Scanning electron microscopy (SEM) has become a powerful and versatile tool for material characterization. This is especially so in recent years, due to the continuous shrinking of the dimension of materials used in various applications. In this blog, we provide an answer to the question ''what is SEM?'' and describe the main working principles of a SEM instrument.
What is SEM?
SEM stands for scanning electron microscope. Electron microscopes use electrons for imaging, in a similar way that light microscopes use visible light. SEMs use a specific set of coils to scan the beam in a raster-like pattern and use the electrons that are reflected or knocked off the near-surface region of a sample to form an image. Since the wavelength of electrons is much smaller than the wavelength of light, the resolution of SEMs is superior to that of a light microscope.
There are two main types of electron microscopes:
- The transmission electron microscope (TEM), which detects electrons that pass through a very thin specimen;
The scanning electron microscope (SEM), which uses the electrons that are reflected or knocked off the near-surface region of a sample to create an image.
Let’s focus on a SEM. A schematic representation of the technology of a SEM is shown in Figure 1 below. In this type of electron microscope, the electron beam scans the sample in a raster pattern. But first, electrons are generated at the top of the column by the electron source. These are emitted when their thermal energy overcomes the work function of the source material. They are then accelerated and attracted by the positively-charged anode. You can find a more detailed description of the different types of electron sources and their characteristics in this guide.
Figure 1: schematic representation of the basic SEM components
The entire electron column needs to be under vacuum. Like all the components of an electron microscope, the electron source is sealed inside a special chamber in order to preserve vacuum and protect it against contamination, vibrations or noise. Although the vacuum protects the electron source from being contaminated, it also allows the user to acquire a high-resolution image. In the absence of vacuum, other atoms and molecules can be present in the column. Their interaction with electrons causes the electron beam to deflect and reduces the image quality. Furthermore, high vacuum increases the collection efficiency of electrons by the detectors that are in the column.
How is the path of electrons controlled?
In a similar way to optical microscopes, lenses are used to control the path of the electrons. Because electrons cannot pass through glass, the lenses that are used here are electromagnetic. They simply consist of coils of wires inside metal pole pieces. When current passes through the coils, a magnetic field is generated. As electrons are very sensitive to magnetic fields, their path inside the microscope column can be controlled by these electromagnetic lenses - simply by adjusting the current that is applied to them. Generally, two types of electromagnetic lenses are used:
The condenser lens is the first lens that electrons meet as they travel towards the sample. This lens converges the beam before the electron beam cone opens again and is converged once more by the objective lens before hitting the sample. The condenser lens defines the size of the electron beam (which defines the resolution), while the main role of the objective lens is to focus the beam onto the sample.
The scanning electron microscope’s lens system also contains the scanning coils, which are used to raster the beam onto the sample. In many cases, apertures are combined with the lenses in order to control the size of the beam. These main components of a typical SEM instrument are shown in Figure 1.
What kind of electrons are there?
The interaction of electrons with a sample can result in the generation of many different types of electrons, photons or irradiations. In the case of SEM, the two types of electrons used for imaging are the backscattered (BSE) and the secondary electrons (SE).
Backscattered electrons belong to the primary electron beam and are reflected back after elastic interactions between the beam and the sample. On the other hand, secondary electrons originate from the atoms of the sample: they are a result of inelastic interactions between the electron beam and the sample.
BSE come from deeper regions of the sample (Figure 2), while SE originate from surface regions. Therefore, BSE and SE carry different types of information. BSE images show high sensitivity to differences in atomic number: the higher the atomic number, the brighter the material appears in the image.
Figure 2: Different types of signals used by a SEM and the area from which they originate
SE imaging can provide more detailed surface information — something you can see in Figure 3. In many microscopes, detection of the X-rays, which are generated from the electron-matter interaction, is also widely used to perform elemental analysis of the sample. Every material produces X-rays that have a specific energy; X-rays are the material’s fingerprint. So, by detecting the energies of X-rays that come out of a sample with an unknown composition, it is possible to identify all the different elements that it contains.
Figure 3: a) BSE and b) SE image of the FeO2 particles
The types of electrons mentioned above are detected by different types of detectors. For the detection of BSE, solid state detectors are placed above the sample, concentrically to the electron beam, in order to maximize the BSE collection.
On the other hand, for the detection of SE, the Everhart-Thornley detector is mainly used. It consists of a scintillator inside a Faraday cage, which is positively charged and attracts the SE. The scintillator is then used to accelerate the electrons and convert them into light before reaching a photomultiplier for amplification. The SE detector is placed at the side of the electron chamber, at an angle, in order to increase the efficiency of detecting secondary electrons. These secondary electrons are used to form a 3D-image of the sample, which is shown on a PC monitor.
