Analysis

• SEM & EDA
• XPS
• Other Analysis Techniques Available

Analysis: SEM & EDA

Scanning electron microscopy (SEM) is a technique for imaging the surfaces of samples which when coupled with energy dispersive analysis (EDA) is a very powerful tool for imaging and elemental analysis.

An SEM consists of a filament from which electrons are emitted, an applied accelerating voltage of typically up to 30kV to target the sample. A series of electron lens focuses and rasters the beam across the sample surface from which an image can be viewed. Magnifications from 20x to 50000x are possible using this technique.

Two sorts of electrons are produced from the interaction of the electron beam with the sample surface, secondary electrons and backscatter electrons. Secondary electrons relate to topographic features. The backscatter image derived shows the contrasts in chemical composition.

A further interaction that takes place between the primary electron beam and the sample is that electrons are knocked out of orbits within the atom. Vacancies are filled by electrons from higher energy outer electron shells with a consequent loss of energy. The energy photon is emitted as an X-ray. The energy (or wavelength) of the X-ray is characteristic of both the element and the energy level transition and provides a direct means of elemental identification at the point of electron beam interaction.

EDA uses a solid-state cooled detector system to detect and measure individual X-ray pulses from the point of interaction. A single spectrum will detect elements from carbon upwards in the periodic table.

XPS

X-ray Photoelectron Spectroscopy XPS, is also known as ESCA (Electron Spectroscopy for Chemical Analysis).

XPS provides quantitative compositional information from the top 10 atomic layers of a sample surface for the elements lithium to uranium. Furthermore, information regarding the chemical states of any elements present can also be obtained.

The sample is irradiated with a beam of monochromatic soft X-rays. Photoelectron emission results from the atoms in the specimen. The kinetic energies of these electrons relates to the atom and orbital from which they originated. The distribution of kinetic energies from a sample is then measured directly by the electron spectrometer.

Atomic orbitals from atoms of the same element in different chemical environments are found to possess slightly different (but measurable) binding energies. These "chemical shifts" arise because of the variations in electrostatic screening experienced by core electrons as the valence and conduction electrons are drawn towards or away from the specific atom. Differences in oxidation state, molecular environment and co-ordination number all provide different chemical shifts.

Photoelectron binding energy shifts are, therefore, the principal source of chemical information. It should be noted that these shifts can be very small and can only be detected using a high performance instrument with suitable software.

By varying the sample orientation with respect to the analyser, the variation of surface chemistry with depths in the range 2-10nm can be probed non-destructively.

A locally destructive technique which measures changes in sample composition as a function of depth (up to 1μm below the surface) by removing surface material with a focused ion beam.

Other Analysis Techniques Available

• SIMS - Secondary Ion Mass Spectrometry
• 3DP - 3D Non Contact Surface Profiling
• TEM - Transmission Electron Microscopy
• EELS - Electron Energy Loss Spectroscopy
• RBS - Rutherford Backscrattering
• FT-IR - Fourier Transform Infra-red Spectroscopy
• GDMS - Glow Discharge Mass Spectrometry
• IGA - Interstital Gas Analysis
• VPD - Vapour Phase Decomposition