Characterisation, Measurement & Analysis
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  • Emission stability in SEM thermionic electron sources: CeB6, LaB6 and W filaments

    Typically, desktop scanning electron microscopes (SEM) make use of thermionic sources, from which electrons are emitted when warming up the SEM filament. Although the working principles are the same, different thermionic sources show a different performance. Phenom SEMs are equipped with a CeB6 source because of its higher brightness and longer lifetime. A parameter that plays a crucial role is the emission current stability. How is the CeB6 source performing in terms of stability? What are the engineering smarts that enable the Phenom source to maximize a CeB6 source's potential? This blog answers these questions.

    Working principle of thermionic sources in scanning electron microscopes
    In an earlier blog on electron sources, we discussed the performance of CeB6 and tungsten sources extensively. CeB6 and tungsten are both thermionic sources with a filament called a cathode, from which electrons are emitted. The emission starts when the electrons are provided with enough energy to cross the potential barrier, given by the work function of the cathode material, which can be either tungsten or CeB6.

    The energy is provided by heating up the cathode, which in turn is done by letting current flow through it. A Wehnelt electrode that is negatively charged with respect to the cathode pushes the unwanted electrons back into the filament, effectively determining the size of the emitting area.

    Below the cathode and the Wehnelt electrode, an anode provides a strong electric field, or a strong lens that makes the electron beam converge into a crossover between the Wehnelt and the anode. Figure 1 shows the schematics of the CeB6 source, consisting of a filament, a Wehnelt electrode and an anode. The filament is at high potential, as well as the Wehnelt, whereas the anode is grounded. The circuitry positioned in between the filament and the anode measures the emission current.

    Fig.1: Schematics of a thermionic source, consisting of a CeB6crystal (the filament), a Wehnelt electrode and the anode. In red, the trajectories of the electrons that are pushed back in the filament thanks to the Wehnelt voltage and the trajectories of the emitted electrons, forming the primary beam.

    Comparison between thermionic electron sources: CeB6, LaB6 and W

    CeB6 is not the only cathode for thermionic sources, LaB6 and tungsten are also used.

    Tungsten cathodes are hair-pin filaments that are bent to reduce the size of the emitting surface. They are typically warmed up to a temperature of 2500-3000 K to achieve high current density, being the work function of tungsten 4.5 eV. At 2800 K, a practical value of current density is 3 A/cm2.

    The lifetime of tungsten cathodes, which can vary between 40 and 200 hours, is limited by the evaporation of the cathode material, resulting in the wire breaking when it becomes too thin. To prevent too much oxidation, a vacuum of 10-3 Pa is kept at the source.

    Hexaboride crystals (CeB6 and LaB6) cathodes are rods with a flat tip, and are typically heated up to 1400-2000 K, as the work function is lower than the tungsten (2.7 eV for LaB6 and 2.5 eV for CeB6). A low work function and low temperatures yield a higher current density than tungsten cathodes, in the range of 20-50 A/cm2.

    Typically, hexaboride cathodes are 10 times brighter than tungsten cathodes, meaning they provide higher beam current in a smaller spot size at the sample. Also, the lifetime of hexaboride cathodes is higher, typically 10 times that of tungsten cathodes.

    However, hexaboride cathodes need a vacuum of better than 10-4 Pa to prevent oxidation. The performance of hexaboride cathodes strongly depends on vacuum and temperature. Studies suggest that CeB6 cathodes are less likely to be affected by carbon contamination than LaB6 cathodes. Also, CeB6 cathodes have a lower evaporation rate at a working temperature of 1800 K compared to LaB6. Therefore, the shape of a CeB6 cathode tip lasts longer.

    The following table summarises the physical properties of the three thermionic sources:

    Emission current stability in the Phenom CeB6 electron source
    The stability of the emission current is a key requirement for thermionic sources. During the operation of the microscope, the emission current is kept stable by adjusting the Wehnelt voltage in a constant control loop. The emission current is measured in the source, by a circuitry between the filament and the anode, as shown in Fig. 1. The Wehnelt voltage is then adjusted according to the read out of the emission.

