X-ray Photoelectron Spectroscopy (XPS)
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. Our experiments were performed using
AXIS ULTRA from KRATOS ANALYTICAL Inc..
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 Ċ
Depth Profiling: Yes, over the top 50Ao ; greater depths require sputter profiling
Depth Resolution: A few to several tens of Ċ, depending on conditions
Lateral Resolution: 5mm-75µm; down to 5µm 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 30Ċ
Instrument Cost: $600,000
Size: 10ft ´ 12ft
