Specific Process Knowledge/Characterization/Element analysis

X-rays, with an energy that is characteristic of the atom they are emitted from, are created when the high energy electrons impinge on the sample.

You can make detailed analysis on the elemental composition and distribution in a sample with 3 instruments at Danchip. The Leo SEM and FEI SEM are both equipped with an X-ray detector that allows you to make elemental analysis by using the technique Energy Dispersive X-ray analysis or EDX. The Atomika SIMS uses a technique called Secondary Ion Mass Spectrometry or SIMS.

Energy Dispersive X-ray analysis

The technique of extracting information from the X-rays generated in a sample that is irradiated with electrons is called energy dispersive X-ray analysis or EDX. (Other acronyms are Energy Dispersive x-ray Spectroscopy, EDS, or Electron Probe Microanalysis, EPMA). The energetic electrons in the incident beam create core level vacancies as they collide with sample atoms electrons in a multiple scattering process. This leaves the atoms in the sample in an excited state. In the process of decaying from this state photons may be emitted. The energy of these photons is determined by the difference in energy of the shells involved. Since atomic shells are unique for every element so will be the transitions between them. Thus, every element has its own characteristic X-ray spectrum that can be used to determine the elemental composition.

Adding an EDX detector to a SEM provides a very powerful tool for elemental analysis. The capability of the SEM to precisely maneuver the electron beam in a number of ways enables us to make point-like analysis with nanometer precision.

Secondary Ion Mass Spectrometry

Atoms that used to make up the surface are sputtered off by the high energy ions; some are emitted as secondary ions.

When a solid sample is sputtered by primary ions of few keV energy, a fraction of the particles emitted from the target is ionized. Secondary Ion Mass Spectrometry consists of analyzing these secondary ions with a mass spectrometer. Secondary ion emission by a solid surface under ion bombardment supplies information about the elemental, isotopic and molecular composition of its uppermost atomic layers.

	Energy Dispersive X-ray Analysis	Secondary Ion Mass Spectroscopy
Technique	Non destructive excitation of X-rays in the sample. The elemental analysis is possible because the energy of these photons is characteristic of the element they emitted from.	Destructive method that sputters off surface atoms with heavy ions.
What elements are detected	Any element heavier than boron/carbon	Any element
Chemical information	None: Only transistions involving inner shell electrons are detected	None
Sample limitations	Vacuum compatible	Vacuum compatible
Spatial resolution	Very precise point-like analysis is possible with SEM electron beam.	Limited to what is visible in a camera
Depth resolution	The size interaction volume depends on the SEM high voltage and sample density: The higher the SEM high voltage the bigger and deeper the interaction volume. The more dense the material is the smaller is the interaction volume	Depth profiling is
Detection limit	Approximately 1 % atomic	Down to 1 ppb for many elements

SIMS is the most sensitive elemental and isotopic surface analysis technique. The secondary ion yields will vary greatly according to the chemical environment and the sputtering conditions (ion, energy, angle). This can add complexity to the quantitative aspect of the technique.

(detection limit down to ppb level for many elements), high lateral resolution imaging (down to 40 nm), and a very low background that allows high dynamic range (more than 5 decades). This technique is "destructive" by its nature (sputtering of material). It can be applied to any type of material (insulators, semiconductors, metals) that can stay under vacuum.

It allows molecular as well as elemental characterization of the first top monolayer in the static SIMS mode. It allows also the investigation of bulk composition or depth distribution of trace elements in the dynamic SIMS mode, with a depth resolution ranging from one to 20-30 nanometers.

This is why SIMS is one of the most widespread surface analysis techniques for advanced material research.