    It is of utmost importance that the current at the sample is kept constant, for given settings. An automated function measures the sample current as a function of the emission current. The emission current is adjusted by varying the voltage on the Wehnelt, thereby regulating the amount of electrons pushed back into the filament, for a constant filament temperature. The current at the sample can be measured indirectly from the signal of an image taken with the BSD detector on a reference material.

    The optimal case, shown in blue in Figure 2, is when the current at the sample has a maximum at 62 µA. If the peak is before or after this value, it means that the temperature of the filament is either too low or too high and needs to be adjusted. The automated function sets the new temperature of the filament and measures the current at the sample for different emission currents again, until the peak falls at the ideal emission current of 62 µA.

    Fig.2: Current at the sample (I spot) as function of the emission current for different filament temperatures.

    Once the temperature is adjusted through the automated function, the voltage on the Wehnelt is set for an emission current of 40 µA, thus on the left of the peak of the blue curve. An emission of 40 µA is chosen as the optimum between resolution and beam current, which translates into image quality.

    The current stability of thermionic sources is typically better than 1% RMS. Measurements on sample current stability of a CeB6 source in a Phenom microscope show that the fluctuation of the beam current is about 0.3 % in the first 5 hours from switching on the source and is 0.2 % from 5 hours up to 15 hours after switching on the source, as represented in Figure. 3. The drop in the first hour is measured to be approximately 10% and is caused by the stabilization of the temperature in the source unit.

    Moreover, the vacuum stability at the source does not affect the emission current of the CeB6 source in a relevant way.

    Fig. 3: Representation of the measured current at the sample over a period of 15 hours.

    There’s much more to discover in Phenom desktop SEMs
    So, the CeB6 source drives the highly reliable and durable character of a Phenom desktop SEM, but it’s hardly the sole contributor. If you take a closer look at Phenom scanning electron microscopes, you will realise that they have a multitude of interesting specifications worth investigating further, like their light and electron optical magnification, resolution, and digital zoom.


    • Introduction to charged particle optics, P. Kruit, Delft University of Technology
    • Scanning electron microscopy, Physics of image formation and microanalysis, L. Reimer, Springer
    • Cathodes for Electron Microscopes, CeBix and LaB6 Filaments Standard Tungsten Loop Filaments, Electron Microscopy Sciences

    Topics: scanning electron microscope, sem filament

    About the author
    Marijke Scotuzzi is an Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. Marijke has a keen interest in microscopy and is driven by the performance and the versatility of the Phenom SEM. She is dedicated to developing new applications and to improving the system capabilities, with the main focus on imaging techniques.



  • SEM and TEM: what's the difference?

    Electron microscopes have emerged as a powerful tool for the characterisation of a wide range of materials. Their versatility and extremely high spatial resolution render them a very valuable tool for many applications. The two main types of electron microscopes are the Transmission Electron Microscope (TEM) and the Scanning Electron Microscope (SEM). In this blog we briefly describe their similarities and differences.

    Working Principle of Scanning Electron Microscopes and Transmission Electron Microscopes
    Let's start with the similarities. For both techniques, electrons are used in order to acquire images of samples. Their main components are the same;

    • An electron source;
    • A series of electromagnetic and electrostatic lenses to control the shape and trajectory of the electron beam;
    • Electron apertures.

    All of these components live inside a chamber which is under high vacuum.

    Now over to the differences. SEMs use a specific set of coils to scan the beam in a raster-like pattern and collect the scattered electrons (read more about the different type of electrons detected in a SEM).

    The transmission electron microscopy (TEM) principle, as the name suggests, is to use the transmitted electrons; the electrons which are passing through the sample before they are collected. As a result, TEM offers invaluable information on the inner structure of the sample, such as crystal structure, morphology and stress state information, while SEM provides information on the sample’s surface and its composition.

    Moreover, one of the most pronounced differences between the two methods is the optimal spatial resolution that they can achieve; SEM resolution is limited to ~0.5 nm, while with the recent development in aberration-corrected TEMs, images with spatial resolution of even less than 50 pm have been reported.

