. lateral position. These carriers are resultant from the incident beam and are confined to a finite volume of the material, referred to as the carrier generation volume. Effectively, the image displayed is the ehp collection efficiency that is extremely sensitive to electrically active defects such as dislocations, grain

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boundaries, inclusions, and anti-phase domains. One application is to use EBIC as a

complementary tool to etch pit density (EPD) measurements. In fact, EBIC can be a superior method to EPD in that it is a non-destructive technique. In addition, it can complement TEM with respect to threading dislocation density measurements, since EBIC performs well below 106 cm-2 density levels where TEM is inapplicable.