As you can see, there are different processes that the electrons must go through before an image can be shown on your monitor - Figure 4. Of course, you don’t have to wait for the electrons to finish their journey; the whole process is almost instantaneous, in the range of nanoseconds (10-9 seconds). However, every “step” of an electron inside the column needs to be pre-calculated and controlled with precision in order to obtain a high-quality image. Scanning electron microscopes are continuously improved, and new applications are still arising, making them fascinating instruments with lots of undiscovered capabilities.
Scanning electron microscopes help researchers optimize their material characterization processes and save valuable time.
Are you looking for a way to optimize your research processes and applications? Would you like to conduct better analyses in less time? Choosing the right microscope for your research can help you to work more efficiently.
Our How to Choose a Scanning Electron Microscope E-guide will assist you in selecting the most suitable scanning electron microscope for your analyses.
Understand the electron source
When you are considering a desktop scanning electron microscope (SEM), it’s important to determine what type of electron source fits your needs since this the electron source you choose has a direct effect on the quality of your output.
The electron source—or cathode, filament, or electron gun—is one of the most important components of a desktop SEM. Its purpose is to provide a stable beam of electrons.
How a thermionic electron source works
The way a thermionic electron source works is this: The electron beam projected onto a sample is created by electrons emitted from the electron source at the anode inside the electron column, with lenses used to control the beam. When any solid material is heated, electrons are emitted by thermionic emission, and the emission becomes significant when the thermal energy of the electrons exceeds the “work function”—or the energy required to withdraw electrons from a given material. The electron source is made from a high melting point material with a relatively low work function required to emit many electrons.
Comparison of tungsten and CeB6 sources
With today’s SEMs, two types of thermionic electron sources are widely used: Tungsten and Cerium hexaboride (CeB6). The question is how do you choose between them?
Of all metals in pure form, Tungsten has the highest melting point, the lowest vapor pressure, the lowest thermal expansion, and a very high tensile strength, which are all ideal properties for making an electron source. Yet Tungsten has some fundamental disadvantages compared to a CeB6 electron source. Consider five ways in which the two electron sources differ:
- Brightness: The lower work function of a CeB6 filament results in higher beam currents at lower cathode temperatures than Tungsten, which means greater brightness at all acceleration voltages. Specifically, a CeB6 cathode provides 10 times the brightnesscompared to Tungsten. This gives the CeB6 source two advantages over a Tungsten source. First, more current is available in the same focused spot, which means a better signal-to-noise ratio at the same spot size. Second, at the same signal-to-noise ratio, the CeB6 spot can be made smaller, which means that a better resolution can be achieved.
- Electron source size: The source size is of Tungsten is elliptically shaped with a dimension ranging from 50 to 100 micrometers (µm), depending on the source configurations and operating conditions. Compared to a CeB6 source, which has a dimension of less than 25 µm, it means that considerable electron optic demagnification is required for a Tungsten source to achieve a small electron probe needed for good resolution in SEM.
- Electron source temperature: The operational temperature of the Tungsten filament lies around 2800 Kelvin, whereas the CeB6 source has an operational temperature of 1800 Kelvin. The difference in temperature has a direct effect on the source.
- Electron beam energy spread: The higher temperature setting of the Tungsten source causes a larger energy spread than a CeB6 source. Typically, the energy spread of a Tungsten source is about 2.5 electron volts (eV). By comparison, the energy spread of a CeB6 source is about 1 eV, resulting in better image quality—especially at lower acceleration voltages.
- Electron source lifetime: A CeB6 source typically provides more than 15 times the service life of a Tungsten source—or 1,500+ hours compared to about 100 hours. A Tungsten filament operates at white-hot temperatures, which thins the Tungsten wire and eventually breaks it during imaging. By contrast, a CeB6 source slowly degrades over time, allowing users to predict its failure and replace it between operating sessions. Another disadvantage of Tungsten is that the breaking of the wire sometime contaminates the upper part of the electron column, requiring these parts to be replaced or cleaned. With a CeB6 source, contamination of the column due to debris isn’t an issue.
For these reasons, we highly recommend a CeB6 electron source. In fact, the only drawback of CeB6 is that it’s more expensive than Tungsten in short term. But when you consider the longer lifespan and the minimal risk of contamination, CeB6 is actually less expensive in the long run. And that makes the investment in a CeB6 electron source even more compelling.