    Which electron microscopy technique is best for your analysis?
    This all depends on what type of analysis you want to perform. For example, if you want to get information on the surface of your sample, like roughness or contamination detection, then you should choose a SEM. On the other hand, if you would like to know what the crystal structure of your sample is, or if you want to look for possible structural defects or impurities, then using a TEM is the only way to do so.

    SEMs provide a 3D image of the surface of the sample whereas TEM images are 2D projections of the sample, which in some cases makes the interpretation of the results more difficult for the operator.

    Due to the requirement for transmitted electrons, TEM samples must be very thin, generally below 150 nm, and in cases that high-resolution imaging is required, even below 30 nm, whereas for SEM imaging there is no such specific requirement.

    This reveals one more major difference between the two techniques; sample preparation. SEM samples require little or no effort for sample preparation and can be directly imaged by mounting them on an aluminum stub.

    In contrast, TEM sample preparation is a quite complex and tedious procedure that only trained and experienced users can follow successfully. The samples need to be very thin, as flat as possible, and the preparation technique should not induce any artefacts (such as precipitates or amorphisation) to the sample. Many methods have been developed, including electro polishing, mechanical polishing and focused ion beam milling. Dedicated grids and holders are used to mount the TEM samples.

    SEM vs TEM: differences in operation
    The two EM systems also differ in the way they are operated. SEMs usually use acceleration voltages up to 30 kV, while TEM users can set it in the range of 60 – 300kV.

    The magnifications that TEMs offer are also much higher compared to SEMs: TEM users can magnify their samples by more than 50 million times, while for the SEM this is limited up to 1-2 million times.

    However, the maximum Field of View (FOV) that SEMs can achieve is far larger than TEMs, which users can only use to image a very small part of their sample. Similarly, the depth of field of SEM systems are much higher than in TEM systems.

    Figure 1: Electron microscopy images of silicon. a) SEM image with SED offers information on the morphology of the surface, while b) TEM image reveals structural information about the inner sample.

    In addition, the way images are created are different in the two systems. In SEMs, samples are positioned at the bottom of the electron column and the scattered electrons (back-scattered or secondary) are captured by electron detectors. Photomultipliers are then used to convert this signal into a voltage signal, which is amplified and gives rise to the image on a PC screen.

    In a TEM microscope, the sample is located in the middle of the column. The transmitted electrons pass through it, and through a series of lenses below the sample (intermediate and projector lenses). An image is directly shown on a fluorescent screen or via a charge-coupled device (CCD) camera, onto a PC screen.

    Table I: Summary of the main differences between a SEM and a TEM

    Generally, TEMs are more complex to operate. TEM users require intensive training before being able to operate them. Special procedures need to be performed before every use, with several steps included that ensure that the electron beam is perfectly aligned. In Table I, you can see a summary of the main differences between a SEM and a TEM.

    Combining SEM and TEM technology
    There is one more electron microscopy technique to mention, which is a combination of TEM and SEM, namely Scanning Transmission Electron Microscopy (STEM). It can be applied to both systems, but its full capabilities are revealed when applied to a TEM tool. Most modern TEMs can be switched to “STEM mode” and the user only needs to alter their alignment procedure. In STEM mode, the beam is finely focused and scans the sample area (as SEM does), while the image is generated by the transmitted electrons (like in TEM).

    When working in STEM mode the users can take advantage of the capabilities of both techniques; they can look at the inner structure of samples with very high resolving power (even higher than TEM resolution), but also use other signals like X-rays and electron energy loss. These signals can be used in spectroscopic techniques; the energy-dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS).

    Of course, EDX is also a common practice in SEM systems and is used to identify the chemical composition of samples by detecting the characteristic X-rays that are emitted from the materials when they are bombarded with electrons.

    EELS can only be realised in a TEM system working in STEM mode and enables the investigation of the atomic and chemical composition, the electronic properties as well as local thickness measurements of materials.

    Choosing between SEM and TEM
    From everything we have mentioned, it is clear that there is no “better” technique; it all depends on the type of analysis that you require. TEM is the choice when you want to get information from the inner structure, while SEM is preferred when surface information is required. Of course, major decision factors are the big price difference between the two systems, as well as the ease of use. TEMs may enable much more resolving power and versatility to the user, but they are much more expensive and larger than SEMs and require more effort in order to acquire and interpret results.

