Image: Variable Pressure SEM

Variable Pressure SEM

Scanning Electron Microscope (SEM)

Zeiss EVO LS 15 SEM: Variable Pressure SEM

The Variable pressure SEM allows the examination of surfaces of almost any specimen, wet or dry, insulating, uncoated and untreated because the environment around the specimen no longer has to be at high vacuum. The environment can be water vapour or other gases in the pressure range from 10 to 3,000 Pa.
When the Pressure Limiting Apertures are fitted in the final aperture of the column, a controllable amount of gas molecules, usually air, can be introduced into the specimen chamber whist maintaining a good vacuum in the electron optical column and the electron gun. Collisions between the high energy electrons of the beam and the gas molecules will cause the gas to ionise. The positive ions that are produced will neutralise the negative charge that has formed on the surface of an insulating material and will enable disturbance free images of the specimen to be obtained. The secondary electrons (SE) and backscattered electrons (BSE) passing through the gas will interact with the molecules and cause photos of light to be emitted. These photons can be collected by a VPSE detector or a BSE detector to give an image of the surface of the specimen similar to that produced by SE and BSE detectors under high vacuum.
The introduction of water into the chamber slows down the dehydration of specimens containing water and helps maintain the shape of the specimen.

Martin Pueschel
© Australian Museum

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