Procurement of a Suitable Scanning Electron Microscope - Assignment Example

Procurement of a Suitable Scanning Electron Microscope (SEM) Electron Microscopy and SEM Electron microscopy relies on accelerated electron beams as probe and then detecting the generated response signals like different kinds of electrons and X-rays for image formation and extracting micro-chemistry and micro-crystallographic information. Electron microscopy has essentially been a leap jump from optical microscopy for investigation of characterization of materials. While resolution in case of optical microscope is limited to about 100 mm (a typical best possible value); electron microscopy stretched it down to sub-micrometers (most common SEM resolution) to even sub-nanometer level (a typical Transmission Electron Microscope (TEM) resolution). Not just that, electron microscopy was further augmented with suitable detectors to even extract quantitative micro-chemistry and micro-crystallographic information not only from the surfaces (in case of SEM) but also from within the bulk of the material in case of TEM. Enhancement in resolution could be possible because of much smaller value of the wavelength of the accelerated electrons (which are the probe in electron microscopy) as compared to that of the visible light (which is the probe in case of optical microscopy). Why electron microscopy could be expanded to provide much more information is because interaction of electrons with matter leads to generation of a variety of signals like different kinds of electrons and X-rays and these signals contain valuable information about not only the topography of the surface but also about chemistry of the material and orientation of the grains. Therefore, it becomes relevant to present a brief introduction to electron – matter interaction for better understanding of electron microscopy in general and SEM in particular. Electron – Matter Interaction When an electron beam strikes with matters it interacts with the electrons – outer shell as well as core electrons and nucleus of the matter. Depending on the energy and intensity of the incident electron beam and thickness of the material being exposed to the electron beam different kind of signals are generated. A schematic diagram showing a typical electron – matter interaction is presented in Figure 1. Figure 1: A typical Electron – Matter Interaction Electron beam can penetrate through the specimen only if its thickness is less than 100 nm and only then the transmitted signals are produced. Transmitted signals are used in Transmission Electron Microscope (TEM) and related analytical equipment like STEM (Scanning TEM), ATEM (Analytical TEM), HRTEM (High Resolution TEM) etc with attachments like EELS, HAADF etc. These signals are not relevant for an SEM and therefore, will not discuss about these signals. Reflected signals like Secondary Electrons (SE), Back Scattered Electrons (BSE) and X-rays are important for SEM. While SE and BSE are used for imaging, X-rays are used for micro-chemical evaluation. Besides, BSE is also used for orientation imaging and hence for micro-texture determination on the surface. Secondary Electrons These are low energy (less than 50 eV) electrons escaping from the specimen when it is struck with the probe electron beam). The origin of these electrons is not very clear, however, these could be the electrons from the primary or probe beam from end of their trajectory within the bulk of the matter [1] and therefore have lost most of their energy in multiple scattering events within the bulk of the matter. These electrons are in abundance and are most commonly used for image formation. These electrons provide topographic contrast as collection of these electrons depend on the local orientation of the surface i.e. topography of the surface. Backscattered Electrons These are high energy electrons. All the electrons with energy greater than 50 eV to right up to the primary or probe beam energy are termed as Backscattered Electrons”. The high energy backscattered electrons result from elastic scattering of the primary beam electrons with atoms of the specimen being probed. Yield of BSE is much smaller than that of SE. Also yield of BSE has direct correlation with atomic number or Z of the matter being probed. Therefore, BSE imaging provides Z-contrast; however it can also be manipulated to provide topographic contrast. Angular spread of the BSE around the original beam and therefore, detector for the BSE has to be arranged around the primary beam to be able to collect most of the BSE signal generated by the material. X-rays Whenever matter is exposed by electron beam it produces X-rays. Depending upon energy of the electron beam and the material being exposed to, two kinds of X-rays are produced – Bremsstrahlung X-rays or continuous X-rays and Fluorescent or Characteristic X-rays. Bremsstrahlung X-rays are produced due to deceleration of electrons in electric field. Characteristic or fluorescent X-rays on the other hand are produced by electronic transitions within the atoms being exposed by primary electron beam. When high energy electrons strike the sample, it knocks out core shell electrons and a vacancy is thus created. This vacancy is filled by electronic transition from a higher energy level shell. The difference in the energy is released in the form of an X-ray. This X-ray is characteristic of the atom it is coming from and therefore is used for microchemistry determination by using wavelength dispersive spectroscopy (WDS) or energy dispersive spectroscopy (EDS). Now that the signals relevant to SEM generated by electron – matter interaction are discussed it is relevant to understand configuration and functioning of a typical scanning electron microscope. Basic Configuration of an SEM A typical SEM consists of the following subcomponents or subsystems: (1) A cylindrical tubular structure that acts containment for all other important subsystems and is capable of withstanding high vacuum. (2) Vacuum system capable of producing and maintaining high vacuum level suitable for electron microscopes, typically of the order of 10-7 m torr. (3) Electron Source for producing high brightness illumination on the specimen (4) Condenser Lenses and Different Apertures (5) Scanning Coils (6) Specimen holder and CNC controlled stage for manipulation