Scanning electron microscope
- SE images
- BSE images
- EDX images
Prof. Dr. Veerle Cnudde, UGent.
(by Darrell Henry, Louisiana State University)
Several of these interactions are used for imaging, semi-quantitative analysis and/or quantitation analysis.
(by Darrell Henry, Louisiana State University)
Signals used for imaging or X-ray generation are generated from different electron interaction volumes => each of the signals has different imaging or analytical resolution.
(by Darrell Henry, Louisiana State University)
Auger and Secondary images have the best imaging resolution, being generated in the smallest volume near the surface of the sample.
(by Darrell Henry, Louisiana State University)
Backscattered electrons are generated over a larger volume resulting in images of intermediate resolution.
(by Darrell Henry, Louisiana State University)
Cathodoluminescence is generated over the largest volume, even larger than Bremsstrahlung radiation, resulting in images with the poorest resolution.
a shape that ranges from a tear-drop to a semi circle within the specimen
its depth and diameter depends on: kV and density of specimen
Secondary electron detectors are common in all SEMs, but it is rare that a single machine would have detectors for all possible signals. Secondary electrons are produced when an incident electron excites an electron in the sample and loses some of its energy in the process. The excited electron moves towards the surface of the sample undergoing elastic and inelastic collisions until it reaches the surface. Here it can escape if its energy exceeds the surface work function, Ew, which defines the amount of energy needed to remove electrons from the surface of a material. One of the major reasons for coating a non-conductive specimen with conductive materials is to increase the number of secondary electrons that will be emitted from the sample (decrease Ew).
The SE that are generated by the incoming electron beam as they enter the surface
High resolution signal with a resolution which is only limited by the electron beam diameter
The mean free path length of secondary electrons in many materials is ~1 nm (10 Å). Thus, although electrons are generated throughout the region excited by the incident beam, only those electrons that originate less than 1 nm deep in the sample escape to be detected as secondary electrons. This volume of production is very small compared with BSE and X-rays. Therefore, the resolution using SE is better than either of these and is effectively the same as the electron beam size. The shallow depth of production of detected secondary electrons makes them very sensitive to topography (Fig. 41b).
The SE that are generated by the backscattered electrons that have returned to the surface after several inelastic scattering events
SE2 come from a surface area that is bigger than the spot ffrom the incoming electrons => resolution is poorer than for SE1 exclusively
In the most common or standard detection mode, secondary electron imaging or SEI, the SEM can produce very high-resolution images of a sample surface, revealing details about less than 1 to 5 nm in size (Fig. 43). In order to obtain SE images, the samples need to be dry and coated with gold or carbon (in order to be electrically conductive) and a SE detector needs to be present.
Due to the low energies of secondary electrons (SE) (~2 to 50 eV) they are ejected only from near-surface layers. Therefore, secondary electron imaging (SEI) is ideal for recording topographical information. To attract (collect) these low-energy electrons, a small bias (usually around +200 to 300V) is applied to the cage at the front end of the detector to attract the negative electrons towards the detector (Fig. 44). A higher kV (e.g. 7 to 12kV) is applied inside the cage i.e. to the scintillator, to accelerate the electrons into the scintillator screen.
Working distance (= the distance between the final condenser lens and the specimen): a short working distance is optimal for high-resolution SE imaging
WD has an influence on the spherical aberration (= the failure of the lens system to image central and peripheral electrons at the same focal point).
A diatom imaged using different WD’s.
Bar is 5µm, Magnification = x 3300, Acceleration Voltage = 5kV
Working distance
beam energy and beam current
a higher accelerating voltage will increase secondary yield from all parts of the specimen due to a greater beam penetration => reduction of the edge effect and therefore a diminishing effect on contrast
Diatom imaged using different accelerating voltages.
Bar is 1µm, Magnification = x 4000, Working Distance = 8mm
Working distance
beam energy and beam current
Atomic number (Z)
The local curvature of the surface (the most important factor)
SE images are used for morphological details in the sample and give a feeling of depth
A fraction of the incident electons is retarded by the elctro-magnetic field of the nucleus and if the scattering angle is greater than 180° the electron can escape fom the surface.
