Transmission Electron Microscopy Dr Heath Bagshaw CMA
[email protected] Slide 2 What is the difference between SEM and TEM?
Scanning Electron Microscope Transmission Electron Microscope In an
SEM, electrons are scanned over the surface of the sample. In a
TEM, electrons are transmitted through the sample. Slide 3
Electrons pass through a (very thin) sample (i.e. are transmitted)
to form an image. Simplistically, In its operation a TEM can be
thought of as analogous to a slide projector:- What is a
Transmission Electron Microscope (TEM)? slide Objective lens
Condenser lens Light source Slide projector Projection screen
Fluorescent screen Objective lensCondenser lens Specimen
(thin)aperture Electron source Electron beam TEM Slide 4 What is a
Transmission Electron Microscope (TEM) cont. In a slide projector
white light from the bulb is focussed onto the slide by a condenser
lens. This produces an image which is focussed by the objective
lens to a point (crossover). The image is then projected onto a
screen and magnified by varying the distance from the crossover to
the screen. In a TEM the electron beam is focussed on the sample
using the condenser lens system. This produces an image which is
focussed by the objective lens to a point (crossover). This image
is then magnified by a series of projector lenses to vary the size
of the image on a fluorescent screen. Changing the current of an
electromagnetic lens alters its focal length altering
magnification. Slide 5 TEMs have basically the same design
independent of manufacturer or age:- Anatomy of a TEM Apertures
Viewing Screen Electron Gun Condenser lens system Apertures Sample
Viewing Screen a)b)c) Examples of Transmission Electron
Microscopes, a)Philips EM420 (1980s), b) Cut through Schematic of
Philips CM 200 (1990s) and c) JEOL 2100 (2000s) JEOL 2100 has
digital imaging and computer assisted operation. Slide 6 main
components of a TEM The TEM can be broken down into a few main
components, these are:- The Gun which produces electrons. The
condenser system which forms the probe. The sample sample
preparation is important, and time consuming. Image formation use
of image plane or back focal plane. Projection of the image
(magnification). Viewing and recording the image. Lets start by
looking at how electrons are produced. Slide 7 Electron Guns Two
main types of gun Thermionic and Field Emission. Thermionic sources
produce electrons when heated. Field emission sources produce
electrons when exposed to an intense electric field. FEGs give much
more brightness than thermionic systems. FEGs give a more
monochromatic electron source and finer probe (i.e. better
resolution). Comparison of the three types of source operating at
100kV Slide 8 The Condenser System The Wehnelt (or 2 nd anode in a
FEG) focuses the beam to a crossover which is accelerated down the
column. The first condenser de-magnifies the crossover to give a
smaller point source this is referred to as C 1 or spot size. The
second condenser lens (C2) is used to either converge or spread the
beam of illumination on the sample (intensity (Philips) or
brightness (JEOL)). A condenser aperture is placed in the beam path
to remove electrons far from the optic axis which would reduce
resolution. The smaller the aperture the better the resolution, but
there is an associated decrease in brightness need to compromise.
Slide 9 The Sample Samples are typically 3mm in diameter and
