Secondary Electrons SEM

Scanning electron microscopy (SEM) is used to visualize materials and give a picture of what they look like. Samples are irradiated with a primary beam of electrons that raster scans the material this excites the atoms in the material so they emit secondary electrons and backscattered electrons. 

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Figure 17.6 (A) Secondary electron SEM image of a group of sulfides and quartz in coal sample...

Figure 4. Secondary electron SEM photograph of same transistor 15,000X.

Figures 5 and 6 show secondary electron SEM images of the electroplated deposits in cross section and plan view, respectively for samples plated in a solution containing 0.01M-0.02M ethylene diamine. The deposition rate increases between 1.2 and 3.2 mA/cm2. Note that the plating time at 1.2 mA/cm2 is 180 minutes, 90 minutes for the samples plated at 1.8 and 2.4 mA/cm2, and 40 minutes for the sample plated at 3.2 mA/cm2. The grain structure of the deposits also varies with an increase in current density. The sample plated at 1.2 mA/cm2 (Figures 5a, 6a) is gold rich and has a smooth surface containing fine pores about 0.1 pm in diameter, while the samples plated at 1.8 and 2.4 mA/cm2 (Figures 5a, 5b, 6a, 6b) exhibit a columnar structure which becomes more coarse with an increase in current density. The deposit formed at 3.2 mA/cm2 appears to have a mixed structure, the bottom two-thirds having a dense, feathery appearance, while the top third has a fine columnar structure.

In addition to detecting backscattered and secondary electrons, SEM instruments offer information on the sample elemental composition when using x-ray detectors. It has been pointed out earher that the ejection of electrons from an atom is accompanied by emission of x-rays. The x-ray spectrum that is produced is a characteristic feature of any given element, and by measming the energy or the wavelength of the x-rays that are produced, its ideutihcation is possible. 

Fig. 3.3 Schematic illustration of two methods for oxidation of M4Tt4 precursors using gaseous HCl evolved from decomposition of NH4CI Reaction in which the precursor and HCl source are at the same temperature (far left), and which the precursor and HCl source are held at different temperatures (middle pane). The latter allows control of the reaction temperature and partial pressure of the oxidation agent (HCl) quasi-independently. Far Right Secondary electron SEM image of an agglomerate of Ba jSi46 particles, prepared by chemical oxidation of the Ba4Li2Si6 precursor. Reprinted with permission from Ref. [86]. Copyright 2011 American Chemical Society...

Fig. 11. Secondary electron SEM micrographs obtained for all Ndo.8Sro.2(Mni-xCox)Q3 compositions prepared by (a) sol-gel (sg) and (b) freeze-drying (fd) methods.

In addition to detecting backscattered and secondary electrons, SEM instruments may also offer information on the elemental composition of the sample. Interaction of the primary beam vhth the sample results in the emission of X-rays. Since the energy of the emitted X-rays is a specific feature of an atom, by measuring the energy or the wavelength of the X-rays that are produced it is possible to achieve quantitative analysis of elemental composition (energy dispersive X-ray spectroscopy or EDS). 

Application High-resolution signal (TEM, STEM) Back-scattering of electrons (BSE signal in SEM) Analytical signal (TEM, STEM, SEM) Emission of secondary electrons (SE signal in SEM)... 

In SEM and STEM, all detectors record the electron current signal of tire selected interacting electrons (elastic scattering, secondary electrons) in real time. Such detectors can be designed as simple metal-plate detectors, such as the elastic dark-field detector in STEM, or as electron-sensitive PMT. For a rigorous discussion of SEM detectors see [3],... 

Edx is based on the emission of x-rays with energies characteristic of the atom from which they originate in Heu of secondary electron emission. Thus, this technique can be used to provide elemental information about the sample. In the sem, this process is stimulated by the incident primary beam of electrons. As will be discussed below, this process is also the basis of essentially the same technique but performed in an electron spectrometer. When carried out this way, the technique is known as electron microprobe analysis (ema). 

Electron Beam Techniques. One of the most powerful tools in VLSI technology is the scanning electron microscope (sem) (see Microscopy). A sem is typically used in three modes secondary electron detection, back-scattered electron detection, and x-ray fluorescence (xrf). AH three techniques can be used for nondestmctive analysis of a VLSI wafer, where the sample does not have to be destroyed for sample preparation or by analysis, if the sem is equipped to accept large wafer-sized samples and the electron beam is used at low (ca 1 keV) energy to preserve the functional integrity of the circuitry. Samples that do not diffuse the charge produced by the electron beam, such as insulators, require special sample preparation. 

One further breaks down the secondary electron contributions into three groups SEI, SEII and SEIII. SEIs result from the interaction of the incident beam with the sample at the point of entry. SEIIs are produced by BSE s on exiting the sample. SEIIIs are produced by BSEs which have exited the surface of the sample and further interact with components on the interior of the SEM usually not related to the sample. SEIIs and SEIIIs come from regions far outside that defined by the incident probe and can cause serious degradation of the resolution of the image. 

Ion intensities up to a count rate of 2 x 10 are measured using a secondary electron multiplier (SEM). When it becomes saturated above that value, it is necessary to switch to a Faraday cup. Its ion-current amplification must be adjusted to fit to the electron multiplier response. 

Photocells and photomultipliers (secondary electron multipliers, SEM) are mainly employed in photometry. These are detectors with an external photo-effect . 

Catalysts were characterized using SEM (Hitachi S-4800, operated at 15 keV for secondary electron imaging and energy dispersive spectroscopy (EDS)), XRD (Bruker D4 Endeavor with Cu K radiation operated at 40 kV and 40 mA), TEM (Tecnai S-20, operated at 200 keV) and temperature-programmed reduction (TPR). Table 1 lists BET surface area for the selected catalysts. 

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