    About the author
    Antonis Nanakoudis is Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. Antonis is extremely motivated by the capabilities of the Phenom SEM on various applications and is constantly looking to explore the opportunities that it offers for innovative characterisation methods.

  • SEM and fibre analysis for filtration systems quality control

    The production of filters and membranes undergoes several quality control steps to ensure that the properties of the product are up to specification. Different tools can be used for such analysis, but only one can provide the best results. Find out in this blog how Scanning Electron Microscopes (SEM) can be used to investigate imperfections in metallic filters.

    Different purpose – different filter
    Whether we are aware of it or not, we use many filters on a daily basis – directly or indirectly. For example, tap water is filtered several times before undergoing chemical treatment, prior to becoming drinking water. This is to remove all kind of particles, from big rocks to small sand grains. In extreme cases, the filters can become so thick that they can catch bacteria.

    Air conditioners also employ filtering systems. They are mostly used to trap dust and small particles, but combined with activated carbons, they can catch odours and purify the air.

    Also, some chemical reactions catalysts are typically shaped as metallic meshes with micropores to let the chemicals flow while, at the same time, maximizing the exposed surface and minimizing the pressure required for the fluid to flow in the reactors.

    Whatever their application, all filtration systems typically require cleaning or replacement, to make sure that they will operate correctly over time. The cleaning operation, combined with normal usage, will eventually determine the end of the life cycle of a filter, which will require a replacement.

    Comparison between BSD (image 1a) and SED (image 1b) image of a metallic mesh. In BSD images the color contrast highlights differences in the atomic weight of the analyzed material, making contamination detection easier. SED signal enhances the sample topography and the surface structure of the sample.

    Deeper inspections with SEM

    The easiest way to investigate a filter is obviously to observe it. Optical microscopes can, in many cases, provide plenty of information on larger filters, but in other cases the resolution that they provide is not sufficient for in-depth inspection.

    Scanning electron microscopes, therefore, prove to be the best analysis technique to inspect any kind of filter thanks to the higher magnification and better resolution. SEM show not just the size of the pores in-between the fibers, but also the surface of the fibers themselves, their roughness and much more. So exactly what kind of data can be obtained?

    One tool for multiple analysis

    An electron microscope is an imaging tool that enables the user to perform a highly detailed visual inspection of the imaged samples. However, modern image processing techniques have turned these devices into automated analysis tools.

    A clear example of this is the automated system to measure the diameter of fibers displayed in the image below. Here, software can reconstruct the direction of the fiber and measure it — an operation that would require tremendous efforts and a huge amount of time from an operator.

    In the case of multi-layer materials or coated fibres, the SEM can be used to determine that the size of each layer has the right size, the distribution of the material on the substrate is homogeneous and the roughness is within specifications.

    Filtration efficiency and product lifetime, as well as resistance to washing processes, can largely benefit from the improvements derived from knowing this information.

    Beyond imaging: chemical composition analysis with EDS

    Most electron microscopes are equipped with an EDS detector. This detector collects the EM radiation emitted by the sample to identify the nature of the element it is composed of.

    This analysis can be used on the fibre’s material to determine, for example, how a different composition can enhance the lifetime of the filter. Even more interesting information can be obtained when analysis is performed on the collected particles, providing precious hints on their nature.

    Image processing for measurement automation

    Image processing technologies have improved dramatically over the last few years and software can now do tedious operations that would steal very precious time from an operator. Applying these techniques to fibre analysis has created a powerful tool to completely characterise not just the fibres diameter and their orientation, but also the dimensions of the holes that are formed in between.

    An example of automatic detection of fibers (image 2a) and results displayed in a diameter size distribution histogram (image 2b).
    An example of automatic detection of pores (image 3a) and results displayed in an area size distribution histogram (image 3b).

    About the author

    Luigi Raspolini is an Application Engineer at Phenom-World, the world’s no 1 supplier of desktop scanning electron microscopes. Luigi is constantly looking for new approaches to materials characterisation, surface roughness measurements and composition analysis. He is passionate about improving user experiences and demonstrating the best way to image every kind of sample.

