Electron Spectroscopy for Chemical Analysis (ESCA), also referred to as X-ray Photoelectron Spectroscopy (XPS), irradiates the sample surface with a soft (low energy) X-ray. This X-ray excites the electrons of the sample atoms and if their binding energy is lower than the X-ray energy, they will be emitted from the parent atom as a photoelectron. Only the photoelectrons at the extreme outer surface (10-100 Angstroms) can escape the sample surface, making this a surface analysis technique.

To produce the low energy X-ray, a 10 keV electron gun (see How an SEM Works) is aimed at an aluminum target. As is explained in EDX analysis, an X-ray is formed. This X-ray, an aluminum K alpha, has an energy of 1486 electron volts (eV). The target is aimed at a quartz crystal which, due to the Bragg's angle, monochromatizes the X-ray (preventing any stray X-rays other than 1486 eV), and focuses it into a spot.

The quartz crystal focuses the X-ray beam into a small spot on the sample surface. The spot size on SEAL Laboratories' ESCA can be varied from 150 to 1000 microns in diameter.

When the X-ray hits the sample surface it excites the sample electrons. If the sample electrons have a binding energy less than the X-ray energy (1486 eV), they will jump out of the sample atom.

Most of these electrons are reabsorbed by the sample, but the electrons emitted from atoms near the sample surface (10-100 Angstroms) can escape the sample surface. This depth is approximately twenty atomic layers.

On a larger scale, the sample surface is hit with a 1486 eV X-ray, focused in a small spot. This X-ray emits sample electrons from approximately the first 20 atomic layers of the sample

1                                                              NEXT