Scanning Electron Microscopy (SEM)

Condenser lens

An electromagnetic lens located just below the electron gun to focus the electron beam emitted by the filament

Electron Backscatter Diffraction (EBSD )

An electron microscope technique used for viewing crystallographic information based on backscattered electrons whose scattering efficiency depends on the crystallographic orientaiton of the specimen. See http://www.ebsd.com/

Scanning Electron Micrograph

An image, normally a digital image, achieved by using a scanning electron microscope. Note that use of the acronym, SEM, for Scanning Electron Micrograph is strictly speaking, incorrect.

Auger electrons

Secondary electrons emitted from atoms in the sample as a result of inner shell ionisations due to bombardment by the primary electron beam.

Angstrom

A measurement of size: 10^-10 meters. One tenth of a nanometer. Also spelled ångström.

X-rays

X-rays are electromagnetic radiation with wavelengths in the range 0.01 to 10 nm.

Charging

A build-up of electrons from the primary beam on the surface of non-conducting samples. The built-up charge can cause deflection of the primary beam, or discharge, which causes instability in SE images.

Stigmator

A collection of small electromagnetic coils within the objective lens that apply additional magnetic fields to the electron beam to make it cylindrical (circular in cross-section).

Wobbler

A control found on an electron microscope that sets up an automatic over focus-under focus loop. If the objective lens aperture is not correctly aligned with the electron beam the image will be seen to wobble back and forth under this regime.

Cryo-scanning electron microscope (Cryo-SEM)

A conventional SEM that has been fitted with specific equipment that allows samples to be viewed in the frozen state.

Everhart-Thornley detector

A detector, mostly used for secondary electrons, used in high-vacuum SEMs.

Detector

A device that can collect and process signals produced by interactions between the primary electron beam and the sample. Different signals are processed by different detectors.

Focused ion beam technology (FIB)

A high energy (typically 30kV) ion beam (typically gallium ions) is directed onto a hard sample and can be used for imaging or milling. Such a beam can be used in combination with the electron beam in a scanning (dual beam) electron microscope.

Thermionic emitter

A material that emits electrons when heated to a temperature sufficient to overcome its work function (the energy required to liberate an electron).

Scintillator

A material that emits light when excited by electromagnetic radiation or a beam of subatomic particles, e.g. electrons.

Conductor

A material through which electrons can move freely. Electrical conductors do not need to be coated for viewing in the scanning electron microscope.

Insulator

A material through which electrons cannot move freely. The surface of electrical insulators need to be made conductive, i.e. coated, for viewing in the SEM.

Semiconductor

A material with electrical conductivity due to flow of electrons between that of a conductor and an insulator.

Microscope

A microscope is an instrument that can be used to see objects, or features of objects, that are too small to be seen with the naked eye.

Freeze drying

A sample preparation technique that refers to the removal of water (in the form of ice) under vacuum.

Critical point drying (CPD)

A sample preparation technique that usually refers to the drying of a sample using carbon dioxide. At a certain temperature and pressure (31.1 degrees Celsius at 1072 psi or 304.25 K at 7.39 MPa) the point is reached where the liquid CO2 turns into gas instantaneously, leaving the sample dry.

Low Vacuum Scanning Electron Microscope (LV-SE)

A scanning electron microscope in which pressure can be adjusted in the sample chamber.

Environmental scanning electron microscope (ESEM)

A scanning electron microscope in which relative humidity, pressure and temperature can be controlled in the sample chamber.

Electron beam

A stream of electrons (cathode rays).

Microanalysis

A technique to analyse composition in regions of a sample in the micron and nano ranges. Techniques include spectroscopy (Energy Dispersive or Wavelength dispersive), Electron Backscatter Diffraction, and Cathodoluminescence

Tungsten filament

A thermionic electron source composed of a hairpin-shaped wire made of tungsten.

LaB filament

A thermionic electron source composed of a small block of a lanthanum hexaboride crystal. It provides greater brightness and a longer lifetime compared with a tungsten hairpin filament.

Electron volt (eV)

A unit of energy equal to the amount of kinetic energy an electron gains when it is accelerated through an electric potential difference of one volt.

Electron High Tension (EHT)

Accelerating Voltage

Environmental SEM

An SEM capable of providing pressure and temperature conditions compatible with liquid water, so that wet samples can be imaged.

Scanning Electron Microscope (SEM)

An instrument which scans or rasters a fine electron probe over a material, and using a variety of detectors reconstructs an image from the signals generated within the sample.

Accelerating Voltage

An operational term describing the voltage applied to the electron microscope that accelerates the electron beam down the column.

Astigmatism

Asymmetrical beam distortions that cause blurring of an image so that it appears out of focus even when focus is correctly set. Astigmatism can be corrected by adjustment of the beam in two directions, at right angles to one another.

