Scanning Electron Microscope - Essay Example

?Scanning Electron Microscope (SEM A Diverse Analytical Instrument Introduction Scanning electron microscope (SEM) is among the most versatile analytical instruments available to materials scientists. This single instrument can perform multiple analyses of differing characteristics like surface imaging at varying magnifications, micro-chemical analysis and micro-texture analysis depending upon the attachments on the detector side. This microscope uses a focused beam of electron as a probe and detects and analyses different kind of responses like secondary electrons, back scattered electrons, X-rays etc. generated by the material. This microscope started its journey as an instrument that done away with limited resolving power of optical microscopes and has evolved considerably over years to become an instrument of versatile capabilities. Today, an SEM has become an indispensible instrument for any R&D or production set up dealing with advanced materials science. In the subsequent sections interaction of electrons with matter, basic principles of electron microscopy, architecture and working of scanning electron microscope, different attachments to an SEM and applications of SEM in materials science will be discussed in brief with special emphasis on specifications of a modern SEM. Interaction of Electrons with Matter Electrons as a probe are extremely versatile as they generate a wide range of signals which can be detected and processed to get useful and meaningful insight about surface topography, microstructure, microchemistry and micro-texture of the material being probed. Different kinds of signal generated as a result of interaction of electron probe are shown in Fig. 1 [1]. Fig. 1: Schematic Drawing Showing Electron – Matter Interaction When thickness of the specimen is less than ~ 100 nm only then the incident electron beam is able be pass through it and generate different kind of transmitted signals. However, the transmitted signals are relevant for Transmission Electron Microscope (TEM) and not for Scanning Electron Microscope (SEM); therefore, we will not discuss about transmitted signals here. Among the reflected signals secondary electrons (SE) and Back Scattered Electrons (BSE) are relevant for SEM for imaging and characteristic X-rays are useful for chemical analysis in SEM. Besides, BSE is also useful in micro-texture analysis using Electron Back Scattered Diffraction (EBSD) attachment. Therefore, we will limit our focus to these signals only. Secondary Electrons (SE) These are low energy electrons (less than 50 eV) generated after the incident beam hits the substrate. Back Scattered Electrons (BSE) These are high energy electrons (more than 50 eV up to beam energy) generated after the incident beam hits the substrate. Characteristic X-Rays When electron beam strikes the specimen, it knocks out the inner shell electrons and the vacancy thus created is immediately filled by an electron from higher shells. This electronic transition leads to generation of X-rays which are characteristics of the element. Thus these X-rays can be used for micro-chemical analysis using Energy Dispersive Spectroscopy (EDS) and / or Wavelength Dispersive Spectroscopy (WDS). Basic Architecture of SEM Basic architecture of an SEM is presented in Fig. 2 [2]. It is a column always under vacuum consisting of different subcomponents. It consists of an electron gun or electron source. This can be different types, which will be discussed afterwards. This is followed by condenser lenses to focus the beam. There are apertures in the path to allow only and an aperture to allow only the useful (central) portion of the beam to the subsequent stages. The focused beam is double scanned and made to pass through an aperture to fall onto the specimen. The beam is scanned over the specimen in a raster and the generated signals – secondary or back scattered electrons are collected, amplified and again scanned in a raster onto a CRT screen in synchronized manner. Thus image is formed pixel by pixel. The magnification is ratio of the CRT screen size to the area of the raster onto the specimen. Because, the CRT size is fixed, therefore, magnification can be increased by scanning lower area onto the specimen and vice – versa. Different components of an SEM will be discussed in the subsequent sections and during this discussion specifications of our SEM will evolve. Fig. 2: A Schematic Diagram of Scanning Electron Microscope Different Components of an SEM 1. Electron Gun or Electron Source This is very important component of an SEM. This determines the quality of the probe beam and therefore, the overall quality of the instrument. It consists of a cathode or a filament, which acts as electron source, a wehnet to restrict the beam size and an anode to pull or extract the electrons emitted from the filament as shown in Fig. 3 [3]. Traditionally, a tungsten filament has been used as electron source. This works on the principle of thermionic emission i.e. when the filament is heated, electrons are ejected from it. In other words the thermal energy supplies the work function to the electrons. Why tungsten has been used is because of its lower work function and higher melting point it can give sizable electron emission and hence sizeable beam current to provide sufficient yield of the response signals like SE, BSE or characteristic X-rays. Fig. 3: A Schematic Diagram of an Electron Gun Although, tungsten filament is a rugged and cheap electron source, the beam quality of a tungsten cathode is not great and with time necessity of better quality electron source was felt and so came other sources like Lanthanum Hexaboride or LaB6, and hot and cold field emission guns (FEG) respectively. These sources offer narrower energy spectrum of the electron beam and better focussability. Therefore, these beams could be focused onto a smaller spot and still generate strong response signal facilitating sharper image and better accuracy and resolution in micro-chemical analysis. Now high end SEMs come with either cold or hot FEG sources and it is recommended that we buy and SEM with hot FEG electron source. 