of the job being examined. (7) Different kinds of detectors and attachments like SE detector, BSE detector, WDS attachment, EDS attachment etc. (8) Computerized console for controlling the SEM, CRT screen to acquire the image, system for digital acquisition and storage of the image and other useful information like microchemistry, orientation image etc. A brief description of these subsystems is presented below. (1) Vacuum system Vacuum level of the order of 10-7 m torr is required for a good SEM. This is usually achieved by employing a combination of turbo molecular pump and rotary pump is series. The main column is under air lock i.e. there is double door for loading and unloading of the sample, so that vacuum level is maintained consistently in the main column, otherwise the electron source and the detectors (SE and BSE) may get spoiled. (2) Electron Source Tungsten filament has been used in conventional microscopes since invention of these machines. These filaments produce electrons by thermionic effect and these electrons are accelerated by application of high voltage applied to an anode plate. The applied voltage is in 20 – 60 kV for modern SEMs and this determines the energy of the primary electron beam. This voltage can be varied. Now a days table top SEMs have also come which use typically 5 – 20 kV accelerating voltage. While tungsten filament is rugged, cheap and have long life; these do not produce high brightness. Therefore, another filament was introduced, which is Lanthanum Hexaboride (LaB6) crystal. This filament produces much higher brightness as compared to the conventional tungsten filament, but requires high vacuum, is much costlier and have much shorter life. Further innovation in electron sources have led to field emission guns, where electronic emission is not due to thermionic effect rather due to application of high electric field. The benefit is that the resultant electron beam is much more monochromatic and the beam can be focused much more tightly to produce very high brightness. These are of two types – Cold FEG (Field Emission Gun) and Hot FEG. FEG is the most sought after electron source in modern times and we should also go for FEG source. (3) Condenser Lenses These are electromagnetic lenses and are used for focusing the electron beam onto the sample. (4) Scanning Coils These are electromagnetic coils imbedded in the condenser lens. These are used for scanning the focused electron beam over the specimen in a rectangular raster. The area of the scan and the rate of scan are controlled by controlling the flow of current through the coils. It is relevant to briefly discuss image formation and magnification in SEM. As the probe beam is scanned over a raster, the corresponding SE or BSE electrons are collected, amplified and scanned over a CRT screen in a raster in synchronized manner. Thus image (relative spatial electron intensity distribution) forms on the CRT screen. Thus the area scanned over the sample is magnified to the CRT screen. This is how magnification comes, which is nothing but ratio of the CRT screen size to the area being scanned on the specimen. As the size of the CRT screen is fixed, the magnification can be increased by reducing the raster size on the specimen. (5) Specimen Holder The specimen should have electrical contact with the specimen holder to prevent build up of electrons. Besides, there should be provision for controlled movement of the specimen stage so that many samples and large specimen can be viewed. Typically CNC travel of 200x200x50 mm is sufficient to accommodate all the specimen we intend to examine in our laboratory. (6) SE Detector The Everhart – Thornley detector is used for secondary electrons. It consists of (i) A collection electrode maintained at +250 V. (ii) A metal-coated scintillator element lying behind the collection electrode maintained at a voltage of +10,000 volts (iii) A light pipe connected to the back of scintillator leading to a photo multiplier tube (PMT). Secondary electron images provide topographic contrast and very useful for fractography and microstructural analysis. (7) BSE detectors BSE detectors are configured around the primary beam for better collection of low yielding BSE. There are many detectors for BSE. Some of important BSE detectors are the following. (i) Everhart-Thornley Detector for BSE (ii) Solid State BSE Detector (iii) Scintillator Type BSE Detector Solid state detectors are the best and we should go for the same for the SEM we intend to procure. (8) Image Acquisition and Storage Computerized system should be there for acquisition, recording and storage of images and other useful information. (9) Additional Attachments Besides, the basic configuration described above one can go for the following attachments (i) EDS or Energy Dispersive Spectroscopy (ii) WDS or Wavelength Dispersive Spectroscopy (iii) EBSD or Electron Backscattered Diffraction Among these attachments EDS is most suitable for our requirements and we must have this attachment in the SEM we proposed to procure. Final Recommendation There are many manufacturers of modern SEM and some of the important and reputed manufacturers are listed below. (1) Apexcorp [2] (2) Camscan [3] (3) EDAX [4] (4) FEI [5] (5) GATAN [6] (6) JEOL [7] (7) OMNIPROBE [8] (8) ZEISS [9] All these companies are reputed manufacturers of the kind of SEM we are looking for and we can do price negotiation with these companies or can invite open tender getting the best offer and then choose from these offers to go ahead with procurement of the SEM we are planning for. CONCLUSION A brief introduction of electron microscopy with special emphasis on SEM has been presented. A typical configuration of a basic but modern SEM has also been briefly outlined and it has been suggested that an SEM with FEG electron source, 200x200x50 mm CNC stage and EDS attachment should be procured. It has also been suggested that the price discovery can be either through negotiations prior to or post invitation of open tender for the suggested SEM. References [1] Goodhew P. J. and Humphreys F. J. “Electron Microscopy and Analysis”, Taylor & Francis, London, New York, Philadelphia, 1988 p. 34. [2] www.aspexcorp.com [3] www.camscan.co.uk [4] www.edax.com [5] www.fei.com [6] www.gatan.com [7] www.jeol.com [8] www.omniprobe.com [9] www.zeiss.com Read More