EDX Analysis with a Scanning Electron Microscope (SEM): How Does it Work?
Electron – matter interaction
The electron beam-matter interaction generates a variety of signals that carry different information about the sample (Figure1). For example, backscattered electrons produce images with contrast that carries information on the differences in atomic number; secondary electrons give topographic information (you can read more about it here); cathodoluminescence can give information on the electronic structure and the chemical composition of materials; and transmitted electrons can describe the sample’s inner structure and crystallography. Another type of signal that is widely used in SEMs is X-rays.
Figure 1. Illustration of the electron-matter interaction depicting its different products.
EDX analysis in SEM: the principle explained
Every atom has a unique number of electrons that reside under normal conditions in specific positions, as you can see in Figure 2. These positions belong to certain shells, which have different, discrete energies.
The generation of the X-rays in a SEM is a two-step process. In the first step, the electron beam hits the sample and transfers part of its energy to the atoms of the sample. This energy can be used by the electrons of the atoms to “jump” to an energy shell with higher energy or be knocked-off from the atom. If such a transition occurs, the electron leaves behind a hole. Holes have a positive charge and, in the second step of the process, attract the negatively-charged electrons from higher-energy shells. When an electron from such a higher-energy shell fills the hole of the lower-energy shell, the energy difference of this transition can be released in the form of an X-ray.
This X-ray has energy which is characteristic of the energy difference between these two shells. It depends on the atomic number, which is a unique property of every element. In this way, X-rays are a “fingerprint” of each element and can be used to identify the type of elements that exist in a sample.
Figure 2. X-ray generation process. (1) The energy transferred to the atomic electron knocks it off leaving behind a hole, (2) its position is filled by another electron from a higher energy shell and the characteristic X-ray is released.
EDX material analysis: how X-ray detection works
Unlike BSE, SE and TE, X-rays are electromagnetic radiation, just like light, and consist of photons. To detect them, the latest systems use the so-called silicon-drift detectors (SDDs). These are superior to the conventional Si(Li) detectors due to higher count rates, better resolution, and faster analytical capabilities. These detectors are placed under an angle, very close to the sample, and have the ability to measure the energy of the incoming photons that belong to the X-rays. The higher the solid angle between the detector and the sample, the higher the X-rays’ detection probability, and therefore the likelihood of acquiring the best results.
Figure 3. Typical EDX spectrum. Y-axis depicts the number of counts and x-axis the energy of the X-rays. The position of the peaks leads to the identification of the elements and the peak height helps in the quantification of each element’s concentration in the sample.
The data that is generated by EDX analysis consists of spectra with peaks corresponding to all the different elements that are present in the sample. You can see an example of this in Figure 3. Every element has characteristic peaks of unique energy, all extensively documented.
Furthermore, EDX can be used for qualitative (the type of elements) as well as quantitative (the percentage of the concentration of each element of the sample) analysis. In most SEMs, dedicated software enables auto-identification of the peaks and calculation of the atomic percentage of each element that is detected. One more advantage of the EDX technique is that it is a non-destructive characterization technique, which requires little or no sample preparation.
Choosing a SEM that best suits your research processes
EDX analysis has now become common practice and is so practical that it is an essential part of a SEM. Imagine always having the ability to know what your sample contains with a very simple experiment!
In this way, SEM can help you optimize your research processes, perform better analyses and save valuable time.
Would you like to conduct better analyses in less time? Choosing the right microscope for your research can help you to work more efficiently.
A spectrophotometer is a device measures the intensity of electromagnetic energy at each wavelength of light in a specified region. A UV-visible-NIR spectrophotometer, such as used in CRAIC microspectrophotometers, operate in the ultraviolet, visible and near infrared regions. As described below, the spectrophotometer consists of a light source, a way to focus light onto the sample, a method to collect the light from the sample, a monochromator to separate the light into its component wavelengths and a detector to measure the intensity of light at each wavelength.
How does a Spectrophotometer Work?
The spectrophotometer is an optical instrument for measuring the intensity of light relative to wavelength. Electromagnetic energy, collected from the sample, enters the device through the aperture (yellow line) and is separated into its component wavelengths by the holographic grating. Simply put, the grating acts to separate each color from the white light.
The separated light is then focused onto a CCD array detector where the intensity of each wavelength (or each color if in the visible region) is then measured by a pixel of the array. The CCD is then read-off to a computer and the result is a spectrum which displays the intensity of each wavelength of light.
