Surface analysis is the use of microscopic chemical and physical probes
that give information about the surface region of a sample. (The term sample
refers to any piece of material, structure, device, or substance that is
under study). The probed region may be the extreme top layer of atoms (the
only true surface, for purists), or it may extend up to several microns
(millionths of a meter) beneath the sample surface, depending on the technique
used. The analysis is done to provide information about such characteristics
as the chemical composition, the level of trace impurities, or the physical
structure or appearance of the sampled region. Such information is of importance
to researchers or manufacturers who must understand the materials in order
to verify a theory or make a better product.
Many of the techniques used to probe surfaces use a beam of ions (SIMS) to strike the surface and knock off atoms
of the sample material. These atoms are ionized and are identified and measured
using a technique known as mass spectrometry. Other probes strike the surface
with electrons (Auger spectrometry, EDS) or X-rays (ESCA,
TXRF) and measure the resulting electron
or photon emissions to probe the sample. Measurements of the way high-energy
helium nuclei bounce off a sample can be used as a sensitive measure of
surface layer composition and thickness (RBS).
Surface structure on a microscopic scale is observed by using electron microscopes
(SEM), optical microscopes, and atomic force
or scanning probe microscopes (AFM/SPM).
Even when one is not solely interested in the surface, sometimes a technique
that happens to probe only the surface can provide better information than
other techniques that probe the entire the sample. A surface analysis technique
is then often used when it is reasonable to assume that a surface measurement
is representative of the rest of the sample. In the case of tiny particles,
such as the flyash from a waste incinerator or a piece of cosmic dust, the
surface region makes up the entire sample.
Sometimes the surface structure is of primary interest, such as the oxide
film on a piece of treated metal from an airplane wing, or the intricate
structures and carefully controlled layers of a semiconductor wafer used
to make integrated circuits. Or it might be that one needs to know about
contaminants on top of a sample surface, such as trace organic residue or
metal impurities on the surface of a silicon wafer ready for processing
in a semiconductor device fabrication facility.