Surface Analytical Instrumentation Technique

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Chapter 5 : Surface Analytical Instrumentation Technique

Atomic Force Microscopy arrow_upward

  • Atomic Force Microscopy (AFM) is a popular method used by many industrial R&D sectors to measure surface topography and material properties.
  • It is used to measure the material properties with spatial resolutions of 5-20 nm and height dimensions in the range from 100 nm down to approx. 5 nm and exceptionally to 0.2 nm.

  • How does AFM work? arrow_upward

  • AFM works by bringing an atomically sharp tip close to a surface.
  • There is an attractive force between the tip and the surface and this force is kept the same throughout the experiment.
  • The force is detected by a deflection of a spring, usually a cantilever.
  • Forces between the probe tip and the sample are sensed to control the distance between the tip and the sample.
  • There are two modes:
    • Contact Mode (Repulsive force)
    • Non-contact Mode (Attractive force)
  • At short probe-sample distances, the forces are repulsive.
  • At large probe-sample distances, the forces are attractive.

  • The AFM cantilever can be used to measure both attractive force and repulsive forces.
  • The cantilever is designed with a very low spring constant (easy to bend) so it is very sensitive to force.
  • Since all this is going on at a very small scale, we can't watch the tip directly.
  • A laser is pointed at the tip and is reflect off the cantilever and onto the sensor.
  • As the tip goes up and down the laser hits different parts of the sensor.
  • With the information the sensor collects, an image of the surface can be recreated.
  • Tip moves across a sample; as the tip moves up and down over the surface, a laser detector records the height.
  • The tip passes back and forth in a straight line across the sample.
  • A topographic image is built up by the computer by recording the vertical position as the tip is raster across the sample.

  • Electron Microscopy arrow_upward

  • An Electron Microscope uses a particle beam of electrons to illuminate the specimen and produce a magnified image.
  • Electron Microscope has a greater resolving power than a light-powered
  • optical microscope.
  • This is due to electron have wavelengths about 100,000 times shorter than visible light (photons) and can achieve better than 0.2nm resolution and magnification of up to 2,000,000x.

  • Types of Electron Microscope arrow_upward

  • Transmission electron microscope,
  • Scanning electron microscope.

  • Transmission Electron Microscopy arrow_upward

  • Transmission Electron Microscopy is an imaging technique whereby a beam of electrons is focused onto a specimen causing an enlarged version to appear on a fluorescent screen or layer of photographic film.
  • A schematic diagram of Transmission electron microscope is shown in the figure below:
  • In TEM electrons are emitted by an electron gun.
  • The electron beam is accelerated by an anode, focused by electrostatic and electromagnetic lenses, and transmitted through the specimen.
  • When it emerges from the specimen,
  • the electron beam carries information about the structure of the specimen that
  • is magnified by the objective lens
  • system of the microscope.
  • Series of electromagnetic lenses (Objective, Intermediate, and Projector) act to illuminate the specimen and focus specimen on the fluorescent screen.

  • Scanning Electron Microscopy arrow_upward

  • In SEM, images are produced by probing the specimen with a focused electron beam that is scanned across a rectangular area of the specimen.
  • SEM is done by scanning electron microscope which is shown in the figure below:
  • Step-1
  • The virtual source at the top represents the electron gun, producing a stream of monochromatic electrons.
  • Step-2
  • The stream is condensed by the first condenser lens.
  • This lens is used for both forms the beam and limit the amount of current in the beam.
  • It works in conjunction with the condenser aperture to eliminate the high-angle electrons from the beam.
  • Step-3
  • The beam is then constricted by the condenser aperture, eliminating some high-angle electrons.
  • Step-4
  • The second condenser lens forms the electrons into a thin, tight, coherent beam and is usually controlled by the fine probe current knob.
  • Step-5
  • A user selectable objective aperture further eliminates high-angle electrons from the beam.
  • Step-6
  • A set of coils then scans or sweep the beam in a grid fashion dwelling on points for a period of time determined by the scan speed.
  • Step-7
  • The final lens, the objective, focuses the scanning beam onto the part of the specimen desired.
  • Step-8
  • When the beam strikes the sample interactions occur inside the sample and are detected with various instruments.
  • Step-9
  • Before the beam moves to its next dwell points, these instruments count the number of electron interactions and display a pixel on a CRT whose intensity is determined by the number.
  • Step-10
  • This process is repeated until the grid scan is finished and then repeated, the entire pattern can be scanned 30 times/sec.

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