Material Science is one of the most important research areas and one of the highly paid jobs in near future. Material Science needs characterization of materials for research. Material Characterization techniques are one of the most important aspects of material science dealing with the determination of material using various methods including Optical microscopy, Metallurgical microscopy, Fluorescent Microscopy, X-Ray Diffraction, Transmission electron microscopy, Scanning electron microscopy, atom probe Microscopy and most advanced material characterization technique and Nuclear spectroscopy.
The Material Characterization techniques that we are going to discuss in this article is Electron Microscopy. But before going to the electron microscopy there is a need to discuss some basic concepts like Resolution, Magnification contrast etc., and Brief history of electron microscopy, Different kinds of electron microscopes, different parts of electron microscope including Electron Sources, Thermionic Emission, Field emission and Effect of Brightness, Coherency and stability on the electron beam, Various sources of electron beams and Detectors. Specimen preparation of Transmission electron Microscope, Scanning electron microscope. Interaction of electrons with Matter, Scattering of electrons.
Applications of Transmission electron Microscope, scanning electron microscope and Limitations of Transmission electron Microscope, Scanning electron microscope.
Basic concepts Resolution, Magnification and contrast
The basics terminologies are important to define before going in-depth of electron microscopy. Resolution is the minimum distance between two points or objectives which can be distinguished as two separate or individual entities. Resolution can also be defined as the minimum distance between two airy discs which can be distinguished as two individual entities. The general formula used to determine resolution is given as Below are the airy discs which are of different types in terms of resolution. The resolution must be as low as possible for good image clarity.
Magnification generally means Enlarging the size of a given object to observe features present in it. The magnification must be as high as possible for better image quality. The wavelength of visible light 350-700 nm, ±500 nm resolution. So, Smaller objects require smaller wavelengths so there is a need for electron microscopes!
Contrast: Contrast means the relative change in light intensity between an object and its background. The general formula used for contrast is given as Contrast = Intensity of object – Intensity of background by Intensity of background.
The magnification of the electron microscope is much higher than an optical microscope, in terms of magnification, resolution and contrast. The high resolution of the electron microscope is basically due to the electrons used for capturing the image. The wavelength of electrons in electron microscopes in much shorter than visible light. This leads to the high resolution which helps determine the very fine details of the specimen. There are mainly two types of electron microscopes.
A brief history of electron microscopy
Let’s discuss briefly the important discovers that lead to electron microscopy.
In 1925: DE Broglie theorized that electrons had wave-like characteristics.
In 1927: Davisson & Germier; Thompson & Reid demonstrated the wave nature of electrons through electron diffraction experiments
In 1932: Knoll and Ruska in a paper proposed the idea of an electron microscope, for which Ruska was awarded the Nobel in 1986.
In 1936: First TEM, Metropolitan-Vickers EM 1 (UK)
In 1939: Successful commercial models developed by Siemens and Halske (Germany), Several commercial models are available now by famous companies like Hitachi, JEOL, Philips (FEI), RCA.
In mid-fifties, Bollman (SUI) and Hirsch (Cambridge, UK) perfected techniques to thin metal foils for electron transparency.
The theory of electron diffraction contrast was developed at Cambridge by Hirsch’s group.
In the 1960s: attempts were made to construct High Voltage Electron Microscopes (HVEM) with accelerating voltages between 1-3MV Rather than pushing the resolution limit, these instruments were utilized to simulate radiation damage.
In the 1980s: only 1 HVEM was built that could go to 1MV
In the 1990s: 3 HVMs, which can go up to 1.25MV were built.
Different kinds of electron microscopes
Different kinds of electron microscopes include Transmission electron Microscope, Scanning electron microscope.
the transmission electron microscope is classified as below types,
a. High-resolution transmission electron microscope: High-Resolution Transmission Electron Microscopes (HRTEMs) operates at an intermediate voltage of 200-400kV.
b. High voltage electron microscope
c. Intermediate voltage electron microscope: Intermediate voltage electron microscope operating at 1MV.
d. Scanning Transmission electron microscop
e. Analytical electron microscope.
Electron microscopes structure is includes
An illumination system, electromagnetic coils, specimen stage, Magnification system, vacuum pump and Image capturing (Detectors). Let’s study the various parts of the transmission electron microscope.
Electron Sources, Thermionic Emission, Field emission
Illumination system has Electron Sources divided into Field Emission Guns (FEGs) and Thermionic Sources
Electron gun contains an electrode, and the electrode contains three parts cathode anode and Wehnelt electrode. Electrons are produced from an electron gun, Example tungsten, Lanthanum Hexaboride.
Electrons are generated from cathode due to high voltage differences between cathode and anode. Wehnelt Electrode is used to protect the electron gun stops fluctuation of voltage while production of electrons. Once electrons are produced, they are condensed using the electromagnetic lens.
Work function (φ): The natural barrier preventing the leak out of electrons. If J is the current density, J = AT2e-φ/KT. When thermionic sources, often temperatures are so high that most of the materials melt or vaporize. Therefore, viable sources are refractory materials or materials with low φ.
Examples of Thermionic guns are tungsten carbide and LaB6.
Field emission guns are classified as Cold Field emission gun and hot Field emission gun.
Tungsten Carbide Electron Gun:
In this electron gun, the tungsten carbide is cathode and copper is the cathode, the temperature difference acts as activation energy the maximum temperature that is attained is 3000-degree centigrade. The work function of W is 5 eV.
It has a work function of 2 eV, it has a high lifetime compared to tungsten carbide due to low-temperature operation and it requires a high vacuum to operate at 10-10 torr.
