Surface analysis by XPS involves irradiation of the sample in
vacuum with mono energetic soft x-rays and sorting the emitted
electrons by energy. The spectrum obtained is a plot of the number of
emitted electrons per energy interval versus their binding energy.
Each element has a unique elemental spectrum, and the spectral peaks
from a mixture are approximately the sum of the elemental peaks from
the individual constituents. Since the mean free path of the electrons
is very small, the electrons, which are detected, originate from only
the top few atomic layers. The depth of the solid material sampled
varies from the top 2 atomic layers to 15-20 layers. Quantitative data
can be obtained from the peak heights or areas and identification of
chemical states often can be made from the exact positions and
separations of the peaks, as well as from certain spectral contours.
The samples can be gaseous, liquid, or solid but a majority of
electron spectrometers are designed to deal with solids. This method
is the least destructive of all the electron or ion spectroscopy
techniques and can be applied to organic, polymeric, inorganic,
organometallic and biological materials through metals, ceramics and
semiconductors. There is always a possibility that other surface
techniques can be combined with XPS to satisfy the individual
requirements. The sample can also be sputtered and different layers
can be analyzed. If the depth of interest is beyond the capability of
sputtering, then the sample can be polished down or sectioned or
etched and then analyzed.
The electrons leaving the sample are detected by an electron
spectrometer according to their kinetic energy. The analyser normally
is operated as an 'energy window', accepting only those electrons
having an energy within the range of this fixed window, referred to as
the pass energy. Electrons are detected as discrete events and the
number of electrons for a given detection time and energy is stored
digitally or recorded using analog circuitry.
Figure : XPS AXIS ULTRA from KRATOS ANALYTICAL Inc.
Range of Elements: All except Hydrogen and Helium
Destructive: No, some beam damage to X-ray sensitive materials
Elemental Analysis: Yes, semiquantitative without standards; quantitative with standards. Not a trace element method
Chemical State Information: Yes
Depth Probed: 5-50
Depth Profiling: Yes, over the top 50Ao ; greater depths require sputter profiling
Depth Resolution: A few to several tens of , depending on conditions
Lateral Resolution: 5mm-75m; down to 5m in special instruments.
Sample Requirements: All vacuum-compatible materials; flat samples best; size accepted depends on particular instrument
Main Uses: Determinations of elemental and chemical state compositions in the top 30
