A microscope that scans a finely focused electron beam across a specimen surface and detects the secondary and backscattered electrons emitted, to observe surface shape and differences in composition.
Japanese name: SEM(走査電子顕微鏡)
SEM (scanning electron microscope) appears in 4 practice questions across 2 topics, including Materials characterization (SEM, TEM, XRD, XPS), Fracture mechanics and fatigue.
In an SEM, an electron beam accelerated to a few kV to about 30 kV is focused down to a few nanometers and scanned across the specimen surface. Displaying the amount of secondary or backscattered electrons emitted from each point as brightness, synchronized with the scan, forms the image. Compared with an optical microscope, the SEM has higher resolution (a few nanometers) and a much greater depth of focus, so rough fracture surfaces and powders can be observed in three-dimensional-looking images that are in focus throughout. Specimens must be conductive; insulators are coated with a thin film of metal or carbon or observed at low accelerating voltage.
Secondary electrons have low energies of 50 eV or less and can escape only from depths of a few nanometers, giving images of shape that strongly reflect surface tilt and edges. Backscattered electrons are incident electrons scattered back out by atoms; they have high energy and come from deeper regions. The backscattered yield increases with atomic number, giving compositional images in which regions of heavy elements are bright, and channeling contrast from crystal orientation can also be obtained. Comparing the two images lets you separate differences in shape from differences in composition.
When the electron beam strikes the specimen, characteristic X-rays with energies specific to each element are also emitted. Analyzing them by EDS or WDS gives elemental distributions as points, lines or maps. Note that characteristic X-rays come from a region of about 1 µm over which electrons spread, so the spatial resolution of elemental analysis is considerably worse than that of the image. In EBSD, the specimen is tilted steeply and the diffraction patterns of backscattered electrons are analyzed to map the orientation of each grain, the character of grain boundaries and the distribution of phases, useful for evaluating texture and recrystallization.
In failure analysis, fracture surfaces are examined to distinguish dimples of ductile fracture, striations of fatigue, cleavage facets of brittle fracture and so on, and to infer the cause of failure. For cross sections, a section is exposed by polishing, ion milling or FIB to examine voids in interconnects, intermetallic compounds in solder joints, film delamination and the like. For foreign particles, EDS confirms the elements present and helps identify their source. Conditions such as accelerating voltage and beam current are chosen with attention to charging, specimen damage and contamination (carbon deposition) by the electron beam.
Answer: A. Secondary electrons mainly show surface topography, and backscattered electrons show compositional (atomic number) differences
Secondary electrons are low-energy electrons of 50 eV or less and can escape only from depths of a few nanometers below the surface. They are therefore strongly affected by surface tilt, topography and edges, producing images of shape with a three-dimensional look. Backscattered electrons are incident electrons scattered back out by the atoms of the specimen; they have high energy and come from deeper regions. The yield of backscattered electrons increases with atomic number, giving compositional images in which regions of heavy elements appear bright. The choice in which the secondary electron image shows atomic number and the backscattered electron image shows topography reverses the roles of the two, and saying their escape depths are the same is also wrong. In failure analysis, the two images are compared to look for foreign particles and precipitates.
Answer: B. WDS has better energy resolution, while EDS can measure many elements simultaneously in a short time
EDS measures X-ray energies directly with a semiconductor detector, so it acquires the peaks of all elements simultaneously in a short time and is easily attached to an SEM. Its energy resolution is only about 130 eV, however, so it has difficulty separating close peaks such as Si Kα and W Mα, or S Kα and Mo Lα. WDS selects wavelengths by Bragg reflection from analyzing crystals, so its resolution is about 10 eV, an order of magnitude better, and it is also superior for detecting trace elements and for quantitative accuracy; but it takes time because the analyzing crystal angle must be scanned for each element. The choices saying that EDS has the better resolution or that WDS is the faster swap the characteristics of the two.
SEM observes surface shape and composition with electrons emitted from the surface, and the specimen may be thick. TEM observes the internal crystal structure and defects at the atomic level with electrons transmitted through a thin lamella.
Coat them with a few nanometers of gold, platinum or carbon to make them conductive, or use a low accelerating voltage that reduces charging, or a low-vacuum mode that admits gas into the specimen chamber.
Instruments with field-emission electron guns reach around 1 nm, but it varies with the specimen, accelerating voltage and detector. The resolution of elemental analysis by EDS is considerably coarser and can be about 1 µm.
EDS measures X-ray energies directly with a semiconductor detector, so it acquires the peaks of all elements simultaneously in a short time and is easily attached to an SEM.
Which correctly compares EDS (energy-dispersive spectroscop…Determining the average composition to a depth of several micrometers is the job of techniques such as EDS in the SEM.
Which correctly describes the information depth of XPS (X-r…EDS (energy-dispersive X-ray spectroscopy) XPS (X-ray photoelectron spectroscopy) Bragg's law Extinction rules Scherrer equation SIMS (secondary ion mass spectrometry) XRD (X-ray diffraction)