Field emission guns:
The working principle is simpler than thermionic sources we have two anodes: First, positively charged by several kV to provide the extraction voltage from the W tip. The other anode is used to accelerate the electron beam to voltages like 100kV or more. Combined fields of these anodes act as an electrostatic lens which controls the effective source size and position, By incorporating magnetic lens into the gun, more controllable beam and larger brightness
Cold Field emission gun operates at room temperature and Hot Field emission gun operates at a temperature greater than room temperature.
The electrons produced from any method should be accelerated by voltage, most SEM’s operating voltage is around 1-50 kV.
Though this process electrons will acquire the energy and this energy can be calculated by E= electron charge X voltage applied. The energy of an electron increases with increase in applied voltage and these electrons will be referred to as high energy electrons. Higher is the energy of electrons larger will be the volume of the material from which the information is acquired.
The basic structure of Electromagnetic coil contains two poles north and south, the case is made of soft electromagnetic material and enclosed by a solenoid. Poles are located at a narrow annular opening of the case. Such an arrangement creates a powerful magnetic field concentrated in the gap between two poles and thus electron beam is more focused and dictated by an electromagnetic field.
The main function of the objective lens is to enhance electron beam.
Interaction of electrons with Matter: Scattering of electrons
Electron Signal from each pixel raster is collected by the detector to generate the corresponding point to point image on display,
When a primary electron interacts with the specimen several different types of signals were generated. When primary electron beam interacts with the specimen elastic and in-elastic scattering occurs due to elastic scattering backscattered electrons are produced and in In-elastic scattering secondary electrons will be produced.
Types of electrons that are generated are:
Secondary electrons are formed due to interaction between primary electrons and loosely bonded electrons of the specimen.
The number of electrons generated per primary electron that is known as the secondary electron emission coefficient and is usually greater than 1.
Secondary electrons are generated near the surface of the specimen, even though SE is generated throughout the specimen because of their low interaction level, they get absorbed by specimen itself before they reach the surface of the specimen, The yield of secondary electrons doesn’t depend on the atomic number of a material, but depends on the angle between the incident beam and specimen surface, So they are used to determine the surface topography of the specimen. The secondary electron energy is as low as 3eV to 200eV but typically range between 10-50eV.
Back Scattered electrons: These are generated by the single large angle or multiple small angles scattering events, BSE depends on the atomic number of the specimen or average atomic number, surface inclination, energy of the primary incident beam and number of out shell electrons. Phases with higher atomic number spear bright and the lower will appear dark in the images.
Another type of electrons or rays which are generated include,
Characteristic X Rays, Auger Electrons, Cathode luminescence and electron beam current.
Effect of Brightness, Coherency and stability on electron beam
flux emanating from the source flux is the current density, therefore Brightness is the current density per unit solid angle of source. Brightness is important in convergent probes (AEM, STEM). In conventional TEM, the large de-focused beam is used.
It is defining how well the electron waves are in step.
Temporal coherency is the measure of how similar the wave packets are. λC = vh/ΔE where λC is the same coherence length, v is the electron velocity, ΔE is the energy spread, For good temporal coherency we need stable voltage supply / high tension and monochromatic means waves of the same wavelength and monochromator + CS corrector à rare + expensive
Spatial Coherency is Electrons all were emanating from some point on the source, Smaller the source è better the spatial coherence.
Apart from the voltage stability for better coherency, we also need the stability of the current coming from the source
The secondary electrons and backscattered electrons pass through the fluorescent screen and are captured by the CCD Camera
Specimen preparation of Transmission electron Microscope
Thickness range of sample must be less than 100 manometers. General steps in Sample preparation is pre-thinning and final thinning. In pre thinning the sample is reduced to less than 3 mm diameter disc for polishing for which a dimple type polisher is used and the sample is reduced to less than 1 mm by 1mm area.
Final thinning is used to reduce the sample size by 1mm by 1mm area to 100-nanometre square area. The various process of final thinning is Ion milling, jet polishing and Ultramicrotome.
Ion milling is used to reduce the sample size by using energy in a vacuum and hitting the sample, by this, we get a sample size around ±20 μm.
Electropolishing is used to reduce the sample size to ±100 μm usually used for metals, semiconductors, ceramics and alloys, we get a simple sample.
Common specimen types •Metals •Ceramics •Polymers •Thin-films •Powders/Nano particle suspensions.
Applications of scanning electron microscope include topographical analysis, morphological analysis, Compositional information, surface fracture information, Surface combination information special variations in chemical composition and crystalline structure info.
Limitations are High cost, large size, area of usage must be free from electric, magnetic and vibration interface, needs steady voltage and cool water supply, special training for operators and risk of radiation exposure to operators.
Applications of Transmission electron Microscope can be used for Imaging, Measuring, Modelling, Manipulating Matter and Kinds of materials that can be studied are Metals, alloys, ceramics, glasses, polymers, semiconductors, wood, textiles, concrete etc
Limitations Thin specimens are needed, thin means relatively thin, electron transparent samples <100 manometers.
Electron beam damage a Problem for polymers, organics, certain minerals and ceramics; The effects are more at a higher kV ~ 400kV. E.g. Quartz samples also can have damage at 125kV.
TEM information is averaged through the thickness, in other words, a single TEM image has no depth sensitivity.
1. MATERIALS CHARACTERIZATION Introduction to Microscopic and Spectroscopic Methods Yang Leng Hong Kong University of Science and Technology.
Further Reading :
1. Diffraction principles
2. Scanning probe microscopy
3. Crystal defects
4. Operational variables
5. Electron spectroscopy for surface analysis.