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SCANNING ELECTRON MICROSCOPEAlper Turhan, Murat Erçelik, Ozan Arslan

Metallurgical and Materials Engineering Dokuz Eylül University

Scanning Electron Microscope

SEM Images Resolution and Contrast Enhancement

Basic SEM Operating Principles

•The SEM can provide a high-magnification image of the surface of a material that is very similar to what would actually be seen if the surface could be viewed by the unaided eye.This feature helps to simplify the interpretation of images.In addition to topographical information, SEMs can provide details of chemical composition of near-surface regions of the specimen.The SEM’s resolution can approach a few nanometers, and it can operate over an easily adjustable magnification range of about 10X to 300.000X.

•The main components of the scanning electron microscope include the electron gun, probe-forming column, electron dedectors, and vacuum system •The electron gun produces a narrowly divergent beam of electrons directed down the centerline of the column.The electron source is a tungsten filament that acts as a cathode.It is 0.25 mm in diameter and is heatedto approximately 2500°C (4530°F).The electrons essentially boil off (thermionic emission)the sharply bent tip of the filament and are attracted to the anode, which consists of a grounded plate with a hole to let the electrons pass.The anode is maintened at a positive voltage relative to the flament, ranging from 5 to 30 kV.This voltage, controlled by the operator, is generally held at 20 kV, but it can be varied for X-Ray and structure analysis.

•A third electrode (the alkaline oxide coated Wehnelt cylinder) has a negative bias of a few hundered volts relative to the cathode.It is introduced to limit the emitting area to the tip of the tungsten filament.•The electron gun and the column must be evacuated to avoid damage to the electron source and high voltage breakdown in the gun.High vacuum is also necessary to minimize scattering of the electrons during their travel from the gun to the specimen surface.•Lens system: Magnetic electron lenses focus the image of the electron source to the final spot size on the sample surface.In most cases, three lenses are needed.Apartures are placed between the lenses to limit the beam diameter. Two sets of scanning coils are placed in the bore of the objective lens to scan the electron beam across the sample. The electron beam scans the the sample in much the same way as a cathode ray tube for image formation on a television screen, and the output of the electron dedectors is displayed on the screen of a CRT.In modern instruments, the analog scanning systems are replaced by digital systems in which the movement of the beam are controlled with a computer.The analog signals from the electron dedectors is digitized and stored as a number for each pixel.

•An SEM produces three principal types of images: secondary electron images, backscattered images, and elemental X-Ray maps.•Secondary and backscattered electrons are produced by different mechanisms, and are conventionally distinguished by their energies. When a high energy primary electron interacts with an atom, it undergoes either inelastic scattering with atomic electrons or elastic scattering with theatomic nucleus.In an inelastic collision, some energy is transfered to the other electron.If the amount of energy transferred is very small, the emitted electron will probably not be energetic enough to exit the surface.However, if the energy transferred exceeds the work function, the emmited electron can exit the solid.

•Secondary Elecrons: Emitted electrons having energies of less than about 50eV are,by convention, called secondary electrons.Most of the emitted secondaries are produced within the first few nanometers of the surface. Secondary electrons are used for showing morphology and topography on samples.•Backscattered Electrons: Backscattered electrons are those that exit the specimen with an energy greater then 50 eV. These higher energy electrons are primary electrons that have been scattered elastically by the nucleus of an atom. Backscatter electrons are used to determine crystal structures and orientations of minerals.•X-Rays: An additional electron iteraction of major importance in the SEM occurs when a primary electron collides with a specimen atom and causes the ejection of a core electron.The excited atom will decay to its ground state by emitting either a characteristic X- ray photon or an Auger electron.X – Rays are used for elemental analysis.

•Each component of a scanning electron microscope has its own characteristics of resolution and noise, which determine image quality. Most instrument parameters are fixed by the designer to achieve maximum performance for the network of components. The operator must know the details only in so far as the instrument allows adjustment of component parameters to optimize image quality for a specific specimen and purpose of investigation. For example, the signal-tonoise ratio of the secondary electrons increases with increasing intensity of the primary beam, with increasing electron yield, and with increasing scanning time. The current of the condenser lenses controlling the spot size, the acceleration voltage, and the scanning time can be set by the operator. Electron yield is determined by the specimen and most other factors by optimum working conditions prescribed by the instrument.

Fig. 4:BE image (acceleration voltage 25 kV) of a plane section showing strong material contrast between the tungsten carbide (light) and the cobalt binder phase (black).

Fig. 5 : SE image of the fracture surface in a fracture toughness test specimen; strongtopographic contrast.

Fig. 6 : X- Ray Maps Fig.7 : Carbon Nanotubes

References

1) ASM Handbook Volume 9 Metallography and Microstructure p 131-149

2) Jeffery B. Bindell AT and T Laboratories Allentown ,Pa ,Advanced Materials & Processes p 20-30 3) David C. Bell Center for Imaging and Mesoscale Structures

Fig.1 : Scanning Electron MicroscopeFig. 2 : Basic Design of the Scanning Electron Microscope

Fig. 3 : Types of electron-beam-excited electrons and radiation used in SEM and the depth of the region below the specimen surface from which information is obtained.

Table 1. Physical effects producing radiation, detector types, and detected signals used for imaging and analyzing metal surfaces in thescanning electron microscope