An example would be a spectral measurement of the visible range which we perceive of as color. White light would enter the monochromator and be separated into a rainbow of each color. This rainbow, with blue light on one end and red on the other, would be focused on to the CCD. Each pixel of the CCD would then measure the intensity of a color.
The results would be a spectrum such as the one shown below. As shown in figure b, the blue pixels emit blue light, the green pixels emit in the green portion of the spectrum and the red pixels emit red light.
Why use a Spectrophotometer?
The spectrophotometer allows the researcher to acquire spectra of by illuminating a sample and measuring the intensity of light returned from the sample relative to its wavelength. With this non-destructive technique, measurements can be made with light transmitted through the sample, reflected from it or even when the sample is made to emit light by processes such as photoluminescence. The UV-visible-NIR range is especially important as more substances...even colorless ones...absorb in the UV than in the visible and infrared regions. Therefore, a UV -visible-NIR range spectrophotometer is very useful for analysis of most samples for any application.
Spectrophotometers are used by analytical laboratories to identify microscopic samples ranging from the kinetics, matching colour (as shown figure c), the qualification of gems and minerals, the determination of colour of ink or paint bu process chemist.
CRAIC Technologies builds microspectrophotometers. These devices integrate the spectrophotometer with a microscope to enable one to measure the spectra but of microscopic sample areas. Microspectrophotometers are also used to measure fluorescence, photoluminescence, thin film thickness, measurements, microcolorimetry, for imaging and much more. Microspectrophotometers are used for diverse applications such as colorimetry of pixels on flat panel displays, reflectometry of vitrinite coal and thin film thickness measurements.
What is a Microspectrophotometer (MSP)?
Spectroscopy of microscopic samples
The microspectrophotometer is a scientific instrument used to measure the spectra of microscopic samples. For example, an engineer in a semiconductor facility will use it to measure the thickness of thin films while a forensic scientist will use one to analyze the dye in a single textile fiber, as shown figure 1 or a chemist will use it to measure the spectrum of a nanocrystals.
CRAIC Technologies™ builds a microspectrophotometer that combines a UV-visible-NIR optical microscope with a UV-visible-NIR range spectrophotometer. While a standard spectrophotometer is designed to measure samples on the order of 1 x 1 centimeters, the microspectrophotometer is able to measure samples on the order of 1 x 1 micrometers...much smaller than the thickness of a human hair.
As shown in the diagram on the Figure 2, the instrument combines a UV-visible-NIR range optical microscope with a UV-visible-NIR range spectrophotometer. In this figure, the instrument is configured for transmission microspectroscopy. The light from the lamp housing is focused onto the sample on the microscope stage (I0). The light that is transmitted through the sample, is collected by the objective (I) and focused onto the spectrophotometer entrance aperture.
The UV-visible-NIR microspectrophotometer is a very flexible instrument and can be configured to measure the transmittance, absorbance, reflectance, polarization, Raman, fluorescence and photoluminescence microspectra™ of sample areas smaller than a micron. Microspectrophotometers are also capable of non-contact microspot thin film thickness measurement and colorimetry. Because UV-visible-NIR microspectrometers are so flexible, they are used in many fields of research and industry.
There are also different types of microspectrophotometers. Some of these instruments are designed to be added to standard microscopes or probe stations while others are fully integrated, purpose-built instruments. As such, microspectrophotometers have greater spectral ranges, better results and several features that are not possible with add-on units.
Why use a Microspectrophotometer?
The microspectrophotometer allows the scientist or engineer to acquire spectra of extremely small sample areas non-destructively and without physically touching the sample. Measurements can be made while light is transmitted through the sample, reflected from it, scattered from it or even when the sample is made to emit light: as shown in Figure 3 the image OLED pixels to the left. The UV-visible-NIR range is especially important as more substances...even colorless ones...absorb in the UV than in the visible and infrared regions. Therefore, a UV microscope spectrometer is very useful for analysis of most samples for any application.
Microspectrophotometers are easily employed in many different fields and are found in both scientific laboratories and production facilities. In the production environment, for example, they are used for quality control of everything from color masks in flat panel displays to the thickness of films on semiconductor integrated circuits.
Microspectrometers are used by analytical laboratories to identify and quantify microscopic samples ranging from the microfluidic kinetics, matching fibers or paints by a forensic chemist, the qualification of gems or coal by a geologist, the determination of the color of ink or paint by a process chemist and even the analysis of great works of art by conservators. As such, the microspectrometer is a highly flexible instrument with many different applications.