Scan coils

Coils within the objective lens that drive the beam off-axis and back again. Two sets of coils are used to deflect the beam in the x and y directions.

Topographic contrast

Contrast from the specimen arising from its surface shape (or topography)

Compositional Contrast

Contrast from the specimen arising from local changes in surface chemistry

Electromagnetic lens

Electromagnetic lenses are made of coils of copper wire enclosed in an iron shell. The magnetic field generated when a current is passed through the coils can be used to focus a beam of electrons.

Elastic scattering

Electron scattering in which the electron changes direction through interaction with the sample but looses negligible energy. This process leads to spreading of the electron beam in the sample. See backscattered electrons.

Inelastic scattering

Electron scattering in which the electron looses energy through interaction with the sample. Inelastic scattering is considered to produce secondary electrons. Note that not all inelastic events produce secondary electrons e.g. Bremsstrahlung is an inelastic event that produces no secondary electrons.

Electron

Electrons are elementary particles, found in all atoms, that are grouped in shells around the nuclei of the atoms.

Electromagnetic radiation

Energy produced by acceleration of an electric charge, and associated electric and magnetic fields.

Load current

Filament current: the current that is passed through the filament in the electron gun to produce the electrons for the beam.

Magnification range

For a scanning electron microscope this is generally in the order of 10 to 300,000 times.

Gate

In an electron microscope this refers to a physical component that blocks off one chamber from another: important in the correct operation of the vacuum system.

Coating

In electron microscopy sample coating refers to the addition of a thin (e.g. 10-20 nm) layer of metal (usually, gold, chromium, platinum or palladium) or carbon, or possibly other materials, onto the surface of the sample. This is usually done to enhance conductivity.

Saturation

In electron microscopy the saturation point is the optimal setting for the filament, at which maximum electron emission is achieved.

Filament

In electron microscopy this refers to the source of electrons for the electron gun: a tungsten wire, a sharpened tungsten crystal, or a lanthanum hexaboride crystal (LaB6).

Aperture

In electron microscopy: a minute hole in a strip of metal that is placed in the path of the electron beam in order to restrict or limit electron progress down the machine column. An aperture stops off-axis or off-energy electrons.

Gun

In electron microscopy: the equipment that produces an electron beam: usually an emitter (electrode), a surrounding cathode (Wehnelt cylinder/ grid cap) and below this an anode with a central hole.

Fixation

In sample preparation for electron microscopy this refers to the stabilisation of biological material by a physical or chemical method that stabilises or cross-links the molecular structure.

Signal processing

In scanning electron microscopy each quantum of signal information (from each beam dwell point) can be changed to some new value that bears a rigorous relationship to the original one, before it is displayed (e.g. to change contrast and brightness of the final image).

Focus

In scanning electron microscopy: The process of bringing the electron beam to its smallest diameter cross section at the sample surface. The production of a sharp, well defined image, rather than a fuzzy, blurry image.

Probe size

In scanning electron microscopy: the diameter of the electron beam at the first cross-over point below the condenser lens. Often used interchangebly with the term spot size because it affects the final size of the electron beam cross section at the sample.

Variable pressure mode

Operating the scanning electron microscope with a gas or vapour in the specimen chamber, generally in the pressure range of 10 to 2500 Pa. Non-conductive samples can be imaged in this mode.

Photomultiplier

Photomultipliers are used to detect and amplify low intensities of light.

Cathodoluminescence (CL)

Photons with wavelengths in the visible light range produced by the interaction of the primary electron beam with semiconductor or insulating samples.

Variable Pressure (VP)

The ability to change the pressure in the sample chamber of a scanning electron microscope.

Backscattered Electrons (BSE)

Produced through an elastic scattering interaction with the atoms in the sample that results in the primary electron being re-emitted from the specimen. The electron is said to have backscattered (or reflected back) from the sample. In backscattering, the electron trajectory is changed by more than 90 deg from the forward direction of motion. This results in the scattered electron propagating back in the general direction of the original beam. Note that between the multiple elastic events (that form Backscattered electrons), the beam electrons may also be involved in inelastic events, and thus reduce the electron's energy. Backscattered electrons usually have energy in the kV range.

Secondary Electrons

Produced through inelastic scattering that results in the ejection of loosely bound electrons from the specimen. Secondary electrons have energies from ~2-50eV.

Brightness

Referring to the appearance of an image: the extent to which all parts of an image emit or reflect light. An image with very low brightness may appear black.

Contrast

Referring to the appearance of an image: the extent to which various parts of an image differ in brightness.

Venting

Refers to the process of letting air into the sample chamber of an electron microscope, usually to allow the specimen to be inserted/removed.

Evacuation

Refers to the process of pumping air out of the sample chamber of an electron microscope.