Back-scattered electrons (BSE) are beam electrons that are reflected from the sample by elastic scattering. Some amount of inelastic scattering does occur so energies are slightly less than the incident beam. Backscattered electrons (BSE) are high energy primary electrons that suffer large angle (> 90°) scattering and re-emerge from the entry surface of a specimen. Most BSE have energies slightly lower than that of the primary electron beam, E0, but may have energies as low as ~50 eV (the upper cut-off for secondary electrons, SE emission is due to inelastic interactions.) (Fig. 45a).
polished surfaces are ideal
give information based on the atomic number of the elements in hte sample
BSE (Fig. 45b) are often used in analytical SEM along with the spectra made from the characteristic X-rays (Fig. 45c). Because the intensity of the BSE signal is strongly related to the atomic number (Z) of the specimen, BSE images can provide information about the distribution of different elements in the sample. In order to detect the BSE, a BSE detector (BSD) is needed in the SEM (Fig. 46). The BSD is mounted below the objective lens pole piece and centered around the optic axis. As the specimen surface is scanned by the incident electron beam, backscattered electrons (BSE) are generated, the yield of which is controlled by the topographical, physical and chemical characteristics of the sample.
Intensity of BSE signal proportional to the average atomic number (AAN) of the specimen, e.g., Gold has a relatively high AAN of 79.0 and appears bright, compared to Quartz (SiO2) which has a low AAN of 10.8 and appears dark
By using EDX analysis (energy dispersive X-rays) element analysis can be performed
X-rays produced from a certain element are characteristic for this element and can be used for its identification.
Figure 47 illustrates a back scattered image together with the mapping of several elements, including Ca, Fe, Si, Na and Ti. The SEM is usually a strictly monochromatic imaging tool, but colour may be introduced based on X-ray energy or backscattered electron energy. Figure 49b shows individual gray scale images from the X-ray signals from a mineral sample imaged in the SEM after scanning in a raster pattern (fig. 48). The X-ray images show the variation in composition of the sample for each of nine elements (Al, Ca, Fe, K, Mg, Na, O, Si, Ti). There are more individual images than the three red, green, and blue display channels, and no obvious “correct” choice for assigning colours to elements. Figure 49a shows a few possibilities, but it is important to keep in mind that no single colour image can show all of the elements at once. Assigning several of these arbitrarily, e.g., to the red, green, and blue planes of a display, may aid the user in judging the alignment of regions and areas containing two or more elements.
By using EDX it is possible to do point analysis, line scanning and point mapping. This allows us to determine the mineralogy of the stone
Bentheimer Sandstone
What are the clay minerals in the centre?
Most of the electron microscope systems have built-in several detector systems. Common signal detectors include backscattered electrons and detectors for characteristic X-rays, such as energy-dispersive (EDS also referred as EDX) or wavelength-dispersive (WDS). In case of EDS, the excited photons are collected as a function of their energy and the spectrum of energy-dependent, photon intensity is analysed to measure the chemical composition of the material (Brandon and Kaplan, 2008). In WDS, the intensity of the excited photons is collected as a function of their wavelength. EDS collectors collect the excited X-rays simultaneously over a wide range of energy and are therefore highly efficient (Brandon and Kaplan, 2008). A disadvantage of EDS is that it has a restricted energy resolution that may sometimes result in overlap of peaks in the X-ray signal. It is possible to obtain several EDS mappings: the electron beam is traversed across a selected region of the sample and the signal is recorded in each step. The spatial resolution of the EDS mappings depends on the pre-set steps per unit length of line. The detection limit will depend on the time of signal acquisition at each step.
Al + Si, no evidence of K => Kaolinite ( Al2Si2O5(OH)4 )
Comparison of resolution of Mo and S spectral lines in EDS (yellow) vs. WDS (blue). In the EDS spectrum the molybdenum and sulfur lines are overlapped, but can be resolved in the WDS spectrum.
When compared to EDS, WDS exhibits superior peak resolution of elements and sensitivity of trace elements
SEM-CL image of sand-size algal cyst that is filled and overgrown with quartz. Inset, BSE image showing the cyst wall (arrow c), quartz infill (q-i), and overgrowth (q-e).
