Surface measurements of films, such as stent films can be compared
to surface measurements of the underlying substrate. If the substrate
surface shape is subtracted from the height of the film surface, then
the resulting data is equal to the thickness of the film, without the
"noise" of the local fluctuations in the two surfaces.
Optical profiling using interference microscopy produces a high-precision
three dimensional measurement
of the surface character
of many materials. Vertical precision may be as precise as fractions
of a nanometer (i.e., in angstroms). Using a wavelength of light as
the standard or measurement, surface peaks, valleys and slopes are accurately
and repeatably measured using NIST-traceable processes - and without
contacting surfaces.
The resulting plot contains reliable two dimensional cross-sections
in any direction across the surface, and can be used to produce precise
calculations of volume,
including voids, wear, pits, grooves
and failure marks, as well as solid
displacement, relative flatness, average (relative) height variations,
frequency and placement of peaks, size and shape of plateaus, tendency
of a texture to depart from a mean (standard deviation), the presence
and shape of surface flaws, the accuracy or precision of manufacturing,
and so forth.
Surface characterization with a 3D optical profiler has been beneficially
used in determining and maintaining quality control in film thickness
of stents and other biomedical
devices , because of the non-contact nature of the test. For example,
in film coating processes, successive measurements can be made of the
exact same location, to determine the cumulative effect or effectiveness
of the coating process. Researchers and scientists have discovered the
value of these recursive measurements in determining the efficacy of
machining, coating, etching
and chemical planarizing processes, as well as optimizing polish, cutting
or time and materials.
In contrast, a stylus may alter the structure of a soft or brittle
surface by rounding peaks. This can prohibit repeat measurement of a
specific location, as the subsequent measurement would measure an altered
surface structure.