  • UKIVA Machine Vision Conference & Exhibition (MVC)

    UKIVA Machine Vision Conference & Exhibition (MVC)

    Wednesday 16 May 2018.

    Arena MK Stadium Way, Milton Keynes MK1 1ST.

    Click here for more exhibition information.

    Learn about the latest technology and applications within the Machine Vision industry, through informative presentations on new and emerging Machine Vision technologies given by industry leading manufacturers and UK distributors.

    See live demonstrations of the latest technologies within the Machine Vision industry, via the exhibition, where Machine Vision component manufacturers, system manufacturers and local distributors will be present.

    Meet industry experts from Machine Vision component and system manufacturers, along with local distributors.

  • Photonex Scotland

    Photonex Scotland - Applied Photonics Technologies.

    Applications, components, imaging, instruments, lasers,  microscopy, optoelectronics and sensors.

    Thursday 14 June 2018.

    University of Edinburgh, South Hall Complex , 18 Holyrood Park Road, Edinburgh EH16 5AY.

    Click here for more exhibition information.

  • UK Semiconductors 2018

    UK Semiconductors 2018. An annual conference on all aspects of semiconductor research.

    Wednesday-Thursday, 4-5th July 2018.

    Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB

    Click here for more information

  • Photon 2018

    Photon 2018.

    The major Optics and Photonics conference organised by the Institute of Physics in the UK and the ninth in the biennial series that started in 2002.

    Monday 03 - Wednesday 05 September 2018

    Aston University, Birmingham.

    Click here for more exhibition information.
  • Rigol’s New Real-time Spectrum Analyser

    Never Miss a Signal

    Today’s RF designs utilise rapidly changing and pulsing signals that are difficult to consistently capture on traditional swept spectrum analysers. The Rigol RSA5000 delivers a class leading 7.45µsec 100% probability of intercept meaning that any pulse with a duration of greater than 7.45µsec will be captured and accurately displayed.

    Isolate & Capture

    IoT Device proliferation means more traffic crowding the available spectrum making identifying and isolating signals of interest critically important. The Rigol RSA5000 includes advanced trigger capabilities making this task easier.

    Frequency Mask Trigger captures and displays data when the signal crosses inside or outside of the masked region allowing the engineer to focus on critical signals and ignore interfering ones.

    Power Trigger allows the engineer to trigger and display only when a power level is exceeded within the real-time span.

    Trigger Input & Output allow the engineer to time correlate RF events with other system elements to quickly find root cause of system problems.

    IF Output recreates the real-time bandwidth on a 430MHz carrier for further analysis.

    Visualize & Analyse

    Complex modulating and pulsing signals require advanced visualisation tools to identify patterns and debug designs. The Rigol RSA5000 provides 7 Realtime Visualization modes to look at signal behaviour versus power, frequency and time. 6 markers, 4 trace types, and trace math capabilities allow engineers to quickly analyse and understand signal behaviour across the entire span.

    7 Display Views

    Normal Display Similar view to traditional analysers. Traces, Cursors for traditional measurement. Simple Data view for initial investigation.

    Density Display Colour map represents signal probability at each frequency and power level. See time varying signals with the Real-time span. Separate superimposed “hidden” signals. Resolution of signals on same frequency band.

    Spectrogram Display Observe signal changes over time. Analyse frequency hopping patterns. Characterise PLL Lock and Settle time. Identify sources of interference.

    Density and Spectrogram Display Combine the insights of the Density Display with changes over time. See pulsing signals inside crowded spectrum. Separate WiFi from Bluetooth and other traffic.

    Power v Time (PvT) Display Display the RF Power for the Real-time span over a defined time period. Evaluate the duration of pulsing signals. Investigate ASK and other amplitude modulations by monitoring power level changes. Determine the timing of pulsing signals.

    PvT and Spectrum Display Display the RF Power for the Real-time span over a defined time period along with the frequency. View not only the RF power, but time correlate the Frequency Data.

    PvT and Spectrogram Display the RF Power for the Real-time span over a defined time period along with the frequency and time. View not only the RF power, but time correlate the Frequency Data. Observe the frequency and power change over time.

