Biomedical Engineering Reference
In-Depth Information
characteristic direction. Sometimes these surfaces are said to have a nondirectional,
particulate, or protuberant lay. Lay is important for optical properties of a surface.
A number of techniques are available to assess the surface roughness. A popu-
lar method is atomic force microscopy (AFM) which uses a small probe attached
at the end of a cantilever to probe surfaces. AFM can be operated either in contact
mode or in noncontact mode. In contact mode, also known as repulsive mode, an
AFM tip makes soft “physical contact” with the sample. The tip is attached to
the end of a cantilever with a low spring constant, lower than the effective spring
constant holding the atoms of the sample together. As the scanner gently traces the
tip across the sample (or the sample under the tip), the contact force causes the can-
tilever to bend to accommodate changes in topography. In constant-height mode,
the spatial variation of the cantilever deflection can be used directly to generate the
topographic data set because the height of the scanner is fixed as it scans.
Noncontact AFM is one of several vibrating cantilever techniques in which
an AFM cantilever is vibrated near the surface of a sample. The spacing between
the tip and the sample for NC-AFM is on the order of tens to hundreds of ang-
stroms. NC-AFM is desirable because it provides a means for measuring sample
topography with little or no contact between the tip and the sample. Like contact
AFM, noncontact AFM can be used to measure the topography of insulators and
semiconductors as well as electrical conductors. The total force between the tip
and the sample in the noncontact regime is very low, generally about 10-12N. This
low force is advantageous for studying soft or elastic samples. A further advantage
is that samples like silicon wafers are not contaminated through contact with the
tip. AFM with receptor molecule tips are also available. Apart from the contact
angle measurements and atomic force microscopy techniques described above, a
number of techniques have been developed to assess the surface characteristics of
a material.
Scanning electron microscopy (SEM) uses a focused beam of high-energy elec-
trons to scan the surface of a material. The beam interacts with the material, which
generates a variety of signals (such as secondary electrons, backscattered electrons,
X-rays) each of which can be used to characterize the material. It is typically used
in analyzing the surface characteristics of materials. Transmission electron micros-
copy (TEM) uses a highly focused beam, which bombards a thin sample to generate
transmittable electrons. The transmitted electron signal is magnified by a series of
electromagnetic lenses and observed through electron diffraction or direct electron
imaging. Using electron diffraction patterns, one can determine the crystallograph-
ic structure of the material. It is typically used in structural and compositional
analyses and high-resolution imaging. Scanning tunneling microscopy (STM) uses
piezoelectric translators that can bring sharp metallic crystalline tips with several
angstroms of the surface. The change in the tunneling current established by a bias
voltage between the tip and surface permits the density and energy maps of elec-
tron states. It is typically used to obtain atomic and molecular resolution images of
conductive and semiconductive materials.
Conoscopy is a novel polarization interferometric technique, which provides
depth information using quasi-monochromatic and spacially incoherent light. It is
based on the interference behavior of doubly refractive crystals under convergent,
polarized light. Since it eliminates the sensitivity and stability problems associated
