Scanning electron microscopy

Scanning Electron Microscope

  File:SEM image.jpg

A scanning electron microscope in use

Classification
Other names
Classification
Hornbostel–Sachs classification
Inventor(s)
Developed
Timbre
Volume
Attack
Decay

Scanning Electron Microscopy (SEM) is a type of electron microscopy that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that contain information about the sample's surface topography and composition.

Principle of Operation

SEM operates by scanning a focused electron beam across the surface of a specimen. The electrons in the beam interact with the atoms at or near the surface of the sample, producing signals that are collected and used to form an image.

Electron Beam

The electron beam is generated by an electron gun, which typically uses a tungsten filament or a field emission source. The beam is focused using electromagnetic lenses and scanned across the sample in a raster pattern.

Signal Detection

The primary signals produced by the interaction of the electron beam with the sample are:

• Secondary Electrons (SE): These are low-energy electrons ejected from the surface of the sample. They provide information about the surface topography.
• Backscattered Electrons (BSE): These are high-energy electrons that are reflected back from the sample. They provide information about the composition of the sample.
• X-rays: Characteristic X-rays are emitted when the electron beam displaces an inner shell electron, causing a higher-energy electron to fill the vacancy. This process provides elemental composition information.

Sample Preparation

Samples for SEM must be prepared to withstand the high vacuum environment and the electron beam. Common preparation techniques include:

• Fixation: Biological samples are fixed using chemical fixatives to preserve their structure.
• Dehydration: Samples are dehydrated to remove water, which can interfere with imaging.
• Coating: Non-conductive samples are often coated with a thin layer of conductive material, such as gold or carbon, to prevent charging under the electron beam.

Imaging and Resolution

SEM can achieve high resolution, typically on the order of 1 to 10 nanometers. The resolution is determined by the size of the electron beam and the interaction volume of the electrons in the sample.

Magnification

SEM can achieve magnifications from 10x to over 500,000x, allowing for detailed examination of surface structures.

Depth of Field

One of the advantages of SEM is its large depth of field, which allows for a greater portion of the sample to be in focus at the same time compared to optical microscopy.

Applications

SEM is used in a wide range of fields, including:

• Materials Science: For studying the surface structure and composition of materials.
• Biology: For examining the surface morphology of cells and tissues.
• Forensics: For analyzing trace evidence and materials.
• Semiconductor Industry: For inspecting and characterizing microelectronic devices.

Advantages and Limitations

Advantages

• High resolution and magnification.
• Large depth of field.
• Ability to analyze surface composition.

Limitations

• Requires vacuum environment.
• Sample preparation can be complex.
• Non-conductive samples require coating.

History

The development of SEM began in the 1930s, with the first practical instrument being developed by Manfred von Ardenne in 1937. The technology has since evolved significantly, with modern SEMs offering advanced imaging capabilities and analytical functions.

See Also

• Transmission Electron Microscopy
• Atomic Force Microscopy
• X-ray Spectroscopy

External Links

• MicroscopyU
• Microscopy Society of America