Scanning electron microscopy
|Scanning electron microscopy|
|Other Names||SEM, In-Line SEM, FE-SEM|
|Equipment||Hitachi SU8000 In-line FE-SEM|
Scanning electron microscopy (SEM) is one of the most versatile instruments available for the examination and analysis of the microstructure morphology and chemical composition characterizations. This examination can yield information about the topography (surface features of an object), morphology (shape and size of the particles making up the object), composition (the elements and compounds that the object is composed of and the relative amounts of them) and crystallographic information (how the atoms are arranged in the object).
- 1 Method of operation
- 2 Electron-specimen interactions
- 3 Energy Dispersive X-ray Spectroscopy (EDS)
- 4 Applications
- 5 Equipment
- 6 See also
- 7 References
- 8 Further reading
Method of operation
In scanning electron microscopy, a narrow beam of electrons with energies typically up to 30 keV is focused on a specimen, and scanned along a pattern of parallel lines. Various signals are generated as a result of the impact of the incident electrons, which are collected to form an image or to analyze the sample surface. These are mainly secondary electrons (SE), with energies of a few eVs, high-energy electrons backscattered from the primary beam (BSE), and characteristic X-rays.
The electron source used in SEMs can be a tungsten filament, a LaB6, a Schottky emitter, or a field-emission tip. Axially-symmetric magnetic lenses are used to accelerate the electron beam down the column. Above the specimen, there are two to three sets of lenses. The lens closest to the specimen is known as the objective lens and its function is to make the diameter of the beam as small as possible (typically ~ 1nm for SEMs with field-emission sources).
The electron beam is scanned horizontally across the specimen in two perpendicular (x and y) directions. The x-scan is relatively fast and known as the line frequency, while the y-scan is comparatively slower and known as the frame frequency. Together, this scanning along x- and y-axes is known as raster scanning causes the beam to scan over a rectangular area of the sample.
Image formation in the SEM is dependent on the acquisition of signals produced from the electron beam and specimen interactions. These interactions can be divided into two major categories: elastic interactions and inelastic interactions. In addition to elastic and inelastic interactions, other interactions that are produced between an electron beam and a specimen include characteristic X-rays, Auger electrons, and cathodoluminescence.
The following section describes the interactions between the incident electron beam and the specimen and the types of characterization signals that are generated.
The most widely used signal produced by the interaction of the primary electron beam with the specimen is the secondary electron emission signal. When the primary beam strikes the sample, loosely bound electrons may be emitted and these are referred to as Secondary Electrons (SE). Since they have an average energy of around 3-5 eV, they can only escape from a region within a few nanometers of the material surface. Secondary electrons give topographic information with excellent resolution.
Detection of backscattered electrons (BSE) provide both compositional and topographic information in samples. BSEs are defined as those that have undergone a single or multiple scattering events and which escapes from the surface with an energy greater than 50 eV. Elements with higher atomic numbers have more positive charges in the nucleus and as a result more electrons are backscattered. For example, in heterostructures, BSE detection can be utilized to interpret the composition of samples. The BSEs have a large energy which prevents them from being absorbed back in the specimen. Hence the lateral resolution of a BSE image is considerably worse than a SE image.
The analysis of characteristic X-rays provide chemical information about the specimen and is the most widely used microanalytical technique in a SEM. When an inner shell electron is displaced by the primary electron beam, an outer shell electron falls into the inner shell. This event is accompanied by the emission of an X-ray photon. The X-ray photon energy is characteristic of the element or elements comprising of the specimen.
Auger electrons are produced following the ionization of an atom by the primary electron beam and the falling back of an outer shell electron to fill an inner shell vacancy. The excess energy released by this process may be carried away by an Auger electron. This electron has a characteristic energy and can be used to provide chemical information. 
Certain materials release excess energy in the form of photons with infrared, visible, or ultraviolet wavelengths when electrons recombine to fill holes made by the collision of the primary beam with the specimen. These photons can be detected and counted using a photomultiplier.
Energy Dispersive X-ray Spectroscopy (EDS)
EDS is an analytical technique used for the elemental analysis or chemical characterization of a sample. It relies on the emission of on X-ray photons when an incident electron beam ionizes a specimen.
There are many different types of SEMs available, tailored to specific needs.
- Image morphology of samples (e.g. view bulk material, coatings, sectioned material, foils, even grids prepared for transmission electron microscopy).
- Image compositional and some bonding differences (through contrast and using backscattered electrons).
- Image molecular probes: metals and fluorescent probes.
- Undertake micro and nano lithography: remove material from samples; cut pieces out or remove progressive slices from samples (e.g. using a focussed ion beam).
- Heat or cool samples while viewing them (while possible in many instruments it is generally done only in ESEM or during Cryo-scanning electron microscopy). Wet and dry samples while viewing them (only in an ESEM)
- View frozen material (in an SEM with a cryostage)
- Generate X-rays from samples for microanalysis (EDS; WDS) to determine chemical composition.
- Study optoelectronic behaviour of semiconductors using cathodoluminescence
- View/map grain orientation/crystallographic orientation and study related information like heterogeneity and microstrain in flat samples (Electron backscattered diffraction).
- Electron diffraction using electron backscattered diffraction.
Below is a general description of the electron beam lithography and associated equipment at the LNF.
Hitachi SU8000 In-line FE-SEM
The Hitachi SU8000 cold field emission scanning electron microscope (FE-SEM) uses a focussed beam of high-energy electrons to generate a variety of signals at the surface of solid specimens.
Denton Vacuum Desk II Sputter Coater
The Denton Desk II sputter coater is used to make electrically non-conducting SEM samples conductive by coating them with a layer of Au-Pd alloy ~10 nm thick.
Complete tool list
- Egerton, Ray. Physical principles of electron microscopy: an introduction to TEM, SEM, and AEM. Springer Science & Business Media, 2006.
- Pease, R. F. W. "Fundamentals of scanning electron microscopy." Scanning Electron Microscopy/1971, Proceedings of the Fourth Annual Scanning Electron Microscope Symposium. IIT Research Institute Chicago, 1971.
- Voutou, B., and E. C. Stefanaki. "Electron microscopy: the basics. Physics of advanced materials winter school." Aristotle University of Thessaloniki, Greece (2008).
- Bogner, A., et al. "A history of scanning electron microscopy developments: towards “wet-STEM” imaging." Micron 38.4 (2007): 390-401.
- Australian Microscopy & Microanalysis Research Facility