2. Accelerating Voltage An accelerating voltage is used between the electron source and the anode plate. This value is typically in 20 kV (lower end) to 60 kV (higher end). This voltage determines energy of the beam and hence the interaction volume and strength of the response signals. While lower voltage allows smaller interaction volume and hence better spatial resolution, the signal strength may not be very good. The reverse is true for higher accelerating voltage. Typically SEMs come with variable accelerating voltage in 20 – 60 kV rang and we should go for such an SEM. 3. Lenses Lenses are required to focus the probe beam. In case of an optical microscope lenses made of glass or quartz do the work. However, for electron microscopes, we need either electrostatic or magnetic lenses. Electrostatic lenses employ electrostatic field to constrict the electron beam. Similarly, magnetic lenses employ magnetic field to do the same. The magnetic field is produced by means of an electromagnet. Force on an electron by magnetic field is proportional to its speed and therefore, to take benefit of higher electron velocity, electromagnetic lenses are invariably employed in SEM except in low kV systems. For our SEM also, we will get SEM with electromagnetic lenses only. 4. Scan Coils These are electromagnetic coils located within condenser lens. By flowing current through these coils deflection is produced in the electron beam. One can control the area of scan by controlling the current through the deflection coil. Similarly scan rate can also be controlled. 5. Secondary Electron Detector Secondary Electrons are very important signal as far as imaging in SEM is concerned. These are low energy electron (less than 50 eV). These are detected by “Everhart-Thornley Detector” (Fig. 4) [4]. Fig. 4: A Schematic Diagram of the Everhart-Thornley Detector This detector was invented by Everhart and Thornley in 1960. This is used for detection of secondary electrons in SEM and every SEM uses the same detector. This 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. 6. Back Scattered Electron Detector Back Scattered electrons are high energy electrons (energy more than 50 eV up to beam energy). These are result of elastic scattering with the atoms and yield of BSE is more with higher atomic number (Z) and vice – versa. Therefore, BSE images produce Z-contrast. These electrons are scattered at low angles with respect to the incident beam and therefore, the detector in annular in design around the incident beam to be able to collect most of the back scattered electrons. Different types of detectors are available for detection of Back scattered electrons. (i) Everhart-Thornley Detector for BSE (ii) Solid State BSE Detector (iii) Scintillator Type BSE Detector Among these detectors, solid state detectors are most versatile and produce good quality images in compositional mode as well as in topography mode. This detector is the most obvious choice for modern SEMs and we will go for Solid State Detector for BSE. 7. Image Recording System The image that we see on the screen should be recorded for further analysis and as archive. There should be provision to record these images electronically so that the same can be stored in a computer. Normally all the new SEMs now come with electronic recording of the image. Other Useful Attachments 1. Energy Dispersive Spectroscopy (EDS) This attachment uses energy of the characteristic X-rays to identify the source atom. The characteristic X-ray signals coming from the sample are dispersed as per the energy and the elements are identifies on the basis of energy of the characteristic X-rays, while intensity of the same is used for quantification. EDS systems are not considered as accurate as WDS or Wavelength Dispersive Spectroscopy; however, the analysis is fast as is useful to serve many purpose. This attachment is very useful with an SEM and it must be in the SEM we are proposing to procure. 2. Wavelength Dispersive Spectroscopy This attachment analyses the characteristic X-rays by measuring the wavelength using Bragg reflection of the same through a known crystal. This is very accurate, however, very slow as well. Besides, this attachment is specific to electron probe micro analysis (EPMA) system. Besides, for our intended applications the accuracy requirements is not so stringent and is good enough to be met by the EDS system. Hence, I recommend not going for this attachment. 3. Electron Back Scattering Diffraction This attachment records back scattered diffraction pattern in the form of Kekuchi lines and thus helps in determining micro-texture of the specimen. This is a very slow process and is not at all useful for us. Therefore, I recommend no going for this attachment as well. Final Specifications: An SEM with the following (i) A Hot Field Emission Gun (Hot FEG) (ii) Accelerating Voltage in 20 – 60 kV range (iii) The Everhart – Thornley Detector for SE (iv) A Solid State Detector for BSE Detection with 4 Sectors (v) An EDS Attachment with ultrathin window to allow detection of light elements (vi) A CNC stage with 200x200x50 mm travel to accommodate and analyze sufficiently large specimen. (vii) A digital recording and storage system for images and micro-chemical analysis results. Summary The basic principles and architecture of a modern SEM system has been presented and specifications have been laid out for procurement of such a system. As there are many suppliers for such a modern SEM unit, it is recommended that a global tender is raised for this instrument and then the offers should be scrutinized for technical and commercial aspects of it to decide on the best SEM we should go for at the best possible price. References: [1] Hesseler-Wayser A., “Intensive SEM/TEM Training: Electron Matter Interaction”, 2009, Retrieved on January 15, 2011 from http://cime.epfl.ch/files/content/sites/cime2/files/shared/Files/Teaching/Doctoral%20School%202009/Chapter%201%20-%20Interaction%20of%20electrons%20with%20matter.pdf [2] Hesseler-Wayser A., “Intensive SEM/TEM Training: SEM”, 2009, Retrieved on January 15, 2011 from http://cime.epfl.ch/files/content/sites/cime2/files/shared/Files/Teaching/Doctoral%20School%202009/Chapter%201%20-%20Interaction%20of%20electrons%20with%20matter.pdf [3] http://aspexcorp.com/products/whatissem/thesource.html [4] http://aspexcorp.com/products/whatissem/sed.html Read More