Vapour fixation

The application of a chemical fixative in vapour form (e.g. osmium tetroxide) to chemically stabilise a biological sample so that it undergoes less collapse than fresh material when viewed with electron microscopy.

Water Chilling System

The application of chilled water (in tubes) around an electron microscope column, used to remove unwanted heat.

Cryo-fixation

The application of low temperature (below the freezing point of water and often that of liquid nitrogen or liquid propane) to stabilise a biological sample so that it can be viewed in its frozen state, or further processed.

Scale bar

The bar (line) displayed on a screen or image which represents an accurate unit of distance.

Electron-beam lithography (E-bea)

The beam of electrons is scanned across the sample in a patterned fashion in order to remove material, not image it.

Column

The component of an electron microscope that houses the electron gun, electromagnetic lenses, and accessory equipment.

Filament current

The current that is passed through the filament in the electron gun to produce the electrons for the beam.

Working Distance (WD)

The distance from the underside of the objective lens to the surface of the specimen.

Sample height

The distance that the specimen (or sample) lies from the underside of the objective lens.

Kinetic energy

The energy that an object, e.g. an electron in a beam, possesses due to its motion.

Edge effect

The enhanced emission of electrons from the edges of the specimen resulting in brighter regions for these features in a scanning electron micrograph.

Magnification

The enlargement of an image, or portion of an image.

Cathodoluminescence image (CL)

The image produced by cathodoluminescence showing otherwise invisible microstructural defects and impurities.

Backscattered Electron Image (BEI)

The image that is obtained from backscattered electrons that are reflected from a specimen. The BEI image is dependent on surface topography and average atomic number of specimen.

Secondary Electron Image (SEI)

The image that is obtained from secondary electrons that are reflected from a specimen. The image contrast depends on surface roughness, profile of sample and sample composition.

Objective lens

The lens nearest to the specimen and used to focus the electron beam onto the specimen surface.

Resolution Limit

The limit beyond which points on an image cannot be recognised as distinct.

Resolution

The limit of resolution is the smallest separation at which two points can be seen as distinct entities.

Atomic number (Z)

The number of protons, and also electrons, that the atoms of an element contain. In the Periodic Table of the Elements, elements are arranged in order of increasing atomic number.

Raster

The pattern of scanning line-by-line on the screen of a visual display, such as a television screen or computer monitor.

Exchange position

The position (X, Y and Z) of a sample stage that is used for exchanging a sample safely, into or out of a scanning electron microscope.

Rotation

The practice of moving a sample in a clockwise or anticlockwise direction while in the sample chamber.

Electron scattering

The process in which a travelling electron is deflected from its original trajectory.

Grayscale

The range of monochrome colours that fit between pure black and pure white.

Resolving Power

The resolution that is achievable by using a particular instrument under optimum viewing conditions.

Topography

The shape or relief of a surface.

Spot size

The size of the beam cross section at the surface of the sample. This affects 1) the resolution of the image and 2) the number of electrons generated from the sample.

Chamber

The space in which the sample is housed in a scanning electron microscope.

Scan speed

The speed with which the electron beam scans across the sample.

Depth of field

The vertical distance above and below a point on a sample that is being focussed that will also be in focus in an image.

Depth of focus

The vertical distance through which the sample can be moved without affecting the sharpness of the image. This will depend on the magnification.

Voltage

The voltage between two points is the electrical force that would drive an electric current between the two points.

Electron beam voltage

The voltage of the electron beam. A scanning electron microscope usually has the capacity to operate in a range from from 0 to 30 kilovolts (kV), and sometimes up to 40 kV.

Interaction volume

The volume of the sample in which electrons from the primary electron beam spread out and interact with the atoms of the sample. The size of the interaction volume depends upon the energy of the electron beam and the average density of the sample.

Wavelength

The wavelength of a sinusoidal wave is the distance over which the shape of the wave is repeated, e.g. the distance between successive peaks or troughs in the wave.

Bremsstrahlung

This is electromagnetic radiation referred to as braking radiation (or deceleration radiation). The term comes from the German bremsen "to brake" and Strahlung "radiation". It is produced by the interaction of electrons with atoms in a sample. Bremsstrahlung has a continuous spectrum of energy (rather than discrete energy peaks). It results in the layer of "noise" underlying the discrete peaks in a spectrum of x-rays when using energy dispersive spectroscopy.

Tilting

Tipping the sample in the sample chamber so that it no longer lies in the horizontal plain.

Field emission source

Usually a single crystal of tungsten shaped to give a sharp point of 100 nm diameter or less. Electrons are drawn from the tip by application of an electric field.

High Tension (HT)

Usually a term used for the control button to turn on the electron beam or accelerating voltage.

Z-axis

Vertical measurement: height. Used when referring to the raising and lowering of a sample stage within an electron microscope.