Physical Principles of Electron Microscopy: An Introduction to TEM ...

76 Chapter 3
resolution (especially if the image is recorded by a camera over a period of
several seconds). But in order to view all possible regions of the specimen, it
is also necessary to move the specimen horizontally over a distance of up to
3 mm if necessary.
The design of the stage must also allow the specimen to be inserted into
the vacuum of the TEM column without introducing air. This is achieved by
inserting the specimen through an airlock, a small chamber into which the
specimen is placed initially and which can be evacuated before the specimen
enters the TEM column. Not surprisingly, the specimen stage and airlock are
the most mechanically complex and precision-machined parts of the TEM.
There are two basic designs of the specimen stage: side-entry and top-entry.
In a side-entry stage, the specimen is clamped (for example, by a
threaded ring) close to the end of a rod-shaped specimen holder and is
inserted horizontally through the airlock. The airlock-evacuation valve and a
high-vacuum valve (at the entrance to the TEM column) are activated by
rotation of the specimen holder about its long axis; see Fig. 3-11a.
One advantage of this side-entry design is that it is easy to arrange for
precision motion of the specimen. Translation in the horizontal plane (x and
y directions) and in the vertical (z) direction is often achieved by applying
the appropriate movement to an end-stop that makes contact with the pointed
end of the specimen holder. Specimen tilt (rotation to a desired orientation)
about the long axis of the rod is easily achieved by turning the outside end of
the specimen holder. Rotation about a perpendicular (horizontal or vertical)
axis can be arranged by mounting the specimen on a pivoted ring whose
orientation is changed by horizontal movement of a rod that runs along the
inside of the specimen holder. Precise tilting of the specimen is sometimes
required in order to examine the shape of certain features or to characterize
the nature of microscopic defects in a crystalline material.
A further advantage of the side-entry stage is that heating of a specimen
is easy to arrange, by installing a small heater at the end of the specimen
holder, with electrical leads running along the inside of the holder to a power
supply located outside the TEM. The ability to change the temperature of a
specimen allows structural changes in a material (such as phase transitions)
to be studied at the microscopic level.
Specimen cooling can also be achieved, by incorporating (inside the sideentry
holder) a heat-conducting metal rod whose outer end is immersed in
liquid nitrogen (at 77 K). If the temperature of a biological-tissue specimen
is lowered sufficiently below room temperature, the vapor pressure of ice
becomes low enough that the specimen can be maintained in a hydrated state
during its examination in the TEM.