Electron Spectroscopy for Chemical Analysis (ESCA) or

X-Ray Photoelectron Spectroscopy (XPS)

Electron Spectroscopy for Chemical Analysis (ESCA) or X-ray Photoelectron Spectroscopy (XPS) is a failure analysis technique primarily used in the identification of compounds on the surface of a sample. 

It utilizes X-Rays with low energy (typically 1-2 keV) to knock off photoelectrons from atoms of the sample through the photoelectric effect.  The energy content of these ejected electrons are then analyzed by a spectrometer to identify the elements where they came from. 

The incident X-Rays used in knocking off the electrons must possess energy that is both monochromatic and of accurately known magnitude.  The X-ray source material must also be a light element since X-ray line widths, which must be as narrow as possible in ESCA, are proportional to the atomic number of the source material. It is for these reasons that commercial XPS systems typically use the K-alpha X-rays of aluminum (Al K-alpha E = 1.487 keV) and magnesium (Mg K-alpha E = 1.254 keV).

Although the X-rays penetrate deep into the sample, only the electrons on the surface of the sample are able to escape without significant loss of energy for analysis. As such, ESCA, just like AES, is basically a surface analysis technique.

ESCA, in fact, is similar to AES in many other ways. The ejected electrons are detected in ESCA in the same manner as in AES, i.e., using a cylindrical mirror analyzer (CMA) detector.  ESCA scans also provide more or less the same information as AES.  They also have a common weakness - their inability to detect hydrogen (H).  ESCA's spatial resolution, however, is poor compared to AES, because X-ray beam diameter is more difficult to make smaller (limit is about 150 microns) than that of electron beams.

Figure 1. Example of an ESCA Equipment from Thermo Electron Corp.

ESCA, however, does offer some advantages over AES, which is why ESCA is a good complementary technique to AES.  For instance, the electron bombardment given by AES is destructive to some materials, but these same materials are left undisturbed by ESCA's X-Ray bombardment.  Electron bombardment also tends to 'charge up' insulative specimens, preventing good analysis.  Charging up is never a problem for ESCA's 'neutral' X-rays. 

ESCA's resolving power for energy (typically at 0.5 eV) is also better than that of AES. Because of ESCA's good energy resolution, it can detect shifts in the binding energy of atoms in a molecular structure with different chemical bonds. This enables ESCA to provide information not only on elemental composition, but on chemical bonding as well.

In the semiconductor industry, ESCA serves as a useful surface analysis tool for studying organics, polymers, and oxides.  It also played a major role in the development of plasma etching techniques.  ESCA is also a good FA technique for resolving issues related to oxidation, metal interdiffusion, and resin-to-metal adhesion. 

See Also:  Failure Analysis;  All FA Techniques;  EDX/WDX Analysis;  SIMS/LIMS; 

Auger Analysis;  Chromatography;  FA Lab Equipment;  Basic FA Flows; 

Package Failures;  Die Failures