    RSA5000 Real-time Spectrum Analysers

    High performance Swept Spectrum Analyser

    • 2GHz and 6.5GHz Models with available TG
    • 1ms Full Span Sweep
    • 1Hz RBW
    • Low Noise Floor as low as -165dBm

    Superior Real-time Performance

    • Up to 40MHz Real-time Bandwidth
    • 45µsec 100% POI
    • 7 Visualization Modes
    • Advanced Triggering

    Flexible User Interface

    • 1” Capacitive Touch Screen Display
    • USB Keyboard/Mouse
    • Knob and Softkey Controls
    • HDMI Out
    • USB/LAN support

    Features and Benefits


    Product Feature Customer Benefit
    DANL as low as -165 dBm with       optional preamp View lower powered signals (harmonics, interference sources) and ease trouble-shooting in swept and real-time mode.
    1 Hz Minimum Resolution Bandwidth (RBW) Provides high resolution to separate signals with close frequencies enabling easier signal identification for characterisation and advanced measurements.
    1 ms full span sweep Utilise the fast 1 millisecond full span sweep to quickly identify signals of interest.
    Up to 40 MHz of Real-Time BW View wider band and hopping signals at one time without additional setup making identifying and analysing signals of interest faster and easier.
    7.45 µs 100% POI Capture pulsed transmissions as short as 7.45 µs with guaranteed capture and power accuracy.
    7 Real-Time Visualization Modes Advanced debugging and analysis with combinations of Normal, Density, Spectrogram, and Power vs Time visualizations.
    Powerful triggering capabilities Identify specific signals of interest with frequency mask, power triggers, or use external triggers to time correlate digital signals for additional analysis.
    Tracking Generator and VSWR measurements Use the optional tracking generator to test cables, antennas, filters, and active components. VSWR automates standing wave parameter measurements.
    Flexible usability options Use front panel buttons, touch screen, keyboard/mouse/monitor, and PC based software control in any combination to match the application and customer preference.
    EMI filters and detectors Standard 6 dB filters for EMI bandwidths as well as Quasi Peak detectors.


    If you would like more information, arrange a demonstration or receive a quote for the Rigol RSA5000 Series; you can contact us via email, through our website or call us on 01582 764334.

    We also supply and service an extensive range of test & measurement products from our list of industry leading manufacturers. Click a manufacturer logo below to see their range of products on our website.

















  • The fastest streaming cameras are now shipping

    Record breaking Ximea cameras of the xiB-64 series: 3,500 fps at 1280 x 864 pixels.

    Awarded with the Vision Systems Design's Platinum Innovators Award, xiB-64 cameras are the fastest and highest performance streaming cameras on the market.

    Key features:

    • 3,500fps at full 1.1 Megapixel resolution
    • 64 Gbit/s PCIe Gen3 x 8 industry standard interface via copper or fibre cable
    • 1280 x 864 pixels, large area, high sensitivity 4/3" global shutter sensor
    • 60 x 70 x 57mm ultra-compact size
    • Fibre cable connection up to 100m

    Further information on our Machine Vision camera series click here.

    To speak with a sales/applications engineer please call 01582 764334 or click here to email

    Lambda Photometrics is a leading UK Distributor of Characterisation, Measurement and Analysis solutions with particular expertise in Electronic/Scientific and Analytical Instrumentation, Laser and Light based products, Optics, Electro-optic Testing, Spectroscopy, Machine Vision, Optical Metrology, Fibre Optics and Microscopy.

  • New 48 Megapixel xiB cameras from Ximea

    The smallest and fastest cameras with a CMV50000 sensor on the market. 20Gbit/s, single cable PCI Express interface. Direct connection without frame grabber.

    Key features at a glance:

    • 48 Megapixels at 30 fps
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    Further information on our Machine Vision camera series click here.

    To speak with a sales/applications engineer please call 01582 764334 or click here to email

    Lambda Photometrics is a leading UK Distributor of Characterisation, Measurement and Analysis solutions with particular expertise in Electronic/Scientific and Analytical Instrumentation, Laser and Light based products, Optics, Electro-optic Testing, Spectroscopy, Machine Vision, Optical Metrology, Fibre Optics and Microscopy.

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