Electron Microscopy

 

 

•         EM (103X to 106X) are developed due to the limitations of Light Microscopes (500X to 1000X).

•         The Transition Electron Microscope (TEM) was the first type of EM developed in early 1930’s.

•         The EM functions exactly as their optical counter part except that they use focused beam of electrons instead of light.

 

Major Types

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The Transmission Electron Microscope (TEM)

•         The TEM works much like a slide projector.

•         It passes light/electron through the slide/specimen.

•         The light/electron passes through it is effected by the structures and objects of slide/specimen.

•         Certain part of light/electron passes through the slide/specimen.

•         This transmitted light/electrons are then projected on the viewing/Phosphor screen, forming an enlarge image.

Block Diagram

Mechanics of TEM

•         The Virtual Source at the top (Electron Gun) produce a stream of monochromatic electrons.

•         The Condenser lenses 1and 2 make the beam small,thin and coherent. It also controlled the spot size of electron beam.

•          The beam strikes the specimen and part of which passes through the sample.

•         The transmitted portion is focused by the Objective lens into an image

•         The image is passed down the column through the intermediate and projector lenses, being enlarged all the way.

•         The image strikes the phosphor screen giving light and real image.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Electron Source (Gun)

•         The Filament is heated to get a stream of electrons.

•         A negative potential (~500 V) is applied to the Whenelt Cap to move electrons to the optical axis.

•         A collection of electrons occurs in the space between the filament tip and Whenelt Cap. This collection is called a Space charge.

•         Due to the positive charge at Anode the electrons from space charge exit the gun area through the small (<1 mm) hole in the Whenelt Cap.

•         These electrons move down the column to be later used in imaging.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Beam Specimen Interaction

 

Application

•         Morphology

   To analyze the size, shape and arrangement of the particles which make up the specimen as well as their relationship to each other on the scale of atomic diameters.

•         Crystallographic Information

                                                                                                                           The arrangement of atoms in the specimen and their degree of order, detection of atomic-scale defects in areas a few nanometers in diameter

•         Compositional Information (if so equipped)

                                                                                                                           The elements and compounds the sample is composed of and their relative ratios, in areas a few nanometers in diameter

The Scanning  Electron Microscope (SEM)

•         SEM works much like the Television in sense that it moves the focused beam of electrons by deflection coils so that a certain region of specimen  can be viewed.

•         It uses Detectors to form an Image by collecting  the  Back Scattered Electrons (BSE), Secondary Electrons (SE) and X-rays emitted from the specimen.

•         In SEM the specimen is made conductive by coating with  a thin layer of heavy metals( usually gold).

•         It uses relatively low accelerating voltages of 15kv to 40kv then TEM.

•         The power of resolution is also smaller but the depth of focus is several orders of magnitude greater. It is there for best suited for low  magnifications.

 

 

Block Diagram of SEM

 

 

 

•         Electron Gun produces beam of monochromatic electrons.

•         First condenser lens forms beam and limits current (Coarse)

•         Condenser aperture eliminates high angle electrons.

•         Second condenser lens forms thinner and coherent beam(fine)

•         Objective aperture further eliminates the high angle electrons from beam.

 

 

 

 

 

 

 

                                                                                                                           

•         Beam Scanned by deflection coils to form image.

•         Final objective lens focuses beam on to specimen.

•         Beam interacts with sample and outgoing electrons are detected.

•         Detector counts electrons at given location and displays intensity.

•         Process repeated until scan is finished(usually 30 frames/Sec)

 

 

 

 

Beam Specimen Interaction

•         Backscattered electrons are caused by colliding incident electrons with the sample atom nucleus and scattering backward (180degree), giving higher atomic number brighter then lower atomic number.

•          Secondary electrons are caused by passing incident electron near sample atom and ionizing its electron, which leaves sample with very small K.E(5ev). It gives topographic information.

•         X-rays are caused by de-energizing of sample atom after a secondary electron is emitted, giving quantitative information.

SEM Detectors

Ø      The Everhart-Thornley detector (ET)

            Most Common detector for SE and BSE.

Ø      Backscattered Detector (BSE)

            Scintillator, Silicon diode, Channel plates

Ø      X-ray Detector

                        Energy Dispersive X-ray Detector  (EDX)

                        Wavelength Dispersive X-ray Detector (WDX).

Ø      Sample Current Detector

                        It is simply a sensitive ammeter which measures the current passing from Sample stage to ground

Ø      Cathodoluminescence Detector

                        Sensitive to Photons emitted form specimen