TABLE OF CONTENTS As we saw in Chapter 1, the TEM is capable of displaying magnified images of a thin specimen, typically with a magnification in the range 10 3 to 10 6. In addition, the instrument can be used to produce electron-diffraction patterns, useful for analyzing the properties of a crystalline specimen. This overall flexibility is achieved with an electron-optical system containing an electron gun (which produces the beam of electrons) and several magnetic lenses, stacked vertically to form a lens column. It is convenient to divide the instrument into three sections, which we first define and then study in some detail separately.
The illumination system comprises the electron gun, together with two or more condenser lenses that focus the electrons onto the specimen. Its design and operation determine the diameter of the electron beam (often called the "illumination") at the specimen and the intensity level in the final TEM image.
The specimen stage allows specimens to either be held stationary or else intentionally moved, and also inserted or withdrawn from the TEM. The mechanical stability of the specimen stage is an important factor that determines the spatial resolution of the TEM image.
The imaging system contains at least three lenses that together produce a magnified image (or a diffraction pattern) of the specimen on a fluorescent screen, on photographic film, or on the monitor screen of an electronic camera system. How this imaging system is operated determines the magnification of the TEM image, while...
