Transmission Electron Microscope

Characterization of Materials

Dr M Israr Qadir

TEM is most versatile tool for microstructural investigation TEM is widely used in industry and academia

Primary objective to investigate the microstructure and composition of semicondrs, alloys, and biological samples

At very high magnification 50x – 150,000x and more

A resolution of 1 Angstrum can be optimized under optimum conditions

1. Alignment of electro-optical system

2. Stability in accelerating voltage

3. Cleanness of the microscope

History In 1924, A French scientist de Broglie “cleared the path”

Electrons can behaves like waves

First TEM was built in 1931 by a German Ernst Ruska.

Principle Working principle of TEM is similar to Optical microscope

Having slight differences such as

1. Photons are replaced by electrons

2. The light source is an electron gun instead of light bulb

3. The lenses are magnetic as opposed to glass

Principle A ray of electrons is generated by a filament

Focused by a illumination lens system on the specimen

Electrons penetrate the specimen “imaging lens systems” are responsible to generate the

images

Pre-requisites Pressure should be very low to reduce the interference

Sample must be very thin, so that the electrons can penetrate the sample

Nanoparticles can be imaged as it is, however, we need cutting, etching and polishing for bulk materials

Price of the microscope is on the order of 1-4M US $

Spherical aberration Spherical aberration means it defocuses the electrons which

are scatter with different angles from source

Unfortunately this is inherent problem of electro-optical lens

New microscopes are equipped with SA corrector

With per piece cost of 1 M € and is complicated in operation

Operator change the focus where the resolution is best

Chromatic aberration Reflects the inability of a single lens to focus

Reason lies with difference in energy of the scattered electron results in variation in energy

The result is an image with slight defocusing.

In early years, chromatic was most damaging aberration