Microanalysis
X-ray signals are generated from a micro volume size specimen of material, 0.2 nm2 x 50-100 nm thick, and provide valuable information about the elemental composition of the specimen. X-rays are formed when the high-energy beam electrons eject inner shell electrons from the atom. An electron from an outer shell fills up the vacancy and the difference in its binding energy at two different orbits is emitted as an X-ray photon. Each atom has a unique X-ray spectral "fingerprint" comprised of emission lines. These lines can be identified with already published emission line tables for all elements. Software programs are also available and widely used to help with emission line identification. There are two main differences between X-ray signals collected on a TEM and those from a traditional SEM: 1) because the TEM specimen is much thinner than a standard SEM sample, the X-ray signal generated from a TEM specimen is much higher in resolution; 2) there is no fluorescence factor to consider when using a TEM for analysis.
Collecting X-rays is more difficult than collecting SEs and BSEs. Because of the specimen chamber configuration, an Energy Dispersive Spectrometer (EDS), which is a semi-conductor based detector, is the only choice of X-ray detection systems. Physically, the EDS detector has a shaft that penetrates the TEM chamber wall to place the sensor very close to the specimen. It is easily recognizable with its liquid nitrogen tank. Liquid nitrogen is used to reduce electronic noise in the detected X-ray signal. This type of detector senses the entire spectrum of X-ray signals of varying energy from the specimen at once. A disadvantage is that it is not sensitive enough to detect trace amounts of elements in the specimen. Another disadvantage of EDS systems is that they cannot separate closely spaced emission lines, which may overlap on the X-ray EDS spectrum.
